Saturday, April 29, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.10


How to Build a 11.5 MHz Crystal Filter

Method #1:

Simply Purchase four 11.5 MHz Crystals at the cheapest price you can. Next build a Ladder Filter using five coupling caps of the same value. For a SSB Filter use 68PF and for a CW Filter use 470 PF. A guess at the in/out impedance would be in the neighborhood of 150 to 200 Ohms. Use 200 Ohms, as a 50:200 match is just a 4:1 transformer.
 
With this method you rely mostly on luck. It will probably not work too well. However if it does then you should immediately go out a buy a single lottery ticket as you are on a path to striking it rich.
 
Here are the shortcomings of Method #1. With only four crystals and making no measurement of their actual frequency you will never know: 1) how well matched they are in frequency 2) what is the filter center frequency and 3) the actual Zin/out. Did I also mention that if you don't test the crystals in an oscillator circuit prior to just installing them in a filter you may not know that one or several are inoperative (you did buy bargain crystals). But many "good enough" filters have been made this way. That said very likely there are substantially more poor filters than good ones that were built using this approach. But hey you built a crystal filter.
 
 

Method #2:

You purchase four crystals at the cheapest possible price and you make a measurement of the crystal frequencies using the G3URR test oscillator. You dutifully note the "loaded" frequency and the spread of each crystal as related to all of the crystals. (A goal is no more than about a 50 Hz spread across all of the crystals). After obtaining this data you simply ignore the information and follow the Method #1 approach. Again if it is perfect, then buy more lottery tickets. But more than likely it will not be. Oh by the way --you will probably need the center frequency info in your Arduino Sketch so you know how much to shift the USB LSB BFO frequencies -- but hey close is close enough. So you get a few dings from the SDR police on 40 Meters --who cares?
 

Method #3:

This is where you find some ham who really knows what they are doing and after an enticement of the standard B^3 (Booze, Bucks and Babes) have them build you the filter. Just sit back and relax and wait for the unit to arrive. This is a lot less stressful and all you need to do is install it in your rig. Now wasn't that easy? Never let the XYL find out you spent $250 for a crystal filter is the real issue.
 

Method #4:

This is where you get serious about homebrewing a crystal filter. The process involves the following:
  • Collecting information on how to actually build a crystal filter. There are several really good sources. First do an Internet Search on Nick Kennedy WA5BDU, as he has prepared an exhaustive tutorial on the steps needed. Also search on Almost All Digital Electronics as they have a computer program that is very handy to design a filter. I also believe that EMRFD has a program on the DVD that is located in the back jacket. Do not overlook You Tube Videos on how it is done. Bottom line you need a disciplined process and resource information.
  • You will have to build some test hardware including the G3URR test oscillator. Basically this oscillator enables you to measure the frequency of a crystal and then by loading that crystal with a small capacitance shifts the crystal frequency. That amount of shift is an important parameter in the final calculations. (It has something to do with pole zero spacing) This is where you need to buy one of those $13 TV SDR Dongles! Get one and modify it so it will work on HF. You should also download the free software program HDSDR. This $13 device will let you precisely measure the crystal frequencies with and without the load. Almost better than a frequency counter. All you do is power up the oscillator and with a short "antenna lead" bring the oscillator near the Dongle and look for the output.
  • Purchase twenty five 11.5 MHz crystal (about $0.30 each at this quantity) from Mouser. When they arrive use a Brother tape label machine on the smallest print size to label every crystal from 1 through 25. Open up a Excel Spreadsheet on your computer and record the loaded and unloaded frequencies for the crystals marked 1 - 25. The first thing that should amaze you is that the crystals while nominally 11.5 MHz are all over the map. Now you can either do it by visual inspection or have Excel do it but rank order the crystals from high to low frequency. Once you do that you should now look for groupings of crystals that are within 50 Hz total spread from low to high. You might get lucky and find five or six that meet this criteria but you need a minimum of four. You will most likely find out of the batch of 25 that you will have several groupings of at least four crystals that are close in frequency. Once you have at least four then you need to follow the process outlined by Kennedy. Look to the manufacturers specifications as you might be lucky to find the "Average" series resistance as this number is needed for the calculation.
  • Most likely the final filter (for SSB) will have low value coupling capacitors (around 100PF) and the Zin/out would be in the 200 Ohm range. But unlike Method #1, you are being precise in the measuring process and WILL have four crystals that are close in frequency.
  • Method #4 is not a 1 or 2 hour process -- it might take several sessions to complete the data analysis and calculations before you start soldering crystal to a circuit board. Be sure to connect all of the cans together and ground that connection. Build the filter over a large ground plane area.
  • At this point Method #3 has a lot of appeal.
Have fun.
 
73's
Pete N6QW



Friday, April 28, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.09

 

Dual Conversion Band Switching


In this posting I want to talk about some of the specifics of the band switching and how to cause the proper Band Pass Filter and Low Pass Filters to be put in line for the band in use.

The Arduino Mega 2560

The first realization I had with the dual conversion multi-band approach was that you needed a lot more pins. There will be those who immediately jump up and say but you can add a pin expander to the standard UNO, Nano and/or Pro-Mini and no need to move up to the larger footprint and more costly microcontroller. There is another requirement that is answered by the Mega 2560 and that is the 10X increase in program size. That perhaps is the bigger driver for the Mega 2560. Undoubtedly there will be more things you want to add to your homebrew rig and then pins is not the issue but programming space will be. The Mega 2560 has 54 digital pins and 16 Analog pins and thus you have many more options available to you. Now as I discovered there are differences from the  Uno, Nano and Pro-Mini so you will need to think about pin assignments and it is not a straight pin for pin compatibility. I will highlight those differences.
 
 

The Band Switching Scheme

My band switch scheme involves two band switches and has the capability for 17 band positions. One band switch has 12 positions and the second band switch has 6 position. So OK you have a blank look on your face. Hey guys Ten Tec did this with some of their transceivers, where you placed the main band switch on 10 Meters and the auxiliary band switch then selected four sub bands within the 10 Meter band.
 
For my Dual Conversion DifX when the main band switch is placed on 60 Meters  that merely connects to the auxiliary band switch where you can select the five channels on the 60M band including one channel that will be tunable as I did in my 60M DifX rig. Moving away from the 60M position in effect disconnects the second band switch.  The process of selecting the operating band with the band switch will automatically trigger a comparable Mega 2560 Pin that has 5 Volts on that Pin to control either relays or Pin Diodes in the Low Pass and Band Pass filters. One switch position changes frequency and selects the proper filters.
 

Mega 2560 Pin Assignments

 
 
 
 
 
The logic of the code is that the digital pins are read, as an example Pin 30, which is designated for 20 Meters when that pin is sensed LOW, then Pin 39 will go HIGH and that switches in the proper Band Pass Filter and Low Pass Filters. On 60 Meters Pin 49 is made HIGH for all of the 60 Meter Pins.
 
Pin A10 and A11 on the Mega 2560 are used for Encoder A and Encoder B. (Thanks Rob!)
 
Keep good notes as we are moving forward with the design.
 
73's
Pete N6QW



Wednesday, April 26, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture Part 2.08

The "Math" of the Frequency Display!


Don't you just hate it when you see information without detail and much like the guy at 1600 Pennsylvania Avenue you hear 'Folks You Will Love This". But it still leaves a lingering question --just how did we get here and how do you insure this is not Fake News!
 
