Tuesday, May 23, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture ~ 2.17


[For more info on the LM373 SSB Transceiver see http://www.n6qw.com/LM373.html)



Reworked Color Display for the Dual Conversion DifX
 

The Crystal Filter Build Out

 
5/24/2017
 
The caps arrived today and I also measured the 150 PF NPO caps I had in stock. One of values needed (4 places) was 151.2 PF. Using my AADE LC meter, I measured 13 caps and here are the results.
 

The results of this test shows that of the 9 caps we have nailed down 5 of them. The two 115.4 PF I think will be asy to achieve with the 100 PF NPO I bought and the 15 PF trimmer. The 430 PF that were to be the basis of the 437.1 PF are so small I can hardly see them so I may have to invoke a work around.
 
I also designed the PC board and it is about 5 inches long and 3 inches wide. There may be a way to compact this but for this 1st run it will be linear. Hope to cut the board tomorrow and start soldering in crystals and caps.
 
73's
Pete N6QW
 
I have been awaiting the delivery of some 1% caps and they will be arriving in the next couple of days so in the meantime I will flesh out the circuit board for the crystal filter. So let us examine what is required. My Dishal filter for the 6 Crystal Filter has a total of nine capacitors and they are as follows: 
 
  • 2 X 151.2 PF
  • 2 X 437.1 PF
  • 2 X 115.4 PF
  • 2 X 151.2 PF
  • 1 X 156.8 PF
Noteworthy is that we really only have 4 values of capacitors including 4 caps of the same value. The precision caps I ordered are 100 PF, and 430 PF and these are 1% and NPO AND Surface Mount. I have a bag of 150 PF NPO caps which I will measure to find values all below 150 PF. (Hopefully finding four that are all below 150 PF but close in value. I have an AADE LC meter and that is my measurement tool.
 
The PC Board will require a total of 11 Pads to accommodate the crystals and the caps. To get me exactly on the values needed I will make the pads large so that there room for the 15PF  air variable trimmer caps in addition to the fixed value ones and the crystals. Playing with these parts before I cut any pads is critical to the final assembly. Thus 437.1PF = 430 PF + 0-15 PF Trimmer
 
I will add to this posting with the layout of the crystal board after I get it laid out.
 
73's
Pete N6QW


Tuesday, May 16, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.16

5/21/2017
I am now documenting the LM373 build on my website so see the
following LM373 Link So for those tired of seeing me gush over my vintage 1970's designed, new rig you will have some relief. Have made about 2 dozen contacts including some at 600 watts. You got to have power to make lots of contacts so get over that QRP stuff! Soon we will be back to the crystal filter build on the DifX.
 
73's
Pete N6QW

5/19/2017 ~ QSL Card from the 1st contact with the LM373 Transceiver. Here is proof! Also noteworthy I was running only 15 Watts and the Mosley 2 Element Beam. What fun! I am now up to 10 Contacts with the LM373 Rig.

Pete N6QW
 


5/17/2016 Some added photos of the innards of the LM373 rig. We have now made six contacts with the new rig. I still need to tidy up the wiring. Yes, I succumbed to an IRF510 in the output which is not my favorite RF device --but it was an expedient. I am redeploying the K5BCQ 5 Watt final to the DifX dual conversion rig. With the outboard amp this will now do in excess of 100 Watts -- more than QRP!!!!! In the last photo you can see the LM373 IC square in the middle of the photo. All that work being done with one IC. To the aide and aft of the 9.0 MHz crystal filter is the 11 VDC power supply and the N6QW solid state power switch. The relay on the back panel only switches the antenna between T and R and one extra set of contacts switches in the outboard amp. In addition to the final wiring clean up I will add some aluminum support structure so that the front/back panels are rigid. This rig has taken about six weeks to build with some of that time awaiting the new Rigol scope.
 
Pete N6QW




5/16/2017

While I gather up some high quality capacitors to build the six pole 11.5 MHz Crystal Filter, I wanted to share more exciting news about the new LM373 20 Meter SSB Transceiver. Yes that is a photo on the masthead --Pretty Cool Blue isn't it?
 
