Voltage controlled oscillator - where is this hideous harmonic coming from?


Joined Nov 4, 2008
Gentlemen .. First of all, many thanks for your various and stimulating inputs.

KL7AJ, blessings be upon you: your very first suggestion (which you edited away?) was to add some capacitance to ground from the base and this, combined with your second suggestion to reduce the gain (I changed the emitter bypass capacitor to make it 10x) has given me a brilliant sinusoidal output, with the detested parasitic ground into the dust, and adjustable between 75 and 135 MHz with remarkably uniform amplitude.
You were all right too much gain.

Nothing happened however (not even parasitic) until I added a 12pF base-to-ground capacitor (alternatively my finger, which was a clue) but HOW does that work exactly?

This circuit was a quick feasibility thing and I apologize for offending you with the ugly board which was, in fact, single sided with an accessible ground plane behind - be assured the final version will be more politically correct.

Also, Bertus, thanks for the referral to the RF circuit design pdf.

I now plan to re-do this same circuit coupled (God help me) to a PD5700 hoping to boost the power to a quarter Watt or so and from that to a 5W power stage, likely using a 2N3375. You may see me here again, cap in hand - please continue to drop in a few gems!

After that I hope to make a graceful exit from VHF power design and resume my normal life - happy to have met you all!

Kind regards and thanks again
Hi Dick:
Glad it worked out for you. First of all, for a beginner, your layout looked great....I've had a lot uglier stuff work for me! Your circuit is FAR from hideous as suggested.
The capacitor to ground "trick" works because it generally results in a lower driving impedance and thus "covers a multitude of sins" when it comes to stability. The reason I deleted that though, is because it's not generally considered elegant. It's a hack that works but could lead to bad habits in more advanced (or higher frequency) applications. Anyway...onward and upward. You will find R.F. design addictive...and soon you will be able to worthily look down your proboscis at mere digital designers. :)