! PPS Source
I have been using 1 PPS from a Motorola M-12 timing module to steer a SRS
PRS-10. I recently heard that a U-Blox ZED F9P module receives both L1 and L2
and can provide much improved positional accuracy.
Would better positions translate into a smoother 1 PPS? Does anyone have
experience with this U-Blox module? Can this be set up with a fixed position as
a timing module?
Is there a better source of 1 PPS at a reasonable cost? The U-Blox is about $200.
Thanks,
Joe, W7LUX
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Hi
Here’s the “whole story”, sorry if it repeats things you already know …
All GPS modules that I have ever seen use a free running clock. The internal
oscillator is not locked to GPS. When they want to generate a 1 pps output
they drop / add cycles from the the internal oscillator to get it “as close as
possible”. That means that you will always have an error in the PPS.
Since they know this is going on, many devices report this error on a
second by second basis. Since the error looks like a sawtooth if you graph
it, this is often called “sawtooth correction”. This correction also takes care
of “hanging bridges” where the sawtooth stays to one side or the other of
“correct” for a long time.
Normally when feeding a PRS-10, the sawtooth correction is not used. That
results in a degraded pps accuracy. The best GPS module to use in this
case is one with a very small sawtooth “window” ( = a fast internal clock).
Right now, the Furuno parts are winning this particular race.
If you do use the sawtooth correction (possibly by feeding a variable
delay line chip), then indeed the F9P and F9T will do a much better job.
Some numbers:
Sawtooth on some older modules can be out around +/- 20 ns On newer
parts it might be down around +/-10 ns. On the F9 parts it is +/-4 ns. The
Furuno parts run half that.
Corrected, on a modern part, and looking at second to second variation,
you can get below 1 ns with various modules. On the F9’s you can get well
below 1 ns.
=====
All of that is looking at short term variation. Your Rb does not move much
short term (unless the temperature changes …). Its stability and aging likely
are quite good.
GPS (as received / uncorrected ) swings around a bit during a normal day.
Swings of 10 to 20 ns are pretty normal. > 50 ns is possible under odd
conditions. That’s more than your Rb is likely to move around over a 4 to 12
hour period.
If you “follow” GPS with your Rb through a conventional loop, you likely
degrade the stability of the Rb. It takes a fairly fancy loop to do a good job
on an Rb.
Bob
On Aug 12, 2020, at 11:44 PM, Joe Hobart nova@npgcable.com wrote:
I have been using 1 PPS from a Motorola M-12 timing module to steer a SRS
PRS-10. I recently heard that a U-Blox ZED F9P module receives both L1 and L2
and can provide much improved positional accuracy.
Would better positions translate into a smoother 1 PPS? Does anyone have
experience with this U-Blox module? Can this be set up with a fixed position as
a timing module?
Is there a better source of 1 PPS at a reasonable cost? The U-Blox is about $200.
Thanks,
Joe, W7LUX
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IIRC the Thunderbolt DOES lock its internal clock to the GPS
David
-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces@lists.febo.com] On Behalf Of Bob kb8tq
Sent: 13 August 2020 14:39
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] ! PPS Source
Hi
Here’s the “whole story”, sorry if it repeats things you already know …
All GPS modules that I have ever seen use a free running clock. The internal
oscillator is not locked to GPS. When they want to generate a 1 pps output
they drop / add cycles from the the internal oscillator to get it “as close as
possible”. That means that you will always have an error in the PPS.
Since they know this is going on, many devices report this error on a
second by second basis. Since the error looks like a sawtooth if you graph
it, this is often called “sawtooth correction”. This correction also takes care
of “hanging bridges” where the sawtooth stays to one side or the other of
“correct” for a long time.
Normally when feeding a PRS-10, the sawtooth correction is not used. That
results in a degraded pps accuracy. The best GPS module to use in this
case is one with a very small sawtooth “window” ( = a fast internal clock).
Right now, the Furuno parts are winning this particular race.
If you do use the sawtooth correction (possibly by feeding a variable
delay line chip), then indeed the F9P and F9T will do a much better job.
