Friday, November 13, 2015

Simpleceiver ~ Part 9

More Investigation of the J310's configured as a Dual Gate MOSFET

Addendum #1: Decoding the Suffixes.
I am glad that every day I have a chance to learn something new and realize that being an "old dog" I can still learn some new tricks! A comment posted on Part 8 by Chris Horner made me realize that my old habits frequently trip me up.
Having started soldering my fingers together early in 1950, many electronic conventions have changed (all for the good) but sometimes they can trip us up. Take for instance how capacitor values were named. A 470 PF cap today was called a 470 micromicrofarfad or 470 MMF. A 100 Microfarad today designated as 100 Ufd, was written back then 100 MFd.. This now leads me to C1 and C7 and their designation on the simulation presented in Parts 8. The values written on the earlier schematic were 100 mF and 10 mf (harkening back to 1950 where I would have said 100 Microfarad or 10 Microfarad). But that is not how LT Spice really recognizes values where  m = million. So thanks to Chris I once again have to wash my brain of the old habits.
His input caused me to look at what I had done and what would be done by those who build the J310 Product Detector. First let me say that I actually built the circuit with C7 using a 100 Microfarad electrolytic and C1 is a 10 Microfarad electrolytic. But this only adds more weight to the value of LT Spice so long as you enter the right values. Here is a short demonstration of why it is important to use the proper values in the circuit. Note C7 is shown as 100 mF and C1 is 10 mF on the earlier schematic.
So then I asked myself what really happens if one were to deliberately change the values of C1 in the simulation.
The first plot is with C1 being changed from a 10 Microfarad (or so I thought) to a 100 NF or 0.1 Microfarad. Wow the low frequency gain has really been impacted. The solid line shows that at the low end the gain is 12 dB less than what we were seeing previously and pretty much ignores the audio range.
 The next plot is where C1 was changed to 10 Microfarad and we see the low end has now popped back up to what we had seen earlier. There is only about a 0.5 dB shift at the low end --so very acceptable.
Next we have the case where C1 is changed to 100 Microfarad and this now also aids in getting the higher frequency to cutoff so that the output is now pretty much in the audio range. This plot now looks like what was presented in Part 8.
Finally here is the revised schematic and I am now suggesting that C1 be changed to a 100 Microfarad electrolytic (10 is very acceptable but for the purists 100 is better). I now think I should be using a "u" instead of an "m" when designating large capacitors. I probably ought to read how to use LT Spice --guilty! I am a button pushers who doesn't read the instructions--but it does get me into trouble and frequently! Thanks Chris as your comment will be helpful to the greater ham homebrew community.
Stay Tuned -- the next post will be on the Arduino Pro-Mini and AD9850 used as the Local Oscillator. If you want to jump ahead we are using the code developed by AD7C. With the Arduino/AD9850 you will have all of the tools to build the Direct Conversion configuration of the Simpleceiver. Love that cool blue color! A simple code change later, will also enable the LO to be used with the Sinpleceiver when we install the crystal filter.
Pete N6QW
Addendum #1
Thanks to one of the blog readers we have the decode that I had I read the LT Spice User's Guide first I wouldn't have goofed up! Then again I usually don't consult the roadmaps until I am really lost!

From the LTspice Users Guide:

Suffix Multiplier
T 1e12
G 1e9
Meg 1e6
K 1e3
Mil 25.4e-6
M 1e-3
u(or ยต) 1e-6
n 1e-9
p 1e-12
f 1e-15

The suffixes are not case sensitive.

Pete N6QW