Coils in series question.

Posted: 5/24/2012 5:19:31 AM

Joined: 5/9/2012

Hello all,

Sorry if this has been answered before, did a search and didn't find it. But I was just curious as to why, in many theremin schematics I've come across, are there 4 seperate 10 mh coils, instead of just one 40 mh coil on the pitch antenna? It seems to be quite frequent this way so I'm sure there's a good reason for it, but I'm still just intermediate at best right now electronics wise.





Posted: 5/24/2012 6:06:50 AM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as . ...................................

Joined: 12/7/2007

One good reason for seperate coils is due to the interwinding capacitance - Effectively, every coil (inductor) is non-ideal - they have series resistance and a parrallel capacitance sumply as a byproduct of wire resistance and capacitive coupling between windings ..

The inductance and capacitance in parrallel form a resonant (tuned) circuit, and one wants this natural resonant frequency to be much higher than the frequency you are operating the theremins oscillator at - Also, even if the self-resonant frequency is higher, one wants the capacitance to be as low as possible, because (for one thing) this capacitance reduces 'antenna' sensitivity.

By having smaller inductors wired in series, one can greatly reduce these problems - Big inductors tend to have lower self-resonant frequencies (higher capacitance) - buliding bigger inductors by wiring small inductors together in series greatly reduces the total capacitance 'seen' at the antenna.

Lots of reasons - but in general one wants the inductors to have low capacitance, and this is achieved by having windings seperated from each other (inductors used in theremins are often wound with several seperate coils on the former, and the winding is specifically done in a way to minimise capacitance).

Posted: 5/24/2012 1:51:51 PM

From: Northern NJ, USA

Joined: 2/17/2012

FredM answered quite completely.  I'll say this: go shopping for coils on-line (DigiKey, Mouser, etc.) and look for ones that have a SRF significantly above the operating frequency of your theremin pitch oscillator.  I believe that you also have to watch the saturation and maximum currents, which can be in the tens of milliamps on a Theremin.  You'll see first-hand how non-ideal inductors really are.

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