Some numbers:
Sawtooth on some older modules can be out around +/- 20 ns On newer
parts it might be down around +/-10 ns. On the F9 parts it is +/-4 ns. The
Furuno parts run half that.
Corrected, on a modern part, and looking at second to second variation,
you can get below 1 ns with various modules. On the F9’s you can get well
below 1 ns.
=====
All of that is looking at short term variation. Your Rb does not move much
short term (unless the temperature changes …). Its stability and aging likely
are quite good.
GPS (as received / uncorrected ) swings around a bit during a normal day.
Swings of 10 to 20 ns are pretty normal. > 50 ns is possible under odd
conditions. That’s more than your Rb is likely to move around over a 4 to 12
hour period.
If you “follow” GPS with your Rb through a conventional loop, you likely
degrade the stability of the Rb. It takes a fairly fancy loop to do a good job
on an Rb.
Bob
On Aug 12, 2020, at 11:44 PM, Joe Hobart nova@npgcable.com wrote:
I have been using 1 PPS from a Motorola M-12 timing module to steer a SRS
PRS-10. I recently heard that a U-Blox ZED F9P module receives both L1 and L2
and can provide much improved positional accuracy.
Would better positions translate into a smoother 1 PPS? Does anyone have
experience with this U-Blox module? Can this be set up with a fixed position as
a timing module?
Is there a better source of 1 PPS at a reasonable cost? The U-Blox is about $200.
Thanks,
Joe, W7LUX
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Hi
The TBolt is a GPSDO rather than a GPS module. It’s a very different beast than a
ZED-F9P. ZED-F9T, or the M-12 the OP is using.
Bob
On Aug 13, 2020, at 12:09 PM, David C. Partridge david.partridge@perdrix.co.uk wrote:
IIRC the Thunderbolt DOES lock its internal clock to the GPS
David
-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces@lists.febo.com] On Behalf Of Bob kb8tq
Sent: 13 August 2020 14:39
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] ! PPS Source
Hi
Here’s the “whole story”, sorry if it repeats things you already know …
All GPS modules that I have ever seen use a free running clock. The internal
oscillator is not locked to GPS. When they want to generate a 1 pps output
they drop / add cycles from the the internal oscillator to get it “as close as
possible”. That means that you will always have an error in the PPS.
Since they know this is going on, many devices report this error on a
second by second basis. Since the error looks like a sawtooth if you graph
it, this is often called “sawtooth correction”. This correction also takes care
of “hanging bridges” where the sawtooth stays to one side or the other of
“correct” for a long time.
Normally when feeding a PRS-10, the sawtooth correction is not used. That
results in a degraded pps accuracy. The best GPS module to use in this
case is one with a very small sawtooth “window” ( = a fast internal clock).
Right now, the Furuno parts are winning this particular race.
If you do use the sawtooth correction (possibly by feeding a variable
delay line chip), then indeed the F9P and F9T will do a much better job.
Some numbers:
Sawtooth on some older modules can be out around +/- 20 ns On newer
parts it might be down around +/-10 ns. On the F9 parts it is +/-4 ns. The
Furuno parts run half that.
Corrected, on a modern part, and looking at second to second variation,
you can get below 1 ns with various modules. On the F9’s you can get well
below 1 ns.
=====
All of that is looking at short term variation. Your Rb does not move much
short term (unless the temperature changes …). Its stability and aging likely
are quite good.
GPS (as received / uncorrected ) swings around a bit during a normal day.
Swings of 10 to 20 ns are pretty normal. > 50 ns is possible under odd
conditions. That’s more than your Rb is likely to move around over a 4 to 12
hour period.
If you “follow” GPS with your Rb through a conventional loop, you likely
degrade the stability of the Rb. It takes a fairly fancy loop to do a good job
on an Rb.
