[time-nuts] Low phase noise VCO

Bill Hawkins bill at iaxs.net
Wed Feb 10 00:24:39 UTC 2010

Which leads me to ask a novice question:

Why not pull a 16 MHz crystal and multiply to 64 MHz?

If you count down from 64 to 10 MHz, isn't the multiplication inside the

Perhaps the noise is multiplied by 4, but would it work for the intended

Bill Hawkins

-----Original Message-----
From: Nick Foster
Sent: Tuesday, February 09, 2010 5:41 PM


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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