The switcher loop compensation may cause peaking of reference or other
noise in this region.
Amplifier supply bypass networks together with coupling capacitors can
cause such peaking.
Bruce
Magnus Danielson wrote:
Hi Bruce,
Switcher, yes. But with a fair bit of filtering on it. See page 18 of
http://www.thinksrs.com/downloads/PDFs/Manuals/FS730m.pdf
Switcher to get +24 V, LC-filtering with two serial 1,28 kHz filters.
Then an LDO (LM1086CS-ADJ) to drop it to +22 V which is actually used.
700 Hz and 2 kHz doesn't sound like modern switching rates.
FLL/PLL? No.
Cheers,
Magnus
On 03/22/2013 10:01 PM, Bruce Griffiths wrote:
Switch mode power supply ?
FLL or PLL loop?
Bruce
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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