[time-nuts] Low phase noise VCO

Nick Foster bistromat at hotmail.com
Tue Feb 9 23:41:21 UTC 2010


Not being an oscillator guru, I thought I'd ask here. I'm building up a fixed-frequency 64MHz PLL oscillator which uses a 10MHz reference. The reference is a homebrew HP 10544A-based GPSDO which seems to work OK. I've built a phase comparator based on a CoolRunner-II CPLD which implements a flip-flop phase-frequency comparator just like the CMOS 4046. Seems to work great, although I haven't put it to the test yet with respect to phase noise. So all I need now is the VCO.

Right now, all I need is 64MHz to clock a software-defined radio, so I was thinking about using a VCXO. I see lots of solutions using LC oscillators, but very few using crystals because overtone crystals can't be pulled very far at all. So my question is, what type of oscillator would you recommend for this project, an overtone crystal oscillator like a Butler, or an LC oscillator? Is there a particular oscillator topology you favor, or (better yet) a schematic of one you like?

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