[time-nuts] Re: Realtime comparing PPS of 3 GPS

[glen english LIST via time-nuts]

Be aware not to confuse the antenna ground plane  (the patch will always 
have its own plane because the top metalization must be fed against a 
plane or counterpoise -  and a ground plane behind the antenna.


I can see the usefulness of the larger ground plane for any purchased 
patch antenna to reduce the likelihood of interference underneath (if 
the feed coax has a good RF contact with the plane), and if the plane is 
coupled well, it may improve the low angle response .


The supplementary ground plane doesnt have to have a galvanic connection 
if the gap between the underside of the patch is low- IE use purely a 
capacitive coupling to tie the patch antenna ground to the large ground 
sheet-


If we consider the patch area to be 10x10mm  = 100uM^2, and the gap 
being air (for simplicity sake) of 0.25mm, the capacitance is 
Epsilon-nought times area, all divided by the distance between the two 
plates


For the above example this is about 3.5pF or (-)j30. Really needs to be 
< j5 .


That means reducing the gap to about 0.05mm  OR increasing the area- 
probably means using a bigger patch.


You might be able to sweat solder the patch antenna (bottom)  to a sheet 
of FR4- that would be my approach.


-glen



On 31/05/2022 1:24 am, Carsten Andrich via time-nuts wrote:

Hi Erik,

have you tried running all receivers off the same antenna via a power 
splitter (make sure to dc block all but one receiver)? That should 
remove the uncertainty due to antenna differences (location, RF 
characteristics, etc.).


Also, are you using ground planes for your puck antennas? These types 
of antennas typically require a ground plane for optimal performance [1].


Best regards,
Carsten

[1] 
https://content.u-blox.com/sites/default/files/products/documents/GNSS-Antennas_AppNote_(UBX-15030289).pdf#page=16


On 30.05.22 13:00, Erik Kaashoek via time-nuts wrote:
Further evaluation did shown the time differences between the 3 GPS 
modules was due to difference in the trigger level setting of the 
timer/counter and difference in length of GPS antenna cables.
After removal of the phase drift due to Rb frequency offset the 
attached image shows the phase differences of the 3 modules versus a 
Rb reference.
The two ATGM modules are very consistent over a 2.8 hours period. The 
NEO-7M varies wildly  with phase errors above 100 ns. Possibly due to 
a somewhat less optimal antenna position.
It seems phase variations over time in the order of 10-20 ns are 
indeed unavoidable, even with a good antenna.

Erik.

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--
Glen English
RF Communications and Electronics Engineer

CORTEX RF

Pacific Media Technologies Pty Ltd trading as Cortex RF

ABN 40 075 532 008

PO Box 5231 Lyneham ACT 2602, Australia.
au mobile : +61 (0)418 975077
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[time-nuts] Re: Build a 3 hat timestanp counter

[Hans-Georg Lehnard via time-nuts]

Hi Magnus, 

I got the E1740 last year on ebay for 80EUR and wrote a quick to dirty
software for it. This is my thread about it:
https://www.eevblog.com/forum/testgear/hp-e1740a-time-interval-analyzer-292384/msg3718660/#msg3718660


It has 2 channels and can timestamp any edge up to 80 Mhz.
It is possible to measure with one channel, with 2 channels and the
difference between them. 
I hope to see a difference between the direct input and the TLV3501. 

I also have 2 Acam 2 channel TDC AS6501 with DDR-LVDS outputs and 10ps
resolution. With this it should be possible to sample up to 12,5 MHz
with a FPGA. This will be my next project. 
There is also the AS6500 which has 4 channels, but can only be read out
via SPI. 

For the LTC6957 I will use the circuit from here:
https://ohwr.org/project/wr-low-jitter/uploads/74305092be60598e9bffd39d59489594/Daughterboard_V2.pdf
Page 4. 

