GPSDO frequency error and Allan Deviation - Page 1
Forum section: electronics / beginners

author: kg4arn
author_rank: Unknown
timestamp: July 17, 2019, 02:25:25 pm
content: Forgive me if this is common knowledge or discussed elsewhere. I can’t seem to find the answer to this question and everything I have web searched seems to stop just short of explicitly discussing it.I have a surplus communications GPSDO : LUCENT RFTG-u. I am using it as a frequency standard for my home lab. The OCXO inside is an MTI 260 series 5MHz.The short term stability of the OCXO is specified 1E-12 for 1 second and 3.5E-12 for 10 seconds.The software polls the GPSDO every 10 seconds.My question is: what is the Allan Deviation graph telling me?1. Is the reference frequency output error for 2 measurements taken 10seconds apart is 3.09E-9 (the tau=10 second Allan deviation value)? And that I have to average 1024 measurements taken 10 seconds apart to achieve an error of 2.38E-12?2. Or I am misunderstanding completely and if the GPSO is allowed to run for 40960 seconds the noise will have averaged out and the error is now limited by the OCXO short term error of 3.5E-12?

author: awallin
author_rank: Frequent Contributor
timestamp: July 17, 2019, 02:51:46 pm
content: from your post I did not understand what your software is measuring. clock measurements are always comparisons - do you have two identical GPSDOs?Quote from: kg4arn on July 17, 2019, 02:25:25 pmroughly: average the frequency data into 'bins'/windows of length tau, and the ADEV is the 'rms' noise of the resulting time-series.

author: kg4arn
author_rank: Unknown
timestamp: July 17, 2019, 10:47:31 pm
content: Thanks awallinThe software documentation states that the Allan Deviation is calculated by comparing:The software documentation states that the Allan Deviation is calculated by comparing the:1pps signal locally generated from the OCXO by means of a frequency dividerWithThe receiver’s 1pps (derived from the satellite constellation).However, when I read (again) further it appears to answer my question.“The comparison of these two PPS signals is the input information to the unit’s frequency standardcontrol loop. Because this input information is dominated by the GPS 1pps jitter (resulting largelyfrom GPS receiver and propagation limitations),do not be fooled into thinking that the Time StabilityMeasure presented here is that of your frequency standard. It is only an internal value whichindicates overall system performance (including the GPS reception). For reliable stabilitymeasurements, you should make independent measurements against a local high stability reference.The task of the frequency standard’s control loop is to ‘marry’ the high short-term stability of theinternal OCXO with the high long-term time stability of the GPS system. For this reason the 10MHzoutput of your unit will have significantly better stability (for short observation times) than thevalues indicated in the Time Stability Measures chart.”[size=0pt]

author: awallin
author_rank: Frequent Contributor
timestamp: July 18, 2019, 11:15:54 am
content: right. you really want two or three clocks to do comparisons..SRS has a graph of how they think the ADEV for GPSDOs with different LOs should behave:https://www.thinksrs.com/images/instr/fs740/FS740_lockedLG.gifso with an OCXO or Rb LO the GPS-steering should take over at time-constants of 1000s (16 minutes) or more.