[time-nuts] retry: PRS10 has spurious frequencies

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Apr 22 18:53:39 EDT 2007


Said
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 
> OCXO's.
>  
> bye,
> Said 
>
>   
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.

Causes
Resonance in bypass/filter/coupling network coupled with slight non 
linearity in an amplifier or AGC detector diode?

Bruce




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