[time-nuts] How to measure phase noise of HCMOS oscillators?

[John Miles]

Your main concern will be injection locking, followed by the additive noise
of the buffer amps that you use to prevent it.  The CMOS output buffers in
the oscillators themselves may provide enough isolation, but don't count on
it.  Injection locking is easy to spot with a scope.  You can adjust the
tuning voltage of one oscillator to match the frequency of the other, and
see if their phases snap into coincidence.  

If necessary, LMH6702 opamps can be used at the frequencies covered by the
CVHD-950 parts.  They will provide plenty of isolation with little or no
effect on noise measurements in the -165 dBc/Hz vicinity, given strong
signals to work with (+10 dBm or more).

If I were driving a mixer with 3.3V HCMOS, I'd try a series 50-ohm
termination first.  Basically, pretend that you have a perfect voltage
source whose output impedance will be that of whatever resistor you place in
series with it.  It won't be, of course, but the effect will be close
enough.  The connections between the oscillators and mixers will not be long
enough to make exact impedance matching necessary.

There should be no need to filter the signal; mixers are switches, and they
like square waves.  If you want, you can use an L-network to handle both
lowpass filtering and matching, of course.  The only filter necessary is the
LPF that follows the mixer.  

-- john, KE5FX
Miles Design LLC

> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-
> bounces at febo.com] On Behalf Of Anders Time
> Sent: Monday, March 10, 2014 8:33 AM
> To: time-nuts at febo.com
> Subject: [time-nuts] How to measure phase noise of HCMOS oscillators?
> 
> A while a go i bought some low phase noise Crystek CVHD-950 and now I
> want
> to meas the phase noise of the oscillators.
> Is it best to try to impedance match the output signal and filter and
> amplify the hcmos before i feed it in to the phase detector? or Just hope
> that the signal is high enough to drive the mixer? What about all the
> harmonics? Does it alter the phase noise result?
> /Anders
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