This transceiver is more than a one hit wonder as I now have made three contacts with the new rig and I am please at how it works. It is also important to note that it is not a bilateral approach and thus truly not a Bitx! By that I mean that signals pass through the IF stage (which is a single IC -- the LM373) and steering through the single direction stage is done with 1N914 signal diodes. On receive the receiver mixer stage (SBL-1) is connected to the front end and the BFO is injected into one port of the LM373. On transmit the output from another SBL-1 acting as the balanced modulator is fed into the front end and now the LO is introduced into the port where formerly we injected the BFO. The port where the audio is recovered on receive is now the transmit output stage. Thus three ports on the LM373 are switched in going from Receive to Transmit --all in a single 14 Pin Dip package!
 
As I often saw with analog VFO's and BFO's the transition from R to T frequently caused a shift in frequency because of the loading effect as the LO and BFO were shifted to other parts of the circuits. That is not the case with the Si5351 --there is no shift. This single pass through an IF stage in older designs employing Analog LO/BFO's was problematic in this aspect. I wrestled with this issue and had some elaborate schemes to mitigate the problem. The bilateral approach where the LO and BFO were not shifted was a solution. Now with the Si5351 this is a solution with a single pass approach.
 
I have challenged myself to use as much stuff as I have in the junk box without resorting to placing orders for parts and that has been very rewarding in that "you use what you have!"
 
Beyond the LM373 I am using my standard Audio Amp stage as well as the bidirectional 2N3904 amp stage and the driver with the 2N2222 and BD139 produces a whopping 400 MW. I now will add the 5 watt K5BCQ RF Power MOSFET board that uses the Mitsubishi RDD series of transistors. Right now the 400 MW is driving an outboard SS amp all the way to about 15 Watts output.
 
 
 
Touring the knobs, the left most smaller knob is the audio gain and the right one the microphone gain. The three switches on the bottom center are for USB/LSB, Tune (Red PB) and the right switch is the MOX. The right hand large knob (ala Drake R4B) is the main tuning knob. There was some careful thought (about 1 minute) to the panel layout relative to ergonomics. Since I am right handed the main tuning and step rate change is the most frequently used so it is to the right not unlike the Drake rigs!

The new Rigol DSO has been very useful as I worked on peaking a tweaking this new rig.

73's
Pete N6QW

Thursday, May 11, 2017

A New Line Of Transceivers ~ DifX

Transceiver Architecture 2.15


News Flash --- The LM373 SSB transceiver is on the air and the 1st contact was with W4DNQ, Ron who is in Florida. The contact took place at 1525 PDST on May 11, 2017 on 14.220 MHz. Here it is "al fresco"




Lets Build the 11.5 MHz Crystal Filter ~ Part III

 
I re-measured the 9 candidate crystals using the G3URR oscillator only this time using the 1 Hz position for fine tuning on my SDR transceiver. The field is now down to 6 crystals with five of those being exactly the same frequency in the loaded state and the 6th is only off by 5 Hz. As we used to say when I worked in Aerospace --Close Enough for Government Work!
 
So now I was ready to go back into the Dishal Software and using the new data, I  re-plotted the curve for 6 crystals. Again the setting of the Band Pass ripple materially affects the Z in/out and the capacitor values. Less of a ripple translates into a Lower Z in/out. So there has to be some reasonability (trade-offs) between Ripple and Impedance. Below is the new plot and we will now highlight some of the factors.
 


First and foremost is that it is a 6 pole filter that is characterized by steeper slopes and a 3dB bandwidth of 2.35 KHz. The band pass ripple is 0.13 dB (pretty flat) and the Impedance is 127.4 Ohms. 127.4/50 = 2.55 and a 8 turn to 5 turn broad band transformer is 64/25 = 2.56 ==so really close.

There are 9 capacitors needed for the 6 pole filter and in looking at the values these are easily achieved using high quality (NPO) caps in parallel with very small air trimmers (0-15 PF which were provided to me by a kind ham located on the east coast --Thanks Bob!)

An old time filter measure was to compare the 6 to 60 dB "shape factor". The BW at 60 dB is only 5.43 KHz --so that should do well with those California Kilowatts located just down the street from me. BTW the math shows 1: 2.225.

My next steps will involve acquiring some capacitors and building the actual filter. Stay tuned.

73's
Pete N6QW

Wednesday, May 10, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.14


Back to the LM373 SSB Transceiver.