In our last posting we advanced the idea that with dual conversion this presents some unique problems because of the frequencies involved and how to accurately display the true transmitted frequency. Essentially we have about five frequencies we must deal with in our display process --and that is just for SSB. By way of recap we have the incoming frequency, then the 1st mixer which is the tunable Local Oscillator (LO) followed by the fixed frequency 2nd mixer and then the two BFO frequencies that address either USB or LSB.
 
Mind you this is Pete's scheme -- there are obviously others and better ones. BUT I understand this and I can be assured that I will not be transmitting out of band. In the final analysis there are many roads to San Diego --the objective is to arrive in San Diego.
 
In my scheme when you shift from USB to LSB the dial will change by 3 KHz without moving the encoder. In going from USB to LSB you will have to move the LO down by 3 KHz to transmit on the same frequency using the opposite sideband. Inconvenient for some -- well not for me. So if this is objectionable to you, then stop here and go write your own code!
 
There are several actions taking place in the background as you change bands and change from USB to LSB which I will now detail. [Because we are using two conversions and the conversion frequency is above the incoming -- the net effect: There is no sideband inversion. Thus the lower BFO frequency will be used for USB and the higher frequency BFO for LSB.]
 
  • Let us say you will start by tuning in 14.2 MHz on 20 Meter Upper Sideband --a favorite spot of mine. As you put the mode switch into the USB mode and turn the encoder to 14.2 there are some behind the scenes steps taking place. The first is that 1500 Hz will be subtracted from the display formula and secondly the LO is preloaded with a frequency that will show 14.2 MHz on the display. But the display is actually the subtraction of 45 MHz and the subtraction of 1500 Hz from the start up frequency. The 1500 Hz is the nominal offset from the center frequency of the SSB crystal filter. [Typically it is +/- 1500 Hz depending on the sideband. ] For LSB on 14.2 MHz we will be adding 1500 Hz.
  • The LO uses a subtractive mix process so the LO - the incoming signal = 45 MHz. Our pre-loaded LO frequency already contained a 1500 Hz add. So our subtractive mix actually resulted in a frequency of 45001500 MHz plus the Voice signal. Since the Bandwidth of the ESC 45 MHz Filter is 7.5 KHz --this is not an issue.
  • The second mixer is at a fixed frequency of 56.5 MHz and the second down mix results in an output of 11.498500 MHz + voice.
  • Feeding this signal into the Product Detector with the USB BFO of 11.498500 leaves only the audio voice signal which is upper sideband.
  • For a LSB signal the preloaded LO will have to be tuned down by 3 KHz to put you on 14.2 MHz LSB. So now the LO is at 59198500 MHz and when we subtract 45000000 we must add in 1500 Hz to make the display read 14.2 MHz. So putting the USB/LSB switch into LSB causes the addition of 1500 Hz to the formula so that the display will read the true transmit frequency.
  • Thus the first mix has the LO at 59198500 - 14200000 + Voice = 44998500 + Voice and this mixed with 56.5 MHz 2nd Mixer signal results in a frequency of 11501500 + Voice.
  • Sending that signal on to the product detector with a LSB BFO of 11501500 results in LSB Voice.
 
 
 
Thus you have the decode on how to use the dual conversion scheme yet have the frequency display read properly for the true transmitted frequency. CW can be done the same way --but involves a lot more code and since I am not a CW person have not done any more with it. But shifting one of the BFO frequencies for CW transmit will get you there as was done in the KWM-4 and use USB for receive.
 
Time to start building your DifX. In a couple of days I will post a link to the Arduino Mega 2560 code and that will be on my website at http://www.n6qw.com
 
73's
Pete N6QW

Sunday, April 23, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.07

So How to Get the Display To Read Right!

4-26-2017 Update

Thanks to Addi dc0dw , here is a link that explains about interrupts. This actually makes sense. LINK

There is a caution here in that the 328 (internal structure and wiring) is different from the 2560 and that is why I was having problems. But a really good treatise on interrupts.

73's
Pete N6QW

4-25-2017 Update

Get Your Heart Racing! --Here are some screen shots of the DifX Dual Conversion display. When you shift from USB to LSB the Display changes by 3 kHz.






The current processor is an Uno or Nano but because of the pin limitations I can only support 7 bands. I have loaded the code on a Mega 2560 and am able to have 15 bands.

But I am asking for some help with the Mega 2560. While I can get the bands to change and USB/LSB to change and the TUNE function and S Meter to work. The Encoder is Dead In The Water. Evidently there is a different code set to make the Mega recognize the Interrupt on Pins 2 & 3. There is some goofy function called attachInterrupt --well I tried the sample code and it just stares at me or nothing happens. So if any kind soul knows how to get the interrupt function to work with digital Pins 2 & 3 so that I can tune the encoder --please let me know.
 
73's
Pete N6QW

I have very much admired Ten Tec and the products they built. Sadly I am not so sure about the future of that company given the number of recent owners and the state of the technology coming from offshore. I have a Ten Tec Triton Model 544 and always thought it one of their best rigs. Some will argue with me about later rigs and how much better crystal mixing was/is over a synthesized rig. But the fact remains they did build some great gear.
 
One anomaly I found in some Ten Tec designs (I think was the Corsair series), where the display would read very properly on the normal emissions for the ham bands id est [that is Latin for i. e.] LSB on 40 Meters but if you switched to Opposite (Ten Tec didn't use LSB or USB but Normal or Opposite), the display would not show the correct frequency. In the Ten Tec documentation there was a small obscure note to this effect.
 
In my DifX designs I have paid particular attention to this issue in my Arduino Code. I always place the LO above the incoming frequency and there is math in the code that for the display what is read is the LO - BFO. So If I switch from LSB to USB on 40 Meters (say 7.2 MHz), and since there is a sideband inversion taking place, the higher BFO frequency is used for USB. Thus the display while reading the true transmit frequency will now display a lower frequency on the LCD. When you shift back to LSB, the lower BFO frequency is used and so now the LCD will read higher. So I do have to move the encoder to 7.2 MHz when shifting from LSB to USB but the true transmit frequency is always displayed.
 
Some would like it so that in switching from USB to LSB you would not have to move the dial --and that is just more code. So I  move the dial and not add the code. But what ever the display and whatever the mode when I read the LCD --that is the true transmit frequency.
 
I addressed this problem in the KWM-4 by using an LO that changed based on the mode. The 1st conversion frequency was 10.7 MHz. Because I simply shifted channels for USB/LSB such that the difference of the LO and incoming would not be 10.7 MHz but either slightly higher or slightly lower than 10.7 MHz and this would easily pass through the roofing filter. The slight difference was then the nominal +/- 1.5 kHz. So then the BFO selected for the Mode would make up that difference. Thus 7.2 MHz for either USB or LSB would read correctly on the LCD and that was the transmitted frequency.
 
In Dual Conversion designs the true reading of the display becomes more complex because of the multiple conversions and the Dual Conversion DifX is no exception. Lets us look at what might be required to get the display to read correctly. We will use 40 Meters as the example.
 
