Hello Bruce, Hello Said

despite the fact that the author is a ham radio colleage of mine I agree
completly to what both of you said.

Who writes something like this has no good idea about the theory! Draw
the tau-sigma of the receiver alone and draw the tau-sigma of the VCXO
alone and look where the lines cross. Then you know EXACTLY which time
constant you have to choose for the control loop without the slightest
guess. And yes, don't forget to throw the Garmin out of the window!

bye
Ulrich Bangert, DF6JB

Well said, Randy.
It wasn't my intention when I asked for comments to trash anybody or
anything. In the somewhat 'rarefied atmosphere' of this forum I doubt
that any comments made were intended to be anything other than
objective.

I guess the ultimate question is whether or not the claimed
performance objective can be met with the hardware/firmware detailed
in the article, regardless of some of the apparent shortcomings
already noted.  If the answer is yes, then I believe the discussion
here should be interpreted as no more than suggested improvements that
could be implemented.

Regards
DaveB, NZ

----- Original Message -----
From: "Randy Warner" Randy@synergy-gps.com
To: "Discussion of precise time and frequency measurement"
time-nuts@febo.com
Sent: Sunday, October 22, 2006 9:35 AM
Subject: Re: [time-nuts] The VE2ZAZ GPSDO

--
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I think it'd be great to see Dr. Griffiths' list of issues in a letter to
QEX, sans a couple of the more, er, "editorial" comments.  This would give
the author a chance to respond, and alert other builders of opportunities
for improvement.

Some of the concerns may not be major issues (e.g., it seems safe to say
that the FLL action will correct for drift caused by the 7805's tempco,
unless you plan to dump a can of R134a on it), but others are worth bringing
to the magazine's attention.  If you'll forward your comments to Doug Smith
at kf6dx (at) arrl.org, he'll most likely print them in the Letters column.

-- john, KE5FX

John,

I agree. I think it was the, er "comments" that seemed a little much.
All EXCELLENT points, and as you say I am sure the author would
appreciate any helpful comments. I'm sure he would like to make his next
version even better.

That's what this forum is all about......

Randy

-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of John Miles
Sent: Saturday, October 21, 2006 2:59 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] The VE2ZAZ GPSDO

I think it'd be great to see Dr. Griffiths' list of issues in a letter
to QEX, sans a couple of the more, er, "editorial" comments.  This would
give the author a chance to respond, and alert other builders of
opportunities for improvement.

Some of the concerns may not be major issues (e.g., it seems safe to say
that the FLL action will correct for drift caused by the 7805's tempco,
unless you plan to dump a can of R134a on it), but others are worth
bringing to the magazine's attention.  If you'll forward your comments
to Doug Smith at kf6dx (at) arrl.org, he'll most likely print them in
the Letters column.

-- john, KE5FX

time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts

John Miles wrote:

A frequency lock loop is not the optimum technique for disciplining a
frequency standard as for periods of less than a few hours measurement
system white phase noise (PPS timing jitter) dominates.
The optimum phase lock technique requires a similar number of components.
Experiments are fine but why try and sell the result of an ill conceived
one?
This "solution" represents a step backwards from existing solutions of
similar cost but better performance.

Since a correctly designed system will have a time constant of around an
hour or so (with a good OCXO), the temperature coefficient of the power
supply is significant as temperature fluctuations with a period less
than this will affect the EFC voltage and hence cause the frequency to
fluctuate. The frequency correction loop will be unable to fully correct
these variations unless the time constant is drastically reduced in
which case the frequency fluctuations due to the GPS PPS pulse jitter
will increase substantially.

Bruce

In message D6D8D27B5219D649A005529978F7913717589A@SBSERVER.Synergy-GPS.sbs, "
Randy Warner" writes:

Seconded.

Beginners should be encouraged, not trashed.

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

"John Miles" jmiles@pop.net wrote:

Having seen some bad vibes unfold through the channels of the
letters column and other communications with editors, I think that
BY FAR the most useful thing to do would be to contribute an
improvement (preferably with schematic) rather than a lengthy
textual or mathematical criticism.

