Build Project: Dewster's D-Lev Digital Theremin

Posted: 2/11/2019 11:38:21 AM
pitts8rh

From: Minnesota USA

Joined: 11/27/2015

Dewster and I have been looking at some pitch noise that occurs at the very fringe of the pitch field, or at least mine does (We may be looking at different things).  My noise has been showing up on the tuner at very low pitches, too low to really be obviously audible unless you are specifically listening for it.  It appears as a constant low frequency (about 4.6Hz) periodic pitch variation, much like a vibrato,  whose amplitude is a function of the tuned pitch oscillator frequency.  By adjusting the antenna length to tune the pitch oscillator, frequencies may be found where the vibrato is nulled or maximized, and these frequencies are not unique.

A while back I had noted sidebands with 60Hz spacing on my pitch oscillator when viewed on a spectrum analyzer set to a narrow resolution bandwidth.  Even though I didn't understand how this was being generated I decided to play with the earth-grounding of the theremin, and lo and behold the harmonics were considerably worse with earth grounding than without.  Grounding the theremin to a large aluminum plate instead showed a cleaner spectrum (although 120Hz sidebands are still present), and the pitch as viewed on the tuner is now nearly perfectly stable down to even 1Hz.  I don't yet understand what is happening here, but I need to revisit my board layout grounding to see if I did something stupid.  In any event the theremin seems to operate quite well merely grounded to a metal plate.  I could have a local ground noise problem as well.

Anyway, here are two plots of the pitch oscillator sidebands over a narrow resolution bandwidth and also one plot of harmonics over a wide band to show up through the 7th harmonic (for Dewster). Comments are noted at the bottom of the plots.

 

Posted: 2/12/2019 2:48:25 PM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

Fascinating Roger!

As an experiment I ungrounded mine and tried various capacitors to ground.  0.01uF (103) gave pretty similar results to hard grounding, smaller values gave a ton more hum.  I believe it would take an enormous plate to get 0.01uF intrinsic (a 180m diameter sphere by my calculations).

I think the large plate reduces the noise because it's flapping around in the same 60Hz environment the Theremin is in.  Too bad it isn't a more physically practical solution.

I don't understand what the source of the 4.6Hz could be, nor why the 120Hz spur series always seems to be larger than the 60Hz series.

Also, thanks for that third analyzer view!  I was expecting no significant energy at even harmonics, but some at odd harmonics due to the square wave drive of the LC tank, which acts like a low pass filter.  This suggests that a 4:1 coil ratio, with the volume axis frequency placed below the pitch axis frequency by a 1:2 ratio would probably work without interference.

Posted: 2/18/2019 10:33:01 AM
pitts8rh

From: Minnesota USA

Joined: 11/27/2015

New AFE2 PCBs and Inductors

Here are some pictures of the new second-generation Analog Front-End (AFE2) boards that I laid out from Dewster's updated design. On the outside they are not very different from the AFE1 boards except for the use of a more compact IDE ribbon connector in place of the previous RJ45 connector.  Although an 8-pin cable connector is used this board requires only four lines, one less connection than the previous design, with the rest of the pins either unused or allocated to ground lines adjacent to signal lines. Where the previous board sent both zero and quadrature outputs back to the FPGA, this one sends only the XOR'd result of these signals with the intention of eliminating any threshold sensitivities that may have been caused by voltage variations within the FPGA itself. So for inputs the new board board requires ground, +5v (locally regulated to 3.3v), the same square wave oscillator drive provided by the FPGA board, and one RF sense signal taken directly from the antenna via the green terminal block connector.  There are two outputs: the inductor drive (also at the green connector) and the XOR output sent back to the FPGA via the ribbon cable.  

PDF schematic of the AFE2.

This is probably the last DIY version of this that I will make since I want to send out for a test batch of both through-hole and SMT boards:

Although not directly related to the AFE2 design change, Dewster is also experimenting with decreased inductance values to push both the volume and pitch oscillators to higher frequencies.

Here is my new 0.5mH pitch inductor made with 26AWG wire, replacing a 2mH inductor.  The pitch oscillator is now running at about 1.89MHz.

And the new 1.0mH volume inductor (previously 4mH).  The volume oscillator runs at roughly 1.23MHz.

I'm still using the AFE1 boards to test these new inductors, but with Dewster's new pitch-tracking hum filter and other changes, the stability at low frequencies is uncanny.  As a test the pitch field can be calibrated to "zero beat" way out past the volume antenna and you can go down to a 1Hz metronome tick with the tuner still showing a steady display.  If you have gorilla hands and you like to play with 12" octave spacings, you can probably set the D-Lev up to do just that, with adjustable linearity as well. 

If this isn't the closest thing to the Holy Grail of pitch-field configurability, you'll have to show me what is.

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