OCXO Stable Reference and Control Voltages?

[lars]

Hi

Just in case anybody else has missed the uBlox "magic code":

A LEA-6 is a nav receiver *unless* it is a LEA-6T. The LEA-6T is a timing receiver. The same convention applies to other parts in their lineup in the same fashion.

The "T" enables a number of features. It is *not* clear that it changes the basic function of the unit. The features it enables make it much easier to use in a GPSDO or other precise timing (sawtooth corrected) application. The net result is that you are below 1x10^-9 at one second rather than up around 1x10^-8 at 1 second.

I enclose two charts showing my results with NEO-6M (naviagation only and about 10USD on ebay) and LEA-6T (about 30USD on ebay). I have tested both with a patch antenna in-house that gave about 30db C/N (=int on chart) and an small patch antenna on the roof that gave about 50dB C/N (=ext). For the LEA-6T I also have set mode 0 that is mobile mode (=nf on chart) and also done a survey and set mode 2 fixed (=f on chart). A note: I have default mask angle 5 degrees and maybe a higher value should have given better results especially as I am in Sweden that is quite far north.

If you use the GPS module standalone the ADEV give a good indication of what you can get if you test for example a counter. If you have 1E-8 at 1sec you can get about 8digits average and 7digits with normal measurement confidence at 1sec gate time (Frequency plots often seems to jump about a factor ten more than the ADEV). At 100 sec ADEV is 1E-10 and for 100sec gate time you get 10digits average and 9digits with confidence.

For using a GPS module standalone I haven´t seen any DIY designs of hardware sawtooth correction. Otherwise sawtooth correction probably gives an extra digit.

I enclosed the MDEV chart as I think MDEV is useful to see possible limits when designing GPSDO controllers with PI-loops and a Pre-filter that take away part of the sawtooth errors.

At short time constants (10-100secs) sawtooth correction probably is very useful for a GPSDO. At long time constants (1000-10000secs) a pre-filter probably is nearly as efficient I think. I have never seen a hanging bridge longer than 300secs. Also at Tau longer than 1000sec the GPS errors (changing satellites, ionosphere etc) starts to get in the same region as the filtered GPS module errors I think.

My own rule of thumb is also that AT cut OCXO´s (and good VCTCXO as DOT050V) need time constants in the GPSDO of about 10-100secs, SC cut OCXO 100-1000secs and Rubidium 5000-20000secs to match good GPS receivers.

Lars

[lars]

GPSDO circuits.
1pps:
Brooks Shera(PLL); http://home.teleport.com/~oldaker/10mhz_construction.htm
http://www.rt66.com/~shera/index_fs.htm
Ve2zaz(FLL): http://ve2zaz.net/GPS_Std/GPS_Std.htm
ZL1BPU(PLL) ; http://www.qsl.net/zl1bpu/MICRO/SIMPLE/SimpleGPS.htm
KE0FF (PLL); http://www.rollanet.org/~joeh/projects/GPSlave_writeup.pdf
10khz input
J.R. Miller(PLL) ; http://www.jrmiller.demon.co.uk/projects/ministd/frqstd0.htm
____________________Heres one I just stumbled upon, will read it later.
https://hackaday.io/post/21322

My observations:

The Shera design is blamed for its 42ns TIC (Time Interval Counter) but gave good results with an HP10811 (not so common available anymore?) if I remember correct.

The VE2ZAZ uses a FLL and 100ns TIC if I understand correct and ADEV curves is not impressive.

ZL1BPU is not open source as I see and also 100nS TIC.

KE0FF seems promising but difficult to see result as no good results presented. Also requires to solder a CPLD and programming.

Nick Sayers also have 100ns TIC and not so impressive ADEV.

The Miller design relies on the Jupiter module being very good, but obsolete? Has anyone showed the same performance with another module with 10kHZ, eg uBlox. I haven´t seen any such results yet.

So except the Miller the Shera still seems best. But some components are obsolete. Another drawback of the Shera maybe that it is only designed for a 45ppb span of the OCXO.

Isn´t it any modern DIY design that uses an optimized design? With a TIC with better resolution than the sawtooth-ripple and a well designed PI-loop and pre-filter as Sheras design?

For a better TIC I have a used my simple design with 1nS resolution that was described in timenuts forum in feb 2014. It uses an Arduino but the software is not enough good for the complete GPSDO (even if I use it to discipline a LPRO and earlier an OCXO).

