theremins

[Paul Perry]

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