[time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
SAIDJACK at aol.com
SAIDJACK at aol.com
Mon Sep 15 19:42:51 EDT 2014
here are some plots from two GPSDOs, one series terminated (CSAC GPSDO),
and one load-terminated (Agilent 58503A) product.
These plots were taken with a 54720D scope and a 1M/50Ohms 500MHz input BW.
The cable was 30 feet of generic cheap RG-58 cable.
The plots are self-explanatory in the filename, and show the following:
* Two plots of the Agilent GPSDO driving an open-ended cable, showing the
oscillations and general pretty ugly looking signal after 30 feet of cable.
One plot is a zoomed-in version of the other.
* One plot of the Agilent GPSDO properly end-terminated by 50 Ohms. This
shows that the oscillations are gone, but for some reason the rise-time is
really smudged out and more than 20ns, and there is a little hump right at
* One plot of the CSAC GPSDO with no load-termination showing a very nice
clean and very fast (<1.5ns) risetime with no over/undershoot. This looks
almost as good as at the source, even with 30 feet of cheap old
single-shielded RG-58 cable.
I don't know what the output circuit of the 58503A looks like, but
assuming that it probably is the same as on the Z3801 units many folks here have
it is quite relevant to this discussion, and comparing these two approaches
really shows the quality difference between these two products' signals.
In a message dated 9/15/2014 16:03:56 Pacific Daylight Time,
hmurray at megapathdsl.net writes:
tmiller11147 at verizon.net said:
> So does adding ~80 pF per meter or 8 nF for 100 meters (RG58) to your
> have any effect on the risetime? Because that is what it will see with
> open cable.
That way of thinking only works if the risetime is long relative to the
length. In this context, long enough is ballpark of 4 to 10 times the
time of the cable.
For long cables, the cable initially looks like the characteristic
of the cable. You can see that on a scope with the classic reflection
In vacuum, the speed of light is very close to 1 ft/ns. Coax varies from
60-90% of that.
So if you have 10 ft of cable, it's unlikely that your driver is slow
for the lumped capacitor approximation to be valid. With shorter cables
old gear using HC chips, you might get there.
These are my opinions. I hate spam.
time-nuts mailing list -- time-nuts at febo.com
To unsubscribe, go to
and follow the instructions there.
-------------- next part --------------
A non-text attachment was scrubbed...
Size: 35987 bytes
Desc: not available
More information about the time-nuts