Antenna tuning.

Posted: 4/28/2012 8:57:52 PM 11

From: Northern NJ, USA

Joined: 2/17/2012

"And yes, it is a grounded base Colpitts oscillator. You have to see the 680p and the 33n in series which makes 666p to which the 200p have to be added - a total capacitance in the parallel tank circuit of 866p. Together with 1mH you obtain a resonant frequency of 171kHz. There is no magic..."

Ah, yes, thank you: 680 || 200 || 30 = 910pF. Why not put the 200pF & trimmers in parallel with the 680pF?

"There is so much metal around the antenna that you can't consider the static antenna capacitance being only 7pf but rather the double. This would explain why the author of this article was happy with only 40mH (and we are even not sure about that!)."

My spreadsheet sim shows about 20pF at the antenna for a 172kHz resonance point. Even with all that metal this seems kind of high to me, but I haven't delved into the antenna math (yet). That, and I guess I'm too busy talking to know anything. ;)

Posted: 4/28/2012 10:02:49 PM 12

dewster

From: Northern NJ, USA

Joined: 2/17/2012

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nickexists, I'd be very interested in a sound bite of your new theremin!

You generally want SRF significantly above the operating point of the circuit, but I'm sure Thierry can add quite a bit of nuance to that when it comes to "linearizing" coils.

Posted: 4/28/2012 10:20:38 PM 13

nickexists

From: US

Joined: 8/17/2011

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https://www.dropbox.com/s/ifl55vvdhospbt5/New%20File%20mix.mp3

as you can tell I don't actually have any idea how to play, but here's a sample.

Posted: 4/28/2012 11:22:51 PM 14

dewster

From: Northern NJ, USA

Joined: 2/17/2012

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"as you can tell I don't actually have any idea how to play, but here's a sample."

Hmm, the MP3 looks like a flat line to me in Adobe Audition.

Posted: 4/29/2012 3:13:34 AM 15

nickexists

From: US

Joined: 8/17/2011

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woops. I'm not sure what happened there. I've replaced it with a new mp3 at the same link that should work.

Posted: 4/29/2012 3:41:39 AM 16

dewster

From: Northern NJ, USA

Joined: 2/17/2012

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Nice low end! Though I'm hearing some 60Hz hum mixed in there. Is that because of your setup or is it coming from the Theremin?

Posted: 4/29/2012 4:11:27 AM 17

nickexists

From: US

Joined: 8/17/2011

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it's from the theremin. I've been looking around for a bad connection or something but can't find one. plus it's still only grounded through the guitar amp.

Posted: 4/29/2012 4:15:09 AM 18

nickexists

From: US

Joined: 8/17/2011

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i've also noticed that touching the volume antenna or the metal casing of the 1/4inch output plug causes the pitch to change drastically.

Posted: 4/29/2012 4:14:01 PM 19

Thierry

From: Colmar, France

Joined: 12/31/2007

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The latter effect is normal. When you touch a metal part, your body is better grounded in relation to the instrument which will result in a higher capacitance seen by the pitch antenna. Thus the pitch increases.

Posted: 4/29/2012 4:17:45 PM 20

Thierry

From: Colmar, France

Joined: 12/31/2007

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dewster wrote: "My spreadsheet sim shows about 20pF at the antenna for a 172kHz resonance point. Even with all that metal this seems kind of high to me, but I haven't delved into the antenna math (yet)."

You are right. A rough "wet thumb" estimation based only on experience tells me that the antenna capacitance would be around 12pF. I guess that the designer picked these 40mH from the EW circuit without understanding what he exactly did...