Bob
On Aug 12, 2020, at 11:44 PM, Joe Hobart nova@npgcable.com wrote:
I have been using 1 PPS from a Motorola M-12 timing module to steer a SRS
PRS-10. I recently heard that a U-Blox ZED F9P module receives both L1 and L2
and can provide much improved positional accuracy.
Would better positions translate into a smoother 1 PPS? Does anyone have
experience with this U-Blox module? Can this be set up with a fixed position as
a timing module?
Is there a better source of 1 PPS at a reasonable cost? The U-Blox is about $200.
Thanks,
Joe, W7LUX
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(At least older) Novatel receivers can be set to lock the internal oscillator.
/Björn
Sent from my iPhone
On 13 Aug 2020, at 18:26, David C. Partridge david.partridge@perdrix.co.uk wrote:
IIRC the Thunderbolt DOES lock its internal clock to the GPS
David
I have often wondered about all this sawtooth correction stuff, and I
think I've asked here too, but never got a definitive answer. Every time
this comes up, there are all sorts of explanations of the
characteristic, and inevitably someone mentions the T-bolt having its
internal GPS clock synchronized with the desired ideal 10 MHz output,
either eliminating or greatly reducing the sawtooth effect. But, to my
knowledge, nobody has said for sure if this is indeed the case, or
whether that's all it takes to achieve perfection, or if more magic is
needed besides synchronizing. I'm also aware that some people have come
up with external sawtooth correction, using programmable digital delay
lines and special control circuits. But, why do all that if you can just
fix the clock instead?
Many years ago I looked at my Motorola Oncore VP (or whatever model is
used in the HPZ3801A), to see what its clock was, and if it was
reasonably possible to synthesize it from the 10 MHz. I think the clock
is a Motorola brand odd looking TCXO, labeled "19096" or something like
that, probably 19.096 MHz, as I recall. That's about as far as I got.
Since then I've just wondered what would happen if it was synthesized
from the 10 MHz, or if it was even worth trying, or if it would result
in other problems. Maybe a certain amount of dither is necessary for
proper operation.
So, here are some questions, that if answered, may go a long way toward
possible improvements in our GPS stuff.
-
If the GPS RX module's internal clock is synthesized (to the same
nominal frequency) from the 10 MHz output of the GPSDO, can that alone
eliminate or substantially reduce the sawtooth effect? -
Does the T-bolt actually do this, and if so, is that all it takes?
If either answer is yes, then I would think the GPS RX makers would have
provisions for external clock reference, at least for certain high-grade
timing type models.
- Do any GPS RX modules have such provision?
That's all I can think of for now.
Ed
Hi
On Aug 13, 2020, at 6:35 PM, ed breya eb@telight.com wrote:
I have often wondered about all this sawtooth correction stuff, and I think I've asked here too, but never got a definitive answer. Every time this comes up, there are all sorts of explanations of the characteristic, and inevitably someone mentions the T-bolt having its internal GPS clock synchronized with the desired ideal 10 MHz output, either eliminating or greatly reducing the sawtooth effect. But, to my knowledge, nobody has said for sure if this is indeed the case, or whether that's all it takes to achieve perfection, or if more magic is needed besides synchronizing.
Properly steering the clock that supplies the pps divider is what is needed. There is no direct need to use that clock for the GPS. A GPSDO is a device
that steers the clock that supplies the pps divider. (Yes, there are other ways to get it done)
I'm also aware that some people have come up with external sawtooth correction, using programmable digital delay lines and special control circuits. But, why do all that if you can just fix the clock instead?
Because you can’t “fix the clock” on a GPS module. That’s what’ this is all about. The GPS modules ( = little PCB GPS receivers ) don’t steer their clock.
Many years ago I looked at my Motorola Oncore VP (or whatever model is used in the HPZ3801A), to see what its clock was, and if it was reasonably possible to synthesize it from the 10 MHz. I think the clock is a Motorola brand odd looking TCXO, labeled "19096" or something like that, probably 19.096 MHz, as I recall. That's about as far as I got. Since then I've just wondered what would happen if it was synthesized from the 10 MHz, or if it was even worth trying, or if it would result in other problems.
