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.

Posted: 3/3/2019 12:05:45 AM
tinkeringdude

From: Germany

Joined: 8/30/2014

For some reason just discovered this building thread. Damn nice case, with the wooden side panels!
What effect does placing the coils close to the antennae have?

You mentioned winding them either on the lathe or by hand.
For what it's worth, for the rare occasions that I wind coils, I use the simple wooden contraption I built, to fix my Li-Ion battery powered drilling/screwdriving thingy (whatever it's called in English) onto, with "analog" speed control on the grip, which is nice for not ripping things apart.
The rod that's inside there instead of a drill, to which a coil base gets fixed, also has a painted-black cardboard disc around it, with one index hole in it. There is an LED on each side, one just shines, the other of same type is abused as receiver, amplified by 1 transistor or so, connected to the counter input of my el cheapo Chinese signal generator - et voila, my short attention span is not a hindrance anymore to make a coil with a certain number of turns
(no, the thing does not produce moon shine)

I saw you etched the PCBs yourself. Have you tried services like JLCPCB? People are very happy with the quality, they're fast & cheap, and they make the vias for you

Posted: 3/13/2019 10:29:46 AM
pitts8rh

From: Minnesota USA

Joined: 11/27/2015

"For some reason just discovered this building thread. Damn nice case, with the wooden side panels!
What effect does placing the coils close to the antennae have?" -tinkeringdude

The presence of the coils within the pitch or volume fields on the D-Lev does not negatively affect playability after the effects are calibrated out.  That is not to say that they don't have a large effect on the oscillator frequencies; they do, but this is the case with just about any theremin that uses compact inductors in series with the antenna capacitance.  The impedance over the length of the inductor itself transitions from low at the drive end to very high at the antenna end, and so you have to make a decision where you place the inductor.  If you put it near the circuit board and run a long wire to the antenna, that wire becomes part of the whole antenna structure and can distort the pitch field because of the horizontal distance before you reach the vertical antenna.  The better choice, although still a compromise, is to place the inductor at the base of the antenna.  In this way the antenna itself is the dominant high impedance element that is influenced by the player and has a predictable pitch field determined by the antenna shape.  The downside is that the low impedance drive wire feeding the inductor is now a fixed capacitance element in the pitch field that lowers the oscillator's frequency and to some extent can desensitize the influence of distant hand movements.

I look at the Etherwave (or generally any compact theremin), where the inductors are on the circuit board and absolutely everything is sitting right in the pitch field (even the volume loop, unfortunately), and I'm amazed that it all works.

I have found the D-Lev to be extraordinarily tolerant to significant changes in the antennas or the theremin's proximity to conductors. You can make pretty major changes to antenna lengths (or change from rod to plate antennas) or set up the theremin near a metal bench, and once you turn it on and perform an "acal" auto calibration, it just works.

"You mentioned winding them either on the lathe or by hand.
For what it's worth, for the rare occasions that I wind coils, I use the simple wooden contraption I built, to fix my Li-Ion battery powered drilling/screwdriving thingy (whatever it's called in English) onto, with "analog" speed control on the grip, which is nice for not ripping things apart.
The rod that's inside there instead of a drill, to which a coil base gets fixed, also has a painted-black cardboard disc around it, with one index hole in it. There is an LED on each side, one just shines, the other of same type is abused as receiver, amplified by 1 transistor or so, connected to the counter input of my el cheapo Chinese signal generator - et voila, my short attention span is not a hindrance anymore to make a coil with a certain number of turns "


A lathe is really overkill, and your setup, or even the hand-cranked fixture is perfectly good for winding these simple coils.  When I was a kid back in the 1960s I hand-wound a high voltage spark coil that required a full pound of #36 magnet wire for the secondary (don't know how many feet that is), and this was entirely wound in single tight layers with paper separators between layers.  This was all done with a simple wooden-crank fixture.  So these theremin coils seem easy!

"I saw you etched the PCBs yourself. Have you tried services like JLCPCB? People are very happy with the quality, they're fast & cheap, and they make the vias for you "

That's coming.  I only do the boards at home because I can have them finished in a couple hours to test the circuits (versus 3 weeks or so).  But the DIY boards need wider traces, larger pads, and have the via problem, all of which prevent making proper layouts with compact routing and ground planes to help prevent noise issues.  The next boards will be commercial prototypes, and I've been working on new layouts over the last couple of weeks.  I have a couple services in mind, but I'll look at the one that you suggested too.  Thanks.

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