[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 
the  beginning.
* 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.

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