Mythbuster Videos 8 and 9 -- The Old Military Radio Net plus "Zero Beat and The Vertical Skirts"

I like to listen to the Old Military Radio Net on Saturday mornings. This week I was listening with the Mythbuster receiver. The AM carriers provided a good opportunity to observe the effects of the steep skirts of the 10 pole crystal filter. We start at zero beat, with the BFO exactly on the carrier frequency. If I turn the VFO dial in one direction, I in effect move the passband in a way that puts the carrier in the passband. And it is no longer zero beat with the BFO, so we hear the heterodyne (the beat!). But if I turn the VFO dial in the other direction, the carrier is now outside the passband. Even though the BFO would produce a tone, we don't hear a tone, because those steep filter skirts are keeping the carrier out. We do continue to hear some of the sideband frequencies, because they remain in the passband. The very sharp drop-off of the carrier tone is a good indication that the steep skirts of the crystal filter are doing the job.

"Zero Beat and the Vertical Skirts" Sounds like the name of a Punk Rock band, doesn't it? Anyway in this video I explain what happened in Mythbuster Video #8 (above). I explain why we can hear the Old Military Radio Net carriers when I tune the VFO in one direction, but not in the other.

Michael Newton Hopkins, AB5L, Author of the FMLA series

Better than anyone else ever has, Michael Hopkins, in his fictional series about Frank Jones and the Five Meter Liberation Army, captures the spirit of homebrew radio.  There is just so much of us in those articles.  I read them some 20 years ago when they first came out; reading them again recently I appreciated them even more.  

Frank was a bit of a curmudgeon:  There are jabs at the appliance operators, Hiram Percy Maxim, hamfests, SSB, the Collins collectors, the QRP movement,  and even Electric Radio magazine.  Howard Armstrong makes an appearance, as do Carl and Jerry.  It all made me want to put a five pin SAW filter on my lapel.  

As I read, I thought about what a great writer Michael was.  When I Googled him, a few of the results led me back to  my own book.  I'd forgotten that Michael was in there, but he is.  On one page he advises me how to power my Mate for the Mighty Midget receiver without using a power transformer (a very Frank Jones approach).  On another page I note that Michael had sent me a kit for the Doug DeMaw "Barbados Receiver."  Wow, that was my first Superhet.  (I also have one that was built by Dale Parfitt.)  Most of the parts were put to use in other projects.  But I still have the board (see above).  Reminded that it came from Michael, I will now have to complete the construction.

Below is a nice article about Michael that appeared in the Flying Pigs newsletter. (Click on the images for an easier read.)   

The articles can be found here: 

http://soldersmoke.blogspot.com/2021/07/summer-reading-for-homebrewers-frank.html

Michael's 2005 Obituary:

https://obits.dallasnews.com/us/obituaries/dallasmorningnews/name/michael-hopkins-obituary?pid=14406223

Thank you Michael.  VIVA EL FMLA! 

Alan Wolke W2AEW's Great Video on Using NanoVNA to Measure Amplifier Input Impedance and Gain

Alan Wolke W2AEW is a true wizard.  We are all lucky to be interested in homebrew radio at the same time that he is sharing his knowledge and wisdom via YouTube. 

The ability of the NanoVNA to measure circuit impedances is, in my mind, one of its most valuable features.  With this, we can MEASURE input and output impedances.  We can put bits and pieces of circuitry together without wondering whether or not we were introducing impedance mismatches.   

But I had trouble getting good NanoVNA impedance readings on my TIA amps. I wrote to Alan about this and he pledged to make a video about how to do it right.  That video was posted to YouTube today (see above).  

Not only did I learn how to get a good impedance reading, I really learned a lot by just watching Alan move around through the various NanoVNA screens.  I want to be able to do that too!  I want to monitor the Smith Chart, and gain, and SWR, all at the same time.  Yes I do!  I also now realize that I have to order a bunch of those cool PC board SMA female connectors from Bezos. 

Thanks a lot Alan.  

Frank Jones's 1936 Radio Handbook

 

https://worldradiohistory.com/BOOKSHELF-ARH/Handbooks/Radio-Handbook-4-1936.pdf

Mythbuster Video #7: Bandswitch, Reverse Polarity Protection, CW with Clarifier Offset

I have the speaker mounted on the front of the board. I kind of like it like that. I now have a bandswitch, and reverse polarity protection (no more living dangerously for me). That Yaesu VFO clarifier circuit might prove useful should I decide to give this rig CW capability. I once again find myself thinking that I might never put this in a metal box. Frank Jones had the right idea.

