Open Theremin v4 - pitch range drift, the incredibly shrinking range issue?!

Posted: 8/17/2022 6:35:23 AM

Joined: 8/17/2022

Hi there - I have an etherwave+ plus for indoor recording etc: but I got the OTv4 to use outside at various locations, recording etc: Inside and linked to a proper earth it is great. Outside having issues. l have soldered a long lead screwed and soldered to the OT earth point, and at the other end connected to a metal peg which is pushed in the earth. I tested various tent pegs and other rods, but this seemed to work - it calibrates and I get a lovely full range about 80cm and deep bass. All good - but after a few minutes the range starts to contract ... and I would say over 5 mins it goes from 80cm down to 30cm for the same range? And yes it's a bugger to compensates tuning wise ?!? - I have tried earthing to me also, but after calibration again pitch range shrinks.

Any solutions?

Posted: 8/17/2022 9:17:43 AM

From: Colmar, France

Joined: 12/31/2007

Please understand that RF oscillators are never perfect. They tend to drift a little with temperature and with humidity. On a 500kHz oscillator, a temperature drift of +500Hz represents an "error" of 0.1% which would be negligible in most technical applications, but which is already crucial in a theremin where it would lead to exactly that field shrinking which you describe.

That's why in most theremins, both pitch oscillators, the fixed one and the variable one are built with identical components and together in a cabinet. Thus, temperature changes would affect both, and even if both went up by 1% or 5kHz, you wouldn't notice it, because the resulting beat frequency (the difference between both) would remain nearly constant.

It must be said that most of the temperature drift in theremins happens during the warm-up period. It takes a few minutes until the current flow through the components has heated them slightly up and they arrive in a thermally balanced and stable state. That's only the moment when the player should tune or calibrate their instrument, because afterwards, the remaining temperature drift is much less.

With the OTv4, things are a little more difficult. First, both pitch oscillators are not identical and won't have common temperature drift since the fixed one is a rock stable Xtal oscillator with almost no noticeable drift while the variable oscillator is (to make it variable through the antenna capacitance) a classic analog circuit, even though it's build with unbuffered CMOS gates, which loves drifting with temperature. That's why temperature and air flow considerations are more important here than with some other theremins.

As you already found out at home, a constant ambient temperature helps greatly for thermal stability. But outside in the wild "with the nude butt in the wind", each slight breeze will de-tune the pitch oscillator and since it is in constant exchange with the ambient air, it will never reach a thermally balanced and stable state. What to do?

First, don't expose the OTv4 to the free air. Build a cabinet around it which will allow it to have its own micro-climate and which will make that after a few minutes of warming up, a thermal balance between the heat from internal power dissipation and the cold from outside will establish itself. Then only, launch the calibration process and play happily afterwards! 

Posted: 8/17/2022 5:46:39 PM

From: Berlin Germany

Joined: 4/27/2016

A very good explanation of this crucial phenomenon, Thierry. Should be pinned here for all the theremin developers! Little remark:
A rock-solid Xtal oscillator and an extremely temperature-stable pitch oscillator can also keep pitch within a few Hz. In fact, this is not easy to achieve, but possible.

Posted: 8/17/2022 7:57:30 PM

From: Northern NJ, USA

Joined: 2/17/2012

"A rock-solid Xtal oscillator and an extremely temperature-stable pitch oscillator can also keep pitch within a few Hz. In fact, this is not easy to achieve, but possible." - JPascal

Air-core solenoids are the key here, though larger values can get unwieldy.

Posted: 8/17/2022 8:11:43 PM

From: Theremin Motherland

Joined: 11/13/2005

Yes, in Paradox-MX I have a drift 22 Hz after 25 min warm up. 

Posted: 8/17/2022 9:04:26 PM

From: Colmar, France

Joined: 12/31/2007

“A rock-solid Xtal oscillator and an extremely temperature-stable pitch oscillator can also keep pitch within a few Hz. In fact, this is not easy to achieve, but possible.”

You know that, I know that. But I didn’t intend to give a 20+ hours lecture about temperature compensated RF oscillator design for free. 😉

The open.theremin v4 is (in my humble opinion) actually the best theremin which you can get in the 100€ price range. Add some Euros for a 3D printed housing which will greatly improve its thermal stability and it will almost be at eyes’ height with the Moog Etherwave Plus (where some exemplars need also a warm-up time of 10-15 minutes) and which is 5 times more expensive. The open.theremin was the very first theremin with heterodyning and wave table synthesis, a long time before the Theremini came to market and still today, its volume/expression control is ways better. 

What started initially as a proof of concept is on the way to a mature instrument. The v4 is already good for its price but I think that Urs Gaudenz and the other contributors will continue to improve it.

Posted: 8/18/2022 12:16:06 AM

Joined: 8/17/2022

Thanks Thierry

But there was no change in temperature or wind on the OTv4 ?

The stable in house was due to me having it earthed in a socket in a room exactly the same temperature as outside ... 

I am not talking about pitch here really but - the distance before the antenna picks up my hand. From about 80cm full range then it drops to about 30cm.

It must be something to do with the earthing, reason being, after calibration - 
1 - no earth a tiny 20cm range, 
2 - earthing to me, calbrated fine, but after 1 minute the range drifted down from 80 to 30, 
3 - one type of steel peg similar to earthing to me
4 - a different type of peg, calibrated and was playable for about 10 minutes before it started to contract again

So not temp related in my view, given no change ... it feels like a leak in the circuit somewhere? 

Also I had it powered outside for easily an hour (and it is in the plastic 3D printed case - not totally air proof but no exposed circuits). Doing several calibrations and grounding tests, and there was not major difference in temp during that time - a sunny cold wintry Australia ... around 8C. So the only other thing I can think of is these instruments not good in the cold?

Thanks Gary

Posted: 8/18/2022 6:01:10 AM

Joined: 8/17/2022

and setup pic 

Posted: 8/20/2022 3:39:32 PM

Joined: 8/17/2022

Update - bought a proper earthing rod (copper plated steel) and alligator clipped it to ground on the OT4 and the rod about 20cm into the ground and ...  it works, holds the field properly. So thankfully not temperature related! 

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