Exorcism Not Quite Complete -- Thinking of Other Options

As often happens, I may have jumped the gun in declaring the exorcism of my 17 meter transmitter to be a success. As readers of this blog will recall, my problem was that when trying to "net" my separate 17 MHz receiver and transmitter, at around 18.116 MHz I could hear more than one tone as I tried to get to zero beat.  The 8th harmonic of my 5.176 MHz carrier oscillator was mixing with the 23 MHz VXO signal and producing a spur.  I could probably knock the level of this spur down below FCC limits, but -- and here is the problem -- I probably could never knock it down to the point that it would not be audible in the sensitive receiver that sits right next to the transmitter.  So this is really a netting problem, not really a spur problem. 

I don't want to try another filter frequency -- I have VXO crystals that really work only with a filter at 5.176 MHz.

So here is my current idea:  Build a receiver board and turn this thing into a transceiver.  Switch with relays the input and output of the 5.176 MHz filter, and use relays to switch to the receiver board the VXO and carrier oscillator signals. 

Making this thing a transceiver would eliminate the need for netting.  This should solve my problem. 

What do you folks think? 

73  Bill 

Roy Lewallen W7EL Gives Us All EZNEC for FREE! Thanks Roy! (Video)

Free download here: https://www.eznec.com/

Thanks to Roy Lewallen W7EL for this great gift to the amateur radio community.  

And thanks to G-QRP for the excellent video (above) of Roy talking about antenna modeling.  

Exorcism Completed! Getting Rid of the Spur in my 17 meter SSB Transmitter using a TinySA (video)

To re-cap:  The problem became evident when trying to "net" or "spot" my transmitter onto my receive frequency.  Around 18.116 MHz, I could hear at least two tones in the receiver as I moved the transmitter frequency.  I needed to get rid of the extra tone. 

First, thanks to all who sent in suggestions.  They came in literally from around the world, and this is a demonstration of the IBEW in action.  I used or at least tried all of them.  They were all good ideas. 

Following Vasily Ivananeko's pseudonymous suggestion I rebuilt the carrier oscillator (apologies to G3YCC).  I used the carrier oscillator/buffer circuit from Farhan's BITX20.   

Henk PA0EME said I should look at the signal level at the input ports of the NE602 mixer.  Henk was right --- the VXO input was far too high.  I lowered it, but the problem persisted. 

At first, I thought that the spur in question was so small that it would not show up on the air.  I could not see it in the TX output using my TinySA spectrum analyzer.  That was good news and bad news:  Good that it was not showing up on the air, bad that I could not see it in the TinySA and use that image in the exorcism. 

At first I thought that the spur was being caused by the 10th harmonic of the carrier oscillator and the third harmonic of the VXO.  This seemed to fit.  So, following VK3YE's sage advice, I built a little 69 MHz series LC trap (using a coil sent by AA1TJ, on a board CNC'd by Pete N6QW).  That trap succeeded spectacularly in crushing the 10 harmonic.  Look at these before and after shots on the TinySA: 

Before Trap

After Trap

Spectacular right? But guess what?  The problem was still there.  

I scrutinized the situation once more. I realized that the spur would be more visible if I put the TinySA on the input of the transmitter's PA (a JBOT amp designed by Farhan) as opposed to putting it on the output.  Watching the spur and the needed signal move in the TinySA as I tuned the VXO, I realized that they were moving in opposite directions.  This indicated that the spur was the result of a carrier oscillator harmonic MINUS a VXO-generated frequency (as the VXO frequency increased, the spur frequency decreased).  Looking at my EXCEL spread sheet, I could see it:  8th harmonic of the carrier oscillator MINUS the main output of the VXO. 

To confirm this, I plugged the values into W7ZOI's Spurtune program.  Yes, the spur popped up and  moved as predicted.  

For further confirmation I shut down the carrier oscillator by pulling the crystal from the socket, and then just clipped in a 5.176 MHz signal from my HP-8640B signal generator (thanks KB3SII and W2DAB). Boom!  On the TinySA, the spur disappeared.  Now I at least knew what the problem was:  a harmonic from the carrier oscillator.  

Following good troubleshooting practice, I turned off the gear and went to bed.  When I woke up, an idea came to me:  Before launching into a lot of filtering and shielding, just try running the carrier oscillator at a lower voltage, seeing if doing so might reduce the harmonic output.   I disconnected the carrier oscillator board from the main supply and clipped in a variable voltage bench supply.   Watching the signal on my TinySA, I watched as the spur completely disappeared as I reduced the voltage from around 13V to 10V  (see video above).  The main signal frequency level did not change much.  I tested this by listening for the hated extra tones.  They were gone.  Exorcised.  

Key lessons: 

-- Spur problems are difficult to troubleshoot.  Armstrong's superhet architecture is, of course, great, but this is definitely one of the pitfalls.  Single conversion makes life easier.  IF selection is very important. Choose wisely! 

-- When looking at the TinySA as you tune the rig, pay attention to which way the spur is moving.  This provides an important clue regarding the combination of harmonic you are dealing with. 