If you multiply 10 MHz directly to GPS band you may have issues ….
Maybe a certain amount of dither is necessary for proper operation.
It’s more an issue of spurs.
So, here are some questions, that if answered, may go a long way toward possible improvements in our GPS stuff.
- If the GPS RX module's internal clock is synthesized (to the same nominal frequency) from the 10 MHz output of the GPSDO, can that alone eliminate or substantially reduce the sawtooth effect?
How are you doing the “synthesized”? Indeed pulse dropping is a version of synthesis. If you phase lock the local clock to the GPS
code clock, that will duplicate what the TBolt does.
- Does the T-bolt actually do this, and if so, is that all it takes?
They phase lock the local 10 MHz OCXO to the GPS code clock.
If either answer is yes, then I would think the GPS RX makers would have provisions for external clock reference, at least for certain high-grade timing type models.m
That sounds wonderful. It turns out to cost money. Since you can get the job done just fine with the software correction, why double (or whatever) the
price of the module? If I was designing a device (like a GPSDO) the lowest cost device with the best accuracy will be the winner Needless
to say, anything that pumps up the price is a major downer. (at least that’s how it worked for the decades I did do this for a living ….).
- Do any GPS RX modules have such provision?
No not on the low cost parts. That’s for the simple reason that OEM users of these modules are quite happy doing it with the sawtooth message.
It’s dirt cheap to implement and has very few downside issues.
Bob
That's all I can think of for now.
Ed
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Hi Ed,
I'm also aware that some people have come up with external
sawtooth correction, using programmable digital delay lines
and special control circuits.
For decades fellow time nut Rick Hambly has done hands-on work with GPS
receiver timing, including sawtooth correction. Here's a recent version
of his classic series of presentations:
https://www.cnssys.com/files/TOW/High-accuracy_Time_and_Frequency_in_VLBI_2019_sem.pdf
His "CNS clock" was an example of a GPS/1PPS source that used the delay
technique to reduce sawtooth effects. The paper is worth a look; lots of
good info packed along with plenty of plots and photos.
Note that his most recent version of the CNS clock no longer uses the
delay technique. I can explain more if you want to go down that rabbit hole.
But, why do all that if you can just fix the clock instead?
Because it is not possible for you or me to "fix the clock instead".
That's a complex business and design decision inside the GPS chip
itself. Almost every DIY or commercial GPSDO uses an off-the-shelf GPS
receiver chip or PCB, so you work with what you can get. Those GPS
modules all use some tiny carefully-specified free-running internal
oscillator for signal sampling, computing, and output pulse generation.
Trimble was in a unique position when the GPSDO era began because they
made both GPS chips and a GPSDO product. For them the integrated OCXO
and GPS receiver design was possible. Even hp couldn't do that.
I think the clock is a Motorola brand odd looking TCXO, labeled "19096"
or something like that, probably 19.096 MHz, as I recall.
That sounds right. See http://www.leapsecond.com/pages/vp/sawtooth.htm
and note Dr Clark's mention of 9.54 MHz, which would be half your 19.096.
- If the GPS RX module's internal clock is synthesized (to the same
nominal frequency) from the 10 MHz output of the GPSDO, can that
alone eliminate or substantially reduce the sawtooth effect?
Yes. But it would also create a list of other technical concerns. I'd
probably rather have to deal with sawtooth than having to deal with DDS
artifacts or phase noise requirements. And now your GPS chip won't even
boot or work right if the user's 10 MHz isn't there, or isn't accurate
enough, or is still warming up, or if the user's 10 MHz has too much
phase noise or instability.
- Does the T-bolt actually do this, and if so, is that all it takes?
Yes, we think so. But remember they had control over everything; the GPS
chip, the firmware, and the specification of the OCXO.
If either answer is yes, then I would think the GPS RX makers would
have provisions
for external clock reference, at least for certain high-grade timing
type models.
Yes, very high-grade timing receivers require, or allow, external
frequency inputs. I've seen some in operation at national timing labs.
