[time-nuts] SRS FS 730

Magnus Danielson magnus at rubidium.dyndns.org
Fri Mar 22 19:27:11 EDT 2013


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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