The 1st IF is at 45 MHz and thus for 7.2 MHz the LO injection would be 52.2 MHz --thus all incoming signals are subtracted from the LO and converted to 45 MHz and in the case of the 2nd mixer which is operating at a fixed frequency of 56.5 MHz the second conversion is to 11.5 Mhz. There is no sideband inversion (we did it twice) and thus to receive LSB our BFO must operate at 11.5015 MHz. So now the problem of how to get the display to read 7.2 MHz LSB. Let us not forget if we shift to USB the BFO is operating at 11.49985 MHz and again how do we insure that the LCD and transmitted frequency is in fact 7.2 MHz USB.

In earlier display designs (1970/80's) there were summing circuits that read the HFO (High Frequency Oscillator), BFO and VFO and the results were then displayed. The DFD-2 (from AADE) did exactly that process. We will do something similar in the DifX. My first encounter with this issue was in 2009 when I built the Tri-Band SSB transceiver that used the frequency scheme AND components from an HW-100. You can read about it here http://www.jessystems.com/2009_xcvr.html

In the description I speak about the summing circuits so that you combine the HFO, BFO and VFO so that the display reads correctly. The innovation from N6QW was to mix two of the components in an SBL-1 and then to use  tuned band pass filters so only the correct mix was used and then that was finally mixed with the VFO signal. Using an EI9GQ huff and puff stabilizer it was possible to shift the display depending on the sideband and thus what was displayed was the true transmit frequency. It was a lot more hardware but 8 years ago I was on frequency!
 
The lazy approach would be to simply display the LO - 45 MHz [Display = LO - 45 MHz] and that would get you close (within KHz) and so in shifting from LSB to USB the display would not change with the mode. But in either case you would be off the true transmit frequency--the 40 Meter SDR Police will have a field day with you -- keep in mind they get gnarly when you are 20 Hz off --imagine being off by kHz. So that is not a good solution -- but a start.
 
Now what if there were one more factor in the math that changed depending on the mode. In case you got lost in the last paragraph we are up-converting the incoming signal to 45 MHz and thus to get the actual signal value we subtract 45 MHz. So now if we rewrite the equation to say the following to account for the  total of 3 KHz in the shift in the spread then we would have Display = LO - 45000000 + Offset. Now we can sign the offset as +/- depending on the mode. So if we were on LSB the Offset would be one value and for USB the Offset would be another. The display would now change by a total of 3 KHz depending on the mode. The second conversion frequencies can be simply ignored as all that is doing is getting the signal to the IF frequency. This is not unlike our single conversion code. Yes you will have to reset the dial when shifting USB/LSB but you will be displaying the correct transmit frequency.

In the next installment I will detail how I did it and the math involved. Maybe if I am lucky I will have code I can share. But the bottom line is that when shifting mode (USB/LSB) the display must account for the BFO shift and display the true transmit frequency. In the old analog VFO days many manufacturers fudged this problem by putting two (yes two) scribe lines on the dial window. One you used for USB and the other for LSB. Amazing solution --like using a rusty spoon to do brain surgery.

73's
Pete N6QW
 
 

Tuesday, April 11, 2017

A New Line of Transceivers - Difx (On Steroids)

4-13-2017 ~ A small Radio story.

Last evening (4/12) my XYL and I went to our favorite Chinese restaurant. Great food and really reasonable prices. As we walked in the door we saw a large party of about 25 people who were celebrating a birthday. I noted an older gentleman seated at the head of the table and our booth must have been only about 5 feet away. I leaned over to another gentleman sitting closely to me and inquired if it was a birthday celebration. He responded back that it was his father-in-law who turned 97 that very day.
 
I wished the celebrant a Happy Birthday and then commented to the other gentleman that his father-in-law experienced the evolution of radio broadcasting, the stock market crash, the end of prohibition, the New Deal, completion of Hoover Dam and the SF Bay Bridge, WWII, TV, space travel and the computer revolution. With that there was a bit of a buzz at the table as the other well wishers suddenly grasped what this man had seen.
 
Then the Birthday Boy spoke up and said I built my first crystal set when I was age 9! Boom that was amazing. I then asked if he wound the coil on an Quaker Oats box and used a piece of aluminum as a slider tuner -- and of course finding the hot spot on the galena crystal. He then said yes and asked how I knew that. I explained that about 18 years after him I built my 1st crystal set. What an experience. Just think anyone in their 90's has seen a dramatic change in our world. Of the 16M who served in WWII only 600,000 are still around. Not many left.
 
73's
Pete N6QW

 

DifX A Dual Conversion Transceiver!


In 2017, VU2ESE announced his uBitx (Micro-Bitx) transceiver which uses an up-conversion technique to a 45 MHz 1st IF and then a lower 12 MHz second IF, which handles the normal transceiver functions. This is a well known Gain and Selectivity approach to minimize "birdies" and to better process a lot of crud showing up on our beloved ham bands.

In theory the higher IF provides the gain and the lower IF the selectivity. But there may be more subtleties with actually having the higher IF to also provide a degree of selectivity much like a "roofing filter". So the first IF must be designed as such as to have  gain and bandwidth parameters in keeping with up conversion and crud prevention.

The magic key in a successful transceiver design is a reasonable gain distribution with attendant selectivity over the entire rig topology. Super high gain front end amplifiers that overload a receiver can and should be eliminated and the gain made up in later stages. When you hop up the signal gain at the front end --you are doing the same to the noise coming into the receiver chain! So what have you gained (that was a pun)?

A dual conversion has other desirable attributes such as the second filter selection. My KWM-4 has a 455 KHz "primo" Collins  mechanical filter embedded as the second filter; but the choice is mostly open to the many of the popular, 8, 9 ,10, 11 and 12 MHz filters. I say mostly as you really have to perform a frequency analysis to determine unwanted mixing products and ones where harmonics of BFO's or LO's end up in the middle of some conversion process. Later you will see such an analysis for this DifX rig. Don't overlook the 4.9152 MHz IF frequency as used in the Elecraft K2.

In fact there is some very strong support for keeping crystal filters in the range of 4 to 10 MHz and avoid those above and below that threshold. The reasons are many especially with the higher frequency filters where  stability is a very major factor. Yes stability -- usually crystal stability ratings are in PPM (parts per million). The inexpensive units (C^3 = Cheapo Chinese Culls) may be rated at 50 PPM or worse. So lets run the numbers at 4 MHz and 50 PPM that is as much as 200 Hz and at 12 MHz (like in the Bitx) = 600 Hz. The better units are 30 PPM (real crystals and more expensive) so our 4 MHz example is now 120 Hz and the 12 MHz versions are 360 Hz. When the 40M SDR Police report you for being 30 Hz low, you can see where this is headed. By the way the lower end filters like say 1600 kHz units (that notably were used in the hallicrafters SR-150) present some issues with regard to image rejection.

Later mention is made of commercial monolithic crystal filters -- the superior characteristics of these filters comes from the use of a common base crystal structure and tight control of the manufacturing processes. That is hard to do with 6 or 8 discreet crystals tack soldered to a PCB board that may be varying in frequency all over the place.
 
Speaking of crud, as I earlier mentioned, I am suspicious that someone in my neighborhood (Southern California) is growing "pot" in their garage, as I hear what appears to be the cycling of "grow lights" especially on 40 Meters. Thus some receiver architectures are better than others in handling such man made interferences. My only hope is that with the recent legalization of pot, my neighborhood grower will soon be out of business. I walk in my neighborhood on a daily basis. Up to now I have been looking for the culprit from among my neighbors who seem very happy (all of the time)  and sporting beer bellies from consuming too many munchies (Overly happy YL's and one's with the onset of mid-drift bulge are included). But I need to be more get scientific and use my compact 20M Transceiver (DifX II) with a battery pack and do some serious direction detection. You all saw that rig on the cover of a 2015 QRP Quarterly --right?