When I heard they cost $20k I decided sawtooth correction wasn't so bad
after all. ;-)
It seems sawtooth bothers you. There are sawtooth effects in many parts
of technology; from printer dots to PC clocks; from calendars to leap
seconds. The goal isn't always to eliminate but rather to fully
understand the effects. The fact that GPS timing receivers output
quantization corrections should be seen as a good thing; it gives the
user maximum information.
/tvb
On 8/13/2020 3:35 PM, ed breya wrote:
I have often wondered about all this sawtooth correction stuff, and I
think I've asked here too, but never got a definitive answer. Every
time this comes up, there are all sorts of explanations of the
characteristic, and inevitably someone mentions the T-bolt having its
internal GPS clock synchronized with the desired ideal 10 MHz output,
either eliminating or greatly reducing the sawtooth effect. But, to my
knowledge, nobody has said for sure if this is indeed the case, or
whether that's all it takes to achieve perfection, or if more magic is
needed besides synchronizing. I'm also aware that some people have
come up with external sawtooth correction, using programmable digital
delay lines and special control circuits. But, why do all that if you
can just fix the clock instead?
Many years ago I looked at my Motorola Oncore VP (or whatever model is
used in the HPZ3801A), to see what its clock was, and if it was
reasonably possible to synthesize it from the 10 MHz. I think the
clock is a Motorola brand odd looking TCXO, labeled "19096" or
something like that, probably 19.096 MHz, as I recall. That's about as
far as I got. Since then I've just wondered what would happen if it
was synthesized from the 10 MHz, or if it was even worth trying, or if
it would result in other problems. Maybe a certain amount of dither is
necessary for proper operation.
So, here are some questions, that if answered, may go a long way
toward possible improvements in our GPS stuff.
-
If the GPS RX module's internal clock is synthesized (to the same
nominal frequency) from the 10 MHz output of the GPSDO, can that alone
eliminate or substantially reduce the sawtooth effect? -
Does the T-bolt actually do this, and if so, is that all it takes?
If either answer is yes, then I would think the GPS RX makers would
have provisions for external clock reference, at least for certain
high-grade timing type models.
- Do any GPS RX modules have such provision?
That's all I can think of for now.
Ed
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Thanks all, for the info on this issue. It does indeed look doable to
experiment with phase-locking the GPS RX module's internal clock to the
ultimate 10 MHz GPSDO output. That is, doable, but not necessarily easy
or without problems.
I dug up my old notes and info from about ten years ago, and found I had
studied it quite extensively, and had some various schemes sketched out
already - I forgot about all this. In the notes I found some pages from
US Patents 4,785,463, and 5,745,741, which are pertinent to the Motorola
Oncore RX units. I also found page 26 of a document "Remote Frequency
Calibration: The NIST Frequency Measurement and Analysis Service," which
talks about it too.
The internal clock appears to be 19.096 MHz nominal, +/- 2 ppm, from a
TCXO, that may or may not have voltage tuning too, depending on the
model. The '463 patent appears to be about the original overall design,
while the '741 is about reducing the sawtooth error by doing coarse
digital corrections in the counter system, while also fine tuning the
VCTXO, to get an integer clock frequency under all conditions. It
mentions the Oncore model as prior art, and the resulting sawtooth error.
As I understand it, the overall process disclosed is to allow for the
TCXO to drift to any frequency in range, but automatically tweak it
slightly to make sure it's an integer (Hz ) value, and set the counter
system to divide by that same integer value, so the 1 PPS output is
consistent, without sawtooth error. So, if the clock is nearly exactly
right on, the counter divides by 19,096,000, and it figures out how to
fine tune the clock to keep it there. If the drift goes beyond the fine
range to say all the way to the upper stated limit of +38 Hz, the
counter is set to divide by 19,096,038, and the clock is again tweaked
to keep it close to that integer Hz. How it does all this is disclosed
in the patent, but I haven't studied it enough to say any more.