-- The TinySA is a very useful tool.  It seems like it is easier to use than the NanoVNA (which is also a fantastic tool). 

-- It can be fun and rewarding to re-visit old projects.  In the years between original construction and the re-look, new test gear has become available, and the skill and experience of the builder has improved.  So problems that once seemed insurmountable become fix-able. 

-- Thinking through a problem and thinking about possible solutions is very important.  It pays to step away from the bench to think and rest.  Rome wasn't built in a day. Here's a rough block diagram that I drew up (noodled!) while trying to figure out this problem: 

1BCG -- The 100th Anniversary of the Trans-Atlantic Test

Thanks to the Antique Wireless Association for this really wonderful video, and for their involvement in the 100th anniversary event.  Special thanks to Ed K2MP. 

On December 11, 2021, the 1BCG team in Connecticut had some technical difficulties.  As we all know, that is part of being a radio amateur. Details of the problems are presented here: 

http://1bcg.org/1BCG/the-special-event-transmitter/

Phil W1PJE managed to hear and record some of the 2021 transmission (Thanks Phil).  Listen here: 

https://drive.google.com/file/d/1uPvD9Qh-VJTnyDzOPPSrYfbksks8sQsx/view?usp=sharing

Phil also sent this spectrogram of the signal. 

Good thing Paul Godley ran into Harold Beverage on the ship going over. 

And imagine me complaining about having to step out into the carport to adjust my antenna -- Godley had to trek one mile THROUGH SEA-WEED to adjust his.  Respect.   

SolderSpace! N2CQR from Geostationary Orbit

 

Farhan VU2ESE kindly invited us to talk to his Lamakaan Amateur Radio Club.  They did a simulcast through the QO-100 Geostationary Satellite.  This picture shows N2CQR being beamed into India from 22,500 miles.   Note the ET-2 and the Mythbuster on the bench.  This was a lot of fun.  Thanks Farhan! 

Straight Key Night 2021/2022 (Videos) -- Happy New Year!

I made a few contacts on 40 meter CW using my old Hallicrafters HT-37 and Drake 2-B. 

Happy New Year to all!  73   Bill 

An Interview with Paul Lutus (Audio)

https://corecursive.com/remote-developer/

Thanks to Bob Scott KD4EBM for alerting us to this wonderful interview. 

Troubleshooting Apollo: 23 MHz Crystals in a NASA Ground Receiver

More amazing Apollo stuff from CuriousMarc. 

Here we see them struggling to find the proper frequency for one of the oscillators in a dual conversion UHF receiver from the Apollo program.  For the VCO, they needed a crystal in the 23 MHz range. They faced the same questions we face:   Series or parallel?  Load capacitance?  Fundamental or overtone? 

It just so happens that at this moment I have on my bench the 17 meter SSB transmitter that I built some 20 years ago.  And the VXO in it uses crystals in the 23 MHz range.  TRGHS. (More on the spur problem with this rig soon.  The solution does involve the 23 MHz VXO.) 

Very cool that CuriousMarc found a manufacturer still willing to produce custom-made crystals. JAN flashbacks!  LapTech Precision in Canada: https://www.laptech.com/index.php

The video above is Episode 8 in the Apollo Comms series.  If you go back one episode, you can watch Marc and his assistant troubleshoot the NASA Apollo UHF receiver.   They use very familiar troubleshooting techniques.  This reminded me a lot of what we do with older, potentially modified gear.  They were able to figure out what was wrong and  how a mod had changed things.  This set the stage for the crystal replacement selection we see in Episode 8.   Here is Episode 7: https://www.youtube.com/watch?v=87qA41A_Ies

Note:  The frequencies in this Apollo receiver were listed in Megacycles, not Mega Hertz. 

Thanks to Bob Scott KD4EBM for alerting us to this. 

McCoy SSB Crystal Filters (1963) -- But Apparently NOT the Real (Lew) McCoy

 

Last month we were talking about this company.  Someone thought it was run by Lew McCoy of ARRL Homebrew fame, but it now appears that our Lew McCoy was not involved in the company. 

Note how they provide TWO carrier oscillator/BFO crystals for each 9 MHz filter, one for USB, the other for LSB. 

They were pricey too:  In 2021 dollars, that Golden Guardian would cost $390. 

Thanks to the K9YA Telegraph for posting the ad. 

My Kind of Chip: A Homebrew Discrete 555 Timer Built on Wooden Boards (video)

This is really beautiful. Radraksha Vegad (Pargrahi) from India built a discrete component version of the venerable 555 timer chip.  He built it on wooden blocks.  This leads to the kind of understanding that even Jean Shepherd would have admired.  No longer is the 555 a little mysterious black box.  No, Pargrahi shows us how it works.   

 I know we could do something similar with the NE602 or the LM386.  But probably not with an Arduino microcontroller or an Si5351.  And that says something about understanding and complexity. 

Thanks Radraksha.  And thanks to Hack-A-Day for alerting us to this: https://hackaday.com/2021/12/20/all-hail-your-new-giant-555-timer-overlord/#more-512230