Oscillator frequency help

[brett]

Hi
the info with this circuit suggests that the frequencies in each oscillator will be about 250kHz.

My brain is dead (it's 37C and Friday arvo), so I need help working out what values I need to change to shift the frequencies up to about 370 kHz.

thanks for any help



PS Also, I plan to use an LM386 with 9V supply as the audio amp instead of the LM389 with 5V.  Any issues with that?

[Cliff Schecht]

Try changing C1 to about 220 pF.

The LM386 datasheet says you're fine with 9V.

[Cliff Schecht]

Err.. Maybe closer to 200 pF.

[brett]

So I halve the C of both oscillators to get the same freq change ?  (ie also change C2 and C3 from 10nF to 5nF)

Is the proportional frequency change equal to the inverse of square root of the proportional change in C ?
(ie halve C gives 1 / sqrt(0.5) = 1/0.71 = 1.41 x increase)  In this case 250kHz to 350kHz ?

sounds about right

thanks

[Cliff Schecht]

It looks like the second oscillator is linked to the first one. Is this circuit supposed to be two oscillators operating at different frequencies, but one syncs off the other? Synchronization of two independent oscillators is a lot of fun, that's why I ask . It looks like you would choose a value for L1 (using 1/(2*pi*sqrt(LC))) to choose the second oscillators frequency (it's something like a Pierce oscillator, just without the crystal). This would give you a cool modulation type effects, which are always cool.

[gez]

Have only skimmed through the circuit, but the schematic shows two oscillators.  The top one is square wave and 'gates' the second (though that could be the other way round - no value shown for the inductor), so you get a chopped/pulsed audio tone.  If the 1st oscillator is slower, then that tone would be in 'blips'. 

What is it that you want from this circuit and what frequencies are you after Brett?  The 1st oscillator is bog-standard and easy to find calculations for.  The second, I'm pretty sure I have it covered in one of my books.

Edited for clarity

[brett]

Hi
it's actually a metal detector, with 2 oscillators running at similar frequencies.  The difference between the osciallators is mixed down to a "beat" frequency.  Bringing a conductive object near the dectector coil changes the frequency of one oscillator, which raises the beat frequency.  I'm looking for theremin-like behaviour from it.  Maybe I'll have to wear a ring to make it work properly.

It might be cool to use this to modulate a guitar signal (e.g. via the Nyquist aliaser).  With a toe ring ovber the detector coil, you coul vary the modulation frequency with your foot.  Or is this just overkill?  But if it doesn't work as a stompbox, I guess I could prospect for gold !

[gez]

Making R1 or C1 smaller increases frequency of 1st oscillator.  Likewise, the caps C2 and C3 increase the frequency of the second if made smaller.  Breadboard it and tweak would be my advice (though if you really need the calculations I have them somewhere).

[gez]

I wonder if you could flip things in that second oscillator.  Use a bog-standard inductor and make one of the caps a sheet of metal or something.  Would be a bugger to setup (and would probably have a really limited range), but if it's on the breadboard might be worth messing about with.