[time-nuts] PLL/GPSDO/etc learning resources for mere mortals

Lars Walenius lars.walenius at hotmail.com
Thu Sep 13 14:02:25 EDT 2018


Hello Bryan,

As you indicate the new timing modules from uBlox like the 8-series and the Venus 838 could be better choices. I prefer the uBlox. I only recommended the NEO7M as it were cheap, available and is easy to use and at the same time gives good performance that I have measured. For simple position receivers with 1PPS I guess the ones from Adafruit is worth looking for even if not state of art for a GPSDO.

I am also curious about the Furano GT87 and hope that somebody will test it and give ADEV and MDEV's.

The nice thing with Lars GPSDO is that almost every GPS module with a 1PPS out can be used if the amplitude is 3.3-5V and the 1PPS is accurate on the positive going flank. The pulse width can be down to 10uS and probably even shorter. A square wave also works.

As you have a Jupiter-T I would definitely use that first. I started with even older and more jittery GPS modules and without timing mode and it worked. Of course a modern timing receiver will give better frequency accuracy and ADEV's but the loop should settle in the same time.

Lars

Från: Bryan _<mailto:bpl521 at outlook.com>
Skickat: den 12 september 2018 09:39
Till: Discussion of precise time and frequency measurement<mailto:time-nuts at lists.febo.com>
Ämne: Re: [time-nuts] PLL/GPSDO/etc learning resources for mere mortals

Hello:

Is there any other recommendations for a GPS receiver module, I note you recommend the Neo7M, but wondering if there are others that can be considered. Is there some better suited to supplying a frequency reference. There is also the 8M/T model(s) and the Venus, but not sure as to what advantages/disadvantages they have. I was going to start with a old Jupiter-T TU60-D120 module that I have as it has a 1pps output.


6.1.1 Pin J1-6: 1PPS output This signal is a positive going pulse with the leading edge being the point of measurement for synchronisation with the GPS satellite transmissions. The pulse width is approximately 26 ms.

Regards

-=Bryan=-

________________________________
From: time-nuts <time-nuts-bounces at lists.febo.com> on behalf of Lars Walenius <lars.walenius at hotmail.com>
Sent: September 7, 2018 12:25 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] PLL/GPSDO/etc learning resources for mere mortals

I heard Jim mentioned my GPSDO. I would not go to the time nuts archive as that describes a far from ready software design. The hardware is almost identical. Last year I published a new design on EEVBlog:

https://www.eevblog.com/forum/projects/lars-diy-gpsdo-with-arduino-and-1ns-resolution-tic/?all

This is a, as I think, a much more robust design and much more tested. By just setting the”gain” it will make a stable 1Hz digital PLL. As the DAC can be set and you have a counter with 1ns resolution you don’t need any extra instruments to do this. The nice thing is that the controller is very flexible and can handle all kinds of 10MHz oscillators as XO, TCXO, OXCO and rubidium. The useful control range of the oscillators is 0.001-6ppm. As only a 1PPS with 3.3-5V amplitude is needed a wide variation of GPS modules can be used. The time constant can be set from 4 to 32000 seconds and stored in EEPROM.

In the 21 pages of instruction I describe the design and also give performance data as both a GPSDO and a time interval counter. As Jim says my GPSDO gives useful information how you can do your own GPSDO. On the first pages I have some discussion of the GPS modules like NEO7M that can be used and also about different oscillators. Note I think the oscillators are the first you should think of as they set the most important specs for the GPSDO. See also the user manual for Stanford FS740 that contains a lot of information about a modern GPSDO. Here the link to Stanford FS740 seems to be broken in my instruction. Today this worked: https://www.thinksrs.com/downloads/pdfs/manuals/FS740m.pdf  See for example pages 25-35 for a good description of the GPSDO part. In the end of the manual you also find complete schematics!

On pages 8-9 in my instruction you find a very short description of my software and it is a standard PI-loop with a low pass filter before just as in the FS740. Paul-Henning has a very simple introduction to the PI-loop that I like http://phk.freebsd.dk/time/20141018.html  . I would also recommend learning from the GPSDO simulator by Tom Van Baak http://www.leapsecond.com/pages/gpsdo-sim/   it also uses a PI-loop + low pass filter and his code tells you a lot.

>From Tom’s GPSDO simulator you can also learn how the time resolution and the DAC resolution work with real time data. Magnus Danielsson in March 2014 published the result of some his simulations with different time and DAC resolutions on time nuts. My takeaway was that a time resolution below 5-10ns was ok and DAC resolutions in the ballpark of the oscillator ADEV was fine, but have a look on his data.

On page 15 in my instruction you find a list of basic links that I have found useful. Some links seems broken so for example for Stanford’s you have to find again on their homepage.

On pages 16-17 you find help commands and description of serial information. By the way Mark Sims excellent LH can read this and use the time, DAC and temperature data that are spit out every second http://www.eevblog.com/forum/metrology/lady-heather-v6-beta-for-windows-exe/?all  . This together with the GPS module of choice serial output data should make this very useful. I am sorry I haven’t used this as I am very weak of chemo and haven’t done anything to my GPSDO for a long time.

Page 18 shows setup to read for example DAC data into Timelab.

On page 19 you find some ADEV results of two of my prototype GPSDO’s. One with a TCXO DOT050V and one with a OCXO 131(that by purpose uses a bad DAC resolution of 1.2E-11 but still gets ADEV’s in the 5E-12 range.

Last two pages show data with the controller used as a time interval counter. By measuring the TIC with a PICDIV PD26 it is possible to enter a few linearization parameters in EEPROM and get ADEV’s in the 8E-10 range at 1sec.

Even if I don’t recommend reading specific about my GPSDO on time nuts I really recommend to search and read the time nuts archive. It is a lot of useful information and a special thank to Bob kb8tq who contributes with so much valuable information. After I published the GPSDO on time nuts I also got good help from Jim H with the hardware interrupt in the ATmega328. With the hardware interrupt the 200ns resolution on timer 1 works well with a jitter of just 40ns p-p. TvB also gave the idea to set the timer1 to an factor that suits the input frequency that in this case is 5Mhz so I choose 50000(-1) (instead of 65536-1). This gives overflow at 10ms and is easy to handle. I could probably thank a lot more people for good advises.

Lars

Ps. Note that Lars GPSDO is more a 10MHz frequency reference, not a time reference and by that I mean it has no jitter free PPS out. This can be added by external hardware.





________________________________
Från: time-nuts <time-nuts-bounces at lists.febo.com> för Jim Harman <j99harman at gmail.com>
Skickat: Tuesday, September 4, 2018 6:28:00 PM
Till: Discussion of precise time and frequency measurement
Ämne: Re: [time-nuts] PLL/GPSDO/etc learning resources for mere mortals

> The challenge seems to me to be

how to temperature stabilize the entire DAC chain without putting it all in
an oven

Hi Forrest,

Generally you don't have to temperature stabilize the DAC because it is
inside the phase-locked control loop and the loop will adjust for
temperature dependencies, just as it adjusts for drift and temperature
dependencies in the oscillator. You will want your phase detector to be
reasonably temperature stable however.

In the time-nuts archives you will find posts from Lars Walenius and myself
with schematics and code for Arduino-based GPSDOs which you might want to
study.
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