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4/10/2016 21:42:57
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LOS MEAS NCO STATUS WEIGHT PBUC FBUC DBUC LBUC IBUC G M TC
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0: 1 0.000e+00 0.000e+00 0x07AC 0 700 700 0 0 2 2 4 29
1: 1 0.000e+00 0.000e+00 0x068C 0 700 700 0 0 0 2 14 226
GPS: 0 -2.082e-09 -6.762e-09 0x0000 1 ---- 1000 1000 ---- ---- 3 6 31
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freq cor = -6.761820e-09
phase cor = 2.000000e-11
gps phase = 3.333333e-09
temp cor = -2.301801e-11
Command Complete
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4/17/2016 20:03:32
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LOS MEAS NCO STATUS WEIGHT PBUC FBUC DBUC LBUC IBUC G M TC
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0: 1 0.000e+00 0.000e+00 0x07AC 0 700 700 0 0 2 2 4 29
1: 1 0.000e+00 0.000e+00 0x068C 0 700 700 0 0 0 2 14 226
GPS: 0 -7.138e-09 -6.458e-09 0x0000 1 ---- 1000 1000 ---- ---- 3 6 31
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freq cor = -6.458316e-09
phase cor = -2.000000e-11
gps phase = 1.033333e-08
temp cor = -3.138324e-11
Command Complete
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4/20/2016 08:25:29
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LOS MEAS NCO STATUS WEIGHT PBUC FBUC DBUC LBUC IBUC G M TC
-------------------------------------------------------------------------------
0: 1 0.000e+00 0.000e+00 0x07AC 0 700 700 0 0 2 2 4 29
1: 1 0.000e+00 0.000e+00 0x068C 0 700 700 0 0 0 2 14 226
GPS: 0 -8.632e-09 -6.277e-09 0x0000 1 ---- 1000 1000 ---- ---- 3 6 31
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freq cor = -6.276511e-09
phase cor = -2.000000e-11
gps phase = 1.533333e-08
temp cor = -1.841605e-11
Command Complete
UCCM-P > SYSTem:STATus?
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090-03861-03 serial number W560935175 firmware ver 1.0.0.2-01 LINK mode
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Reference Status __________________________ Reference Outputs _______________
Ref 8KHz 0: [LOS]
TFOM 2 FFOM 0
UCCM-P Status[ACTIVE]
>> GPS: [phase: 1.27e-08]
ACQUISITION ................................................ [ GPS 1PPS Valid ]
Tracking: 4 ____ Not Tracking: 5 ________ Time ____________________________
PRN El Az C/N PRN El Az GPS 20:17:52 17 APR 2016
7 12 329 39 10 16 153 GPS Synchronized to UTC
8 17 275 35 16 76 255 ANT DLY +5.000E-08
20 23 50 44 18 34 117 Position ________________________
21 59 71 35 26 59 168 MODE Hold
27 48 279
LAT N 51:25:23.472
LON W 0:17:26.922
HGT +2.70 m (MSL)
ELEV MASK 5 deg
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Command Complete
If *frequency* accuracy is of interest, then ADEV (especially at 1 second) is probably not a good measure on a GPSDO. At 1 second, you are looking at the free running performance of the OCXO. You get great numbers, but the accuracy might only be as good as the OCXO's. You need to wait long enough that the GPS section has had time to get the OCXO settled down in order for the accuracy of the GPS system to transfer in a meaningful way to the OCXO.
A simple experiment:
Yank the antenna on the GPSDO and look at the ADEV. The numbers inside 10 seconds look no different than the numbers with the antenna connected. The OCXO *could* be off anywhere.
Leave the antenna off and watch the ADEV plot of the OCXO. As the unit stabilizes (over days / weeks), the curve past 10 seconds flattens out. The longer it is on, (in a stable environment) the flatter it is likely to get. That may continue out past 1,000 seconds or it may start to turn up a bit. The flatter that curve is, the better the short term frequency of the OCXO will be compared to the long term.
