[time-nuts] 58503A VS SR625

Bob kb8tq kb8tq at n1k.org
Mon May 28 12:22:04 EDT 2018


The key question on sat’s is - do you always have 4 locked? If so the device should  never go into holdover. 
Some GPSDO’s will stay out of holdover with as few as one sat locked. It’s best not to count on that. 

Simple setup for 10 MHz to 10 MHz is to hook one to the external reference input and the other to input A on the
counter. This works for most counters, not just the SR620. You often do need to dig out the manual to make sure
the external reference is being recognized and locked to. In some cases you discover that your counter needs a 
bit of service ( like re-adjusting the internal reference) for this to work. Set the gate time to something like 10 seconds
and look at the frequency. For fun change the gate time and watch the measured “error” get worse at the shorter
gate times. 

For 1 pps measurement on GPSDO’s things are a bit more complicated. The PPS signals both arrive at the same
time. Counters do not like to deal with this sort of thing. What I do is to dig out the manual on the GPSDO and find 
how to use the cable delay setting on one of the GPSDO’s. You set the cable delay to 500 ns or 1 us. That moves
the pps output of the GPSDO by 500 or 1000 ns. The two pps signals never collide when things are set this way. 
You are measuring a very short time period, so the time base on the counter does not matter much.

Getting the counter to properly start on channel A and stop on channel B is also a “back to the manual” thing for 
each counter you own. None of them are quite the same. Once you get it going, it should be quite clear what is
going on. That is another nice thing about the “round number” offset between the GPSDO’s. You know approximately
what the data is going to look like. 

Again - this process works for any dual channel counter and any GPSDO’s. It’s not restricted to one particular
brand or model. I keep one GPSDO around with the PPS offset simply to do this sort of work. It is much easier 
to just grab that signal than to go set things up every time. If you have several GPSDO’s you can measure them
all against the same offset PPS signal. You can then compare data between them all. 

Lots of fun !!!


> On May 28, 2018, at 11:52 AM, Anton Moehammad <moehanton at yahoo.com> wrote:
> Hi All,
> Thank You for respond.
> Yes I will make 58503A to 58503A comparison but my problem are I got difficulties found the suitable setting for my SR620 to do it if any one can give a hint about proper setting for SR620 time interval setup I will happy to follow it. I like to have 10MHz out comparison and 1PPS comparison too.
> my Antena is 30m above ground and according to Lady Heather I my worst S/N is 35dB all time 
> Pada Senin, 28 Mei 2018 21.17.25 GMT+7, Bob kb8tq <kb8tq at n1k.org> menulis:
> Hi
> If you are seeing 0.3 ppb delta between various HP GPSDO’s the first question would be “on for how long? / under what conditions?".  The HP devices take a *long* time 
> to settle in ( like weeks ) to their final best performance loop settings. To fully settle, they need to be locked the whole time. Staying locked means an antenna that has a 
> good satellite view and the unit having a solid survey location. If they are dropping out of lock from time to time, they will never really settle. 
> Once settled, you still have a noise process. ADEV is one way to plot this sort of noise. Your results will still depend on the gate time on your counter(s) since that impacts 
> how you “see” the noise.  A very short gate will give you more variation than a longer gate. Put another way, “tau” is part of any frequency measurement, even if ADEV is 
> not being calculated. 
> Bob
> > On May 28, 2018, at 8:25 AM, Anton Moehammad via time-nuts <time-nuts at febo.com <mailto:time-nuts at febo.com>> wrote:
> > 
> > Hi All, I have 5 pcs 58503A with various firmware and 3 of them have 8 channel recv and 2 have 6 channel receiver all is working but I found none of 5 has same result I mean when I compare it with my SR625 use SR620 or my CNT-90 use timelab there is a variation in the output.the variation about 3 E-9 Hz. yes use same cable and same antenna, I also monitor the temp and I sure the temp variation is very minimal.any one has explanation about this ?Thank YouAnton
> > 
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