Discussion:
Stable32 Input (Multiplier)
(too old to reply)
Chris Burford
2018-08-08 14:10:47 UTC
Permalink
I'm using Lady Heather to write data from an RFS. In looking at the data file it appears the values are written as ppb. My question is how to best accommodate this with the correct Stable32 multiplier value. Should the multiplier value be +1.0000000000000e-09 or +1.0000000000000e-10?

It seems I have thoroughly confused myself as to which value will scale the data properly.

_______________________________________________
time-nuts mailing list -- time-***@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
Magnus Danielson
2018-08-09 10:27:41 UTC
Permalink
Hi,
Post by Chris Burford
I'm using Lady Heather to write data from an RFS. In looking at the data file it appears the values are written as ppb. My question is how to best accommodate this with the correct Stable32 multiplier value. Should the multiplier value be +1.0000000000000e-09 or +1.0000000000000e-10?
ppm, parts per milion, 1E-6
ppb, parts per billion, 1E-9
Post by Chris Burford
It seems I have thoroughly confused myself as to which value will scale the data properly.
Hope it cleared up.

Cheers,
Magnus

_______________________________________________
time-nuts mailing list -- time-***@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
Tom Van Baak
2018-08-11 15:32:00 UTC
Permalink
Post by Chris Burford
Should the multiplier value be +1.0000000000000e-09 or +1.0000000000000e-10?
I worked with Chris off-list on this one. It turns out he was measuring his new PRS10 and trying to match his ADEV plots against the published plots from SRS. Hence the request about 1e-9 (which is ppb and should be correct) and 1e-10 (which is not ppb, but seems to give a much closer match of the plots). It's an interesting mystery and I'm glad he posted his question. He's new to the list, but clearly has gone well down the time nuts path already.

He was using Heather to monitor the PRS10, extracted the "FC" column from the log file, and used Stable32 to scale and plot the ADEV. Can you spot the problem?

The integer FC (Frequency Control) values from the PRS10 is a pair of DAC settings; the DAC used to steer the internal OCXO to the rubidium resonance. Heather scales these two numbers by 1.5e-9 and 1.5e-12 (per the manual) [1] and then scales them by 1e9 (so they logged as ppb). Stable32 (or TimeLab) then has to unscale them back by 1e-9. So far so good.

But the actual problem is that the FC value is *not* a measurement of PRS10 performance. It's simply telemetry that shows how the DAC is operating; how much steering the OCXO needs to generate a Rb-locked 10 MHz output. In order to actually measure the output of a PRS10 one needs a rather high-res counter as well as a better-than-PRS10 frequency standard as a reference. A PRS10 can't measure itself, and Heather is a reporting tool not a measurement tool.

I'm guessing that the reason scaling by 1e-10 looked closer to spec is that by tau 100 s the Rb is about an order of magnitude better than the OCXO. But let's find out for sure.

Two questions for time-nuts:

1) Do any of you have log files of FC values from your PRS10? I'd like to compare yours with Chris's to see if they are in the same ballpark. A couple of hours or a day is all that's needed. Either raw ascii from the PRS10 itself or processed values via Heather is ok.

2) Have any of you ever logged the FC values while at the same time taken precision measurements of PRS10 output phase? I ask because that would allow one to sort of back out the DAC adjustments and make ADEV plots of both the OCXO and the Rb. It would then nicely show the effect of the Rb disciplining the OCXO, similar to how we make plots of GPS disciplining a OCXO. [2].

