[time-nuts] Data collection cycle -- multiple sources to one counter?

Brooke Clarke brooke at pacific.net
Sat Jun 18 22:47:16 EDT 2005

Hi John:

If you have Time RAIM turned on then using GPS as the start pulse is 
good, that way if the RAIM shuts off the GPS no measurement will be 
started.  Using GPS as the stop pulse would result in an extra long 

I'm using 1000 seconds per reading on the SR620.  I like it in that you 
get new readings fairly often so if something changes you don't have to 
wait forever.  The sigma is almost always in the 9 ns area, which is the 
saw tooth size for the M12T+.

I moved an HP 59307A Dual 4-way HP-IB coax switch here to do just what 
you describe, but am not happy with the performance of the FTS4060.  The 
longer you measure each source the fewer relay cycles the switch needs 
to do, making it last longer.

When working with the HP 4380S Balanced Network Analyzer system I found 
that using flexible cables resulted in an unstable system.  My first 
improvement was to use 0.141" semi rigid coax (it's much more on the 
rigid side) and got a mandrel to bend it.  Later I discovered you can 
get flexible 0.141" coax that's not "rigid" and it worked great, no 
mandrel needed.  The point is that you probably will need to have good 
control on all the coax lines.  The lines will change length with 
temperature, so the longer they are the more temperature variation there 
will be.

Tom or Doug probably have much more experience in recording over long 
time frames.

Have Fun,

Brooke Clarke, N6GCE
w/Java http://www.PRC68.com
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml

John Ackermann N8UR wrote:

> I want to monitor all my atomic devices over the long term against GPS. 
> Rather than devoting one counter to each source and gathering a lot more
> data than is useful, I'm going to use a GPIB-controlled coax switch to
> route the 1pps from each source into the counter START input.  That way
> I can alternately read each source for X minutes against GPS.
> My question is whether there is an optimum "X" for the time to dwell on
> each source.  I've been thinking that 15 minutes might make sense.  That
> means I'd start with device A, average it against GPS for 15 minutes of
> 1pps samples, record that to the log file, then switch the input to
> device B, dwell there for 15 minutes and record, then to device C for 15
> minutes and record, then repeat the cycle.
> With three devices, that would give me averages of 900 readings and a
> tau of 45 minutes.  Seems reasonable, but I suspect a lot of labs do
> something similar and I wonder if there's tribal knowledge of the
> optimum cycle.
> Any thoughts?
> John
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