 
 
In 2012 I built a dual conversion SSB transceiver known as the KWM-4 which of course, was a DifX (Different than a Bitx.) Now we are moving to another DifX (on steroids) which uses that earlier topology but takes advantage of newer technology not readily available just a short five years ago. Essentially I will be strapping on a mixer conversion stage ahead of the basic single conversion DifX stages that I have been building for some time and most recently in the 60M Rig, the Big Kahuna and the DifX II.

I guess my 1st use of a DifX topology was in  the Spring of 2010, when I built the 20M MMIC Based SSB Transceiver which graced the cover of QRP Quarterly. Now that was a bit of innovation --MMIC IC's used in bilateral amplifier stages. For my friend N2CQR, that rig started with an analog VFO. But as you can see, soon morphed to the "Digi" stuff --- 7 years ago.

 
 
For my newest rig, the directed approach is to take advantage of a packaged 45 MHz, 7.5 kHz wide commercial monolithic crystal filter available from ECS (Digikey). While the uBitx employs a 45 MHz homebrew multi-pole filter, unless one is extremely lucky it will be difficult to match the performance of the ECS unit. Yes I know, there are the purists who want to homebrew everything and certainly that is commendable. But when I have an opportunity to utilize a device with known specifications at the outset, then it is a simple engineering decision.

I liken this to building homebrew double balanced mixers. Was I successful building DBM's  and did they work --yes? BUT an SBL-1 beat the pants off of the six that I homebrewed. It is hard to match components built on a manufacturing line using precision parts and tight process control while attempting to do the same with non-precision stock components, in a cold garage with an 80 Watt Radio Shack Soldering iron with a "Fat Tip"!
 
At the end of the day I want a top notch transceiver not just one that works. The packaged ECS filter is less than $17 USD and the cost differential of what would be spent building a multi-pole filter is not so great as to warrant a non-consideration of the commercial "black box". Keep in mind you might need to buy two dozen 45 MHz crystals just to find six close enough in frequency. With a matching circuit provided by the manufacturer the 45 MHz four pole filter with a 7.5 KHz bandwidth has a Z in/out of 50 Ohms which is perfect for insertion into many of the receiver topologies. The Si5351 third clock provides the 2nd mixer frequency. The matching circuit is shown below and L1 and L2 can be 11 Turns of #24 enameled wire on a T-30-6 core. (T-30-6 ~ Al = 36)

The first task I tackled was coming up with a PCB layout for the filter (really small) and the matching network. I simply took the manufacturer's pad layout which was in mm and I made a drawing at 10X in the normal inches (forget that metric BS). Once I had that done I used the design program (G Simple) to scale everything by 1/10 and then using the fact that 1 meter = 39.37007 inches (1 inch=2.54 CM or 25.4 mm) I had the design program further scale the design down to mm. I made a test cut on my CNC and was very close. I then had the design program scale everything by 1.10 (10% Increase) and it was dead nuts on. The alternative for those without a $250K CNC machine --you can just turn over the filter and wire it dead bug.

Below is a quick and dirty first cut prototype after the 10% upward adjustment; and the overall width is less than 5/8" so you can see fairly small. I have dxf files for this pad layout and the  ADE-1L DBM which I will be happy to share. I used a dull engraving bit for the prototype and when I make the final unit the lines will be crisp and sharp. It is envisioned that the conversion board will be about 2X3 inches and contain the 1st and 2nd Mixers (ADE-1L's) the matching network and the ECS filter. In discussions with the manufacturer the whole assembly will be shielded.



 
Just think the space you'll conserve by not building the VU2ESE multi-pole filter. Four poles of filtering having a 7.5 KHz bandwidth at 45 MHz is very in keeping with the gain and bandwidth  parameters. The specifications for this filter are 30 dB down at the half power points ( 3dB) and an ultimate  stop band   of 80 dB. The stability will exceed the homebrew filter!
 
 
 
.
 
 
 
Lets us start with the premise that the 1st LO (tunable) would up convert everything to 45 MHz. Thus in going from 1.8 MHz to 30 MHz the table below shows the conversion frequencies. My first thought was to use some packaged crystal filters for the 2nd IF such as those from the GQRP Club or INRAD. These filters are at 9 MHz. If you do the math the 5th harmonic of the BFO is right in the middle of the ECS filter pass band (5X9 = 45). So that choice is not a good one. If you use the uBitx approach, you avoid that problem. But this being a DifX I picked 11.5 MHz for the second filter frequency.
 
My frequency analysis shows you avoid the problem of having harmonics of the BFO end up in other parts of the rig. In checking the current price of the 11.5 MHz crystals in the large can form factor, 25 pierces can be had for 30 cents a piece form Mouser. I suggest getting the 25 pieces so that you have enough stock to find at least four that have no more than a 50 Hz spread across all four crystals. Most likely you will end up with several filters so not all bad.  If you can find five or six within that frequency constraint then you have the makings of a superior second IF crystal filter
 
 


 

The above table looks like it will avoid the problem of a tuning range sitting on the same frequency as the second mixer so we should be good.  I think we can now move forward


We now can take a peek at a first cut of a block diagram shown below. In a more detailed view all interfaces and matching are to 50 Ohms. So if you don't know --time to get smart on broad band matching and turns ratio squared! The block diagram below was based on the original thought of using 9.0 MHz filter; but is essentially the same with the 11,5 MHz homebrew filter. So where it says 9 substitute the 11.5 and the appropriate USB/LSB frequencies. Since we are using broad band amps for the bilateral amps no other frequency adjustments are necessary. CLK1 will now be 56.5 MHz and CLK2 will be either 11.4985 MHz or 11.5015 MHz. There are no changes to CLK0.

 
 
So lets examine closely what is happening with our frequency scheme and why does this and how does this arrangement work. Keep in mind ahead of this is a broad band amp stage (2N3904) that is adjustable and can provide up to 10 dB of gain. Typically I run these at about 1/2 the gain --enough to perk up the signal but not overload the downstream stages. That amp feeds a bank of relay switched band pass filters.
 
When the 42IF123 transformers were available on the market this would have been the tool of choice --so now you are stuck with buying a stock of TOKO transformers or building the BPF's from discrete components. If you are lucky enough to own a copy of the SSDRA (Solid State Design for the Radio Amateur) you can hand calculate the BPF's.
 
There may be a computer program in the EMRFD that will let you do the same; but I have never used the CD that came with my EMRFD. As you can tell, even though I have a copy, EMRFD is not my first choice for a reference document. Right now my EMRFD is a pretty expensive book end. After hand calculation, using LT Spice you can run your BPF's and fine tune them. The hand calculation process coupled with LT spice let's me really get inside my BPF's and when it is necessary to make a new one, I know how to do it!
 
So now the signal entering the block diagram will be relay selected and covers just the ham band we picked. So already there is some signal clean up in play. The 1st LO from CLK0 in the Si5351 up-converts the ham band signals using the difference frequency (59.0 MHz -14.0 MHz = 45 MHz or 59.2 MHz - 14.2 MHz = 45 MHz) The sum frequency would be (59 + 14 = 73 MHz ) way out of the pass band. Now for the first bonus -- the ECS filter has a 7.5 KHz bandwidth --so anything 3.75 KHz +/- away from the desired frequency is also filtered out! You are screening out (attenuating is a better word choice) the California KW station 10 KHz away from your rig!
 
