Re: [time-nuts] Cs standard stability
Bob
You should to use the same measurement bandwidth when comparing ADEV measures.
The measurement bandwidth of a Timepod is usually less than 1kHz whereas the 5370A has a trigger system bandwidth of 100MHz or so. If the high phase noise of your GPDSO isnt attenuated by a filter it may cause significant measurement noise.
Bruce
On 05 March 2017 at 14:03 Bob Stewart bob@evoria.net wrote:
OK, but how does the phase noise of the GPSDO make the ADEV for the Cs worse than that for the GPSDO? I find it hard to believe that the phase noise from two separate units would cancel out for two GPSDOs but add for the Cs.
Bob
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From: Bruce Griffiths <bruce.griffiths@xtra.co.nz>
To: Bob Stewart <bob@evoria.net>; Discussion of precise time and frequency measurement <time-nuts@febo.com>
Sent: Saturday, March 4, 2017 6:03 PM
Subject: Re: [time-nuts] Cs standard stability
Bob
We are certainly not off track.
The phase noise of the GPSDO affects the trigger jitter of both the 5370A and the jitter of the GPSDO PPS output and consequently the ADEV measured by your setup.
NB ADEV depends on the measurement bandwidth which is relatively small when using a Timepod.
Bruce
On 05 March 2017 at 12:31 Bob Stewart bob@evoria.net wrote:
Bruce,
Are we getting a bit off track? The question I asked is in regards to the Cs, not the purity of the output of the GPSDO. The two ADEVs were done under the same conditions, yet the ADEV of the Cs is obviously not as good as that of the GPSDO. Also, the ADEV of the GPSDO pretty much tracks what is to be expected when run on a good 5370, isn't it?
Bob
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From: Bruce Griffiths <bruce.griffiths@xtra.co.nz>
To: Bob Stewart <bob@evoria.net>; Discussion of precise time and frequency measurement <time-nuts@febo.com>
Sent: Saturday, March 4, 2017 5:22 PM
Subject: Re: [time-nuts] Cs standard stability
Bob
The Phase noise of that GPDSO seems quite high and some form of filtering (bandpass or low pass) of its output may be necessary to reduce the resultant jitter when connected to the 5370A input.
There are commercial TDCs that claim that sort of performance (~10ps) but a comprehensive test of their actual performance appears as yet unavailable.
HP used to sell a TDC with 5ps resolution but detailed specs are unavailable.
Higher resolution and lower noise should be achievable but would require a lot of number crunching for each measurement.
Bruce
Bruce,
For the moment, let's forget about the Timepod plot and just concentrate on the screenshot of the two Timelab runs on the 5370: one on GPSDO Unit 2, and one on the Cs. Given that the test conditions were identical, down to the the clock source for the 5370, how can the phase noise on the GPSDO supplying the clock and 1PPS signals (Unit 1) benefit the ADEV of the GPSDO under test (Unit 2) while making the ADEV of the Cs worse? I would expect that two units with similar phase noise would cause a worse measurement, rather than a better one.
Bob
From: Bruce Griffiths <bruce.griffiths@xtra.co.nz>
To: Bob Stewart bob@evoria.net; Discussion of precise time and frequency measurement time-nuts@febo.com
Sent: Saturday, March 4, 2017 7:14 PM
Subject: Re: [time-nuts] Cs standard stability
BobYou should to use the same measurement bandwidth when comparing ADEV measures.The measurement bandwidth of a Timepod is usually less than 1kHz whereas the 5370A has a trigger system bandwidth of 100MHz or so. If the high phase noise of your GPDSO isnt attenuated by a filter it may cause significant measurement noise.Bruce
On 05 March 2017 at 14:03 Bob Stewart bob@evoria.net wrote:
OK, but how does the phase noise of the GPSDO make the ADEV for the Cs worse than that for the GPSDO? I find it hard to believe that the phase noise from two separate units would cancel out for two GPSDOs but add for the Cs.
Bob
From: Bruce Griffiths bruce.griffiths@xtra.co.nz
To: Bob Stewart bob@evoria.net; Discussion of precise time and frequency measurement time-nuts@febo.com
Sent: Saturday, March 4, 2017 6:03 PM
Subject: Re: [time-nuts] Cs standard stability
BobWe are certainly not off track.The phase noise of the GPSDO affects the trigger jitter of both the 5370A and the jitter of the GPSDO PPS output and consequently the ADEV measured by your setup.NB ADEV depends on the measurement bandwidth which is relatively small when using a Timepod. Bruce
On 05 March 2017 at 12:31 Bob Stewart bob@evoria.net wrote:
Bruce,
Are we getting a bit off track? The question I asked is in regards to the Cs, not the purity of the output of the GPSDO. The two ADEVs were done under the same conditions, yet the ADEV of the Cs is obviously not as good as that of the GPSDO. Also, the ADEV of the GPSDO pretty much tracks what is to be expected when run on a good 5370, isn't it?
Bob
From: Bruce Griffiths bruce.griffiths@xtra.co.nz
To: Bob Stewart bob@evoria.net; Discussion of precise time and frequency measurement time-nuts@febo.com
Sent: Saturday, March 4, 2017 5:22 PM
Subject: Re: [time-nuts] Cs standard stability
BobThe Phase noise of that GPDSO seems quite high and some form of filtering (bandpass or low pass) of its output may be necessary to reduce the resultant jitter when connected to the 5370A input.There are commercial TDCs that claim that sort of performance (~10ps) but a comprehensive test of their actual performance appears as yet unavailable.HP used to sell a TDC with 5ps resolution but detailed specs are unavailable.Higher resolution and lower noise should be achievable but would require a lot of number crunching for each measurement. Bruce