Let's Design and Build a (mostly) Digital Theremin!

Chill Out!

Just did a simple tempco test on two coils.  The first was coil #3 from the Q testing:

Stuck it in a -20 C freezer for an hour, then connected it to my LC meter.  As you can see on the meter in the photo, the frosty coil measured 1.693 mH, and this didn't change by even one count over the course of warming up to 17 C room temperature.  Which means the coil is stable to better than:

  0.001 / 1.693 / (17 - (-20)) = 16 ppm / C

I'd have to connect it to the D-Lev to measure with greater resolution and get an actual number here.  For reference, a typical crystal oscillator is good for around 0.25 ppm / C.

The second coil tempco tested was the Bourns 6310-RC RF choke which employs a ferrite form (testing shown here: http://www.thereminworld.com/forums/T/28554?post=224024#224024).  It measured 50.32 mH @ -20 C and 51.36 mH @ 17 C:

  (51.36 - 50.32) / 51.36 / (17 - (-20)) = 547 ppm / C

So the ferrite Bourns tempco is at the very least 547 / 16 = 34 times worse than the air core solenoid!  So on the basis of this simple experiment alone, I will not be using ferrite coils in any of my designs (famous last words).

T-COIL V2

Slightly newer t-coil program posted: https://d-lev.com/research/tcoil_v2_2024-04-25.zip

You could always do any of the supported forms using the "choke" command, but breaking them down into brooks, solenoid, and donut helps the user by eliminating some of the necessary input flags.  Now flat spirals are similarly simplified via the "spiral" command.  And -awg & -b can now be used instead of -wcd & -wd, which makes for less back and forth with the "wire" command.

Theremin Drift

Interesting paper on air dielectric capacitors: https://www.osti.gov/servlets/purl/1504063

Two graphs from it:

The top graph is capacitance change in uF/F or ppm vs. relative humidity, the lower graph is ppm vs temperature C.  The lower one in particular is rather astonishing, with 25 ppm or more per degree C.  I suppose one could put a temperature sensor in the Theremin and try to compensate for this, but it would need really fine grained control so as not to disturb any performance going on, as well as a good ambient location far from heat sources / heavy breathing.

The commonly dispensed wisdom is that ferrite transformers in both the fixed and variable Theremin pitch oscillators will drift together, and therefore largely cancel.  But this can only be true if either:

1. There is no series EQ coil (with very likely much higher temperature drift ferrite composition than the tank IF coil)
 - or -
2. The fixed oscillator also employs an EQ coil (which is never the case AFAIK).

Anyway, the temp / humidity drift of air is much smaller than that of the Bourns coil tested above, and much larger than the D-Lev coil drift.

A Brief History of Ferrite

https://ethw.org/Milestonesevelopment_of_Ferrite_Materials_and_Their_Applications,_1930-1945

The colon & capital D get turned into .  Even sticking it in a link or code is messing up here.

TDK patented the electronic use of ferrite in in 1932, and started commercializing it in 1937.  As usual, war screwed up progress for a long while, then Philips developed more exotic formulations in the 1940's.

So Theremin couldn't have used ferrite in his early designs as they pre-dated its widespread general usage.