The signal is now passed on to the second mixer stage operating at 56.5 MHz and so the down mix is at 11.5 MHz. In both of our mixing processes the mixing frequency was above the signal (most desirable) and in effect we have two sideband inversions -- so the lower BFO frequency will yield USB and the higher BFO frequency LSB. Again all interface matching is at 50 Ohms and three ADE-1L's are used as the double balanced mixing devices. The ADE-1L's are 3 dBM devices so low drive requirements -- set the drive level in the Si5351 to 2 Ma.
 
I will now outline some of the performance specification for the new DifX rig.
  • Dual Conversion ~ 1st IF @ 45 MHz and 2nd IF at 11.5 MHz
  • 10 Band Operation: 160, 80, 60, 40, 30, 20, 17, 15, 12, and 10 Meters 
  • USB/LSB Operation
  • 15 Watts Output
  • Color TFT Display
  • PIN Diode and Some Relay Switching
  • Si5351 PLL Clock Generator
Some words here about the VFO. Forget building a quality rig like the DifX with an Analog VFO as you are really limiting yourself. Time to move up the Big Boy's sandbox.  The use of the Arduino in addition to providing the digital VFO capability also provides the color display, Tune function, USB/LSB, band selection and on and on. Analog VFO's can be fun in some rigs, but not this one! This is a DifX so move on up.

When I build a transceiver I always start at the back end and as I progress forward each part of the completed build is now part of the test system that will be used to test new additions. If I build a circuit and install it into the test systems 99% of the time it is what was just installed is where the problem lies as everything up to that point works! Thank you Heathkit
 
Thus starting at the back end is the audio amplifier. The audio stage deserves some respect so use the low noise version of the NE5534 driving an LM-380. Forget that crap about building a discrete component amplifier with 2N3904's/2N3906's --yes building one using that approach is like earning a Boy Scout Merit badge. It is also like doing brain surgery with a rusty spoon; thus it is time to move on to something more robust. Another NE5534 can be used as the microphone amplifier. This is an uptown rig and so move on  past the low rent district.
 
 
This is enough to start your heart pumping and the insatiable to desire to heat up the soldering iron. Welcome to the world of the DifX.
 
73's
Pete N6QW

Tuesday, April 4, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.06


So What Would You Do If You Were the Designer?


OK! OK! Not everyone is a rig designer and there probably are far more hams who are fully capable of building rigs than  than those who can design a rig from the ground up. But as hams we are all users and we can certainly articulate what we would like to see in a rig.
 
Even more so today as the new technology available to us for literally pennies, can  bring a whole new dimension of features to home built rigs. While there are many builders who like analog VFO's and homebrew dial mechanisms, there are many of us who want digital VFO's with their high accuracy and amazing stability. Many of us also like the colorful displays where lot of information is readily displayed--with much of it being in real time. Thus designs must be flexible so all quarters can participate in the project.
 
So as users we can influence what we want to see in a rig and in effect become the rig designer by what we ask our rigs to do.
 
Here is a laundry list of what users might like to see in a homebrew rig:
 
  • Frequency agility and frequency stability (more difficult with analog VFO's)
  • Sensitive receivers and great sounding transmitters
  • Costs that don't involve taking out a home mortgage or selling off one of the kids
  • Ease of construction using common parts
  • Circuit boards or building block techniques for the final assembly
  • Readily available tech help and a strong user support community
  • Multi-band Operation
  • Low power drain
  • Reasonable power output in the 10 to 15 watt range
  • Ability to handle strong signals and AGC
  • S Meter
  • Selectable sidebands
  • Compact size and low weight
  • 12 VDC Operation
  • Let us not forget the digital modes and interfaces needed for the digital programs
  • RF Pre-Amp and/or 10 dB attenuator
  • Well designed Band Pass and Low Pass Filters paying attention to 2nd Harmonic reduction
  • Use the Arduino Mega 2560 because of the large number of Input/Output Ports
  • Fused Input and Reverse Voltage protection
  • DFM ~ Designed For Maintenance
  • CW Capable and Audio Filtering
  • Painted Juliano Cool Blue
  • IF Shift/ RIT
  • Tone Function for Tune Up ( Standard feature of the DifX)
  •  
  • By making this list we are now starting to define our design criteria. Some may gravitate to a single conversion design but the more sophisticated rigs very likely will be a dual conversion design.
 
What did you have on your list?
 
73's
Pete N6QW

Sunday, March 26, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.05

The Big Kahuna

 

3/31/2017 ~ All Back Together

Only a hint of burnt wiring but working FB. Made several contacts and all back to normal. My ham friends this too is a DifX!
 
73's
Pete N6QW
 



 
 
 

3-30-2017 --- A Case of Smoked Parts!

 
Just thought I'd share a moment of grief when I was working on one of my rigs (LBS-II). Today I decided to convert the LBS-II back to 20 Meters. The rig originally was on 20M and then I moved it to 40M and now decided back to 20M.
 
The boards and wiring are really compact and while I was making an adjustment some wiring came into contact with ground. Wow a vapor cloud similar to a nuclear explosion was emitted from the rig and you could smell the burnt wiring. My heart sank!
 
Well as it turned out just the wiring harness got melted --yes melted, more like welded wiring.  When I replaced the harness -- I actually found a better way to do the wiring. About 1/2 half hour and the rig was back working. I just plain got lucky.
 
 
Below is a photo of the welded wiring.
 
 
 
It could have been a lot worse! I too occasionally smoke some parts.
 
73's
Pete N6QW


3-29-2017

 
 
 
This is our mark -- DifX. Stay tuned for some exciting new transceivers -- a new day is dawning.
 
 
3/27/2017
 
Some feedback from the contest operation. I am not a contester and never have been. But this past weekend convinced me that I could have really used this rig in a contest and scored a lot more points.
 
  1. First observation is that it hears really well! I was hearing DX stations and with 750 watts into a beam they were hearing me!
  2. The signal handling capabilities were quite good. That is a measure of a homebrew radio -- does it hear well or does it crumble in a contest environment where I swear some of the 6 Land stations appeared to be running 6 KW. It did well. The GQRP Filter for such a little cost did a yeoman's work. Somewhere buried in a box is an 8 pole KVG crystal filter dating back to the 1980's Now there was a  gold standard of a filter. If I can find that jewel it will soon have a new home.
  3. There were several comments from contester's to me like beyond 5X9 --your signal is really nice sounding. Now those are words you like to hear
  4. So get off your duff and start building a DifX.    
73's
Pete N6QW
 
So Ok guys now I will write Ad Nauseum (nice Latin word meaning sick to your stomach) about the Big Kahuna a two band (but five band capable in the code) SSB transceiver hot off the bench from N6QW.
 
A friend in Australia, Greg, asked if it was called the Big Kahuna because it was initially laid out Al Fresco on a surf board versus a chopping board. As I explained to Greg it is called that because it has a very large LCD display thus a Big Kahuna! No, it was initially laid out on the work bench!
 