Lars

[DimitriP]

I have tested both with a patch antenna in-house that gave about 30db C/N (=int on chart) and an small patch antenna on the roof that gave about 50dB C/N (=ext).

Which  10Mhz output did you measure? Did you use the 10Mhz out of the LEA or was it disciplining an external OCXO?

If the former, it would be interesting to compare ADEV plots for the LEA (M & T ) producing  10KHz

The Miller design relies on the Jupiter module being very good, but obsolete? Has anyone showed the same performance with another module with 10kHZ, eg uBlox. I haven´t seen any such results yet.

Now that would be something!
I'm hoping someone with equipment capable of producing ADEV plots will build one someday and publish the results.

[lars]

I have tested both with a patch antenna in-house that gave about 30db C/N (=int on chart) and an small patch antenna on the roof that gave about 50dB C/N (=ext).

Which  10Mhz output did you measure? Did you use the 10Mhz out of the LEA or was it disciplining an external OCXO?

If the former, it would be interesting to compare ADEV plots for the LEA (M & T ) producing  10KHz

I have only compared with 1PPS out from the NEO/LEA against a divided down rubidium. My setup with the HP5370 is not enough fast to compare 10kHz signals. If so I probably have to use an XOR (that is build a Miller GPSDO

Lars

[ve7xen]

GPSDO circuits.
1pps:
Brooks Shera(PLL); http://home.teleport.com/~oldaker/10mhz_construction.htm
http://www.rt66.com/~shera/index_fs.htm
Ve2zaz(FLL): http://ve2zaz.net/GPS_Std/GPS_Std.htm
ZL1BPU(PLL) ; http://www.qsl.net/zl1bpu/MICRO/SIMPLE/SimpleGPS.htm
KE0FF (PLL); http://www.rollanet.org/~joeh/projects/GPSlave_writeup.pdf
10khz input
J.R. Miller(PLL) ; http://www.jrmiller.demon.co.uk/projects/ministd/frqstd0.htm
____________________Heres one I just stumbled upon, will read it later.
https://hackaday.io/post/21322

My observations:

The Shera design is blamed for its 42ns TIC (Time Interval Counter) but gave good results with an HP10811 (not so common available anymore?) if I remember correct.

The VE2ZAZ uses a FLL and 100ns TIC if I understand correct and ADEV curves is not impressive.

ZL1BPU is not open source as I see and also 100nS TIC.

KE0FF seems promising but difficult to see result as no good results presented. Also requires to solder a CPLD and programming.

Nick Sayers also have 100ns TIC and not so impressive ADEV.

The Miller design relies on the Jupiter module being very good, but obsolete? Has anyone showed the same performance with another module with 10kHZ, eg uBlox. I haven´t seen any such results yet.

So except the Miller the Shera still seems best. But some components are obsolete. Another drawback of the Shera maybe that it is only designed for a 45ppb span of the OCXO.

Isn´t it any modern DIY design that uses an optimized design? With a TIC with better resolution than the sawtooth-ripple and a well designed PI-loop and pre-filter as Sheras design?

For a better TIC I have a used my simple design with 1nS resolution that was described in timenuts forum in feb 2014. It uses an Arduino but the software is not enough good for the complete GPSDO (even if I use it to discipline a LPRO and earlier an OCXO).

Lars

I have been toying with the idea of using the TDC7200 TIC (55ps resolution TIC designed for ultrasonic flow measurement) to design a GPSDO. The part should be able to measure the 1PPS vs. 10MHz directly without any external dividers, though I am not sure this is a good idea from a performance point of view... Trying to learn more of the control theory and think it will be an interesting project, though I don't really have any good way to measure the loop performance. If I get around to implementing a working design I guess it will be time to set up a collection of Rubidiums and TICs to log data... Only in the planning stages at this point, but I think this is a viable and relatively easy / low part count option to get my feet wet.

I had previously considered re-implementing the design of the PICTIC on something other than PIC and using it as the phase detector for a GPSDO, but that complicates the hardware quite a bit.

Sorry for the noise .

[DimitriP]

My setup with the HP5370 is not enough fast to compare 10kHz signals.

