Advice Request

Posted: 3/30/2009 3:07:17 PM

Joined: 10/2/2008

i'm basically using the information given by someone elses project. the schematic and their write up is found here:
link (

as i've said, i've put this tough just on breadboard so as to do various tests. so as to check the antenna i've been looking at the signal on an oscilloscope of the output of just the one oscillator (the one with the antenna).
i'm sure whatever i'm doing wrong is probably really simple to solve but i'm confused none the less

[i]fixed link tw-s[/i]
Posted: 3/30/2009 7:59:34 PM

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

Ok - I am not familiar with the oscillator in this schematic, but there are some tests you can make.. check the waveform and frequency on the collectors of Q1 and Q2 and be sure these are running at the same frequency when hand is away from the antenna (trim C15 and/or C25 to achieve this).. Then place hand close (10cm ish) to the antenna and check the frequency on Q2 - The frequency should have reduced by at least 1kHz when using a short wire antenna, and more for a larger antenna area.
If you are not seeing this change in frequency, it is most likely that something is 'loading' the antenna circuit - is your PCB sitting on a grounded ESD mat? Is the layout such that there is a big capacitance coupling to ground or some other signal?
C22 is quite a huge capacitance in parrallell with the antenna (capacitance) - Reducing this should increase sensitivity and frequency.. If only the antenna circuit is being loaded, this may help - but you will need to retune both oscillators and probably need to change the value of C12 as well.
There is no linearity compensation, so you can expect the major frequency changes to occur quite close to the antenna.

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