Tim.

shoppa@trailing-edge.com (Tim Shoppa) wrote:

To elaborate a little bit:

Writing for a magazine is not something anyone will ever make
money at. It's about as profitable as sitting around all day
posting to Usenet and mailing lists :-).

Editors typically chose articles based not on technical merit
compared to commercial products or state-of-the-art lab merits
but instead on FIRST on how it looks to sell the magazine and
then secondarily on reproducibility/constructibility. The order
is actually often reversed by the best of the ARRL editors, to
their credit.

I ghost-authored some articles for hobbyist electronics magazines
in the 80's. It was a memorable experience. It had little to do with
academic knowledge or product superiority and everything with
shine and pizazz. It seriously soured me on salesmanship - I can
literally get disgusted when I see the techniques used to push
a crappy product in a sales presentation etc. In some ways
I wish I could recover the naivete of my youth and be the
"happy wanderer" that buy all the crap being sold out there!

Tim.

Tim Shoppa wrote:

Since the disciplining technique is far from optimum, this may only
serve to further sidetrack the less knowledgeable from the best
engineering solution which can achieve a far better performance for a
similar cost. A comparison of the performance of this method with the
optimum method using similar receivers and crystal oscillators would
have been helpful in assisting newcomers in selecting a more appropriate
method.

Perhaps someone should set up a webpage detailing how one should go
about disciplining a high stability crystal oscillator.

Allan deviation vs tau for various GPS receivers
Allan deviation vs tau for various crystal oscillators.
Various phase measurement techniques and associated tradeoffs
Effect of various phase measurement techniques on the system Allan
deviation vs tau.
Filtering phase measurements and discarding outliers.
Choosing the appropriate loop time constant.

Miscellaneous:
Use of synchronisers to reduce probability of generating runt pulses and
metastability problems.
Don't use same IC to buffer different frequencies
Resynchronising divider chain outputs to reference frequency clock to
reduce jitter.

DAC requirements and tradeoffs
Monotonicity
Resolution
Noise
Short term stability
Cost etc.

Monotonic high resolution DAC techniques
Multibit Audio DACs
PWM
etc.

Bruce

thanks for volentering sounds like you know what it takes to make a good one
and set up the webpage..............
tom w0kgw

----- Original Message -----
From: "Dr Bruce Griffiths" bruce.griffiths@xtra.co.nz
To: "Discussion of precise time and frequency measurement"
time-nuts@febo.com
Sent: Saturday, October 21, 2006 7:20 PM
Subject: Re: [time-nuts] The VE2ZAZ GPSDO

OK, here is my problem. I do not think it is a unique problem, based on
recent mail :-)

I have read about the Allan Deviation and I understand the principle,
even though the nuances between the 3 basic Allan deviations escape me
at the moment, but I am sure it will come once I re-read the Help file
that comes with the the AlaVar software , and I have downloaded and
installed AlaVar, a free software that can compute the various flavors
of the Allan Deviation.

I have a working HP 5370A, which I believe is required (even though
maybe other counters, such as the HP 5334 or HP 5316, both of which have
a TI function that  might be used for that purpose) to gather the data
that will be fed into AlaVar.

I have a working GPIB interface (actually several types) and a computer
attached to it, and I can write a Visual Basic programs to talk to the
counter and download data (I have already written Visual Basic/GPIB
programs to control signal generators, power meter, spectrum analyzers
and other instruments).

I have several HP 10811 oscillators (with EFC input), and a couple of
Ovenair (also with EFC input for at least one of them), some are inside
working HP instruments, and a couple are spares.

What I do not have is a procedure. What data do I need to feed the
software and how do I actually collect the data?

I assume the 5370 should be set to measure TI between 2 oscillators.
Should I use the built-in averaging function? What sample size and
resolution should I use? Should I try to use the 5370 in raw mode (much
faster, 6000 samples/sec) or in formatted mode (10-20 samples/sec)? Does
it make a difference?
What if the oscillators are not phase locked and show frequency drift?