You could , instad of measuring 1PPS coming out of the LEA, set it to produce 10KHz, and divide it down to 1PPS.
Other than the divide by 10 the rest of the setup would be the same as what you have already measured.

[uncle_bob]

Hi

Just in case anybody else has missed the uBlox "magic code":

A LEA-6 is a nav receiver *unless* it is a LEA-6T. The LEA-6T is a timing receiver. The same convention applies to other parts in their lineup in the same fashion.

The "T" enables a number of features. It is *not* clear that it changes the basic function of the unit. The features it enables make it much easier to use in a GPSDO or other precise timing (sawtooth corrected) application. The net result is that you are below 1x10^-9 at one second rather than up around 1x10^-8 at 1 second.

I enclose two charts showing my results with NEO-6M (naviagation only and about 10USD on ebay) and LEA-6T (about 30USD on ebay). I have tested both with a patch antenna in-house that gave about 30db C/N (=int on chart) and an small patch antenna on the roof that gave about 50dB C/N (=ext). For the LEA-6T I also have set mode 0 that is mobile mode (=nf on chart) and also done a survey and set mode 2 fixed (=f on chart). A note: I have default mask angle 5 degrees and maybe a higher value should have given better results especially as I am in Sweden that is quite far north.

If you use the GPS module standalone the ADEV give a good indication of what you can get if you test for example a counter. If you have 1E-8 at 1sec you can get about 8digits average and 7digits with normal measurement confidence at 1sec gate time (Frequency plots often seems to jump about a factor ten more than the ADEV). At 100 sec ADEV is 1E-10 and for 100sec gate time you get 10digits average and 9digits with confidence.

For using a GPS module standalone I haven´t seen any DIY designs of hardware sawtooth correction. Otherwise sawtooth correction probably gives an extra digit.

I enclosed the MDEV chart as I think MDEV is useful to see possible limits when designing GPSDO controllers with PI-loops and a Pre-filter that take away part of the sawtooth errors.

At short time constants (10-100secs) sawtooth correction probably is very useful for a GPSDO. At long time constants (1000-10000secs) a pre-filter probably is nearly as efficient I think. I have never seen a hanging bridge longer than 300secs. Also at Tau longer than 1000sec the GPS errors (changing satellites, ionosphere etc) starts to get in the same region as the filtered GPS module errors I think.

My own rule of thumb is also that AT cut OCXO´s (and good VCTCXO as DOT050V) need time constants in the GPSDO of about 10-100secs, SC cut OCXO 100-1000secs and Rubidium 5000-20000secs to match good GPS receivers.

Lars

Hi

If you use the sawtooth correction out of the 6T to take out the sawtooth issues, the ADEV plot drops down well below 1x10^-9 at 1 second.

Bob

[lars]

Here is my ADEV and MDEV with sawtooth correction on the LEA-6T module. I used the HP5370B and subtracted the values I got from uCenter for TPerror afterwards.  I also did ADEV and MDEV curves for the values on TPerror  from uCenter as I saw wiggles on the sawtooth corrected curve. The wiggles is also seen in the correction data, I don´t know why.

Lars

[DimitriP]

Great stuff! 
As for the wiggles, we had them appear at another thread (Hi Ed and VGkid) and we couldn't get rid of them no matter what was suggested and tried .. Consider yourself lucky they only appeared once ! Hopefully you'll never see them again.

[uncle_bob]

Here is my ADEV and MDEV with sawtooth correction on the LEA-6T module. I used the HP5370B and subtracted the values I got from uCenter for TPerror afterwards.  I also did ADEV and MDEV curves for the values on TPerror  from uCenter as I saw wiggles on the sawtooth corrected curve. The wiggles is also seen in the correction data, I don´t know why.

Lars

Hi

Don't ask how many times I've run down this list:

1) Sawtooth is either on the edge I just got or on the next one

2) Sawtooth either subtracts from the reading or adds to the reading

Yes, one would *think* that going through all 4 combinations would not be required to figure out which combo was correct....

Bob

[Vgkid]

Those aren't wiggles, these are wiggles  . Actually it is a pretty good oscillation. 

[edpalmer42]

I'll fix those stupid wiggles.  Where's my C-4? 

[uncle_bob]

Those aren't wiggles, these are wiggles  . Actually it is a pretty good oscillation. 

Hi

Those wiggles are from a spur at 0.09 Hz off carrier.

Bob