[time-nuts] Brooks Shera's GPS locking circuit

Brian Kirby kirbybq at bellsouth.net
Thu Dec 14 02:43:14 EST 2006


I would like to share my experience using the Brooks Shera controller 
with a rubidium oscillator.  3 Years ago, I decided to discipline a 
Efratom FRS-C rubidium.  For a 1PPS timing reference, I am using a 
Motorola M12+ GPS receiver.

 

Starting with the rubidium, which was new old stock on EBay and part of 
an AT&T timing sub-system, I burned it in for about 3 months to 
stabilize it and to find its center control voltage point, with respect 
to UTC.  When centered on UTC (we'll it was somewhere like -8.8x10-13), 
I found that dialing the EFC up one volt and down one volt (reference 
the UTC setting) gave a plus and minus change of 5x10-10.  UTC in my 
case was around 2.1 volts on the EFC line.  So I made the following 
assumptions on EFC: 5x10-10 for a 1 volt change, 5x10-11 for a 100mv 
change, 5x10-12 for a 10mv change, 5x10-13 for a 1mv change, 5x10-14 for 
a 100uV change, etc.

 

Using the original Shera controller (because of its voltage sensitivity 
design) , I knew that I would need an amplifier, an offset voltage 
generator, and a summing amplifier and a buffer.

 

I built an variable voltage high stability reference, designed to give 
me a voltage from 1.5 to 2.5 volts.  I built a variable gain amplifier 
that gave voltage gain from x9 to x20.  This unit was constructed using 
the LM299 voltage reference and OP07 op amps.

 

After some early data, I found the 30 second update rate for the DAC 
output in the controller, was too short for the rubidium.  I contacted 
Brook's and we worked on some details, and he was able to change the 
gain of the controller to a voltage sensitivity of 1x10-9.  We also 
programmed the PIC for a 120 second update rate for the DAC output, and 
to average 120 input samples.

 

I was able to remove the voltage gain amplifiers from the circuits.  The 
circuit was further refined to have a fixed voltage referenced of 
2.000x... volts and a single summing amplifier (one input for the Shera 
controller, the other input for the 2 volt reference.  I used a LM399 
voltage reference and LTC1002 op amps for the interface circuitry.   
These circuits use 0.1 percent resistors and a lot of attention was paid 
to, stability, low thermal EMF/Kelvin type problems, grounding and 
induced noise, etc.  It was quite a learning curve.

 

 I found early during the test, that the power supply had to be very 
stiff and quiet.  My final units start with +/- 24 volt regulators, then 
some noise filters, then some precision +/- 12 volt regulators (using 
precision references), and they feed Shera's board which uses some 
common LM2940/2990 regulators.  My notes show the voltage drift was 
maintained to around 1 mV.

 

I also learned the limitations of my test equipment, trying to test 
these precision oscillators.  For references, I have a FRK-L that is on 
UPS power and its in a temperature controlled environment, I also have 
two HP Z3801A GPSDOs, 1 Motorola M12+ GPS receiver, and 4 Motorola 
Oncore VP GPS receivers.  I use a HP53131A time interval counter, with 
TAC32 Plus software, as the measurement system.  What I found, is the 
53131A noise floor dominates the measurements below 100 seconds Allan 
deviations when trying to measure the precision oscillators.  As this is 
a serious hobby and I have financial obligations, I cannot afford a dual 
mixer test set and a real primary frequency standard.   So I have to 
test with what I can work with, and realize that I cannot achieve 
certain gains.  So, if you plan to become a "time nut", baseline your 
time interval system, and make sure you can work in and report real 
world results (within your realized limitations).

 

And I also had fun baselining all the GPS receivers, which are above the 
noise baseline of the 53131A.  You may find one that stands out and I 
found some minor quirks between same units.

 

Because I did not have a dual mixer test set and I realized that I was 
needing instrumentation that would handle below 100 second variances, I 
decided on using a simple phase detector, by using balanced mixers, lo 
pass filters and a strip chart recorder.  As a visual guide, one channel 
of the recorder was feed from the disciplined rubidium and the other 
channel was feed from another Z3801A and both were referenced to another 
Z3801A.  So I could see perturbatations relative to each other.  I found 
the 120 second PIC seemed to have a temperature problem, and the 
disciplined rubidium was much more noisy compared to an undisciplined 
rubidium and the Z3801s in the short term.  At that time, I had no way 
to digitally record any data from the phase measurement system.  I now 
have a Fluke 45 multimeter, that has RS232 outputs and I can record the 
phase detector voltages and convert the voltage phase data back to 
meaningful results.  Characterizing the Fluke 45 is also another story....

