[time-nuts] Low phase noise VCO

Lux, Jim (337C) james.p.lux at jpl.nasa.gov
Wed Feb 10 00:18:32 UTC 2010

> -----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. 

But you don't need to move very much.. A VCTCXO or VCXO might be the ticket for you.  A lot of TCXOs have a "trim" input that might be usable.  Check the Mouser or DigiKey catalogs.

How good does the performance need to be (outside the loop... I assume inside the loop it's disciplined by your 10 MHz reference)  A FOX924E has a voltage input and is available as a 16MHz unit (which could be x4 to your 64)

Have you considered a straight multiply/add type scheme?  Multiply 10 to 60, divide 10 to 1, quadruple to 4, mix 4 with the 60 to get 64. (or some other combination of convenient frequencies..)

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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