While I take the time to re-measure the 9 crystals for the Dishal Filter, I thought I would update the LM373 SSB Transceiver project. This rig qualifies as a DifX as it is not a Bitx and the subject matter that will be covered has direct applicability to the DifX Dual Conversion project.
 
W5BAA's LM373 SSB Transceiver (Ham Radio November 1973) used 11.0 VDC as the supply voltage for his rig. If you look a the photo of his rig in the article you will see it connected to a 12 VDC Gel Cell. So connecting the dots, this gave him some lee way as the Gel Cell discharged. My evaluations of his approach actually demonstrated that his design did indeed "love" 11 VDC and so I followed his direction.
 
But now you have to have a source for 11 VDC (at least for the lower level stages (which is most of the rig) and you also needed a means of efficiently switching 11 VDC to the steering diodes to change the application of the circuit elements.
 
So task #1  was to come up with a 11 VDC regulator circuit. For this I went to an LM317 TO-220 style that is good for 1.5 amps. My circuit has the element of being adjustable which you will shortly see the why. The Internet abounds with regulator calculators and favorite circuits. I simply lifted one of those circuits and used values from one of the voltage calculators.
 
Essentially the LM317 samples the output voltage and using a voltage divider feeds that back into the V adjust pin. The ratio of the feedback resistor and the resistor from V adjust to ground sets the final output level. The LM317 can handle input voltage up to 40 VDC in and will regulate voltages up to 37 VDC out. The delta difference is needed for the regulator to work properly.
 
In my design the two resistors were 220 Ohms and 1715 Ohms. The 220 Ohm is a stock value but 1715 is not! So I homebrewed a 1715 resistor by placing in series a 1K connected to a 200 Ohm pot wired as a variable  resistor and that in turn is connected to a 680 Ohm resistor to ground. Thus with the pot a "0" we have 1680 Ohms and with the pot at 200 Ohms we have 1880 ohms. The pot also compensates for the tolerances of the 1K and 680 Ohm. The circuit works very well and this is shown below.
 
 
 
So now we have a 11 VDC source and as was mentioned when I first covered the LM373 Transceiver, W5BAA used steering diodes to route signals through the single direction IF block and it is through the application of the steering voltages that the LM373 rig goes from Receive to Transmit. Long ago I designed a circuit to do this. It was out of necessity as I was using an analog VFO (Yes Bill one of those old technology devices) and a power relay to switch voltages. Well the back EMF was so large (even with a snubber) that the VFO would jump frequency. This resulted in my solid  state switch design shown below to replace that pesky relay and has been updated for the 11 VDC. This same switch will be used in the DifX Dual Conversion rig although at +12 VDC.
 
Stop! I can see all of those who are into power FETs that will quickly make a posting about doing this with power FETs --you know will supply the arguments  that no current is drawn and how when you take this to the field battery life is precious and how your design is so much better. Well good for you. I am not taking this to the field, I have a stock of parts and it works just fine.
 
So how does this work. First I thought it would be cool to use an optoisolator like the 4N35. That device is also important as the next element in the circuit is quad NAND Gate and a bouncing input signal will give a lot of false triggering --the 4N35 acts as a buffer between your mechanical PTT switch and the 7400 IC. Two of the NAND Gates are wired as inverters with one Gate triggering the second Gate. With the PTT in an OFF state the upper 2N3904 has a signal applied to the base which powers on the TIP32C and we have juice for the receive condition (R). When the PTT is engaged the 7400 is toggled so that the HIGH output is not on Pin 6 but Pins 3,4,5 and thus the upper 2N3904 is now OFF and the lower 2N3904 is ON and we have juice for the Transmit stages.
 
You can build your self a truth table to verify what I said. Since there is a pull up resistor on Pins 1 and 2 that means it is HIGH (with the PTT disengaged) which results in Pin 3 being Low. But Pin 3 is bussed with Pins 4 &5 which make them low and now Pin 6 is High --driving the upper 2N3904 to ON. Now when you trigger the 4N35 with the PTT the output goes low which means the 10K is to ground and that drives Pins 1 & 2 to a Low state. Now Pins.3.4.and 5 are high and Pin 6 is Low. But since we are tapping off of Pins 3,4,and 5 and in a  HIGH condition we now see a signal on the lower 2N3904 and we have a transition from R voltage to T voltage. No relays are involved for this part of the circuit. Below is the build.
 