First and foremost it is a DifX (Different than a Bitx) and the photo taken below on 3/25/2017 shows the Big Kahuna as it participated in the WPX contest. I made 12 contacts running 750 watts with the external homebrew amp--so cool that it usually only took one call boom they were back to me. What was also cool was a totally homebrew KW input station --no MFJ Ameritron amps in my shack.
 
Full details about this rig and how it differs from the Bitx can be found on my website at this link http://www.n6qw.com/Big_Kahuna.html  Schematics are provided as well as the Arduino sketch code. There is but one page that needs detailing and that is the miscellaneous wiring. The 60M rig previously showcased on the blog has many similar boards. The rig almost looks commercial but is all homebrew.
 
You too can build a DifX!
 
73's
Pete N6QW
 



Thursday, March 23, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.04

N6QW on 60 Meters!

 

Today we have a post about an exciting new 60 Meter transceiver from N6QW and of course it is a DifX. The 60 Meter band must be the world's best kept secret as operating on this band is like a breath of fresh air. With but five channels it would seem to be almost a non-starter. But that soon will change with the addition of more channels.
 
The 2015 WARC approved the new channels and effective January 1, 2017 they are indeed a reality BUT and double BUT so far they have not been actually authorized by the FCC for use in the USA --
 
Essentially the new channels center around what is now Channel 3 [5.357 MHz] and includes specific allocations for data and CW as well as specialized applications. But the bulk is for USB operation. Essentially Channel 3 is expanded by about +/- 7 kHz with the reasoning that you end up with 4 additional USB channels. But that also comes with a price of a power limitation of 15 Watts ERP. While not a problem for the QRP enthusiasts, already you hear grumbling from the California Kilowatt stations who always run 100 watts (Only 100 watts?). One recently heard QSO on 60M revealed the op might be running 350 watts --so he could be heard. Hmm what happened to 100 watts into a dipole?
 
I am very  aware that avid homebrewer's like Bill, N2CQR and Don, ND6T have successfully moved a VU2ESE Bitx40 to 60 Meters and we say a huge Congratulations and Bravo to Bill and Don. Thus the legacy of the Bitx lives on and embodies VU2ESE's concept of making the Bitx40 a springboard for hacks and changes. The changeover to 60M involves minimum surgery to the Bitx40 board thus that will make it a real incentive to get on 60M.

Keep in mind $59 buys you a nearly complete transceiver and at that price, two radios for around $120 would give you optimized radios for two of our ham bands. Making the high power modifications (to 20 Watts) would certainly make the radio easily heard out to the 1000 mile range. But that power level would have to be scaled back when the new regulations go into effect. (Well theoretically as probably some hams might just ignore the law --like running 350 watts.)
 
But I wanted more from my 60M endeavor and thus the N6QW rig is scratch built and not a modified kit. Again I want to stress that there are other designs and  features available to the homebrewer which are not found in the Bitx40 board and thus my approach was to not cobble up the Bitx40 I previously built. Let us explore these additional features that are found in my DifX. A good place to start is the display as shown below:
 
 
 
 
 
Starting with a 160 X 128  Color TFT display enables the homebrewer to show a lot of information. At the very top is the channel frequency, the actual channel number, the mode of operation (LSB/USB is panel selectable) an S Meter and some additional functionality such as  TUNE function and a reminder that channel 3 is tunable the +/- 7 KHz It is the only one and built into the code. To be safe I set my band swing limit to +/- 5 KHz. When the rig is placed in TUNE the word TUNE appears in red on the display right between the USB and Channel Number. It is a pulsed 988 Hz  tone and lasts for about 10 seconds.
 
 
This is a photo of the front panel as of March 22, 2017.
 

 
[So OK the dial plate, black with white fill, is a bit of overkill; but since I have a CNC mill just wanted to show off what  about 15 minutes of work can do to dress up your rig. Next I will work a bit on engraving programs, as it would have been nice to just engrave the whole front panel. How cool would that be -- a DifX with an engraved front panel? 

Starting at the left the small phone jack is the audio when using headphones and below that is the standard microphone jack. The two smaller black knobs are for volume and to the right of that is the encoder that only works on Channel 3 to tune +/- 5 KHz which is the SSB portion of the proposed allocation. The toggle switch below the two knobs is to select USB/LSB and the red button is the Tune button. The large knob in the center selects the 5 channels. An additional feature is the installation of a small miniature toggle switch to the right of the display and its purpose is MOX operation. For the non old timer MOX was a way of turning on the transceiver manually like in Manual VOX = MOX.  While this DifX does not have VOX (neither does the Bitx40), but that is something that could be added in a future iteration. That is a project for later this year.
 
This rig was originally on 40M and I modified it to work on 60M. I am sure glad I did.
 
By way of recap this DifX features include:
  • Five Channels selected by the main band switch
  • Channel 3 is tunable based the proposed 60M band changes
  • USB/LSB Selectable
  • Tune Function (988 Hz pulsed tone)
  • Power Output is 10 Watts (meets the new proposed band changes)
  • S Meter on the display (Still working the sensing electronics)
  • 160 X 128 Color TFT Display
  • Arduino Nano with Si5351
  • Selectable step tuning rate on Channel 3 with a 1 KHz default step
  • 9.0 MHz IF using the GQRP Club Crystal Filter
  • IRF510 in the Output Stage.
  • Single 2N3904 transistor Microphone Amp stage
  • A 2N3904 driving a LM386-3 Audio Chip for the Audio Amp stage
  • Plessey Bilateral amp stages (From EMRFD, so they have a pedigree)
  • N6QW 2N3904 Bi-Directional Stage ( Rx RF Amp and Tx pre-driver stage)
  • A 2N2222 / 2N3866 driver stage. (From EMRFD, so it has a pedigree)
  • MOX Operation (Manual VOX)
  • Hybrid, SMD and Leaded component build.
For those who would ask, the architecture is not unlike what was found in the Let's Build Something (LBS) transceiver that was showcased in a two part article by Ben, KK6FUT and myself in QRP Quarterly. It is also a very close cousin to the Big Kahuna. BUT it is a DifX!
 
In several days of casual operation, starting last weekend,  where I was actually just on the air testing of the rig, I made fifteen* contacts with stations in California, Nevada and Arizona. The best DX was 400 miles and the signal reports were excellent. I should mention that some advice given me was to get an amp so I could register 40/9 on their S Meters and that using my droopy dipole was not optimal.
 
But hey I was fully copied albeit only S5 and the audio quality was quite good --but not 40/9. Most of the contacts were on Channels 3,4 and 5 and a couple on Channel 1. Channel 5 is the DX channel and where I heard the stations in the mid-west. Channels 3 & 4 seemed to have the PSK operations. But that may be just the left coast.

Imagine my surprise that in the early evening hours I was hearing station out past 1000 miles in the heart of the mid-west. If they were truly running 100 watts, then there is a real possibility of nabbing one of these stations for a QSO with my puny S5 signal. Most had pretty good antennas and so that is the other half of the equation --got the rig now need a better antenna.

There is yet a bit more work on this 60M rig, mostly on the S Meter sensing circuits. Are homebrew rigs really ever finished? The Arduino code really adds much to making this work on 60M. One of the 60M stations worked almost harkened back to the 40M Police using SDR radios to somewhat "pick apart" your homebrew rig creation. However in this case the report was from the station using one of the smaller versions of the Apache Labs SDR who stated "your spectrum looks clean". Now those are words I like to hear!
 
This is a DifX!
 
