[time-nuts] Yet another GPSDO project.

Wed Mar 20 19:57:02 UTC 2019

Hi Bernd,

awesome that you still remember this :-)
sure, I believe you since my primary requirement was phase noise. This is because I'd like to use the OCXO as reference for my spectrum analyzer and also for my HP 8663 signal generator to do phase noise measurements - the signal generator can be improved quite a bit if a proper external reference is used, compared to its internal OCXO! (a HP 10811A)

What you mention is very interesting: you said " As it is from an older production lot, the aging may meanwhile be much lower than specified." does this mean that the aging of an OCXO does improve, even if the OCXO is unpowered in its box?

Do you think the stability of the OCXO improves a bit if I use some thermal isolation, as I did in my GPSDO, or is it (as Achim points out) perhaps contraproductive.

I'll contact you off list for the measurement - indeed this woud be awesome, and if you can offer me an even more stable OCXO this would be fantastic!

Best
Tobias
HB9FSX

________________________________________
From: time-nuts [time-nuts-bounces at lists.febo.com] on behalf of Bernd Neubig [BNeubig at t-online.de]
Sent: Wednesday, March 20, 2019 19:12
To: 'Discussion of precise time and frequency measurement'
Cc: bneubig at axtal.com
Subject: Re: [time-nuts] Yet another GPSDO project.

Hi Tobias,

Indeed the OCXO which you got from us (AXTAL) was an item from stock, which
was not particularly designed for low ADEV. As it is from an older
production lot, the aging may meanwhile be much lower than specified.

If you want, you may ship your OCXO back to us. We will be glad to measure
ADEV. Maybe we can also offer you one with lower ADEV even.
Best regards
Bernd Neubig DK1AG

-----Ursprüngliche Nachricht-----
Von: time-nuts [mailto:time-nuts-bounces at lists.febo.com] Im Auftrag von
Tobias Pluess
Gesendet: Mittwoch, 20. März 2019 08:54
An: Discussion of precise time and frequency measurement
<time-nuts at lists.febo.com>
Betreff: Re: [time-nuts] Yet another GPSDO project.

Hi,

no, I don't want the OCXO to closely follow the 1PPS - this would make no
sense. Instead I thought the "right way" of doing it is perhaps:

a) measure the phase difference between the GPS PPS and the PPS derived from
the OCXO.
b) put that phase difference into a loop filter.
c) the loop filter's output goes to a DAC.

Of course I don't want to have an analog phase detector, nor an analog loop
filter due to the reasons you already mentioned (small phase detector output
voltage, insance capacitor values etc.). Instead both the loop filter and
the phase detector should be digital, and this is my current step where I
struggled - how to do that.
But thanks to the loop filter I would expect the phase jitter of the GPS PPS
to be averaged out. Of course the loop filter's averaging time should be
chosen such that it lies approximately at the point there the ADEV of the
OCXO becomes worse than the ADEV of the GPS (however, I have no ADEV info
available about my oscillator, *sigh*).

But for example, if my OCXO's ADEV begins to increase at, say 1000s, the
perhaps easiest loop filter would be a FIR filter which averages the last
1000 measurements of the phase difference between GPS PPS and OCXO PPS,
right? (afterwards one could improve this to IIR or whatever to save filter
taps...).

The OCXO I have is perhaps not the most stable one, but also not a low cost
device. It has an aging of perhaps 1ppb per day and 100ppb per year, but it
is not a double oven device, unfortunately... perhaps I should look for an
Oscilloquartz 8663 or so?

Tobias
HB9FSX

________________________________________
From: time-nuts [time-nuts-bounces at lists.febo.com] on behalf of Bob kb8tq
[kb8tq at n1k.org]
Sent: Tuesday, March 19, 2019 23:35
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Yet another GPSDO project.

Hi

Ok, *if* you phase lock the OCXO to closely follow the GPS PPPS, the OCXO
will be every bit as noisy as the GPS. Actually it will be worse since you
now have 1Hz noise running around that needs to be taken care of ( = the
output of the phase detector is a series of pulses at a 1 Hz rate).

Net result is that you still need a a long time constant loop to do any
good. There also will be some interesting “analog” issues in trying to deal
with the ouput of a normal phase detector at this slow a rate (leakage,
giant capacitors, weird noise issues …).  Bottom line - it’s going to be an
exciting design in a number of ways.

You might say - up the output rate of the GPS. That’s fine, but the
correction information you need only comes out once a second. The rest of
the time you are running with (noisy) uncorrected pulses.  You took one step
forward and two back ….