 
Yes there is a combo of leaded and SMD parts and the layout is close to the schematic. This is a keeper and no power FETs were involved in the build! When I connected the regulator and solid state switch to my LM373 I measured the voltage at the R and T ports and then adjusted the 200 Ohm pot so that they read 11 VDC at the R and T pins (collectors).
 
After some additional tweaking I now find that with my single 2N3904 pre-driver stage it will produce 10 Millwatts into a 50 Ohm load - (2 Volts Peak to Peak) . Now we will proceed with the driver and final stages. This is really moving along.

Tuesday, May 9, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.13

Lets Build that 11.5 MHz Filter Part II

 
In Part 1, (2.12) we shared how we measured all 25 crystals and found a grouping of 9 that were essentially "carbon copies" of each other. I never play the lottery because of that exact situation. I suspect all lotteries are rigged and the likelihood of having nine exactly the same crystals is somehow rigged. So the first thing I will do is to re-measure all of those nine in hopes of finding 6 out of that batch that are exactly the same. That may happen yet today.
 
So once you have the crystals and the two frequencies that were measured on the G3URR oscillator proceed to the WA5BDU tutorial on how to calculate Lm and Cm. I made an assumption that the crystal holder capacitance was 3 PF --probably not good or bad. Regardless, using the equations provided by Nick Kennedy I found the Cm to be 16.356 Femptofarads (10 to the -15) and using that value you can calculate the Lm which is 11.7 MHy.
 
Now for the actual crystal filter calculator go to the ARRL QEX download page and for the year 2009 find the Zip file 11/09_Steder_Hardcastle and download to your computer. Do NOT place it in the Program Files or Application Files!!!!!!!! I have a directory called N6QW and in that directory I have a folder called Dishal --extract it to that folder. Milton Dishal wrote a piece of software to simulate crystal filters. Steder and Hardcastle take that software and turn it into "ham speak" so it can be used by people like me.
 
Now I had a problem with the QEX download --my computer kept telling me it contained a virus and promptly removed it from my computer no less than 3 times. The fourth time on the Zip file I used a tool called compatibility --it seems when I now use the Dishal software my computer thinks it is a windows XP machine and all works good.
 
Here is where the dishal software can save you lots of effort --if you enter the shifted frequencies it will actually calculate the Lm and Cm --and they match my hand calculation. After playing with it a bit I can see that it would be ideal to have six crystals in the filter and here is why. See below.
 
 
 
 
The first plus is the 3 dB bandwidth which is about 2.2 KHz and at 30 dB down is probably less than 3.0 KHz. The 20 dB numbers and 40 dB numbers look OK. It appears to have a more symmetrical shape ( a factor that improves by having more crystal in the filter), thus getting away from the idea of strictly an LSB filter.
 
I found out the pass band ripple is linked to the input impedance. In this case it is 102 ohms. 102/50 = 2.04 and if we did a 7 turn and 10 turn winding on a FT37-43 core we would have 100/49 = 2.04 --so a perfect match. But if you drop the ripple to 0.2 dB then the input impedance raises to 127 ohms and this is a ratio of 2.54 when transformed to 50 Ohms. So if we use an 8 turn and 5 turn winding on a FT37-43 core that transforms to 64/25 = 2.56 ---really close.
 
The software has a built in  LC matching network calculator so that you can essentially design "L Networks" for the matching. I prefer the broadband match.
 
The 0.2 dB pass band ripple also "twizzles" the caps so that the values are much nearer to close tolerance standard value caps. I have some low value air trimmer caps (0-15 PF) that could be paralleled with the standard values so that the filter will be dead on.
 
I also believe this filter will have to be built in such a way where the crystal cases are flat to a ground plane and the crystal can soldered to that plane. I will use SMA connectors to the filter and it will be totally enclosed in a metal box. This will be bullet proof.
 
My plan is to update this posting with the re-measuring of the 9 crystals --to see if I should play the lottery today.
 
73's
Pete N6QW




Friday, May 5, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.12

Let's Build That 11.5 MHz Crystal Filter!

