[GPS_Standard] vcxo control averaging?

Sat Feb 17 19:04:03 EST 2007

Hi Arie,

I agree that the longer you average out, the better the filtering will be.
The S parameter does exactly that: it averages out 16-second frequency
samples for as many as 65535 samples. This should remove most GPS
disturbances. Of course I wish we could sample forever....but there is a
tradeoff between accuracy and integration time.

As for the statistics window, you are the first one to report a potential
problem with the maths. Which field are you reporting as problematic,
Standard Deviation or Average Frequency Offset? When I get a chance, I'll
have a look at the formulas.

Regarding the graph, there is one bug I am aware of, and it is that you must
change the graph type back and forth to update it. So load a Log file, and
click on a graph type other than the default one, then click back to the
graph type you want. The graph should now display all the points. Note that
the tool only displays the points for which there was a DAC update. For
plotting graphs, you may want to use MS Excel or OpenOffice Calc. You'll get
more flexibility...

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: Saturday, February 17, 2007 3:19 PM
Subject: Re: [GPS_Standard] vcxo control averaging?

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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>
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