PUFF Screen Grab of VHF Oscillator is it ok

Jun 13, 2005 3 Replies

Hi Kevin,

S22 should be closer to S11, whether it be 50 ohms or at some other common value -- or your simulated open-loop gain results will be inaccurate. And you must also plot your phase, which must be at or near zero degrees at your peak S21 gain. And for maximum loaded Q -- and it looks like you are already doing this -- keep 'c' (the series inductor) high in value, and 'd' (the series capacitor) low in value (the narrower the S21 slope the higher the Q). And your biasing resistor 'h' is too low in value (unless you have a high current or low Vcc oscillator); but if you have calculated that that is the bias you want, then you will have to add a choke (an RFC) in series with the resistor, as well as a decoupling cap between the battery and the RFC, to stop the RF from entering the supply.

Best,

Bill

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Hi all, I put a screen grab of a VHF oscillator I am designing, onto my web-space for you to look at.



I'm also using Cotter W. Sayer "Complete Wireless Design" book but there isn't any pictures or explainations of what should appear on the smith chart for a stable oscillator.



Can someone tell me if my design looks ok and what values of S11,21,12,22 I should have.



Cheers, Kevin.


Hi Kevin,

It depends on your Pout requirements for your oscillator, but from a thermal-created frequency drift point of view, it is typically better to go with a low-ish output power (but phase noise will degrade somewhat over an oscillator that has higher Pout).

-Bill

Kev> Hi Bill,

Hi Bill, Thank you very much for your great reply. I'll try to re-gig my circuit a bit.

Q. Is it better to go with a lo current design? One of the models uses 5v for Vce and 15mA for Ic.

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