73's
Pete N6QW
 
*W6ANR (3), K7RFI, N7GZZ, N6FUZ/M, WB6PGJ, WB6JNN, K6DNS, WA6JBZ, WA6KDW, W6DAX, W6PJD, W7IRS, KM6CQ. Locations include Placerville, Reno, Carson City, Visalia and Yuma.




Tuesday, March 21, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.03

The Dual Conversion Scheme an evaluation of Frequencies.

In the KWM-4 I was presented with a real engineering problem that being how to take advantage of the Collins Mechanical Filter capabilities yet deal with its low frequency of 455 kHz. The path quickly leads to a dual conversion scheme, where you can manage the gain at the higher IF and manage the selectivity at the lower IF.
 
This is a clue in that at the higher IF the filter does not have to be 2.1 KHz wide but something on the order of 7.5 KHz will certainly keep down the crud ending up in your receiver band pass. The lower IF (Collins Filter) can do all the heavy lifting as now signals in the pass band coming into the filter are very narrow and the mechanical filter is essentially slicing that down to about 25% of the bandwidth -- 2.1 KHz. Below is the frequency scheme for the KWM-4.
 
 
 
 
 
 
Five years ago this is how the frequency mixing was handled. This exemplifies the concept of gain at the higher IF and selectivity at the lower IF. A filter at 10.7 MHz was chosen as the higher IF because given what I had at hand five years ago this worked with the COTS (Commercial Off The Shelf) components that could be easily and readily purchased. This design has a shortfall in that because of the high IF chosen, operation on 30 Meters was questionable. For some this is a deal breaker.
 
But today were I building this I would have chosen another IF frequency as frequency translation with an Arduino + Si5351 is a far easier task. The uBitx uses a homebrew multi-pole filter at 45 MHz  MHz and that is a good choice of frequency as it avoids the ham bands and does provide for 30M operation. However I would stick with a packaged 45 MHz crystal filter from ECS that does cost about $17 (Digi-Key) but has some really desirable and predictable parameters. It is good for 30 dB of attenuation for +/ 3.75 KHz and the stop band is 80 dB. The Z in/out is 350 Ohms and that is easily matched to 50 Ohms with a 3 Turn to 8 Turn broadband transformer (350:50 = 7:1, 3^2 = 9 and 8^2 = 64, 64/9 = 7.111 --close.). The injection frequency into the 2nd mixer is a matter of what you have and also close examination of the mixing by products.
 
In the KWM-4 the 10.7 MHz filter with a 7.5 KHz bandwidth resolved the subtractive mixing issue where 10.245 - .455 was out of the filter pass band. So lets say you wanted to use a commercial crystal filter such as the INRAD Model #351 which has a center frequency of 9.0 MHz. Thus the injection frequency would be 45 + 9 = 54 MHz which still puts it outside the ham bands --a bonus. Then the third clock would provide the normal 8.9985 and 9.0015 MHz BFO frequencies --OR a third BFO at 9.000 MHz for CW. The KWM-4 CW scheme would work perfectly here.
 
The Arduino and Si5351 would provide a far easier approach in a today build of a KWM-4 --but with some noodling I worked with what I had five years ago.
 
In a recap of a DifX version of a Dual Conversion Transceiver I would use the following:
 
  • 1st IF at 45 MHz using the ECS packaged crystal filter ~ 7.5 KHz wide
  • CLK0 on the Si5351 would provide injection frequencies above the 1st IF ranging from 46.8 to 75 MHz --all with in the capabilities of the device to give 160-10 Meter all band coverage. A quick math analysis shows that the injection frequencies required for all of the ham band avoids the second mixing frequency. The LO injection frequency for 7 MHz would be 52 to 52.3 MHz and the LO frequencies for 10 MHz would be in the 55 MHz range
  • CLK1 on the Si5351 would provide a 54 MHz fixed inject frequency to convert the signals to 9.0 MHz (54 - 45 = 9 MHz)
  • CLK2 would provide the USB/LSB and CW Carrier Oscillator frequencies
  • The INRAD Model #351 is a 4 pole 2.3 KHz wide filter with a Z in/out of 200 Ohms an easy 4:1 match to 50 Ohms. A word here about homebrew crystal filters. It is not a simple matter to build a good quality crystal filter especially for someone who has never done one. You do need test equipment and you do need to understand what you are doing. There are some excellent tutorials on how to do it --but excellent  results for the neophyte with no test equipment would be like winning the Mega Millions having only played once and having only bought one ticket. The two filters suggested will cost you about $50  and one of them is a surface mount. But their specifications are well known as is their performance. Let me not discourage you from homebrewing a filter --but I am for a sophisticated project like this. A six pole 45 MHz filter may require the purchase of 20 to 30 individual  crystals before you find six that are close in frequency. You can't simply buy 6 crystals and think you are there. If by happenstance you are after buying only six, then I suggest you go out and buy one lottery ticket --for you are indeed a lucky person.
  • For the Bi-Lateral Amps you have a choice of the TIA or the Plessey Amps.
  • For DBM's I would use the ADE-1L (low drive requirements at 3 dBM).
Thus the Dual Conversion scheme offers some real advantages but also brings about the need to be careful in frequency selections.
 
73's
Pete N6QW

Sunday, March 19, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.02

In our last post we hit the highlights of the KWM-4 and in the closing paragraphs I mentioned the band switch decoder and how using the three digit code provided by the K5BCQ digital VFO that automatic band switching was possible. Today we have with the Arduino far more efficient means of doing this; but this was how it was done before the uBitx.

Basically the circuit detects the BCD code and translates it into decimal outputs from 1 to 6 (Six Bands). With each output is a PFET that is "switched on" to provide power to the appropriate Band Pass and Low Pass Filter banks. An Arduino Mega 2560 having lots of pins could provide the 3 digit code or if you wanted to waste 10 digital pins you could do it directly. Also shown is how you could switch the bands using 3 toggle switches.

 
 
Another innovation was how the "Push To Talk" was handled including how to key the transmitter for CW. This was a total "in house design" and I think one of the special features that are found in the KWM-4 again pre-dating the uBitx.
 
Shown below is the schematic for the control board. Noteworthy some of this same circuitry is found in the Big Kahuna. On SSB the PTT trips the 4N35 opto-isolator and that toggles the SN74LS000 so that the trigger signals shift from the receive side to the transmit side. It is all DC switching, so no big clunky relays with back emf. There is a separate solid state switch to key an external linear amplifier. [The 4N35, SN7400, 2N3904's and the TIP32C's form the basic control functionality in the Big Kahuna.  If the CW capability were added to the Big Kahuna, then virtually the entire same circuitry shown below would be required.]
 
CW on the other hand is a more complex process and this involves the use of an NE555 timer --not for timing per se but to supply a voltage for a fixed duration. The 4.7 Ufd cap is part of the timing circuit so that the CW oscillator is held on for a period of time determined principally by the value of this cap. Most high speed CW ops prefer a shorter cycle and so values down to 2.2 or 1 Ufd would be used.
 
