Metal detector (handheld using 2 oscillators and a mixer)

[fanciepants]

I've managed to create an LC oscillator that produces a 200mVpp sinewave signal @384kHz.

For the other I have a 384kHz crystal but all attempts to make a circuit for it have failed.
Also I plan to run the entire thing off a 9V battery.

Also for the mixer I need to get the sum and difference of the two frequencies, however when fed into a BJT all i get is the average of the two out of the collector.

Can anyone offer advice or how to resonate a crystal into a sinewave?

And/or advice on how to mix 2 frequency signals of relatively the same frequency into the sum and difference?

Many thanks!
David Fancie

 

[alec_t]

For the other I have a 384kHz crystal but all attempts to make a circuit for it have failed.

Without knowing which circuits you've tried it's difficult for us to help . There must be loads of suitable circuits on the web.

when fed into a BJT all i get is the average of the two out of the collector.

Are you buffering the oscillator signals before mixing, to reduce oscillator-pulling effects?

 

[fanciepants]

Are you buffering the oscillator signals before mixing, to reduce oscillator-pulling effects?

I have a test simulation where I duplicated the working LC oscillator and fed them both into the mixer as a test.
Each oscillator has a polarized capacitor at the output before entering the BJT. I'm unsure however if that's a proper buffer.

Without knowing which circuits you've tried it's difficult for us to help

Attached is the simulation I've been running for the mixer.
The filter is unattached and the schmitt triggers are something I was toying with based on a teachers suggestion.

View attachment 63617

As well, here is one of my crystal resonanter sims. Multisim didnt have the same crystal that I have so I used a different value, the frequency doesnt read however and this is the waveform I get

View attachment 63618

 

[alec_t]

Each oscillator has a polarized capacitor at the output before entering the BJT. I'm unsure however if that's a proper buffer.

It's not a buffer. The cap more or less 'shorts' the two oscillators so each will affect the other; which is not what you want. Each oscillator should drive a respective buffer amplifier (i.e. one having a high input impedance and a low output impedance). The outputs of the buffers would then go to the mixer.

 

[colin55]

The 7414 is a 5v device