[time-nuts] Low phase noise VCO

[Lux, Jim (337C)]

> -----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?
> 

Does it need to be phase coherent with your 10MHz reference, or just frequency locked?

You could use a run of the mill XO with a small heater on it, and drive the heater with a frequency control loop.  Granted, this limits you to loop bandwidths in the sub hertz range, but that might be all you need.

Also,  If you are using it in a software defined radio, you don't necessarily need *control* of the oscillator frequency.  What you might need is *knowledge* so that you can set the correct number into a DDS/NCO.  

Knowledge (especially against an external 10 MHz source) is a lot easier to come by than control: all it takes is a counter.

This lets you choose an oscillator with great performance in the short term (high Q resonator) without having to worry about temperature compensating or adjusting (both of which require a lower Q, so you have something to adjust).

APL has built some "ultra stable oscillators" (USOs) using this approach.  And it's also similar to the MCXO from places like Q-Tech.


OTOH, if you need phase coherency or phase continuity, it's a bit trickier.