[time-nuts] GPSDO using 100Hz

Ulrich Bangert df6jb at ulrich-bangert.de
Mon Nov 24 10:15:07 UTC 2008


Warren,

James Miller, G3RUH, also uses the 100 Hz signal. So it's neither
specifically new nor the philosopher's stone that you may perhaps
believe to have found. The PPS and the 100 Hz signal come from the SAME
oscillator. The jitter in the PPS comes from the fact that the receiver
logic decides WHICH slope of the oscillator signal comes next close to
the TRUE point of time where the PPS should be generated. The key error
in your idea is the assumption that every 100 positive slopes of your
100 Hz is identical with the start of a new second, which is wrong when
you look at it at a second to second base. What is true is that the MEAN
frequency of the 100 Hz is locked to the GPS and that is why your fast
pll works as well as James Miller's.

Best regards
Ulrich Bangert   

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von WarrenS Email
> Gesendet: Montag, 24. November 2008 10:48
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] GPSDO using 100Hz
> 
> 
> Bruce Griffiths answered:
> 
> Its difficult to make much useful comment as you provide few 
> measured results.
> 
> With an M12+T or equivalent the ADEV of the PPS output 
> (without sawtooth
> correction) goes below 1E-10 at  Tau > 200 sec or so.
> Thus with an optimized GPSDO it wont take an hour or so to 
> achieve 1E-10 stability. However a single shot phase error 
> measurement system resolution of around 1ns or so is usually 
> required. Take a look at the GPSDO ADEV plots at:
> 
> http://www.leapsecond.com/pages/gpsdo/
> 
> Where the ADEV for various GPSDO remains below 1E-11 over the 
> Tau range of [0.1s, 100,000s]
> 
> The achievable performance also depends a great deal on the 
> quality of the OCXO used in the GPSDO.
> 
> If the "digital" phase error measurement techniques you have 
> been comparing your system with have inadequate resolution it 
> will tale longer for the measured ADEV to fall below 1E-10. 
> If the OCXO used has a relatively high ADEV at low values of 
> tau then it may well required averaging over very long time 
> intervals to achieve an ADEV below 1E-10.
> 
> More detail is required before an analysis of the performance 
> of your system is possible.
> 
> Bruce
> 
> *********************
> 
> Bruce, Thanks for the feedback, Good information to know, 
> but you seemed to missed my point and question.
> 
> Yes I am comparing to SIMPLE, and home built type, non optimized, 
> trackers like  Brooks Shera's "10 Mhz GPS Frequency Standard". 
> I understand, and even wrote, NIST reports about  a 10ns uncertainty 
> with a 10 minute average, which would give a 1e-8 / 600, or  
> 1.5 parts in 1E-11 
> possible in 600 seconds, 1e-10 in 100 seconds.  Its safe to 
> assume NIST is 
> not using Shera's unit, which I believe adds an additional  
> 24 ns or is 
> it 41.7 ns uncertainty to each 1 second reading. 
> Besides asking if anyone is using the 100Hz output, I would 
> like to ask 
> why don't the generally available GPSDO use the 100Hz, which can give 
> about 1 ns of certainly with a simple PLL and analog RC 
> filter, instead of 
> the using the 1 sec which has more like 100 ns of uncorrected 
> uncertainty 
> in it and must use a processor?
> 
> Also I should comment that on  LeapSecond.com 
> http://www.leapsecond.com/pages/gpsdo/
> you stated "where the ADEV for various GPSDO remains below 1E-11 
> over the Tau range of [0.1s, 100,000s]. This Tau has NOTHING 
> to do with 
> the tracking time constant that the GPSDO is set to, which is usually 
> recommended to be set for a TC of several hours for optimal 
> results. BTW most of what they have plotted is the results of 
> NOT setting the GPSDO 
> tracking TC slow enough. This is why the Allan Deviation 
> increases in the tracking 
> mode at mid averaging times. The 1 second GPS tracking signal 
> is adding noise, 
> which pretty much makes my point that 1 PPS signal is not so 
> good to use 
> if you want good fast results.
> 
> Warren
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