 

I decided to change to a 60 second controller - and at this point I 
really started keeping some real detailed data and records.  I ran 
variance test for weeks at a time using one second data and started 
running variances curves above 100 seconds to 86400 seconds.  Some of 
these test took 12 weeks to get meaningful data on the "daily" variance 
- and I have to wonder about little introduced biases, such as the 
weather.  Then we learn that our 1 PPS reference dividers also have 
temperature drifts....  Did I tell you the basement has it own air 
conditioner and heating system, all this equipment is running on AC 
voltage regulators and UPS systems ?  And with the recent energy cost, I 
really need to shut this project down....

 

OK, here is some TIC data from the project, using 128, 1024, 8192, and 
65536 second variances:  And the baseline noise floor.  All are modified 
Allan variances.

 

Noise floor  3x10-12, 4x10-13, 5x10-14, 7x10-15

FRK-L rubidium  vs M12 GPS receiver 1x10-10, 1x10-11, 2x10-12, 1x10-12

FRK-L vs FRS-C (undisciplined) 3x10-12, 6x10-13, 7x10-13, 6x10-13

FRK-L vs FRS-C disciplined 4x10-12, 7x10-13, 9x10-13, 9x10-13

FRK-L vs Z3801(A) 4x10-12, 3x10-12, 1x10-12, 1x10-12

FRK-L vs Z3801(B) 4x10-12, 4x10-12, 1x10-12, 2x10-12

 

Noise floor 3x10-12, 4x10-13, 5x10-14, 7x10-15

Z3801(A) vs M12 1x10-10, 1x10-11, 2x10-12, 2x10-13

Z3801(A) vs FRK-L 4x10-12, 3x10-12, 1x10-12,3x10-12

Z3801(A) vs FRS-C 3x10-12, 2x10-12, 1x10-12, 2x10-12

Z3801(A) vs FRS disciplined 4x10-12, 3x10-12, 9x10-13, 1x10-13

Z3801(A) vs Z3801(B)   4x10-12, 6x10-12, 2x10-123x10-13

 

Noise floor 3x10-12, 4x10-13, 5x10-14, 7x10-15

FRS-disciplined vs M12  1x10-10, 1x10-11, 2x10-12, 2x10-13

FRS-disciplined vs FRK-L 4x10-12, 7x10-13, 1x10-12, 1x10-12

FRS-disciplined vs Z3801(A) 4x10-12, 4x10-12, 2x10-12, 3x10-13

FRS-disciplined vs Z3801(B) 4x10-12, 2x10-12, 1x10-12, 2x10-13

 

It appears my limitation is the references, mainly the Z3801s.

 

I am in the process of recording 1, 10, and 100 second phase data with 
the balanced mixer and voltmeter system.  The 1 second data is 
completed, but the 10 second and 100 second test are in progress.

 

FRS-disciplined VS Z3801(A) is 1.6x10-10 for 1 second.

 

Its been fun, but I have also learned to be patient, and I consult with 
several friends who use to be in the test equipment and instrumentation 
calibration business.  Let me point out, that a lot of us time nuts are 
"amateurs" and in my case, I cobbled up what I could afford and find.  I 
was lucky that I got a head start in precision timing as I got to use 
cesium beam clocks when I was a younger man in the Air Force - and at 
that time, getting real knowledge seemed to be limited - it was books, 
reports and who you knew - the internet has changed our world, as we 
knew it ! My later experiences came from the telecommunications 
industry, which is very dependent on precision timing.  I use to head up 
a chronic problem team on high speed data circuits and one of the first 
things you check is the timing configurations of the circuits.  I later 
worked with NASA at a mission control facility and again, precision 
timing for the instrumentation and communications.

I have somewhat retired and because of the 401K rapes and corporate 
failures, I have went back to work and I maintain radio systems - about 
the only timing I run into nowadays is a GPSDO for base station 
references.  I sure miss having the precision gear to test with.

 

It boils down too now having precision frequency references that I 
thought I would never be able to afford in my lifetime.  I attribute it 
to the availability of the GPS system, and the constant improvement and 
reliability of electronics.  What do I use the systems for ? - I drive 
several frequency synthesizers and frequency counters on the bench.  I 
believe that I can warrant my frequency generation and measurements to 
reflect the x10-10 level minimum, right now, instantly, and x10-12 level 
per 24 hours.   And this is reference to UTC for real.  Not too long 
ago, you would be lucky, if your  counters time base could hold x10-9 
performance over a year and you had everything slaved to it.  UTC 
reference - well, when it was sent off and calibrated....

 

 

I look forward to our discussions on the Time Nuts list server - we have 
got a great crowd here and its small, tight and right.

 

Brian - N4FMN

 

 

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