Yes, it would be nice if the ADEV curve dropped off as 1/sqrt(tau). That's not what happens in an OCXO. The physics don't work that way.
Not sure how I would get ADEV?
But I think that's exactly my question... At startup you effectively get the OCXO performance and then the OCXO becomes gradually disciplined by the GPS PPS signal. So when am I confident it is at least accurate to 1mHz in 10MHz. Which I presume is 1e-10?
Looking at the DIAG:LOOP numbers they don't seem to change much over several days.
Not sure how I would get ADEV?
But I think that's exactly my question... At startup you effectively get the OCXO performance and then the OCXO becomes gradually disciplined by the GPS PPS signal. So when am I confident it is at least accurate to 1mHz in 10MHz. Which I presume is 1e-10?
Looking at the DIAG:LOOP numbers they don't seem to change much over several days.
You get ADEV by sending the data from your counter to a computer and then giving it to an ADEV program. A program like Timelab combines those and lets you use either RS-232 or GPIB to collect the data and generates the graph automatically - if your counter is supported.
If your goal is 1 mHz accuracy, it shouldn't take very long. Unfortunately, we don't have specs for these units. However, the HP/Symmetricom Z38xx units use the same terminology and commands (mostly). The Z3801A specifies a 10 MHz error of < 1e-9 averaged over 1 day (See note below). They say that the spec is reached when the FFOM is <= 2. GPS has improved since that manual was written (2000) so the performance of even an old GPSDO will be significantly better than spec. But without measuring it, I can't say how much better or when you'd reach your 1 mHz goal. I suspect that if you wait for an FFOM of 0 you'd be fine.
If you've had your unit running for a week or two, the loop parameters might have settled almost as much as they're going to. Some GPSDOs remember the OCXO's behaviour even over a power cycle, so you might not be able to see a lot of variation even if you cycled the power.
Ed
Note: I just looked at the specs for an HP 58540A Time & Frequency Receiver. It's in the same family as the Z38xx units. The manual, also dated in 2000, states that the 10 MHz accuracy is <= 1e-11 averaged over one day. I don't understand why there would be so much difference in specs for the two units. I doubt that there's much real difference between the Z3801A and the 58540A.
Not sure how I would get ADEV?
But I think that's exactly my question... At startup you effectively get the OCXO performance and then the OCXO becomes gradually disciplined by the GPS PPS signal. So when am I confident it is at least accurate to 1mHz in 10MHz. Which I presume is 1e-10?
Looking at the DIAG:LOOP numbers they don't seem to change much over several days.
You get ADEV by sending the data from your counter to a computer and then giving it to an ADEV program. A program like Timelab combines those and lets you use either RS-232 or GPIB to collect the data and generates the graph automatically - if your counter is supported.
If your goal is 1 mHz accuracy, it shouldn't take very long. Unfortunately, we don't have specs for these units. However, the HP/Symmetricom Z38xx units use the same terminology and commands (mostly). The Z3801A specifies a 10 MHz error of < 1e-9 averaged over 1 day (See note below). They say that the spec is reached when the FFOM is <= 2. GPS has improved since that manual was written (2000) so the performance of even an old GPSDO will be significantly better than spec. But without measuring it, I can't say how much better or when you'd reach your 1 mHz goal. I suspect that if you wait for an FFOM of 0 you'd be fine.
If you've had your unit running for a week or two, the loop parameters might have settled almost as much as they're going to. Some GPSDOs remember the OCXO's behaviour even over a power cycle, so you might not be able to see a lot of variation even if you cycled the power.
Ed
Note: I just looked at the specs for an HP 58540A Time & Frequency Receiver. It's in the same family as the Z38xx units. The manual, also dated in 2000, states that the 10 MHz accuracy is <= 1e-11 averaged over one day. I don't understand why there would be so much difference in specs for the two units. I doubt that there's much real difference between the Z3801A and the 58540A.
