[time-nuts] Low phase noise VCO

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Feb 10 01:13:19 UTC 2010


If you must roll you own 64MHz crystal oscillator you can eliminate the 
problems associated with parasitic oscillation in the sustaining 
amplifier by using a low phase noise unconditionally stable MMIC (eg 
ERA-5SM) as the sustaining stage.

A diode limiter in the feedback path that includes the crystal together 
with a bandpass filter (to select the required overtone) avoids the 
increase in phase noise that occurs if one relies on MMIC saturation to 
limit the crystal dissipation.

One then only need ensure that attenuation through the feedback path is 
sufficient to preclude oscillation at any frequency other than the 
desired crystal overtone.

Designing and optimising the required matching networks is simplified by 
the fact that the amplifier is unconditionally stable.
However the phase noise floor is limited by the power available at the 
MMIC input.

A splitter can be used at the MMIC output to isolate the crystal 
feedback network from the output.

Bruce


John Miles wrote:
> 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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