It looks like this improvement was in a later model, or was perhaps
never actually used, since this was around the time that Motorola was
departing the GPS business. I assume the older Oncores like mine do not
have any of this improvement, so are subject to both the clock drift and
the sawtooth. But, one thing I got from this, is that if the nominal
clock and divide numbers match, and are fixed at 19,096,000, then
replacing the original clock with a sufficiently clean synthesized
external clock should work too.
I have a number of possible options, depending on the actual original
TCXO. If it's also voltage-tuned, but that isn't utilized, then it can
stay, but needs circuit mods to release and access the tune signal. The
clock signal is accessible at the TCXO, or possibly less risky in 2X
form 38.192 MHz from the downconverter IC. This would be the best
option, to make an external PLL to tweak it. BTW I have a spare GPS RX
unit - I would not risk taking the Z3801A out of commission and messing
around with it.
If the TCXO has no tuning ability, then a new clock signal needs to be
made. Of all the schemes I sketched out back then, the most
straightforward seems to be simply adding 4.096 MHz from a VCXO, PLLed
to 10 MHz, and 15 MHz derived directly from the 10. These can all be
scaled up in frequency in various arrangements, and use standard binary
frequency XOs. The next notch up for instance, is 8.192 MHz + 30 MHz
giving twice the clock, and so forth. Unfortunately, this method is
additive in frequency, which I hate - I prefer to take the difference of
two much higher frequencies, which is so much easier to filter. I don't
yet see any ways to do a difference method without using special
frequencies, so for now I assume regular old standard XO frequencies
will have to do, and the filter designs will need to be fancier. The
PLLing seems to be straightforward. I can get a decent 16 kHz comparison
frequency with simple integer dividing, like 4.096 MHz/256 = 10 MHz/625,
but would like to get it as high as possible, without resorting to
fancier schemes. Fractional-N is not in the cards here, I don't think.
It would be nice if for some reason a 19.096 MHz VCXO module - or even
just a crystal - was available for cheap, but I doubt such exists
outside of the Oncore line. Maybe a TCXO from a junker would do, but
again, it has to have voltage tuning too, and I don't know if any had
that. (One of my more far fetched schemes even pictured thermal tuning -
TEC heating and cooling the TCXO module - but imagine the nightmare of
characterizing the part and the dynamics, and the mechanical and control
loop issues.)
So anyway, I think I can do it with the additive VCXO combo, but doubt I
ever will - there are too many projects to worry about without reviving
this one, but it's fun to ponder.
BTW one last thing is that in my collection, I have parts of an ancient
(ca 1990), huge Trimble L1/L2 GPSDO. I was going to share some info on
it, but it will have to wait for another time. I find it very interesting.
Ed
Ed,
Good work digging deep into that. I remember hearing about someone
playing with the Oncore oscillator. It was Robin Giffard, one of the key
architects behind the hp SmartClock series (58503A, Z3801A, etc.). A
copy of his paper:
"Estimation of GPS Ionospheric Delay Using L1 Code and Carrier Phase
Observables"
https://apps.dtic.mil/sti/pdfs/ADA497270.pdf
You'll see the Motorola Oncore VP mentioned in several places. He used a
5061B and a simple M/N PLL to give 3590 / 188 MHz = 19.0957 MHz.
I don't recall any other papers describing a similar experiment. In that
era the Oncore VP was one of the favorite timing receivers. You would
think if there was any merit to the clock hack then lots of people or
products would do it. But AFAIK, none did, not even hp. All of these
GPSDO had microprocessors and TIC onboard so using the GPS receiver
generated sawtooth correction message is all they needed.
/tvb
On 8/17/2020 1:53 PM, ed breya wrote:
Thanks all, for the info on this issue. It does indeed look doable to
experiment with phase-locking the GPS RX module's internal clock to
the ultimate 10 MHz GPSDO output. That is, doable, but not necessarily
easy or without problems.