Hi
Here's the issue:
The "averaged over a day" number simply indicates that the phase lines up to some degree over 24 hours. The "frequency" being used is really frequency as delivered by GPS time. For practical use, it is not the number you think it is. The OCXO could swing +/- 1 ppm and hit that spec.
The more important spec if you are interested in frequency is -- how close is it if I use a gate time of ____ ? Normal numbers are 1, 10, and 100 seconds. The gotcha is that there is no closed form solution to that measure. (Which is why we have ADEV). About all you can say is: If I want it to be correct 99% of the time, what is the limit? Consider that with thousands of samples in a day, with 1 or 10 second gates, having 1% out still is a lot of errors.
You don't see the 99% spec on the cell tower GPSDO's. It was not part of the master specification on the units. The frequency limit in the master spec was 1x10^-8 which is pretty silly in this context. Over the first day a typical unit will hit < 1 ppb on a 99% basis. That will evolve to about 0.1 ppb pretty quickly (day 2 or 3). It will progress into the 0.1 to 0.01 ppb range, but probably will not exceed 0.01 ppb unless you have a *really* good OCXO.
Bob
Not sure how I would get ADEV?
But I think that's exactly my question... At startup you effectively get the OCXO performance and then the OCXO becomes gradually disciplined by the GPS PPS signal. So when am I confident it is at least accurate to 1mHz in 10MHz. Which I presume is 1e-10?
Looking at the DIAG:LOOP numbers they don't seem to change much over several days.
You get ADEV by sending the data from your counter to a computer and then giving it to an ADEV program. A program like Timelab combines those and lets you use either RS-232 or GPIB to collect the data and generates the graph automatically - if your counter is supported.
If your goal is 1 mHz accuracy, it shouldn't take very long. Unfortunately, we don't have specs for these units. However, the HP/Symmetricom Z38xx units use the same terminology and commands (mostly). The Z3801A specifies a 10 MHz error of < 1e-9 averaged over 1 day (See note below). They say that the spec is reached when the FFOM is <= 2. GPS has improved since that manual was written (2000) so the performance of even an old GPSDO will be significantly better than spec. But without measuring it, I can't say how much better or when you'd reach your 1 mHz goal. I suspect that if you wait for an FFOM of 0 you'd be fine.
If you've had your unit running for a week or two, the loop parameters might have settled almost as much as they're going to. Some GPSDOs remember the OCXO's behaviour even over a power cycle, so you might not be able to see a lot of variation even if you cycled the power.
Ed
Note: I just looked at the specs for an HP 58540A Time & Frequency Receiver. It's in the same family as the Z38xx units. The manual, also dated in 2000, states that the 10 MHz accuracy is <= 1e-11 averaged over one day. I don't understand why there would be so much difference in specs for the two units. I doubt that there's much real difference between the Z3801A and the 58540A.
Hi
Here's the issue:
The "averaged over a day" number simply indicates that the phase lines up to some degree over 24 hours. The "frequency" being used is really frequency as delivered by GPS time. For practical use, it is not the number you think it is. The OCXO could swing +/- 1 ppm and hit that spec.
The more important spec if you are interested in frequency is -- how close is it if I use a gate time of ____ ? Normal numbers are 1, 10, and 100 seconds. The gotcha is that there is no closed form solution to that measure. (Which is why we have ADEV). About all you can say is: If I want it to be correct 99% of the time, what is the limit? Consider that with thousands of samples in a day, with 1 or 10 second gates, having 1% out still is a lot of errors.
You don't see the 99% spec on the cell tower GPSDO's. It was not part of the master specification on the units. The frequency limit in the master spec was 1x10^-8 which is pretty silly in this context. Over the first day a typical unit will hit < 1 ppb on a 99% basis. That will evolve to about 0.1 ppb pretty quickly (day 2 or 3). It will progress into the 0.1 to 0.01 ppb range, but probably will not exceed 0.01 ppb unless you have a *really* good OCXO.
