[time-nuts] SRS FS 730

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Mar 22 17:01:09 EDT 2013

Switch mode power supply ?
FLL or PLL loop?


Magnus Danielson wrote:
> Fellow time-nuts,
> I got the oppertunity to measure a cesium with my TimePod, I hooked up 
> on the FS 730/1 and noticed a bump in the phase-noise. I by-passed it 
> and the bump was not there. Turns out that it had a 20 dB increased 
> phase-noise at 700 Hz. So, we tried the other FS 730/1 they had, and 
> it showed the same 20 dB increase, but at 2 kHz. It was not changing 
> with load. There was a fair amount of 50 Hz and 150 Hz in the spectrum 
> too.
> It was a fairly wideband rise of noise, rather than the spikes.
> I did look in the schematic, but did not have the time to measure 
> inside them.
> Have people seen this before? A fair idea of what aspect in the design 
> which is to blame? Let's learn from this, right?
> My ideas so far:
> The input has a low-Q LC filter, could it be the noise shaped as it 
> goes into the input limiter, considering that the LC filter might be a 
> little detuned from 10 MHz.
> Another theory would be a resonance in the power, but I have not found 
> an obvious place in power design that would be a give-away.
> So, thoughts? Bob? Bruce?
> Cheers,
> Magnus
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