Problems connecting the oscillator with the mixer

Posted: 12/11/2015 8:59:19 PM

Joined: 12/11/2015


We are trying to do a theremin, but we have problems when we connect the two blocks i have mentioned. The blocks alone works well (the mixer with a function wave generator). But with the oscillator the mixer just give the generator signal. The signal of the oscillator just disappear.

Thats a problem of the circuit, or maybe the protoboard or the impedance?

The circuit is this:




PD: sorry for the spelling if i made any mistake.

Posted: 12/11/2015 9:42:15 PM

From: Northern NJ, USA

Joined: 2/17/2012

Haven't simulated, but off the top of my head R7 and R11 seem like their resistances may be too high.  Try somewhere around 10 to 100 ohms here.  The values of R1, C1, and C2 seem suspect also, you don't want to load your oscillator down too much.  I'd recommend an emitter follower between the oscillator output and the mixer input for isolation. 

Posted: 12/11/2015 11:35:00 PM

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014

Just my opinion:

Get rid of R8 & R12, R7 & R11, C9 & C13. Take your signal off of the two "emitters" through a 15pf capacitor to mix. I do not understand your mixer. Just use a single 1N914 diode or feed the two signals into the base of the first stage of a transistor for audio amplification.

Maybe even scrap C5 & C10.

Add a 1K Pot in series with either R2 or R3 as an excellent method of tuning. Varying the transistor junction voltage will change the circuit capacitance.

Good Luck


Edit: If not battery operated, I agree with dewster, R7 & R11 would be nice to keep, but at 100 ohms for noisy power supply smoothing.

Posted: 12/12/2015 4:27:34 PM

Joined: 12/11/2015

Thanks both of you!

dewster, we try an emitter follower but didn't work. But we did it rush, so maybe wrong. We will try it again.

oldtemecula, the mixer is based from here: multiple diode mixer. We try just a Shottky diode conected to the ground, but didn't work neither.

Anyway, the simulation did work, so theoricaly our circuit is ok. The problem is in the real world haha.

Thanks again!



Posted: 12/12/2015 5:20:30 PM

From: Northern NJ, USA

Joined: 2/17/2012

If you have the voltage swing at the oscillator pick-off point (what is the voltage swing here?) then as oldtemecula suggests a couple of small pF caps going to a single diode mixer should work - indeed this is what the vast majority of Theremins employ.  

You might want to get something tried and true running before experimenting around too much, the hardware equivalent of a "hello world" program.

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