 
 

Sunday May 7, 2017

 I have completed the first round of crystal measurements and have found several groupings of crystals that have potential for a filter. One group has nine crystals and my next step is to re-measure all 9 to verify that indeed that all have the same specifications. I used different colors to highlight the crystals with the same specs. See the table below:
 
 
 
 
Stay tuned for the next round of measurements.
 
73's
Pete N6QW
 
___________________________________________________________________________________

 
 
See video at end added today on measuring the loaded crystal frequency --yes being done with a Raspberry Pi3 and a SoftRock V6.3 SDR radio.
 
My bag of 25 crystals arrived (May 4th) and in case you are wondering what I bought here is the Mouser Part Number (See below) The one thing nice about the manufacturers specifications is that the ESR (Equivalent Series Resistance) is provided and that number is 35 Ohms.(That could be good or bad but at least I know what they say it is,)

449-LFXTAL026687BULK

 

 

It is amazing how crystal filters are suddenly at the top of everyone's list. The man himself, W7ZOI, Wes Hayward on May 1, 2017 just released an exciting document that details the "innards" of our beloved crystals as used in filters. You can find that document here  Essentially Wes describes the crystal model and how today's crystals present some issues when we try to build crystal filters. Download this and keep it under your pillow at night.
 
In Transceiver Architecture 2.10, I mentioned that there are some excellent you tube videos on the subject of homebrewing crystal filters and w0qe, Larry Benko, in his part 13 and 14 has taken up that task. Part 13 deals with some of the front end analytical work and Part 14 gets to the nuts and bolts.

One tool used in Part 13 was the use of Elsie (free Student Version from Tonne Software) to simulate a crystal filter. The "free version" will only let you simulate a 4 crystal filter but that is good enough for our purposes. The professional version lets you add many more crystals.  Elsie will not design a filter for you --BUT it will let you "twizzle" the parameters so that you can visualize how changing parameters impact the shape of the filter curve and the SWR. Interestingly the Elsie parameters match closely what you will finally need for a good filter. The Zin/out looks like less than 200 Ohms and the coupling caps are in the range of around 100 PF.
 
Using the Elsie Software (with help from Jim Tonne, the developer) I was able to simulate what a crystal filter might look like using my 11.5 MHz filter frequency. If I could match this model in practice, we would have a pretty good second filter for our DifX Dual Conversion Transceiver. It is really interesting to see how changing the bandwidth and Zin/out materially affects the shape of the curve. My latest run at this suggests that the Zin/out should be about 100 Ohms and the bandwidth 2.8 KHz which of course will place me in the league of the enhanced SSB guys. But those numbers are not the final values --just data.The combined plot below shows the filter shape and the SWR. Don't overlook the SWR as that impacts the filter capability to pass signals without distortion!
 

 
 
In Part 14 Larry, presents a plan on how to go about actually building a filter and it starts out with "get a bag of lots of crystals" and then "measure them and record the results in an Excel Spreadsheet" -- hmmm sound familiar?

The Elsie Software presents an idealized filter with crystals that are exactly alike (you'd like to see to 10 decimal places).  But that is not the real world as the Elsie software only captures just a few decimal places-- like out of the box maybe two or three places. As filter builders we in essence TUNE each crystal so that it will match the Elsie model. This is not a five minute job! But it can be done. In Part 14 Larry, show how he kept changing parameters and how that fine tuned the filter. I suspect the first time through the process will be like being 7 years old and waiting for Christmas and it is only June.

My first step is to identify and then  measure each crystal and finally record the results. Fortunately long ago I built a G3URR oscillator and will use my Raspberry Pi SDR radio (with Quisk) as the measurement tool or I might use my Omnia with HDSDR --so I have some choice here. Or my new Rigol Scope that has a built in frequency counter.

But having just run some tests using the Rigol Scope  "counter function option" has shown that will not work. The reason is that the scope (or my lack of knowledge on how to have this do something different) only takes you to 4 decimal places. I ran a few crystal into to the G3URRR test oscillator and the scope counter read 11.4998. This is not enough resolution! They tutorials all sing the same song. If you use an external oscillator method to excite the crystal for measuring Lm and Cm then you need to have a generator with a 1 Hz resolution. So you will need to have something as good to measure the crystal response.