========

The normal way the lockup issue is handled is via software and by “cheating”
a bit while the device locks up. Just how you do this depends a *lot* on how
your system is set up. With a pure PLL you don’t have quite as many choices
as with a phase sampling ( TICC counter-like) setup. With phase samples, you
can look at frequency and lock to that early on. Then switch to a true phase
lock later on. Most high performance GPSDO’s go through multiple steps
before they are in their final lock condition.

========

Stepping back a bit - Whats good?

A $100 telecom Rb off of eBay will hold 0.1 ppb for a very long time with
very little help. Yes you might buy two or three to get one good one. You
also can do a bit of digging and improve the odds. Second to second wise
they should hold 0.01 ppb.

An fairly good ( = double oven) OCXO might hold 1 ppb for a month after it
warms up for a while. If you buy it on eBay. Second to second wise it should
be around 0.002 ppb

A low cost OCXO might struggle at the 50 ppb level over a month. Second to
second wise ….
who knows … maybe 0.1 ppb (yikes …).

A GSPDO (while locked) will have zero net frequency error (it’s phase
locked).  It’s short term frequency can / will / yikes wander. The net will
be zero. Second to second wise it should be about the same as it’s “core”
device (Rb or OCXO).

Based on all this, I’d suggest that a GPSDO needs to be in the < 0.1 ppb
range in terms of “how good" to beat out the unlocked Rb.

0.1 ppb is a nanosecond over 10 seconds. It is a phase error of 360 /
1,000,000,000 degrees at 1 Hz over that time period. That’s a pretty small
phase change and not a very big time change.
At 100 seconds you are up to 10 ns (so a bit easier to spot). You are also
at 10X the degrees, still pretty tough.

To turn the phase into other units:  if your phase detector has a 1V /
radian slope (not uncommon on an analog phase detector), you are after 2*PI
/ 1,000,000,000 volts on the output. That’s just a bit over
6 nV. Ideally you would want your detector drift to be < 20% of this (or
about 1 nV) to get the job done …. yikes ….

Lots of fun !!

Bob

> On Mar 19, 2019, at 4:43 PM, Tobias Pluess <tobias.pluess at xwmail.ch>
wrote:
>
> Thanks for the replies so far.
> Unfortunately I used digest mode for this mailing list, so perhaps my
answers are not well-formed - please excuse; I have changed my settings to
non-digest (even though this is perhaps a bit less convenient).
>
> @Jim Harman
> Indeed I know Lars' GPSDO. Can you tell what performance you achieved with
one of those?
>
> @ Bob
> wow. That's quite a pile of information you put in your post!
> Indeed I have some fun with the GPSDO topic, but not yet particularly with
my unit, its performance is, in my oppinion, currently way too poor. So I
need a better one ;-) Cs or Maser would be awesome, but I don't even have a
Rb. I once thought of buying one from the 'bay, but they are often of
doubtful pedigree and nobody knows how much life is perhaps left in the Rb
valve, if I am unlucky it will only last for a few months, so I think I will
not risk that.
>
> Of course I know the PPS from the GPS module is jittery. This is why
> we want to synchronize our OCXOs, isn't it ;-) However the STAR4 GPSDO I
have here has also an 1PPS output, which is derived from the OCXO. I used
this PPS on my HP 5335A. I am looking for a HP 53131A or 53132A since months
for a decent price - this would allow for higher resolution.
> I have made an ADEV plot for my GPSDO, but I didn't save it because it was
disappointing.
>
> By the way, this is the unit I built:
>
> https://hb9fsx.ch/wordpress/index.php/2018/11/14/my-own-gpsdo-part-i/
>
> I have also a homemade antenna for it, since I read somewhere that the
quadrifilar helix antennas are better than patch antennas, I made one by
myself. A choke ring antenna would perhaps be even more awesome, but this is
for a later project!
>
>
> @ Achim.
> Yes indeed, I encounter the problem that my measurement resolution is only
100ns. So it takes ages until the frequency locks and it takes longer and
longer, the closer the frequency is... and I exactly assumed what you have
pointed out: perhaps my steering becomes so sparse in time, that the OCXO's
aging rate is faster.
> 2e-11 at 1s is not bad, but hoped for something with e-12 or even better -
I think this should be possible, not?
>
>
> So I think I would really like to go the PLL route, I hope to achieve
faster lock time and a bonus would be that the PPS pulses derived from my
OCXO would be aligned to those of the GPS module.
> But how to interface the PFD to a microcontroller and implement the
> loop filter is still a mystery for me :-)
>
> Thanks for all your hints
> Tobias
> HB9FSX
>
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