Theremin pitch changing over time

Posted: 5/22/2020 1:23:24 PM
ILYA

From: Theremin Motherland

Joined: 11/13/2005

BTW,
does anyone know a freq meter application for PC with automatic logging feature?

Posted: 5/22/2020 1:48:52 PM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

"Well, I quite admit  that the drift varies from one to another."  - ILYA

It's a good exercise to quantify drift!  I wonder how much is internal (components heating), and how much is external (room heating / cooling, changes in the electrical nature of the air)?

Posted: 5/22/2020 3:20:47 PM
austin

Joined: 5/17/2020

I do extracted the data from austin video and presented it as a graph.

This is an awesome presentation with curve, thanks!

You are right, the lower pitches will go much sharper as it warms up. The higher pitches will be more resilient to drift. A better experiment would be run it three times, setting the starting pitch at the lowest note, and octave above that, then another octave above that. Then I can compare drift.

I also wonder if the drift is changing the frequency linearly, but because an octave = doubling the frequency in Hz, it sounds like the pitch drift is bigger at lower octaves.

Posted: 5/22/2020 4:18:11 PM
oldtemecula

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014


Hello Everyone,

I like this friendly gathering of theremin explorers. Though Pitch Drift occurs over time its cause has no relevance to time.

Let me go into greater detail: In solid-state drift is caused by the slightest imbalance between the two RF oscillators, to be more specific mostly the difference in the behavior of PN junctions.

The drift effect is exaggerated by temperature change caused by poor design internally that allows a heat source like transformers, regulators and too much design current flow; this is what causes the “unnecessary” warm up period. Then there is the effect of the outside room temperature changes. Solid-state theremin circuitry should always be at room temperature On or Off.

All graphs should choose the same single note like A4 440 Hz and graph frequency change against a highly accurate thermometer. This should be done “without” the Pitch Antenna connected as you only want to monitor the circuitry for stability. The antenna will add its own effect and why using a telescoping antenna is a better idea though I would not use one as I think they look too whimpy. There are other ways to add small changes to the length of the Pitch Antenna to balance it against the environment background.

Austin there is nothing you can do to improve your theremin’s drift behavior other than keep adjusting. Also Thermal Drift will affect note spacing but has “no” effect on linearity

Christopher
www.Hwy79.com

Posted: 5/22/2020 7:27:39 PM
JPascal

From: Berlin Germany

Joined: 4/27/2016

Today I did a simple test - analogous to Austin and Ilya - from my experimental theremin. The pitch was measured with PC program audacity over 1.5 hours. I switched the theremin on and left the room. After 2 minutes I came in and goes out.

The Logged wave-file has 500 Mb. To get the frequency I marked always some seconds and used the internal spectrum analyzer to get the frequency maximum. I could not see a drift.  Started with 62 Hz, the coming in and go out after 2 minutes led to a jump at 72 Hz because of minimal mechanical movements of the construction. At the end after 1:30 h the frequency was 73 Hz.

What I not have investigated is the behavior by change of the room temperature and the effect of housing my stuff in a closed wooden box. But what I sure can say is - as Ilya also mentioned: without high inductive air core coils that wouldn't be possible. And additionally, the used oscillator design is definitly crucial.

Posted: 5/22/2020 7:41:30 PM
ILYA

From: Theremin Motherland

Joined: 11/13/2005

 At the end after 1:30 h the frequency was 73 Hz. 

Interesting.
Do you use two identical oscillators?

In Paradox MX one is LC, other is crystall.

Posted: 5/22/2020 7:54:53 PM
JPascal

From: Berlin Germany

Joined: 4/27/2016

Princip of oscillators is identical, crystal using is one of the conditions. The second is, how to eliminate internal capacitances and the third how to eliminate influence of voltage supply.

Posted: 5/22/2020 9:43:44 PM
oldtemecula

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014


JPascal Said: led to a jump at 72 Hz because of minimal mechanical movements of the construction. At the end after 1:30 h the frequency was 73 Hz.

To say your thermal drift was only 1 Hz after 1.5 hours only makes sense if the two oscillators are locked onto one another. This needs a better explanation as possibly your results are excellent or fantasy.

Christopher   
www.Hwy79.com

Posted: 5/23/2020 7:35:54 AM
JPascal

From: Berlin Germany

Joined: 4/27/2016

Yes, Christopher. Results are excellent. I have explained many and partially new basics in my experimental thread. The reactions are often close to zero. Sometimes there is response to my things and thoughts, thanks to those members here. I do my theremin investigations now over some years and share the most results. Long time ago, during my diploma thesis, I  developed and built for an old n.m.r. Spectrometer the current stabilization, which at that time replaced the tube technology with transistors. The requirements for the short and long term drift of the current through the magnet was less than a millionth. This was only possible by using a differential amplifier that was integrated on a chip with internal temperature control.

But you can calm down here. This is not necessary for a common theremin. My test was made with the pitch and volume functionalities and antennas, of course. 

Posted: 5/23/2020 3:21:28 PM
oldtemecula

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014


Just before the roll-over

JPascal, those are nice words and explanation of your excellent results. We both have been around for a long time. Unlike ten years ago theremin activity on the design and construction side has dwindled. We theremin designers all seem to do our own thing, which is good as it adds diversity. Maybe one day that enlightened kid will come along and view the work of all of us and pull it together. There needs to be a way each of us can put what we know in storage (100 mb) and accessible to the future generations. My own web service bluehost would not allow me to buy ten years in advance.

Christopher
www.Hwy79.com

The reason I add my web address, it is the only way google can find a webpage today, in the old days google found everyone as the web was small. Google scans TW daily.

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