Thanks,
/tvb

[1] https://www.thinksrs.com/downloads/pdfs/manuals/PRS10m.pdf
[2] http://www.leapsecond.com/pages/gpsdo/


----- Original Message -----
From: "Chris Burford" <***@austin.rr.com>
To: <time-***@lists.febo.com>
Sent: Wednesday, August 08, 2018 7:10 AM
Subject: [time-nuts] Stable32 Input (Multiplier)
Post by Chris Burford
I'm using Lady Heather to write data from an RFS. In looking at the data file it appears the values are written as ppb. My question is how to best accommodate this with the correct Stable32 multiplier value. Should the multiplier value be +1.0000000000000e-09 or +1.0000000000000e-10?
It seems I have thoroughly confused myself as to which value will scale the data properly.
_______________________________________________
time-nuts mailing list -- time-***@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
Magnus Danielson
2018-08-11 16:05:15 UTC
Permalink
Hi,
Post by Tom Van Baak
Post by Chris Burford
Should the multiplier value be +1.0000000000000e-09 or +1.0000000000000e-10?
I worked with Chris off-list on this one. It turns out he was measuring his new PRS10 and trying to match his ADEV plots against the published plots from SRS. Hence the request about 1e-9 (which is ppb and should be correct) and 1e-10 (which is not ppb, but seems to give a much closer match of the plots). It's an interesting mystery and I'm glad he posted his question. He's new to the list, but clearly has gone well down the time nuts path already.
He was using Heather to monitor the PRS10, extracted the "FC" column from the log file, and used Stable32 to scale and plot the ADEV. Can you spot the problem?
The integer FC (Frequency Control) values from the PRS10 is a pair of DAC settings; the DAC used to steer the internal OCXO to the rubidium resonance. Heather scales these two numbers by 1.5e-9 and 1.5e-12 (per the manual) [1] and then scales them by 1e9 (so they logged as ppb). Stable32 (or TimeLab) then has to unscale them back by 1e-9. So far so good.
But the actual problem is that the FC value is *not* a measurement of PRS10 performance. It's simply telemetry that shows how the DAC is operating; how much steering the OCXO needs to generate a Rb-locked 10 MHz output. In order to actually measure the output of a PRS10 one needs a rather high-res counter as well as a better-than-PRS10 frequency standard as a reference. A PRS10 can't measure itself, and Heather is a reporting tool not a measurement tool.
The FC values would be how the OCXO long term performance compares to
the rubidium, and we expect it to be worse than the rubidium, or else
the rubidium reference isn't very helpful in the long-term.

Actually, you could use the PPS lock mechanism and read out the
time-errors from that.
Post by Tom Van Baak
I'm guessing that the reason scaling by 1e-10 looked closer to spec is that by tau 100 s the Rb is about an order of magnitude better than the OCXO. But let's find out for sure.
1) Do any of you have log files of FC values from your PRS10? I'd like to compare yours with Chris's to see if they are in the same ballpark. A couple of hours or a day is all that's needed. Either raw ascii from the PRS10 itself or processed values via Heather is ok.
2) Have any of you ever logged the FC values while at the same time taken precision measurements of PRS10 output phase? I ask because that would allow one to sort of back out the DAC adjustments and make ADEV plots of both the OCXO and the Rb. It would then nicely show the effect of the Rb disciplining the OCXO, similar to how we make plots of GPS disciplining a OCXO. [2].
Now, that would be a worthy lab-setup to do. Sounds like a fun exercise.
I'll see if I can swing something together as I'm down there anyway.

Cheers,
Magnus

_______________________________________________
time-nuts mailing list -- time-***@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
Mark Sims
2018-08-11 16:35:15 UTC
Permalink
Also note that Heather can only get/update the FC value (usually( every two seconds. Occasionally, due to vagaries in the PRS-10 response timing, you can see two consecutive values in the log that are not the same.
Post by Tom Van Baak
2) Have any of you ever logged the FC values while at the same time taken precision measurements of PRS10 output phase?
I think that Heather can do that if you use a TICC or counter as an auxiliary input device (/ei command). In that mode Heather gets the "PPS" and/or "OSC" values from the counter. Since the FC value is treated as the Heather "OSC" value, the PPS value would come from the counter (chA on a TICC). I'll try to do that next week... my TICC and 5370 are doing something else right now.
_______________________________________________
time-nuts mailing list -- time-***@lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.

Loading...