theremins

[Arthur Harrison]

Hi Paul, Grant, and everyone,

Monolythic RC oscillators tend to be too noisy for heterodyne
theremins.  The Exar emitter-coupled multivibrators, however,
are a bit better than a 4046 scheme, especially in terms of tempco
(about +/-20ppm/deg.C at optimum).  Still, inductance-based
oscillators such as a dual-differential pair type
(http://home.att.net/~theremin1/circuitlibrary/differentialpairosc.html)
or even a thermally stabilized Colpitts will provide much better service
than any of these approaches.  BTW, once the temperature drift in the
variable oscillator is sufficiently tamed, then crystal-controlled LOs are
perfectly reasonable, and very inexpensive in the form of those
ubiquitous "TTL" oscillator modules.

One thing I've been intending to do is trying the above-referenced circuit
with a transistor array to see if the added Vbe tracking further enhances
temperature stability.  It is also possible to add thermostatic regulation,
if there are enough transistors left over in the array.  (This doesn't
address
inductance tempcos, of course, but I've found the Miller 4652 phenolic-core
jobs to be pretty good for the purpose.)

-Art


-----Original Message-----
From: Paul Perry <pfperry at melbpc.org.au>
To: synth-diy at mailhost.bpa.nl <synth-diy at mailhost.bpa.nl>
Date: Saturday, December 12, 1998 6:31 PM
Subject: Re: theremins


>At 01:31 PM 12/12/98 -0600, Grant Richter wrote:
>
>>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.
>
>this hasn't been stable enough for me.
>OK as a 'special fx' woo-woo sort of thing, but not musically useful.
>Without a good (or any) specrtum analyser I don't know where the prob is,
>the v+ is stabilised (but, it would have to be stabilised to a few
>millivolts at least).
>
>I remember seeing ambient RF changing the characteristics of fets (try
using
>a mobile phone
>and putting the antenna near a desk phone while it is ringing.. the
>electronics in the desk
>phone go haywire & the local ring sound generator sounds wacky! a good
laugh
>on a slow day)
>
>I have tried replacing the antenna with a fixed cap & it is still unstable,
>so the
>interference (if that is the prob) is not coming from the antenna.
>
>paul perry melb aust
>