[time-nuts] Re: FS740 Thoughts?

[Bob kb8tq]

Hi

This is one of those classic examples of what marketing writes and what
(once the beer is poured) engineering actually knows. If you dive into the
grubby details of the filtering. Been there / done that with a number of 
these guys. 

It’s very much *not* set up for GPS. The hanging bridge stuff PHK mentions 
in a previous post is one example. The need to adapt the filter to the monitored 
results is another weak area. Just how and when this or that pops up varies, 
but it will pop up.  

Bob

> On Nov 18, 2021, at 2:02 PM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
> 
> Bob, I seem to remember reading in the manual on the PRS-10 where they
> expounded
> at length about how they took great pains to make the unit work well with
> typical noisy
> GPS PPS input.
> 
> I'm using a CNS Clock II as my primary GPSDO, checking phase slippage with
> respect
> to my PRS-10 to make decisions about when (or when not) to manually tweak
> the PRS-10.
> The CNS seems awfully noisy to me, even though the mfr says that it has
> strong hanging
> bridge correction.  I run the two 10 MHz signals through a quadrature
> demodulator whose
> I & Q outputs go into a 2-channel DSO running in extremely low "roll
> mode".  I take a glance
> at it several times per day, with each glance showing me the most recent
> ~4-hour history.
> When I do tweak, I strive to set the PRS-10 about 1E-11 low in frequency,
> which yields a
> couple of months of hands-off operation before it drifts to 1E-11 on the
> high side.  For
> now, that has to suffice.  But I'd rather not have to pay so much attention
> to it, which is
> why I dream about buying a new PRS-10 with PPS locking capability.
> 
> Dana
> 
> 
> On Thu, Nov 18, 2021 at 8:47 AM Poul-Henning Kamp <phk at phk.freebsd.dk>
> wrote:
> 
>> --------
>> Bob kb8tq writes:
>> 
>>> The 'filter' in the PRS-10 is not really set up for a GPS sort of signal.
>> 
>> It is particularly bad at handling GPS's because of the "hanging bridge"
>> phenomena, and the better the GPS, the worse the result...
>> 
>> I tried injecting the "negative sawtooth" via the serial port to my
>> PRS10 but firmware features/bugs prevented that.
>> 
>> The offset could only be changed permanently in the saved configuration,
>> you could not change the running value on a second to second basis.
>> 
>> --
>> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
>> phk at FreeBSD.ORG         | TCP/IP since RFC 956
>> FreeBSD committer       | BSD since 4.3-tahoe
>> Never attribute to malice what can adequately be explained by incompetence.
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