[GPS_Standard] vcxo control averaging?

[Bert, VE2ZAZ]

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
>
> ______________________________________________________________
> GPS_Standard mailing list
> Home: http://mailman.qth.net/mailman/listinfo/gps_standard
> Help: http://mailman.qth.net/mmmain.htm
> Post: mailto:GPS_Standard at mailman.qth.net
>
>
>