> -----Original Message----- > From: [email protected] [mailto:[email protected]] On > Behalf Of Nick Foster > Sent: Tuesday, February 09, 2010 3:41 PM > To: [email protected] > 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. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