My friend KV4QB, DuWayne was so kind to send me an SNA Jr. board and it will produce the 1 Hz resolution for the DDS generator. I have an Omnia SDR and with HRSDR can read the signals to 1 Hz. Keep in mind we want no more than a 50 Hz spread for a SSB filter and as I found out no more than 10 Hz for a CW filter -- the Rigol scope will work to get you into the ball park but will not tell you the seat number.

w0qe suggested several actual crystal filter design programs (including the AADE one I mentioned). But did indicate several that were more user friendly and so I will look to using one of the ones suggested.

For now I will have to take a short respite while I measure the crystals and record the results. Stay tuned. Yesterday using my handy Brother tape label machine I punched out 25 numbers and will start first by affixing the numbers to each of the crystals. Then I will need to clear a spot off of my work bench so I can do the measuring. I may get a chance to video the crystal measuring process as that may be helpful t those wanting to build the 11.5 MHz filter.



Too many projects and not enough time --but I will continue to work on the LM373 transceiver and keep you posted on its progress.

73's
Pete N6QW

 

Monday, May 1, 2017

A New Line of Transceivers ~ DifX

Transceiver Architecture 2.11

5~04~2017 1St Transmitter Tests of the LM373 Transceiver. Pure Joy.



A Way Forward is to Look Back.
Soon it will be time for another Solder Smoke Podcast (#196) and Bill, N2CQR and I hope to spend some time on "Old Time Homebrew Transceivers". Not necessarily that the transceivers themselves are old for they are both new; but that they involve older technology. BTW this is the 3rd year that I have been doing the podcasts which started with #161 in May of 2014.
 
[I am taking a short detour from the dual conversion DifX as I await my bag of twenty five 11.5 MHz crystal to arrive and then I will share (using Method #4) how I built the filter.]
 
In SS #195 I casually mentioned that my first solid state SSB transceiver used an amazing single IC from National Semiconductor (now a part of Texas Instruments) called the LM373 and that was in the early 1970's. My rig was on 20 Meters. My first introduction to this device was from an article in a 1970's QST by Tom Sowden, W6KAN and today a friend of mine. His project used a 9.0 MHz crystal filter and covered two bands with a single VFO -- the old 20/80 Meter trick. If you take the sum frequency of a 5.0 MHz VFO and the IF you get 14 MHz, and of course the difference 9 - 5 = 4 which is 75 Meters. In one case the VFO tunes forward and in the latter case backwards -- but hey two bands. That receiver circuit formed the basis of my transceiver project. My transceiver worked as did one built by my then next door neighbor, Bill K6ACJ. I think Bill still has his rig.
 
 
 
Below is the LM373 datasheet. It was good through 30 MHz which of course is 10 Meters.
 
 
 
But a real uptown version of an LM373, 20 Meter SSB Transceiver was built by Charles Hill W5BAA as detailed in a November 1973 Ham Radio Article.
 
 
 
It should be mentioned that I contacted Charley, W5BAA at the time this article came out and he sent me some additional data not in the article. Today Charley and I are still in regular contact and I even bought my first SDR kit from him which was the subject of a QRP Quarterly Article.
 
Charley's transceiver was absolutely amazing. His rig used three LM373's as this versatile IC had many applications. The primary use was as the main circuit element of the transceiver where it was used as the IF amplifier wrapped around a crystal filter and as the product detector on receive and as the transmit mixer on transmit. A second LM373 was wired as the Receiver Mixer stage. In W6KAN's receiver it used a 40673 dual gate MOSFET for the mixer stage. The third LM373, in W5BAA'a rig was used as the Balanced Modulator.
 
In most of my rigs I use double balanced mixers such as the SBL-1 or, TUF-1 or even the ADE-1L for the front end Rx Tx mixer stage and a second DBM is used as the Product Detector on receive and as the Balanced Modulator on transmit. This approach facilitates making a singular connection of the LO to one DBM and the BFO to the second DBM. So there is no switching of these signals as you go from receive to transmit. This saves some wiring and the need to do switching of signals. What we have is the bilateral approach where the signals are sent left to right in the IF for receive and right to left on transmit.
 