The 5370 has a 10811 oscillator for its time base, so it is good but no
better than any of the oscillators I want to check. Do I use it as a
reference, or do I compare two stand-alone oscillators?

How do I know which oscillator I am measuring when the two oscillators I
am comparing are the same models? Should I compare 3 or more?

Regarding the GPS receiver, I thought most modern GPS receivers
automatically switch from nav mode to survey mode when they stop moving.
I would probably be mistaken to believe this is comparable to a true
time-keeping GPS receiver, but how bad is it? Tom Clark wrote previously
on Time-Nuts that his experience with the Jupiter was good, with +/- 13
nS jitter, other than the fact the receiver will not return the timing
error on the next pulse, which prevents from writing smart software that
can compensate for it.

I have a Jupiter GPS receiver which I intend to use to discipline one of
the 10811 oscillators. The Jupiter receiver has a 10kHz output, which
would simplify the phase lock loop a little (even though it would not
allow to speed up the loop). Is there any disadvantage in using it
instead of the 1PPS output? It seems the 10 kHz would be easier to
filter,  and maybe allow to speed up the loop following power up
(assuming it is set to the normal, longer time constant once phase lock
is achieved), but what do I know?

I also have a modified distribution amplifier to distribute the good 10
MHz to my lab without affecting the master oscillator.

So I am anxious to use the AlaVar software and the toys I have listed
above to do the following:

1. select the best OCXO to be the basis of my GPS disciplined frequency
   standard
2. find the best placement for the GPS antenna (the one that gives the
   most stable GPS signal)
3. fine tune the phase lock parameters and estimate the quality of the
   end product

Any further information and guidance (with practical tips) would be
greatly appreciated.

Didier KO4BB

Dr Bruce Griffiths wrote:

Hi Didier --

I have written a bunch of simple programs for GPIB data gathering, using
perl and the linux-gpib libraries under Linux.  However, the code is
pretty straightforward and it should be easy to extract the command
strings to send to the counter.  From that, you should be able to
re-write for any platform/language that can talk to a GPIB card.

Check http://www.febo.com/time-freq/tools/gpib and look for obvious
names like "hp5370-tint.pl".  I will warn you that these programs are
not examples of good programming, but most of them work.  Sort of :-)

John

Didier Juges said the following on 10/22/2006 01:29 AM:

Hi John,

I have written a number of Perl programs under Linux, so your offer of
the scripts was great. However, permissions on your server do not allow
me to download the scripts. I get a directory though...

Regarding programming style, we can share then :-)

Thanks

Didier KO4BB

John Ackermann N8UR wrote:

Ah, yes, the server tries to protect me by not allowing executables to
run on the machine.  Let me see if I can fix that for that directory.

John

Didier Juges said the following on 10/22/2006 01:19 PM:

I couldn't figure out how to enable downloading of .pl files, so I just
made each file into a .zip so you should be able to grab that.  Sorry
for the inconvenience.

John

John Ackermann N8UR said the following on 10/22/2006 02:04 PM:

Hi John,

I got zip (does not work either :-)

I remember going through something like that a while back on one of my
servers, I have no recollection how I fixed it :-(

The files look pretty small, can you email them to me?

Interestingly, on my ISP's server (Linux too), I have no problem getting
.pl files in and out of my cgi-bin directory, but I cannot get the same
file from the main httpdocs directory as a text file, even though the x
bit is turned off, even zipped just like you did. mmm! Try
http://www.ko4bb.com/Userdb-log.pl.zip

Didier

John Ackermann N8UR wrote:

Didier Juges wrote:

Reading between the lines on the Jupiter GPS receiver datasheet it would
appear that the 10KHz output is phase modulated at 1Hz to realign it to
successive PPS output pulses. As the PPS jitters about so does the 10KHz
signal.
Most GPS receivers with higher frequency outputs than 1Hz, phase
modulate the high frequency output in this way and the datasheets
explicitly indicate this.

Thus there would appear to be little advantage in phase locking to the
10KHz signal with a short loop time constant.

To be absolutely sure you will need to use an oscilloscope to observe
the synchronous jitter in the 10KHz waveform.

Bruce