[time-nuts] Low phase noise VCO

John Miles jmiles at pop.net
Wed Feb 10 00:54:06 UTC 2010

IMHO it'd be best not to roll your own crystal oscillator unless you have
the equipment to measure its noise and other performance parameters, and the
time/skills/interest to optimize it.  That's a tunnel in the rabbit hole
that will just take you away from the one you're headed down now.  For a
one-off project you can often find good parts on eBay just by searching on
"mhz oscillator" (without the quotes).  Remember that you have the luxury of
not being married to a particular frequency, in an SDR application.

For new parts, Crystek makes some good VCXO parts that are easy to get from
DigiKey, and you could also look at Valpey-Fisher's ovenized VFOV110 line
for better performance.  Watch the residual noise specs of the ADCs and
DDSes to make sure you're not paying for a low noise floor that you can't
take advantage of.

-- john, KE5FX

> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
> Behalf Of Nick Foster
> Sent: Tuesday, February 09, 2010 3:41 PM
> To: time-nuts at febo.com
> Subject: [time-nuts] Low phase noise VCO
> Gentlemen,
> 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?
> Thanks,
> Nick
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