[time-nuts] Broken Ovenaire OSC 85-50

Pascal Nguyen vk2ihl at gmail.com
Sun Jul 5 09:47:33 UTC 2009


Hi Joe

I am just a radio amateur and I build a lot of oscillators for my need,
including a GPSDO "tunable" with CW25-TIM GPS receiver chip set. Please
allow me to share my usual  trouble shooting when oscillator is not working
properly.
I see an oscillator is just only a high gain amplifier with  positive feed
back via "narrow band pass" filtering to sustain an oscillation in a
controllable manner.
I had a look at your circuit with all Vcbe of all transistors transcribed,
it seems to me all transistors are within normal limit (bias # 0v7).
Please look at the "narrow filter", my suggestion is to check this circuit.
1) 22pF in parallel with a serial (10uH - 30pF) is a  diplexer, high
impedance to ground for 10 MHz and low impedance to ground for unwanted
spectrum.
2) 80 pF feed back capacitor.
3) 10 MHz Xtal

Now I can perform a differential diagnosis:

All transistors are correctly biased, can eliminated fault from power supply
and transistors.

A quick and dirty approach is to do a loose coupling (capacitor or inductor)
with the 10 uH inductor, connect to an external 10 MHz variable output
source, check gain of all transistor stages to the last output

Check high impedance to ground for 10 MHz, disconnect the "diplexer", check
magnitude of output signal, signal is increasing, hurray we see the problem.

Same signal level, put the diplexer back to the circuit, check the 80 pF
cap, all else failed, the last component of the "narrow filter" is the Xtal.


Good luck

Pascal Nguyen
vk2ihl / xv2pn


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