[GPS_Standard] vcxo control averaging?

[Arie Dogterom]

Hallo Bert,
I do fully agree with you. Did not realize that some aspects of the PIC 
limit possibilities. But probably a good solution would be to have a 
sampling time larger than 16 seconds, say 16^x seconds. It would require 
some adaptation of the decision level, but the GPS receiver jitter would 
normally be averaged out. According to DF6JB ( an expert in the time 
science) GPS only has a better allen variation than the HP10811 for t> than 
about 5000 seconds, so a value of x in the order of  3 would be sufficient.

Another matter, but that in fact is another thread, is the monitor 
programme. I like it very much but the statistics part puzzles me. The value 
of the average deviation in general has an exponent e-10, sometimes e-11 en 
very seldom e-12 but then the next values have exponents e-25 or e-26. 
Nothing in between. Is there a slight error in the calculations?
I have not yet been able to use the programme to arrive at a graphic with 
the control voltage as a function of time. It only shows with me a southwest 
to notheast straight line.
73
Arie


----- Original Message ----- 
From: "Bert, VE2ZAZ" <ve2zaz at sympatico.ca>
To: "Arie Dogterom" <pa0ez at amsat.org>; <gps_standard at mailman.qth.net>
Sent: Friday 26 January 2007 1:24
Subject: Re: [GPS_Standard] vcxo control averaging?


> Hi again Arie,
>
> This is the reaction we have when we see it happening. We think that one
> sample or two out of 500 screwed up everything. Statistically, if after 
> 498
> samples, the accumulated frequency difference is 0, we can say that the
> system is quite close to nominal frequency, BUT we cannot claim it is 
> right
> on frequency. I'll bet you that the 497th sample did not give an 
> accumulated
> difference of 0...Right? So we are interested in the "general trend" over
> several samples. Ideally, it would have been better to perform a true
> average calculation (sum of all samples divided by the number of samples).
> Unfortunately, this requires floating point calculations and the PIC micro
> is not designed to easily do this. Note that the Montrol software performs 
> a
> true average in the statistics window. Have a look for fun...
>
> Now, in order to make sure that a couple of samples do not impact a long
> averaging cycle, I have added parameter N, the Frequency Change Negate
> threshold. Please read the definition carefully in the user manual. With
> this, you will deny a frequency change if the accumulated frequency
> difference is too close to nominal frequency. By the way, I recommend 
> using
> the voting mode on long averaging cycles.
>
> One more comment, the Holdover Limit (H) parameter is used on each and 
> every
> sample, not on the accumulated frequency difference.
>
> I hope I have answered your question(s).
>
> Best Regards,
>
> Bert, VE2ZAZ
>
>
> ----- Original Message -----
> From: "Arie Dogterom" <pa0ez at amsat.org>
> To: "Bert, VE2ZAZ" <ve2zaz at sympatico.ca>; <gps_standard at mailman.qth.net>
> Sent: Thursday, January 25, 2007 8:04 AM
> Subject: Re: [GPS_Standard] vcxo control averaging?
>
>
> Dr OM Bert,
> I think I did not formulate the point correctly.
> In fact I did all you suggested.
> But my point is different: After a certain number of samples, albeit 50 or
> 500 the difference counter can give any value between 0 and the holdover
> threshold, although the long term average is nearly or exactly zero. For
> example I see in a random case ( and this is not much different in the two
> modes) with a 500 sample period that after 498 samples the cumulative sum 
> is
> 0. After 499 it is -1 and at 500 it is -3. Well that results in a change 
> of
> the dac output. But the real average deviation is very near to zero.
> Or do I see this incorrectly??
> 73
> Arie
>
> ----- Original Message -----
> From: "Bert, VE2ZAZ" <ve2zaz at sympatico.ca>
> To: "Arie Dogterom" <pa0ez at amsat.org>; <gps_standard at mailman.qth.net>
> Sent: Thursday, January 25, 2007 1:20 PM
> Subject: Re: [GPS_Standard] vcxo control averaging?
>
>
>> Hello Arie,
>>
>> You seem to have left your parameters to the initialisation mode. Once 
>> the
>> FLL has reached equilibrium, you must set your parameters for a much
>> longer
>> averaging period. As a minimum, perform two things:
>>
>> Your Ageraging Sample size (S) is set too small. 10 samples are not
>> enough.
>> To really try to measure to a better accuracy, you must set your S
>> parameter
>> to something, say, more than 100. A one hour long period is a good start.
>> Set S to 225.
>>
>> Also, to decrease the impact of one "way off" sample, set your FLL
>> Sampling
>> mode (M) to Voting.
>>
>> Please read my website and the user manual. I explain all of this in more
>> detail in there.
>>
>> Regards,
>>
>> Bert, VE2ZAZ
>>
>> ----- Original Message -----
>> From: "Arie Dogterom" <pa0ez at amsat.org>
>> To: <gps_standard at mailman.qth.net>
>> Sent: Thursday, January 25, 2007 6:55 AM
>> Subject: [GPS_Standard] vcxo control averaging?
>>
>>
>> Hello,
>> I have built the gps controlled standard and am very impressed by the
>> intelligent control and monitoring software.
>> But one question remains:
>> Say one sets a period of 10*16 seconds to average out the "noise"on the
>> one
>> sec impulses. If the vcxo is about correct on frequency the result of the
>> counter jumps around 0 but I see deviations of +-4 at times. The PC
>> programme statistics picture indicates gradually an average which goes
>> down
>> and reaches values around 10^-11. But at the end of the averaging period
>> it
>> is possible that just at that moment the result of the 16 sec measurement
>> indicates not 0 but + or - 3. In that case, although the frequency is
>> almost
>> correct, the control voltage suddenly is changed and the measuring period
>> starts again. But now it is possible that the vcxo is slightly set away
>> from
>> the correct figure.
>> I would think a better solution would be that not the momentary deviation
>> but the long time average is used to set the control voltage.
>> Or do I see this incorrectly?.
>> Arie
>>
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>
>
>