[time-nuts] yet another GPSDO design, or so
EWKehren at aol.com
EWKehren at aol.com
Sun Jun 27 21:27:39 UTC 2010
Stanley
I am not an expert but it is not only the jitter it is the fact that since
the two sources are not linked the independent drift of the 100 MHz causes
a distribution of the count. Again even with the same resolution the count
error will be four X and the subsequent correction will be to scale, with
out changing the code. Bert
In a message dated 6/27/2010 4:00:59 P.M. Eastern Daylight Time,
stanley_reynolds at yahoo.com writes:
Without knowing your 100Mhz clock would it not need less jitter to average
out +- one count at 100Mhz vs one count at 24Mhz?
The GPS error has improved with better hardware/software in the receivers
as well as turning off SA. So I'm not so sure the GPS error will wash out
the counter's error. Even if it does, as a want to be time nut I need a
better counter which is why more is better, a faster count with a recipical
counter equals better unless the faster counter no longer averages out the +-
count.
Stanley
----- Original Message ----
From: "EWKehren at aol.com" <EWKehren at aol.com>
To: time-nuts at febo.com
Sent: Sun, June 27, 2010 1:17:51 PM
Subject: Re: [time-nuts] yet another GPSDO design, or so
Going from 24 to 100 MHz only gives you smaller steps (resolution) every
thing else stays the same.
If the he saw 2 to 3 nsec should be more like 8, going to 100 MHz will
improve it by a factor of 4. In a redesign of the total system I would
have two
sample sizes maybe stay with 30 or go to 50/60 and in the Rub. mode 200
maybe 300 sec. Let us not forget what we start out with the GPS signal
does
not allow us to take advantage of the full resolution.
Do not forget I did this to get smaller D/A steps and am not able to
rewrite the code, basically fooling the controller that the error should
call
for a 1.7 E-13 correction when in reality the error is 4.3 E-14 and the
resulting step is also 4.3 E-14 per D/A bit.
Bert
In a message dated 6/27/2010 1:43:40 P.M. Eastern Daylight Time,
stanley_reynolds at yahoo.com writes:
Yes I see your need for a reduced range with smaller steps. But I was
looking for smaller steps to improve the tracking accuracy without a loss
of
the benefit of averaging. From the QST article:
"Interestingly, it is desirable to have the frequency of U7 drift
slightly
rather than being synchronized with the VCXO. A
slight random drift averages out the count ambiguity that is inherent in
any pulse-counting device. My measurements
indicate that the simple phase-measuring circuit I use is consistently
accurate to 2 or 3 ns (for a 30-second measurement), while
without drift, the resolution would be limited to 42 ns. The $5 crystal
oscillator module drifts adequately"
So the drift should just cover the area of uncertainty that is one cycle,
too much drift would reduce accuracy, not enough and the average is of no
benefit.
One extreme no jitter, average doesn't work as it doesn't distribute the
samples over the range of uncertainty. The other case too much jitter and
the best to expect is an average weighted to one side or the other (+-1
count) with the extreme producing multiple counts of error. Something
about this
makes me nervous maybe the part about "slight random drift" what is
slight
at 24 Mhz is it also slight at 100 Mhz ? An average of 30 samples does
have a limit to what it will correct.
Stanley
----- Original Message ----
From: "EWKehren at aol.com" <EWKehren at aol.com>
To: time-nuts at febo.com
Sent: Sun, June 27, 2010 8:58:55 AM
Subject: Re: [time-nuts] yet another GPSDO design, or so
Stanley
the faster counter also has the jitter, no change, as long as it is not
tied to the input frequency. The 24 MHz is not unique, the 100 MHz is
same
technology just four times faster and thus gives me smaller steps on the
D/A
and since I use it on Rub. the full range of the 18 bit covers the full
tuning range of the Rub.
Bert
In a message dated 6/27/2010 9:05:12 A.M. Eastern Daylight Time,
stanley_reynolds at yahoo.com writes:
I have been thinking about a faster counter also but the Shera board was
depending on the jitter in the 24 Mhz clock to average out the +- count.
The
faster clock would reduce the need for this but without the right amount
of jitter we lose the benefit of this average.
Stanley
<snip>
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