[time-nuts] SRS FS 730

[Bob Camp]

Hi

The hump sounds a lot like power supply noise.

Bob

On Mar 22, 2013, at 7:43 PM, Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:

> 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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>>> 
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