[time-nuts] retry: PRS10 has spurious frequencies
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Sun Apr 22 18:53:39 EDT 2007
SAIDJACK at aol.com wrote:
> Hi Henk,
> I have a expensive 10MHz ultra-low-noise Wenzel oscillator that we bought
> some months ago that has similar very nasty noise on it's output.
> See the attached HP8563E plot, you can see the worst spur at 280MHz at
> -56dBm, which is about -50dBc/Hz!
> The Wenzel has a measured phase noise floor of -169dBc/Hz at 10Khz.
> In comparison, see our Jackson-Labs Fury GPSDO output measured exactly the
> same way - one can only see the second and third harmonic everything else is
> killed by the seventh-order low pass filter.
> I asked Wenzel about this noise, and they said "we see it too, it's caused
> by the HP8563E spectrum analyzer itself, we are not worried about it".
> Well, Wenzel went silent when I sent them the attached plots of an MTI-230
> OCXO raw output, and our Fury GPSDO output both measured on the same 8563E
> setup. Never heard from them since.
> I think the measurements show that it's pretty important to put a low pass
> filter on the output of OCXO's.
> I wonder what is generating such high frequency, high energy spurs in these
Actually a bandpass filter is even better, particularly for higher
frequency OCXOs, but don't get too aggressive in lowering the bandwidth
or the phase instabilities of the filter will degrade the performance.
Rather a combination of a low pass and a high pass filter with reasonbly
wide bandpass so that the effect of minor component instabilities only
have a small effect on the phase of the desired OCXO output frequency.
If there are any spurious output frequencies relatively close to the
deired frequency then a bandstop filter tuned to the unwanted frequency
has better phase stability at the desired frequency than anarrow band
bandpass filter tuned to the desired output frequency. For the ultimate
in phase stability avoid ferrite core inductors, air core or powdered
iron core inductors have lower (close in) phase noise.
Resonance in bypass/filter/coupling network coupled with slight non
linearity in an amplifier or AGC detector diode?
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