[time-nuts] SRS FS 730

Magnus Danielson magnus at rubidium.dyndns.org
Fri Mar 22 16:20:31 EDT 2013

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 

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?


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