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RockMite Sidetone Spectrum: Why That QRP Beep Sounds So Square

 One of the first things I realized about my RockMite, (the tranciever at the heart of Project TouCans, our low power, Morse code only, ham radio), is that the sidetone output is, shall we say, LOUD!

This works for me since my hearing isn't great and I'm frequently operating from urban environments where there's lots of ambient QRM in the air around me: horns, crowds, sirens, and so on.

I'm often reminded of that the sidetone is loud by other posts I find00 about working on old RockMites. The author usually starts by mentioning that theyauthor turned down the sidetone volume by changing the circuit. The post that comes to the top of my mind is Taming the Wild Rockmite by G. Forrest Cook. It has a great section on boosting the output power Rockmite's that I've put to good use.

In any event, while workking on training an AI to decode Morse code, I started looking at spectograms of the Project TouCans audio output. Here's what I saw 

Those vertical bands that cover the entire (vertical) frequency spectrum are the RockMite's sidetone. I also used an audio processing app, included in Davinci Resolve, to look at the frequency spectrum of the rig's sidetone:


Check out the harmonics all the way out to the edge of the analyzer's frequency cutoff. This is exactly what I'd expect to see if an op-amp was being overdriven and clipping.

At this point, I was joined by Tawnse, the 10 year old.

When Tawnse and I looked at the RockMite's schematic, we found out that the sidetone doesn't pass through an op-amp. It's created using an unfiltered, unamplified square wave from the keyer's controller chip!



We looked up square waves on Wikipedia and found out they can be described by Fourier series with odd harmonics of a waveform. 



Next question. Was the spectrum we saw actually a square wave?

We had the graph from DaVinci Resolve, but we needed finer frequency measurements, so we used an overlaid chart on a Google Sheet that we set up to match the scale and then added 400 data points to it, all with value 1. That gave us a straight line where we could read off the frequency with the mouse!


Here are our measurments:

MeasuredHarmonicsError%
Odd
680
2040204000.00%
3400340000.00%
Even
1360136000.00%
27002720-20-0.74%

 Sure enough, mostly it's a square wave. There are even harmonics, but they're not as prominent as the odd ones. It was also really cool that the harmonics lined up as well as they did.

This is all a lot of fun! I set out to train an AI to decode Morse code. I still hope to accomplish that. In the mean time though, while waiting for it to train, Tawnse and I got to study square waves and Fourier series. Will she remember every single detail? Probably not, but I think she'll remember a few, and she'll see the whole thing again as work through all of this.


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