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 > _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts