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

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Hope there's enough cake here for everyone!

May the new year bring us all good health and happiness!

Thanks for your good wishes, Eric.
A happy new year to TW and all of you!

(There is (was) something missing on your cake.)

"There is (was) something missing on your cake."  - André

Ah, very good!  You're way ahead of me André!

EW+ Antenna Voltages

I just modded an EW+ (http://www.thereminworld.com/forums/T/34017?post=224378#224378) and was curious as to what the antennas operate at in terms of voltage swing.  A neon bulb near the antennas fails to light up, and my DIY high voltage scope probe reports the following:

Pitch: 290kHz @ 64Vpp
Volume: 494kHz @ 22.5Vpp

These voltage swings are considerably lower than I was expecting.  The drive oscillator LC tank can provide up to 24Vpp, and the Q's of the coils are likely around 80, so hundreds or even thousands of volts (24Vpp * 80 = 1920Vpp) seemed possible.  I presume it's due to a parallel LC driving a series LC, and the two "fighting" each other at their combined resonances.  My probe loads them with 2pF or so and that's probably throwing things off somewhat too.

I was feeling kinda bad that the D-Lev can "only" develop 220Vpp or so at its plates (measured with my HV probe) but if 64Vpp is sufficient for a mass-produced semi-pro instrument, then I guess it's doing OK in comparison.  Modifying the D-Lev drive circuitry could probably boost this to around 300Vpp given the AFE regulated supply of 3.3V.

The modded EW+ seems fairly linear, with open/closed hand producing ~1 octave pretty much throughout the field.  The mod didn't require any re-tuning of the oscillators either.

RF Chokes

Looking at RF chokes again and seeing some rather common features.

Evan's 1mH, 2mH, 10mH, and 20mH chokes are air core on a 6mm diameter former.  I believe the 1mH & 2mH are wound with 34AWG double coat, and the 10mH and 20mH are wound with 37AWG double coat.  

Also looking at a Bourns 50mH on a 7mm diameter ferrite core, which I believe is wound with 37AWG single coat.  

All of the individual "donuts" that make them up are suspiciously close to Brooks coils, where the OD is 2x the ID and the width is equal to the inner radius, which gives a square cross-sectional winding area and maximum inductance for a given wire length.  Donut spacing is between 0.8mm and 1.7mm.  

If the Bourns were wound the same but instead on an air core, then the inductance would be around 21mH, so the ferrite is concentrating things 50 / 21 = 2.4 times.

So I guess the RF choke design process goes something like this: 
1. Pick a former material based on how much mH magnification is desired;
2. Divide the target mH by that; 
3. Using this new target mH, design an air core with 3 or 4 donuts, adjusting the former diameter and wire gauge so that the donuts are more or less Brooks coils.
4. Iterate until whatever parameter you're targeting is optimized, hopefully patterns emerge from the chaos.

Using double coat on an air core makes sense as it will slightly decrease the proximity effect.  I assume it would help ferrite cores too?

I believe the Brooks dimensions form a rather broad maxima, so nailing the 1:1:1 ratio isn't too critical.

Hi Eric,

On my D-Lev, I applied your suggestions about coils and wires positions (see : http://www.thereminworld.com/forums/T/28554?post=224366#224366).

The coils are about 8 mm from the top and side wall.
The drive wire is outside the coil and at some distance, in the corner.
The sense wire is as short as possible and away from the drive wire.
I didn't make enough tests before this modification to feel a difference but at least, it still works !
 
I also changed the three 4.7k resistors of the tuner driver with 6.8k.
The LEDs are less bright but I still have to leave the brightness at minimum (1).

I have a squeak noises, not very loud, but still annoying when playing.

I suspected the 5V switching power supply.
I have half a dozen of these in my drawers. I tried all of them and could only find one which gave minimum perturbations but is not fully satisfactory.
I think I'm going to make a 5V linear power supply with the negative output connected to earth. Unless you have a better idea...

A few pics here : https://u.pcloud.link/publink/show?code=kZSmRJ5ZuAR3wmy95gXJ3FX4AeefU59UPvEX

"The coils are about 8 mm from the top and side wall.  The drive wire is outside the coil and at some distance, in the corner.  The sense wire is as short as possible and away from the drive wire.  I didn't make enough tests before this modification to feel a difference but at least, it still works!"  - André

That looks much better!  I might have pulled the drive wires even farther away, and the DACs are somewhat close to the pitch coil, but leave it like it is for now. 

"I also changed the three 4.7k resistors of the tuner driver with 6.8k.  The LEDs are less bright but I still have to leave the brightness at minimum (1)."

This is to be expected unfortunately given the limited dynamic range of the analog adjustment.  I keep mine at 1 as well.

"I have a squeak noises, not very loud, but still annoying when playing."

Not sure what you mean here?

"I suspected the 5V switching power supply.  I have half a dozen of these in my drawers. I tried all of them and could only find one which gave minimum perturbations but is not fully satisfactory.  I think I'm going to make a 5V linear power supply with the negative output connected to earth. Unless you have a better idea..."

Check the 5V at the PWB, USB cables and interconnects can have significant resistance and voltage drop, and if you get below the LDO points of the linear regulators the current slugs from the synth can start modulating things.  Switching noise could be a thing I suppose, chargers tend to be kinda crap here so I stick to those provided with reputable equipment, though I haven't seen any direct issues AFAIK.  Instead of a linear supply you might use a switcher set to a volt or so above the dropout point of a 5V LDO, itself set a bit high to help compensate for any USB voltage drop.

Good grounding is really essential too.

You should also check the voltage swing at the "C divider" points on the AFE's with a scope.  You want to see a healthy 2Vpp or so sine wave here with sufficient headroom given the ill-defined inverter threshold voltage and the 3.3V supply.

"A few pics here"

Very nice!  Is it OK if I post a couple to the D-Lev web site?

I'm definitely up for a video chat to help diagnose things / get your fields set up!

Squeak noise : I mean a funny noise similar to what you get when you touch the volume antenna on analog theremins, but much lower volume.

I made some measurements.

Supply voltage :
At the output of the 5V supply : 5.04V and 0,44 A.
At the main board : 4,92V. This means a voltage drop of 0.12V and a cable resistance of 0.27 ohm.
Ripple voltage of 40mV at 200Hz approx. Complex wave form. Other frequencies are present but I couldn't identify them.

C dividers :
1.4Vpp, limits 0.6V and 2V, clean sine wave for both.
800kHz for pitch and 1280kHz for volume.

Of course you can post a couple of pictures on the D-Lev site! Thanks for that!
(By the way, I should change my avatar...)