But another non-bilateral method is to send signals a single way through the rig and at either end you change the elements. On receive the input side has the receiver mixer and on the back end is the product detector. In transmit the front end has the balanced modulator and the back end the transmit mixer and RF chain. In this case circuit elements  must be steered to the front and back ends of the IF block depending whether you are in receive or transmit.
 
In W5BAA'a rig his signal steering involved the use of a property of diodes to act as switches. When diodes are biased properly they appear as a closed switch. Change that bias and it is like an open circuit. In the receive mode the VFO is "steered" to the LM373 acting as the Receiver Mixer and the BFO is steered to the product detector port on the main LM373. In the transmit mode the BFO is now steered to the 3rd LM373 that acts as the Balanced Modulator and the VFO is now steered to the port where the BFO was connected. Interestingly the output pin that connected to the audio chain on receive is now connected to the transmit circuitry in the transmit mode. [An Italian ham built a vacuum tube transceiver ( I think it had five tubes) and the audio output tube became the final RF amp stage on transmit]. Charley's rig put out a whopping 5 watts --so certainly QRP.
 
So based on my utterance on SS#195 I decided to recreate my first solid state transceiver circa 1970 using the LM373. I thought this would be blast although my first easy decision: NO Analog VFO and No Crystal BFO. Going back in time is fun; but using an Analog VFO is like doing brain surgery with a rusty spoon!
 
I also decided that I would use SBL-1's for the receiver mixer and the balanced modulator. It wasn't until I had finished cutting a PC Board and began installing parts that I realized that I really only needed one SBL-1 as it could have functioned as the Receiver Mixer on receive and as the Balanced Modulator on transmit. Should have noodled more on this one. But true to Charley's design I have used diode steering. Well if I ever build a version #2 that would be a change.
 
I also decide to use two separate Band Pass Filters as this simplified switching. My board at this time has only the receive one installed but the second unit is on today's work list. The Rx RF amp is a single 2N3904 as will be the transmit pre-driver stage.
 
So where is my project now? Well after wiring it up I discovered it didn't work and then right in the middle of doing some trouble shooting my Hantek 200 MHz Digital Storage Scope shot craps and it was toast! I always thought it acted flaky. So I had to get a new scope which I settled on a Rigol 100 MHz DSO --good move and works much better (Model #DS1102E).
 
Initially I had chosen to build the LM373 rig using a mix of mostly surface mount parts and a few leaded parts and my board layout was designed for that purpose. I should mention that I had a small stash of LM373's left over from 1970 and some were the 10 Pin TO-5 style while others were the 14 Pin DIP. 10 is less than 14 and so the board was designed using the 10 Pin can style. When the rig didn't work, my first thoughts were bad devices and so I made an adapter board so that the 14 Pin Dip style (using sockets) was retrofitted to the board. Still didn't work --thus had to wait on the new scope arrival.
 
Well 5 minutes with the new scope convinced me the problem was N6QW. What I hadn't mentioned was this was the first time I used a new soldering iron and what I had done is applied too much heat to many of the SMD parts and they were burnt "open". If you put the scope probe on one side of a cap, say on the LO side, you get a nice signal and plenty of juice. Put the Scope on the other side and you get almost nothing. So step one: change out all of the SMD coupling caps (where I found several others that were open) to leaded versions and boom it worked. At this point just the receiver is working as I am awaiting some parts (coming with the bag of crystals) to finish up the transmitter stages and then we can have another go with my 1970's SSB transceiver. Maybe before the next podcast
 
Here for your listening pleasure is the LM373 SSB transceiver vintage 1970's
 
video
 
 
 

 
 
 
As was stated the way forward is to look back -- signal steering would not be my 1st choice today; but in 1970 it was the right thing to do. The LM373 has a built AGC circuit and it can even be made adjustable --that certainly is a desirable feature in today's rigs. I have no good explanation why there was not more widespread use of the LM373. But for a low part count rig --it is perfect.
 
Oh you will have a terrible time finding LM373's (I happen to have some in the bin's) so for those who want to replicate this rig -- just buy a Bitx40 kit and move on.
 
When I get the transmit side working I will share that with you. Lets have a cheer for W6KAN and  W5BAA and the amazing 1970's --remember the  even / odd gas days.
 
73's
Pete N6QW