I dug up my old notes and info from about ten years ago, and found I
had studied it quite extensively, and had some various schemes
sketched out already - I forgot about all this. In the notes I found
some pages from US Patents 4,785,463, and 5,745,741, which are
pertinent to the Motorola Oncore RX units. I also found page 26 of a
document "Remote Frequency Calibration: The NIST Frequency Measurement
and Analysis Service," which talks about it too.
The internal clock appears to be 19.096 MHz nominal, +/- 2 ppm, from a
TCXO, that may or may not have voltage tuning too, depending on the
model. The '463 patent appears to be about the original overall
design, while the '741 is about reducing the sawtooth error by doing
coarse digital corrections in the counter system, while also fine
tuning the VCTXO, to get an integer clock frequency under all
conditions. It mentions the Oncore model as prior art, and the
resulting sawtooth error.
As I understand it, the overall process disclosed is to allow for the
TCXO to drift to any frequency in range, but automatically tweak it
slightly to make sure it's an integer (Hz ) value, and set the counter
system to divide by that same integer value, so the 1 PPS output is
consistent, without sawtooth error. So, if the clock is nearly exactly
right on, the counter divides by 19,096,000, and it figures out how to
fine tune the clock to keep it there. If the drift goes beyond the
fine range to say all the way to the upper stated limit of +38 Hz, the
counter is set to divide by 19,096,038, and the clock is again tweaked
to keep it close to that integer Hz. How it does all this is disclosed
in the patent, but I haven't studied it enough to say any more.
It looks like this improvement was in a later model, or was perhaps
never actually used, since this was around the time that Motorola was
departing the GPS business. I assume the older Oncores like mine do
not have any of this improvement, so are subject to both the clock
drift and the sawtooth. But, one thing I got from this, is that if the
nominal clock and divide numbers match, and are fixed at 19,096,000,
then replacing the original clock with a sufficiently clean
synthesized external clock should work too.
I have a number of possible options, depending on the actual original
TCXO. If it's also voltage-tuned, but that isn't utilized, then it can
stay, but needs circuit mods to release and access the tune signal.
The clock signal is accessible at the TCXO, or possibly less risky in
2X form 38.192 MHz from the downconverter IC. This would be the best
option, to make an external PLL to tweak it. BTW I have a spare GPS RX
unit - I would not risk taking the Z3801A out of commission and
messing around with it.
If the TCXO has no tuning ability, then a new clock signal needs to be
made. Of all the schemes I sketched out back then, the most
straightforward seems to be simply adding 4.096 MHz from a VCXO, PLLed
to 10 MHz, and 15 MHz derived directly from the 10. These can all be
scaled up in frequency in various arrangements, and use standard
binary frequency XOs. The next notch up for instance, is 8.192 MHz +
30 MHz giving twice the clock, and so forth. Unfortunately, this
method is additive in frequency, which I hate - I prefer to take the
difference of two much higher frequencies, which is so much easier to
filter. I don't yet see any ways to do a difference method without
using special frequencies, so for now I assume regular old standard XO
frequencies will have to do, and the filter designs will need to be
fancier. The PLLing seems to be straightforward. I can get a decent 16
kHz comparison frequency with simple integer dividing, like 4.096
MHz/256 = 10 MHz/625, but would like to get it as high as possible,
without resorting to fancier schemes. Fractional-N is not in the cards
here, I don't think.
It would be nice if for some reason a 19.096 MHz VCXO module - or even
just a crystal - was available for cheap, but I doubt such exists
outside of the Oncore line. Maybe a TCXO from a junker would do, but
again, it has to have voltage tuning too, and I don't know if any had
that. (One of my more far fetched schemes even pictured thermal tuning
- TEC heating and cooling the TCXO module - but imagine the nightmare
of characterizing the part and the dynamics, and the mechanical and
control loop issues.)
So anyway, I think I can do it with the additive VCXO combo, but doubt
I ever will - there are too many projects to worry about without
reviving this one, but it's fun to ponder.
BTW one last thing is that in my collection, I have parts of an
ancient (ca 1990), huge Trimble L1/L2 GPSDO. I was going to share some
info on it, but it will have to wait for another time. I find it very
interesting.
Ed
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