Hans-Georg  

Am 2022-05-29 18:35, schrieb Magnus Danielson via time-nuts:

> Hi Hans-Georg,
> 
> On 2022-05-28 17:04, Hans-Georg Lehnard via time-nuts wrote: 
> 
>> Hello Magnus,
>> 
>> I understood that simply sampling 3 channels fast and averaging does not
>> solve all the problems ;-).
> Sure, I just want to illustrate how various approaches could allow you to get 
> the most out of the hardware you have. 
> 
>> I have a HP E740A time interval analyzer that I might use for my
>> oscillators.
>> The HP-TIA has 50ps resolution, can sample 10 Mhz directly but has only
>> 512K sample memory. For longer recordings I can use the histogram
>> function. I still need to do some work on my software for that, but that
>> might be the fastest way to get results.
> Does the E1740A have a high speed time-stamping port? The 5371A and 5372A 
> have that as an option, so if one could have some suitable hardware process 
> on those time-stamps, it would be something. 
> 
>> Attached is a picture of my HP10811 and another one that someone made
>> for me as a comparison to other TIAs.
>> 
>> The third pic shows another meter i have  with TLV3501 and TDC7200
>> without averaging compared to an FA2.
>> 
>> I will keep trying with the TDC7200 and maybe better with the LTC6957
>> and only one channel.
> 
> Yes, to try different approaches. The LTC6957 is a very cool little chip.
> 
> Cheers,
> Magnus
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[time-nuts] Re: Realtime comparing PPS of 3 GPS

[Magnus Danielson via time-nuts]

Erik,

The NEO-7M may have sawtooth correction output, have you checked that 
and made compensations?


Since the oscillator is not steered and free-floating, the 
cycle-assignment of the PPS may be less than optimal so just measuring 
that without the compensation can cause a wider range of PPS than the 
actual receiver time stability represents.


In particular, check chapter 12 and the TIM-TP message of [1].

[1] 
https://content.u-blox.com/sites/default/files/products/documents/u-blox7-V14_ReceiverDescriptionProtocolSpec_%28GPS.G7-SW-12001%29_Public.pdf


Do notice that the TIM-TP message is documented to be issued before the 
(PPS) pulse it report on.


The variations you report is consistent with what the datasheet report 
for the pulse assignment, which may not be representative of the 
receivers performance.


Cheers,
Magnus

On 2022-05-30 13:00, Erik Kaashoek via time-nuts wrote:
Further evaluation did shown the time differences between the 3 GPS 
modules was due to difference in the trigger level setting of the 
timer/counter and difference in length of GPS antenna cables.
After removal of the phase drift due to Rb frequency offset the 
attached image shows the phase differences of the 3 modules versus a 
Rb reference.
The two ATGM modules are very consistent over a 2.8 hours period. The 
NEO-7M varies wildly  with phase errors above 100 ns. Possibly due to 
a somewhat less optimal antenna position.
It seems phase variations over time in the order of 10-20 ns are 
indeed unavoidable, even with a good antenna.

Erik.

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[time-nuts] Re: Realtime comparing PPS of 3 GPS

[Bob kb8tq via time-nuts]

Hi

The variation you see is dependent on a number of things. One of them is space 
weather. If you do your run during a very active period ( typically peak sun 
spots)
you may see some very dramatic swings on a single band device. 

Bob

> On May 30, 2022, at 3:00 AM, Erik Kaashoek via time-nuts 
>  wrote:
> 
> Further evaluation did shown the time differences between the 3 GPS modules 
> was due to difference in the trigger level setting of the timer/counter and 
> difference in length of GPS antenna cables.
> After removal of the phase drift due to Rb frequency offset the attached 
> image shows the phase differences of the 3 modules versus a Rb reference.
> The two ATGM modules are very consistent over a 2.8 hours period. The NEO-7M 
> varies wildly  with phase errors above 100 ns. Possibly due to a somewhat 
> less optimal antenna position.
> It seems phase variations over time in the order of 10-20 ns are indeed 
> unavoidable, even with a good antenna.
> Erik.
> <3GPS_phase_difference.png>___
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[time-nuts] Re: LORAN C receiver

[paul swed via time-nuts]

Glenn good afternoon.
Unfamiliar with that receiver. If its a location receiver for a plane or
boat then it has no value as they depended on 3 sites to generate a
location. Today the eLORAN tests have been pretty much single site that
fakes a secondary transmitter.
Things like Austrons and SRS can leverage the single site for very accurate
frequency.
But those receivers were intended for frequency or timing.
I am staying clear of the timing aspect because I am honestly not sure
the old method is actually being transmitted.

Eloran provides an additional bit in the pulses that actually does contain
timing and correction information. Though all of the existing receivers to
my knowledge do not produce this data as an output.

