[time-nuts] Low phase noise VCO

Ulrich Bangert df6jb at ulrich-bangert.de
Wed Feb 10 08:02:30 UTC 2010


altough you have been asking for advice for the VCO I would like to draw
your attention on how you can improve your phase detector. Edge-triggered
phase detectors are not bad, the even can work as frequency detectors if you
are far from lock and help you to gain lock where a XOR would not. However,
once you are CLOSE to a phase lock, a simple XOR makes the better phase
detector with less phase noise. There is a VERY easy circuit that marriages
the best of both worlds. Google for the AD9901 frequence/phase detector and
mimic their schematic of the first page into your CPLD. Have done this with
different CPLDS from XILINX and ALTERA and has always worked very well for
me in terms of frequency locking range and low phase noise.

Best regards
Ulrich Bangert  

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Nick Foster
> Gesendet: Mittwoch, 10. Februar 2010 00:41
> An: time-nuts at febo.com
> Betreff: [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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