[time-nuts] SRS FS710 spurs at 50Hz and harmonics?
magnus at rubidium.dyndns.org
Fri Oct 2 15:54:27 EDT 2015
I did measure the phase-noise of a FS710 with my TimePod and saw the
same as you. Cross-correlation is a nice tool in the toolbox.
It was really clear that the OSA cesium was much cleaner than the FS710
buffered variant of the signal.
The sawtooth is expected, and the caps just makes sure you don't hit
rocks bottom. The regulators will damp thus further, by burning of the
difference as heat, but it's not really perfect. If you put a
DC-blocking cap in, you will be able to amplify the AC signal and hear
it, it's there for sure.
The FS710 isn't really built to be ultra-quiet.
I think some of these noises is to be expected, so it is a matter how
much, and that depends on how much effort you put in cleaning up power.
Traditional grid-transformer-rectifier-caps-regulator setup will create
this 100 Hz, and all we can do is to toss caps (yeah, and inductors) and
regulators on this. A switcher can change this equation, even if the the
50 Hz rectification process is still there.
No wonders some go for battery options, but then batteries is pretty
On 10/02/2015 08:29 PM, Anders Wallin wrote:
> Hi all, thanks for all the suggestions.
> I am starting to think this is a feature not a bug.
> I tinkered with the FS710 a bit today. It shows about 600mVpp 100Hz
> sawtooth on the +/-13.9VDC input to the voltage regulators.
> I changed the big electrolytic caps and the smaller tantalum caps around
> the 7805/7905 regulators - but no change to the sawtooth.
> With a 1M 10x probe and a cheap scope there is no visible 100Hz ripple on
> the +/-5V regulator outputs.
> some new pictures in my blog:
> Maybe my earlier result with strong spurs was a result of a ground loop
> between REF-distribution-amp, 3120A, and this F710-distribution amp. They
> were probably plugged in to different AC outlets...
> On Thu, Oct 1, 2015 at 1:52 PM, Bob Camp <kb8tq at n1k.org> wrote:
>> It can also be a blown diode in the bridge. When that happens the ripple
>> way up. A quick check with a scope should tell you what’s happened. 50 Hz
>> wave = blown diode. 100 Hz triangle wave = blown cap.
>>> On Oct 1, 2015, at 4:04 AM, Esa Heikkinen <tn1ajb at nic.fi> wrote:
>>> Anders Wallin kirjoitti:
>>>> I seem to get very strong spurs at 50Hz and harmonics with an old
>>>> second-hand SRS FS710:
>>>> feature or bug? Anyone looked at the powersupply and figured out what
>>>> to change?
>>> If you live in a country with 50 Hz mains network then this sounds like
>> a dead capacitor in the power supply. Usually this means that it has old
>> fashioned linear power supply (with classic iron transformer). In that
>> case, replace the secondary capacitors after the rectifier bridge(s). Check
>> voltages with oscilloscope if you wanna see which ones, but it might be
>> good idea to replace them all (there should't be many of them).
>>> Should be very easy to fix.
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