theremins

[Arthur Harrison]

Grant Richter wrote, re: theremins:

>One trick I used successfully comes from a hobby magazine circuit (don't
>remember which one).
>Take a CD4046 PLL chip and tie the VCO control line to +V. Program the RC
>values to give a fixed
>oscillator in the range of 100kHz to 400KHz. Attaching an antenna to one
>leg of the timing capacitor
>will give a usable range of six inches or so from the antenna. Another 4046
>can be used to turn the
>varying oscillator frequency into a control voltage.

Hi Grant,

Yes, I've seen some 4046-based theremins as well.  My experience with these
circuits
was also about a half-foot range, which isn't too good for a stand-alone,
voiced theremin
unless constrained to only an octave or two.

>The problem is the coupling to the antenna. The theremin design uses large
>inductors to decouple
>the antenna from the oscillator and give a much smoother and longer control
>range.

There is an article by Robert Moog in which he gives a one or two line
explanation
of the function of the series inductance.  (The original patent kinda talks
around the
specific theory of operation of this inductance.)  In practice, I've usually
avoided the
series inductance to conserve temperature drift and cost. Depending on the
circuit
topology, sufficient sensitivity is readily available without this inductor.
To his credit,
Theremin did quite well without the luxury of the signal processing
techniques
readily at our disposal, and the series inductance was one such method of
adding
gain without the cost and bulk of additional active circuitry.

>As I recall
>the output variation from the 4046 circuit running at 350kHz was about
>25Khz from full contact
>to about six inches away. The Big Briar Etherwave runs about 162?Khz and
>changes about 2Khz for twenty four inches of motion near the antenna.

Yes, this seems correct.  Moog specifically chose frequencies in a
seldom-used
part of the LF band, not coincidentally around some popular IF frequencies,
so it would seem.


>Two 4046's can be used to make a direct conversion unit using the
>EXOR gate in one as the RF mixer. Nasty sounding square wave though,
>and the same problem of excess range over too short of a distance.

I endorse the EXOR detection scheme; I believe it preserves a maximum
amount of signal information.  A two-pole Sallen-Key filter will remove the
upper products, and you can actually obtain a useful timbre in the audible
beat with such a filter.  The appropriate ratio of hand-to-static
capacitance
in the oscillator should be a way to tailor the sensitivity.

>It doesn't get much cheaper or simpler than a 4046 as a good place to
>start experimenting with a home built proximity controller. The same
>principle of
>decoupling the antenna with large L values (60 millihenry?) would apply.

Yes, that series inductance does need to be rather large to rhyme with the
small static antenna capacitance and hand offset capacitance (only a few pF
at best!).  The cost of such inductors is quite high.  Moog strings a bunch
of them in series in his Etherwave, and that adds a lot to the unit's cost.

>Even a 2Khz variation in RF oscillator output could be scaled with the 4046
>circuit to give 0-10V over a usable range.

Temperature drift, however, as well as phase noise, will prevent the 4046
approach of providing the same quality as an inductance-based oscillator
such as in the Etherwave.  The key to a good detection oscillator is a Q-
factor as high as possible without sacrificing the required
hand-capacitance-induced deviation.  Of course, if the objective is simply
to provide a few spacial triggers, these RC oscillator schemes may be just
fine.

Much of my work, actually, is in the techniques of frequency-to-voltage
conversion
needed for a heterodyne-to-voltage-to-VCO approach.  The popular method of
a simple RC averager following a monostable works, but is lossy and
non-linear.
Tachometer approaches are much better.

>Electronotes issue #111 page 12 has an article by John Valente with two
>designs
>for Body Proximity controllers.

I have to look into that.  I wrote a "circuits ideas" entry with a fellow at
Maxim
Integrated Products earlier this year that was put in an industry mag.  It
used
a phase-detection scheme, as opposed to a heterodyne scheme.  (I am under
the impression that Electronotes is a print subscription only, so I'll have
to do
some looking.  I have a contact who might loan me 111.)

Thanks for getting in touch.  I will be putting some more circuits on my
page in
early '99, if all goes well!

-Art