If the US were to invest in eLORAN then I honestly believe the 9th bit
could be recovered from the SRS and Austrons. Decoded and used for time and
propagation corrections. I have seen the actual eLORAN receivers and they
seem to be SDR like.
But nothing a cheap time-nut could get your hands on.
Regards
Paul
WB8TSL

On Mon, May 30, 2022 at 10:57 AM Glenn Little WB4UIV via time-nuts <
time-nuts@lists.febo.com> wrote:

> I have a Si-Tex KODEN 797 receiver.
> I this usable with eLORAN and can I get useful timing data from the data
> out BNC connector?
>
> Can this be used for anything other than parts?
>
>
> Thanks
> 73
> Glenn
> WB4UIV
>
> --
> ---
> Glenn LittleARRL Technical Specialist   QCWA  LM 28417
> Amateur Callsign:  WB4UIVwb4...@arrl.netAMSAT LM 2178
> QTH:  Goose Creek, SC USA (EM92xx)  USSVI, FRA, NRA-LMARRL TAPR
> "It is not the class of license that the Amateur holds but the class
> of the Amateur that holds the license"
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>
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[time-nuts] Re: Realtime comparing PPS of 3 GPS

[Carsten Andrich via time-nuts]

Hi Erik,

have you tried running all receivers off the same antenna via a power 
splitter (make sure to dc block all but one receiver)? That should 
remove the uncertainty due to antenna differences (location, RF 
characteristics, etc.).


Also, are you using ground planes for your puck antennas? These types of 
antennas typically require a ground plane for optimal performance [1].


Best regards,
Carsten

[1] 
https://content.u-blox.com/sites/default/files/products/documents/GNSS-Antennas_AppNote_(UBX-15030289).pdf#page=16


On 30.05.22 13:00, Erik Kaashoek via time-nuts wrote:
Further evaluation did shown the time differences between the 3 GPS 
modules was due to difference in the trigger level setting of the 
timer/counter and difference in length of GPS antenna cables.
After removal of the phase drift due to Rb frequency offset the 
attached image shows the phase differences of the 3 modules versus a 
Rb reference.
The two ATGM modules are very consistent over a 2.8 hours period. The 
NEO-7M varies wildly  with phase errors above 100 ns. Possibly due to 
a somewhat less optimal antenna position.
It seems phase variations over time in the order of 10-20 ns are 
indeed unavoidable, even with a good antenna.

Erik.

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[time-nuts] Re: Realtime comparing PPS of 3 GPS

[Erik Kaashoek via time-nuts]

Further evaluation did shown the time differences between the 3 GPS 
modules was due to difference in the trigger level setting of the 
timer/counter and difference in length of GPS antenna cables.
After removal of the phase drift due to Rb frequency offset the attached 
image shows the phase differences of the 3 modules versus a Rb reference.
The two ATGM modules are very consistent over a 2.8 hours period. The 
NEO-7M varies wildly  with phase errors above 100 ns. Possibly due to a 
somewhat less optimal antenna position.
It seems phase variations over time in the order of 10-20 ns are indeed 
unavoidable, even with a good antenna.

Erik.
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[time-nuts] LORAN C receiver

[Glenn Little WB4UIV via time-nuts]

I have a Si-Tex KODEN 797 receiver.
I this usable with eLORAN and can I get useful timing data from the data 
out BNC connector?


Can this be used for anything other than parts?


Thanks
73
Glenn
WB4UIV

--
---
Glenn LittleARRL Technical Specialist   QCWA  LM 28417
Amateur Callsign:  WB4UIVwb4...@arrl.netAMSAT LM 2178
QTH:  Goose Creek, SC USA (EM92xx)  USSVI, FRA, NRA-LMARRL TAPR
"It is not the class of license that the Amateur holds but the class
of the Amateur that holds the license"
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[time-nuts] Re: Build a 3 hat timestanp counter

[Magnus Danielson via time-nuts]

Hi Hans-Georg,

On 2022-05-28 17:04, Hans-Georg Lehnard via time-nuts wrote:

Hello Magnus,

I understood that simply sampling 3 channels fast and averaging does not
solve all the problems ;-).
Sure, I just want to illustrate how various approaches could allow you 
to get the most out of the hardware you have.

I have a HP E740A time interval analyzer that I might use for my
oscillators.
The HP-TIA has 50ps resolution, can sample 10 Mhz directly but has only
512K sample memory. For longer recordings I can use the histogram
function. I still need to do some work on my software for that, but that
might be the fastest way to get results.
Does the E1740A have a high speed time-stamping port? The 5371A and 
5372A have that as an option, so if one could have some suitable 
hardware process on those time-stamps, it would be something.


