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-boun...@febo.com [mailto:time-nuts-boun...@febo.com]on
Behalf Of Nick Foster
Sent: Tuesday, February 09, 2010 3:41 PM
To: time-nuts@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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