[time-nuts] yet another GPSDO design, or so

Stanley Reynolds stanley_reynolds at yahoo.com
Sun Jun 27 19:17:02 EDT 2010


My last statement is not quite correct.

"If the drift is too big we will over shoot to the next higher integer number and this will bias the average up too high."

In the case where the 100Mhz clock drifts both up and down the number could average to the correct fraction.  But now the rate of drift as well as it's magnitude would need to be correct. 

Accuracy is not important here just the precision, as the number is not important just the rate of change of the number indicates error.

I need to think more about this ....

Stanley



 


----- Original Message ----
From: Stanley Reynolds <stanley_reynolds at yahoo.com>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
Sent: Sun, June 27, 2010 5:23:13 PM
Subject: Re: [time-nuts] yet another GPSDO design, or so

Jitter may not be the correct word. I also don't know how accurate or repeatable the averaging effect is as described in the article.  But I do believe the amount of drift is important for this to work as stated before. Independent is required, if locked then this is the extreme case of no drift and average value will be the same as all the samples are the same integer number. To see the fraction between the integer we need our data set to have some number of samples above and below and hopefully in a ratio to indicate the real measurement. If the drift is too small then we will tend to calculate an average too small, as more samples will fall to the low side the high side is never reached. If the drift is too big we will over shoot to the next higher integer number and this will bias the average up too high.

Stanley




----- Original Message ----
From: "EWKehren at aol.com" <EWKehren at aol.com>
To: time-nuts at febo.com
Sent: Sun, June 27, 2010 4:27:39 PM
Subject: Re: [time-nuts] yet another GPSDO design, or so

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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