Attached is a picture of my HP10811 and another one that someone made
for me as a comparison to other TIAs.

The third pic shows another meter i have  with TLV3501 and TDC7200
without averaging compared to an FA2.

I will keep trying with the TDC7200 and maybe better with the LTC6957
and only one channel.


Yes, to try different approaches. The LTC6957 is a very cool little chip.

Cheers,
Magnus
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[time-nuts] Re: Optimizing GPSDO for phase stability

[Magnus Danielson via time-nuts]

Hi Erik,

On 2022-05-28 10:29, Erik Kaashoek via time-nuts wrote:

Hi Magnus,

I've insufficient understanding of PLL's to grab the full meaning of 
your remark on "shift of the resonance"


OK, so a PI-controlled PLL has two basic characteristics, it's resonance 
frequency and it's damping factor (reciprocal of Q-factor).


You will get a frequency where there is a positive gain, giving 
jitter-peaking, as the phase-noise (aka jitter) from the reference port 
get's increased gain over to the output. The I factor of the PI-looped 
PLL is proportional to the square of this characteristics. The P factor 
is then proportional to the resonance frequency times the damping factor.


Now, this peak of noise energy will have a tell-tail in the ADEV plot as 
being similar to the wavey pattern you get from a pure sine of the same 
frequency as the mid-point of the jitter-peaking. What I was observing 
was how that peak moved in the ADEV plot, and suggested that a better 
view could be given in the phase-noise domain.


For jitter-peaking, see for instance Wolaver "Phase-locked loop circuit 
design".


Attached are the 3 phase PSD plots from stable32. Is that what you 
where looking for?

Tick_01 is for Kp=0.1, Tick_004 is for Kp=0.04, etc...
With Kp=0.01 there seems to be a peak at 3e-3Hz, for the other Kp it 
seems to be less evident if there is a resonance peak in the phase.
Also attached are the Frequency PSD plots (Freq_001, Freq_004, 
etc...)  and these show a clear shift of the peak.

Indeed, as I suspected

Does this shift imply the loop is not yet tuned optimal?


I wonder how your model and parameters work.

I tend to label the phase-detector to EFC gain factor as P and the 
phase-detector into the integrator (who's output is added to the EFC) 
gain factor as I.


VD = PhaseDetector output
VI = VI + VD*I
VF = VI + VD*P
EFC = VF

I tend to model it as analog continuous time, but similar enough 
properties occurs in digital discrete time.


In such a model, the steering parameters is resonance frequence f0 and 
damping factor d.


I = KI * f0^2
P = KP * f0 * d

The fixed constants KI and KP can be derived from loop and scaling 
parameters.


Notice that there is no single gain-point which will only dial for f0, 
but both I and P need appropriate scaling.


To keep jitter peaking reasonable, the damping factor d should be 3 or 
higher. However, for test purposes it can be set lower to make jitter 
peaking and thus resonance frequency easier to observe.


Cheers,
Magnus


Erik.


On 27-5-2022 21:30, Magnus Danielson via time-nuts wrote:

Dear Erik,

On 2022-05-27 18:02, Erik Kaashoek via time-nuts wrote:
The GPSDO/Timer/Counter I'm building also is intended to have a 
stabilized PPS output (so with GPS jitter removed).
The output PPS is created by multiplying/dividing the 10MHz of a 
disciplined TCXO up and down to 1 Hz using a PLL and a divide by 
2e8. No SW or re-timing involved.
The 1 PPS output is phase synchronized with the PPS using a SW 
control loop and thus should be a good basis for experiments that 
require a time pulse that is stable and GPS time correct.
As I have no clue how to specify or evaluate the performance of such 
a PPS output I've done some experiments.
In the first attached graph you can see the ADEV of the GPS PPS (PPS 
- Rb) and the 1 PPS output with three different control parameters 
(Tick - RB)
As I found it difficult to understand what the ADEV plot in practice 
means for the output phase stability I also added the Time Deviation 
plot as I'm assuming this gives information on the phase error 
versus the time scale of observation.


The ADEV plot is the frequency stability plot, so it can be a bit 
challenging to use it for phase stability.


The TDEV plot is the phase stability plot, so it is more useful for 
that purpose.


