[time-nuts] yet another GPSDO design, or so
stanley_reynolds at yahoo.com
Sun Jun 27 17:43:22 UTC 2010
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.
----- 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
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.
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.
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