SOT: can someone identify this type of VLF oscillator for me?

[brett]

Hi

I'm building something using this oscillator.  It's from a design in a metal detector.  However, I can't find any information about it.  It doesn't seem to be the typical layout for a Colpitts or other common oscillator. 



The original schematic was for an 80kHz oscillator, and it conformed to the Colpitts rule that f0 = 1/(2.pi.(L.C1.C2/(C1+C2))^0.5 (the caps were 5.6nF (e to c) and 15nF (e to gnd))

I want it to oscillate at 17kHz, and have used the Colpitts rule to choose the caps (2x 0.22uF).  Do you think that will work? Is it ok that they have equal values?
At times like this I wish I knew a lot more about phase angle and EE maths.

Thanks for any help!

[R.G.]

Any LC oscillator which works for one given L-C combination should work with a different L-C combination by scaling the caps given that:
(a) it actually worked with the first L-C setup
(b) the new caps have parasitics which do not poison the oscillation with (for instance) too-high ESR or ESL

However, that does look funny to me. I believe that it's intended to be running in grounded-base mode because the feedback is applied to the emitter. However, the base is not "grounded".  If your question is prompted by having built one and it not running, try a cap from base to ground. I'd start with a 0.22 as you're using for the output capacitance. Also, it will not work if the power supply is not a good bypass, so use something like a 10uF-100uF paralleled by another 0.22 to LOCALLY bypass the power supply from V+ to ground.

[brett]

Thanks very much RG.  You've answered my questions again, for which I'm very grateful.

RE: bypass
I've got a 22uF electro and a 0.1uF film cap in close proximity.  Hopefully that, and the low freq will help me.  The original schematic just used a 0.1uF film cap.

RE: grounded base design.
Yes, I've noticed that many RF osciallators use a PNP transistor in a common base arrangement.  However, the original schematic is as I've shown, except for a mistake in the resistor from V+ to base (which is 100k).  I'll post the original schematic tonight or tomorrow.

thanks again!

[brett]

Hi again

Here's the original schematic:



and description of how it works:



I'll build the 17 kHz (maybe) version on the weekend and see how I go.
cheers