There is a technical difference between these beyond the difference 
of frequency vs phase stability, and that is that ADEV is the 
frequency stability for a Pi-counter where as TDEV is the phase 
stability for a Lambda-counter, where MDEV is the frequency stability 
for the Lambda-counter. There is no standardized phase-stability for 
Pi-counter. For a nit-pick like me it is significant, but for others 
it may be mearly a little confusing.


Lastly a plot is added showing the Phase Difference. All plots where 
created using the linear residue as the Rb used as reference is a 
bit out of tune.

Also the TIM files are attached
The "PPS - RB" and "Tick - RB Kp=0.04" where measured simultaneously 
and should show the extend to which the GPS PPS is actually drifting 
in phase versus the Rb and how this impacts the output phase of the 
stabilized output PPS.
My conclusion is that a higher then expected Kp of 0.1 gives the 
most stable output phase performance where the best frequency 
performance is realized with a Kp = 0.04
I welcome feedback on the interpretation of these measurements and 
the application of output phase stabilization.


Since Kp is 

[time-nuts] Realtime comparing PPS of 3 GPS

[Erik Kaashoek via time-nuts]

During evaluation of a GPS module there was some concern over the 
stability/correctness of the PPS and, as the NIST paper on PPS accuracy 
mentions there could be substantial variation in PPS accuracy due to 
atmospheric conditions, an experiment was set up to measure the PPS of 3 
different GPS modules simultaneously.

The experiment uses one NEO-7M and two ATGM336H GPS modules
The PPS of each module is send to 3 separate timer capture inputs of one 
timer running at 200 MHz providing 5 ns measurement accuracy.
The 200 MHz timer was clocked using a 10 MHz Rb output up converted to 
200 MHz using a PLL.

A plot showing the timing variation is attached.
Vertical axis is in ns, horizontal axis is in seconds
All GPS modules had separate simple puck antenna all with almost free 
sky view and all using at least 10 GPS satellites.
The differences between the PPS of the 3 modules was surprisingly large, 
two modules where in average 10 ns apart and a 3rd module was in average 
39 ns apart from the first two modules. More investigation is needed as 
the HW paths between PPS output of the GPS modules and the timer capture 
inputs of the 3 timers are not fully identical
One GPS module showed little variation in PPS timing versus the Rb 
reference, only a slow drift of about 15 ns over 400 seconds. The other 
modules showed a periodic variation of up to 20 ns. The worst variation 
was observed from the NEO-7M module.
The measurement is at time of posting of this message still running to 
see if over a longer period the differences reduce.
If the observed short term (< 400 s)  PPS variation in the order of 
10-20 ns are common and unavoidable it would make sense for the 
requirement for phase stability of a GPSDO to be aligned with this level 
of  PPS phase uncertainty.

Erik.

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[time-nuts] Re: Ublox M6T -M8T

[Marek Doršic via time-nuts]

I just come accross this application note directly from u-blox.
https://content.u-blox.com/sites/default/files/products/documents/Timing_AppNote_%28GPS.G6-X-11007%29.pdf
 

They evaluated 8 kHz output.
 

   .md

> On 28 May 2022, at 19:05, R Putz via time-nuts  
> wrote:
> 
> Has anyone done anything with the Ublox GPS timing receivers? As it appears 
> the 
> Navman with the 10khz outputs seem to be drying up, I'm wondering about the 
> Time Pulse 2 output being set to 10 Khz or 100 Khz. Thoughts anyone?
> Rich
> W9ENG
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[time-nuts] Re: Ublox M6T -M8T

[Hal Murray via time-nuts]

Matthias Welwarsky said:
> Only frequencies that are even multiples of the internal crystal frequency
> (48MHz) are "clean". Everything else is, as Bob puts it, "drop a pulse, add a
>  pulse" approximations. However, that's normally easy to filter. 

How clean?  It's a GPS, not a GPSDO, so I'd expect the 48 MHz is free running 
rather than locked to GPS time.

Take the PPS case.  There will be adds/drops going on to correct for the 
frequency error in the 48 MHz crystal.  The pattern will change as the 
frequency changes with temperature or whatever.  That's how you get hanging 
bridges.  :)

Assume it is off by roughly 1 PPM.  That's well within specs for a crystal.  I 
just want a number for the back of an envelope.

For 10KHz, 1 PPM is an add/drop every 100 cycles.  I'll bet you can catch that 
on a scope running in persistance mode.



-- 
These are my opinions.  I hate spam.


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