Antenna length and sound frequency of a theremin.

Posted: 11/11/2017 1:33:56 PM
JPascal

From: Berlin Germany

Joined: 4/27/2016

The relevant input parameters in your measurements are then: L=0,505 µH inductance of the tank coil; the pitch oscillator frequency with antenna is round about 2,400 MHz.

So you must have a resulting static capacitance parallel to coil (from rod, coil self, buffer coupling, circuit itself, padder...) of Cstat =  8,71 pF. A change by additional hand influenced capacitance of 0,69 pF results then in a frequency decrease of 90 kHz at hand distance of 5 cm. Values come from using the LC resonance formula.

Assuming all the static capacitors are remaining nearly constant. We only change the coil value to L=10 mH and add 10 pF for higher self capacitance of coil. Then the oscillator frequency decreases to 368 kHz and is reduced by 7 kHz at 5 cm hand distance. Is this that what you have done to get your data set?

Your "far field null frequency" describes the offset between fixed oscillator and pitch oscillator in case of hand absence. This parameter is very sensitive and usefull for getting linearity at the lower frequencies of theremin audio signal.

Posted: 11/11/2017 3:15:02 PM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

JPascal, yes, exactly.

Also, C padding at the antenna obviously lowers the absolute sensitivity to total C change with the hand, but since the ear response to pitch is logarithmic this only serves to shift the perceived pitch change to hand movements downward, and doesn't reduce the perceived rage itself.

In other words, C padding won't alter how many octaves you hear when you move your hand from, say 0.25m to 0.35m.  The padding will only shift this octave change downward.  I've dubbed this effect the invariant, and I discuss it with FredM here.

Also, C padding shifts the perceived pitch more than you might imagine given the LC resonance equation (so is is a fairly sensitive adjustment to physically add to a Theremin, though the original Theremins, as well as Bob Moog's earlier designs, employed it - it is the best possible location to adjust for environmental C when a series EQ coil is also used).  

Posted: 11/12/2017 4:22:47 PM
JPascal

From: Berlin Germany

Joined: 4/27/2016

A coupling condensor to the antenna shortens and shifts the frequency range influenced by hand to higher frequencies. This is similar to the bandspread tuning in broadcast receivers. Influence on linearity (means logarithmic scaled frequency is linear to hand distance) is here of interest. I think also it is not high, but should to deal with this parameter. Further more with scaling dependences of those theremin circuits with two inductances.

Posted: 11/12/2017 10:43:42 PM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

C padding actually shifts the range down, after the instrument is re-nulled.

For Theremins with series EQ inductors there are three resonances: 1) the fixed local oscillator, 2) the variable oscillator tank LC, and 3) the LC formed by the EQ inductor and the antenna C.  If 1 & 2 are relatively constant, then one can use variable C padding at the antenna to tune the Theremin.  Otherwise, one needs to be able to vary both 1 & 2 to compensate for different environmental C.  This is all theoretical though.  The Etherwave only has a convenient operator control to tune 1, which is rather problematic as 2 is a ferrite slug inside the cabinet, and 3 is a bent wire over a ground plate, also inside the cabinet, and removing the cabinet top profoundly influences the tuning (wood is not C neutral).  The Etherwave is kind of a mess, really.

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