Mythbuster Video #6 -- On to 20 Meters (But With Bandpass Filter Woes). Please help solve the mystery!

Here's how I started with the Elsie program. Note that to get a 50 ohm match on both ends it needs an impractically low value for the coils (.064 uH). 

But Elsie lets you specify the coil value.  So I then I went with 1 uH.  But with this value you don't get 50 ohms at either end.  You need a matching network.  Elsie provides this too!  

I asked Elsie to match my BP filter to 50 ohms.  It provided several options to do this --  I went with a simple capacitive impedance divider.  But alas, I was now bumping up against the 7 limit of the free version of Elsie so I had to reduce the number of LC elements from 4 to 3.  Bummer. 

With 3 LC tuned circuits and matched to 50 ohms the plot looks OK.  But I would have preferred 4 LC circuits. 

 

The rftools website created a BP filter for me with 4 LC elements, and matched to 50 ohms.  Very useful. https://rf-tools.com/lc-filter/

But here's my problem:  With both the filter designed by Elsie and the one designed by rftools, I found that the filter passband was too low.  It was in the 12 - 13 MHz range.   I found that by removing 3 turns from the 1 uH coils I could move the passband up to the desired range.  But why the discrepancy?  I was measuring the coils and the caps with an AADE meter.  I was testing the passband both with a NanoVNA and with a combination of an HP8640B sig gen and a Rigol oscilloscope (with the filter terminated into a 50 ohm resistor).  Any suggestions on why these filters should have passbands lower than predicted would be appreciated. 

Mythbuster Video #5 -- More Listening on 75

Mythbuster Video #4 -- First Signals, 75 meter Bandpass Filter, Yaesu VFO output

This receiver required almost no coaxing or tweaking, probably because I had been so careful about testing and measuring each of the stages. 

I have been pleasantly surprised at how well the receiver works without an RF amplifier ahead of the first mixer.  But I need to know how much AF gain I have in order to understand how/why the entire receiver works so well.  I think I have about 35 db of gain (combined) through the two TIAs and the crystal filter.  That would mean that all of the remaining gain is provided by the AF amplifiers (with some loss in the product detector).   I haven't really measured the gain of the AF preamp/LM386 combo, and I had some trouble measuring the input impedance of the pre-amp with the NanoVNA.    

The 75 meter LC filter to the left of the VFO is actually a bandpass filter, not the lowpass filter. And what I call "the mixer" to the right of the VFO is really the Product Detector/BFO.

For the 75 meter bandpass filter, I used the ELSIE program. 

75 meter Bandpass Filter designed in Elsie.  10 turns on a T50-2 toroid yield .46uH.

Here's the plot from Elsie on the 75 meter BP filter.  

Alan W2AEW asked for a picture of the VFO output. 

On this shot I had the probe between theVFO and the 

outboard booster amp that I built to bring it to 7dbm. 

Mythbuster Video #3 -- Using the VFO from a Yaesu FT-101

FT-101 VFO

I used LTSpice and Wes's FBA program (from LADPAC) to come up with a circuit that would provide the needed gain. I needed to get the 290 mV rms signal (across a 50 0hm load) up to the 500 mV rms signal needed by the ADE1 mixer.  Above is the amplifier that I came up with.  The key here is to adjust R2 and R1 to get the required gain. 

Mythbuster Video #2 -- 10 Pole Crystal Filter

This is a 5.2 MHz crystal filter. I used the G3UUR method for determining the crystal's motional parameters. I then used Dishal and AADE software to design a 10 pole Cohn Min-Loss filter. I tested the bandwidth with an Antuino Scalar Network analyzer (thanks Farhan!) and a NanoVNA. I found the passband to be a bit tight for SSB, so I replaced the capacitors with caps of a slightly lower value -- this broadened the passband. It is still a bit tight, but the SSB audio -- while not enhanced or Hi-Fi -- sounds fine.  

Passband filter shape as seen in the Antuino and in the NanoVNA. The -20 db line in the Antuino actually corresponds to no loss. 

The schematic provided by the AADE software.  Dishal software may have come up with better, more correct values for the capacitors. 

The passband as predicted by AADE.  Skirts so nearly vertical as to strike fear in the hearts of SDR owners! 

Filter under construction -- waiting for the caps from Mouser. 

First scan with the NanoVNA.  Insertion loss looks very high but that is only because I am terminating the filter with resistors -- I just wanted to see the passband shape.