Here is the CW sequence. You tap the key and two actions take place the first of which is to start a timing cycle by placing a "high voltage" on the NE555 pin #3. This closes two relays the first of which is a relay whose contacts are in parallel with the PTT switch and the second is the relay that takes the 1st Bi-lateral amp which is normally connected to the Collins filter for receive and SSB transmit but now is connected in the transmit mode to the output of the buffer amplifier. You will recall that the CW signal on transmit does not go through the Collins filter. Tapping the key also keys the buffer amp and keeps the NE555 in the On state. Let up on the key and let the timing cycle complete and the rig is back in the receive mode. Voltage off of Pin #3 also is the source voltage for the CW oscillator. This is quite a complex switching and control system and distinctly feature rich and perhaps far different than other rigs.
 
It is a N6QW design! Also keep in mind this was designed  many years ago and while the Arduino capabilities today would render some of these design elements as being antiquated --it was and is a successful control system. Certainly not QSK; but that was not the design intent. The main design problem was CW with offset and how to simply tap the key and transmit CW all automatically. As with the Collins KWM-2, the panel mounted Mode Switch had to be set to CW -- this also accomplished setting the CW receive mode to USB. The elegance of the KWM-4 was not just casual happenstance but careful and reasoned thought -- five years ago!
 
 
73's Pete, N6QW.
 

 


Saturday, March 18, 2017

A New Line of Transceivers --- DifX

Transceiver Architecture 2.01

Please note the DifX is not a singular transceiver (like the Big Kahuna) but instead is a concept to demonstrate that successful transceiver projects can be achieved with something other than the Bitx20 footprint. The Bitx is a long standing successful design; but is not the only approach to homebrewing a rig. We are now at a point where with the aid of low cost technology we can build in many new features from the outset. The DifX series of radios will provide some insight into the "how to do it."

 


As promised in the previous post, I will explore homebrew transceiver architectures that are different than a Bitx. I can think of no better place to start than with my KWM-4 design which began in late 2012 and resulted in a completed transceiver in early 2013. This project was published in an 2013 article in QRP Quarterly.
 
What is significant about this project was that it is a dual conversion transceiver and covered six amateur bands and for my good friend N2CQR had a digital VFO. Just announced and mentioned in the SolderSmoke Podcast 194 and just appeared in Hackaday is the Micro-Bitx or uBitx from VU2ESE. The uBitx is a dual conversion transceiver and has created quite a stir in the homebrew community. While I can't comment on the design details of the uBitx as I simply don't know, I can share with you my concerns about a dual conversion architecture and what I had to "noodle my way through" on the KWM-4 rig.
 
Let us start with a high level block diagram of the KWM-4 and identify some of the key areas of noodling. This is shown below and highlighted in green shading.
 
 
 
 
Let us first start with the two blocks identified as BLA --yes Bilateral Amplifiers. But these amps are based on a design by Ron Taylor, G4GXO and appeared in the GQRP SPRAT #128. Essentially the amplifying devices can be either a Dual Gate MOSFET such as a BF991 or two J310's connected in a cascode circuit (source of one connected to drain of the second). The stage gain is 17 dB and the signals are diode steered. Z in/out is matched to 50 Ohms. This BLA is not the Bitx BLA! So Ok you want to see it. Noteworthy I first used this BLA in a 2007, 17M/40M Transceiver and later in the 2009 Tri-Band transceiver that used the HW-101 frequency scheme.
 
 
 

 
 

 
 
This circuit was selected as the key element over the Bitx or Plessey BLA for one specific reason, that being Gain Control. This circuit has the capability for either manual or Automatic Gain Control at the IF stage. Yes the W7ZOI Hycas AGC circuit provided the gain control in the KWM-4. The Bitx does not have that as a direct capability. There was a second reason for this approach which is shown later in a detailed block diagram and that is CW. In the KWM-4 the output/input of the 1st BLA ahead of the Collins Mechanical filter is relay switched so that in SSB it is always connected to the Collins Filter. But on CW transmit that connection is switched over to a keyed buffer amp that is supplied a signal from a third BFO frequency centered on 455 KHz.
 
Thus CW does not go through the Collins Filter on Transmit. For CW receive USB is used. But had I used an additional panel mounted switch the operator could have chosen either LSB or USB for receive --CW reverse. This is not found in the Bitx. Optionally if you had a Collins CW filter --then the ne plus ultra for the CW enthusiast -- selectable receiver filter bandwidths. W7ZOI in the SSDRA shows how to diode steer two filters and even provided an extra gain stage for the CW filter.
 
Now to the dual conversion part of the KWM-4. This project started innocently with my acquiring a 455 KHz Mechanical Filter as used in the KWM-2. I also had managed to scrounge up the USB and LSB crystals as well as a crystal smack on 455 KHz. There was a siren's call to "use me in a transceiver". But if used only with a single conversion (and dual conversion too as you will see) becomes problematic because of image and frequency mixing issues as you go higher in frequency.
 
Here is an example. I generate a SSB signal (we'll use 455 kHz as it will make the math simple) at 455 KHz and I mix that with 20.845 MHz LO. Normally the sum frequency would be 21.300 -- a really great place to be on 15 Meters! But the difference frequency is 20.845 - .455 = 20.390 -- That is really close to the 15 Meter band and unless you had some really brick wall Band Pass Filter --difficult to knock down. As you go even higher in frequency the spread becomes even less. So LOW IF frequencies tend to need some way of distancing themselves. Thus dual conversion.
 
In the three shaded blocks that show the LO, and 1st and 2nd mixers, I will now detail how I resolved that matter. Touring through the Mouser catalog, I found that they carried crystal filters at 10.7 MHz and these came in two bandwidths --15 KHz and 7.5 KHz. I also discovered that one of the stock computer crystals was 10.245 MHz and the light bulb went on. Initially I bought the 15 KHz filter but then changed that out for the 7.5 KHz filter and one crystal at 10.245 MHz. So here is why the light went on. If you add .455 MHz to 10.245 MHz, the sum is 10.7 MHz (hold that number) Now if you subtract the 0.455 MHz from 10.245 MHz the sum is 9.79 MHz. The 7.5 kHz bandwidth of the 10.7 MHz crystal filter will reject the difference frequency. Thus anything coming through the 10.7 MHz filter would only be he SUM frequency. That eliminated the undesired frequency product.
 
The K5BCQ frequency generator thus converted the 10.7 MHz SSB/CW signals to the appropriate ham bands. The only negative to this scheme is the 30 Meter band is too close to this 10.7 MHz conversion frequency and thus was not included -- not a problem for me but a problem for some I am sure.
 
The K5BCQ frequency generator has some 900 channels (900 VFO's) and I used that for assuring that what the dial says is the real transmit frequency. Thus the LO frequency is offset (switched to a different channel and different offset frequency) depending on the mode. There is about a 3 kHz spread in BFO frequencies between USB/LSB, thus by changing the LO injection frequency by that amount accounts for the mode. In the old days of analog VFO's this was done with a varactor diode somewhere in the VFO box that would shift the VFO frequency for the same analog dial setting.
 
Today using an Arduino with a Si5351 would eliminate the three BFO frequencies and the need to switch channels -- all done in software --so 4+ years does make a difference in the technology.
 
Now for the detailed block diagram of the KWM-4.
 
 

 
 

 
One other noodle problem is how to change the band pass and low pass filters at the same time you change bands? Luckily the K5BCQ frequency generator also had built in a 3 digit binary code that could be linked to the band change. I simply decoded that information and that provided the appropriate set of Band Pass and Low Pass filters for the band in use. I had to learn how to decode the information and steer the signals --more on that in a subsequent posting. This is a DifX!
 
73's
Pete N6QW