Bob
Yes, the 'one day' figure by itself is open to a lot of interpretation. I think the Z3801A manual is just a reissue of an older manual and so doesn't spec things very well. The 58540A manual however, also specs the ADev as <= 5e-10 @ 1 sec. one hour after initial lock, so frequency swings of even 1 ppb are unlikely. But you're correct that they don't give a 99% spec.
But that still doesn't help us with this Symmetricom unit. I don't have one so I have no measurements to share. Measuring a GPSDO can be a challenge. Tau values of > ~1000 sec. aren't necessary because GPS sets the performance. In any case, it's easy to make those measurements. But measuring at short tau values at these levels requires some effort and equipment.
Ed
Unfortunately 1 second ADEV tells you only that the standard deviation of the frequency change second to second (after drift is removed) is some number. It tells you nothing about the average change at all. The system can be cycling 1 ppm up and down and still hit a 5 x 10^-10 ADEV. It will have to do it very slowly. That's exactly what GPSDO steering does (change things slowly). Is it likely (or even possible) that it swings a ppm? No, it's not and it doesn't do that. The point is simply that it *could* do that and a 1 second ADEV would not catch it.
If you are going to evaluate a 1 second 99% spec at the 1x10^-11 level, you pretty much must have a Hydrogen Maser to compare it to. Anything else (including a high performance tube in a 5071) isn't going to do the job. There are a few other bits and pieces you will need, but the maser rules out most of us common folk.
Another approach is to compare two GPSDO's of different designs against each other. That can lead to all sorts of tangles. Yet another approach is to take the gate time out to > 100 seconds and a 5071 *might* do the job with some hand waving involved. Needless to say, you don't see many GPSDO's with a 1 second 99% confidence point at 1x10^-11 guaranteed by 100% testing.
Bob
Simplistically, So if I wait 10^4 seconds (presumably after hold reached) then I should have got a good chunk of quality out of my box? 10,000s is about 3 hrs?
Simplistically, So if I wait 10^4 seconds (presumably after hold reached) then I should have got a good chunk of quality out of my box? 10,000s is about 3 hrs?
Simplistically, So if I wait 10^4 seconds (presumably after hold reached) then I should have got a good chunk of quality out of my box? 10,000s is about 3 hrs?
Hi
GPSDO's go through a series of filter adjustments as the OCXO warms up and the unit stabilizes. You only get through the first part of the process 24 hours. Running for at least three days is about the minimum for reasonable accuracy.
Bob
Simplistically, So if I wait 10^4 seconds (presumably after hold reached) then I should have got a good chunk of quality out of my box? 10,000s is about 3 hrs?
Hi
GPSDO's go through a series of filter adjustments as the OCXO warms up and the unit stabilizes. You only get through the first part of the process 24 hours. Running for at least three days is about the minimum for reasonable accuracy.
Bob
Hi Bob,
Can you give an estimate of what 'reasonable accuracy' would be? We're flying blind with this Symmetricom GPSDO. We have no docs, no specs, and no clue what the design requirements were. But if it was telco equipment, the start-up phase might have been very loosely defined since it was likely intended for 24/7 operation. It appears to be from WiMax equipment. And remember that NivagSwerdna is only looking for 1 mHz accuracy for the 10 MHz output.
Ed
Is the unit capable of doing position hold to a prior survey?
Bottom line - it's just a happy coincidence that these gizmos put out accurate frequency. We all use them that way, but they were not designed with that as a primary goal. The specs don't talk about it and the firmware is not optimized to do it.
The simple answer it to let your GPSDO run continuously.
Bottom line - it's just a happy coincidence that these gizmos put out accurate frequency. We all use them that way, but they were not designed with that as a primary goal. The specs don't talk about it and the firmware is not optimized to do it.
Are you talking about this Symmetricom unit specifically or all GPSDO's in general?
OK, I just got the sense you were implying(well you said it) that it is only a happy coincidence they generate an accurate frequency. Considering several HP models say "Frequency reference" on them I'd like to think they were specifically designed with accurate frequency in mind. One of mine says "reference clocks" and doesn't supply any 1 PPS etc so the only thing it is doing is providing an accurate frequency output.