[time-nuts] Looking for good SR620 setup to compare GPS and rubidium

[James Maynard]

David Forbes wrote:
> Folks,
> 
> We have a rubidium oscillator in a laboratory here that is ripe for 
> calibration against a primary standard. We have installed a Datum 9390 
> GPS receiver next to it as well as an SR620 counter.
> 
> Can any of you recommend a good operating mode to make the SR620 reveal 
> the rubidium drift rate in a reasonably short time a couple days is OK) 
> so that we may adjust it to near zero?
> 
> We have both 10 MHz and 1PPS available from both sources.
> 
> We don't currently have a logging computer connected to the SR620, but 
> we can do that if needed.
> 
> 
> 
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> 
> .
> 

This is a similar measurement problem to what I'm doing at home.

Count the rubidium oscillator's output down to 1 pulse per second, and 
feed that 1 PPS into a length of 50-ohm coax. (I use a PTS-50 
distribution amplifier for this; it has a divided-by-N output, for which 
I set N such that it provides 1 PPS.) Attach that 50-ohm cable to an 
oscilloscope, uwing a 50-ohm terminator at the oswcilloscope input. 
Observe the waveform, and pick a voltage level that will be convenient 
for triggering the scope (soon, the SR620 counter) such that the trigger 
point will not be susceptible to any errors from ringing on the 
waveform. Now set Set the SR620's "A" trigger level to that same 
voltage, and move the cable from the oscilloscope to the SR620's "A" 
input. (You can remove the 50-ohm terminator now, and use the SR620's 
internal 50-ohm terminator.)

Using a similar procedure with the 1 PPS output of the GPS receiver. Use 
the oscilloscope to determine a suitable trigger level, then connect the 
1 PPS from the GPS receiver, via a length of 50-ohm coax, to SR620's "B" 
input. Set the SR620's "B" trigger level to the level you determined 
with the oscilloscope, and enable the "B" input's 50-ohm terminator.

Set the SR620 to its "+time" mode, to measure the time interval between 
the reference 1 PPS (counted down from your rubidium oscillator) and the 
1 PPS from the GPS. This is your relative phase measurement.

Set the sample size (I'll call it N) on the SR620.  For a quick look, 
set N=1, and configure the SR620 to graph its output on your 
oscilloscope. (The SR620's rear panel X and Y outputs go to two channels 
of your oscilloscope, with the oscilloscope set to display the output in 
XY mode, with 1 volt per division on X and 1 volt per division on Y.) 
Set the SR620 display its "strip chart" of relative phase. With N=1, it 
takes 250 seconds to fill the strip chart, after which it starts 
re-painting the strip chart.

Now increase N, to give you a longer time to obswerve the trend of the 
relative phase measurements. The slope of the graph will tell you the 
frequency error.  If your rubidium oscillator is high in frequency, the 
slope will be positive; if low in frequency, the slope will be negative.

My rubidium oscillator (a Stanford Research Systems model PRS-10) has a 
10 MHz output. The 10 MHz from the PRS-10 goes to the 10 MHz input of 
the PTS-50 distribution amplifier. One of the PTS-50's 10 MHz outputs 
goes to the rear panel external frequency reference of the SR620.  And, 
as I mentioned before, the 1 PPS output of PTS-50 goes to the "A" input 
of the SR620.  If your rubidium oscillator has a 5 MHz rather than 10 
MHz output, it can still be used as an external reference for the SR620; 
just configure the SR620 accordingly.

I hope this helps. I didn't describe everything, as the SR620 manual 
tells you how to do all this.

-----
James Maynard, K7KK
Salem, Oregon, USA