Burt, Great insight thanks. You nailed it: out with the old oscillator and in with one that doesn't have that problem.
Btw the mechanical tuning issue you mentioned is essentially the same exact problem: even the slightest turn will make the frequency jump too high or too low. It can drive you (and the loop) crazy trying to get it on-frequency. Bye, Said Sent From iPhone > On Oct 21, 2014, at 8:54, "Burt I. Weiner" <b...@att.net> wrote: > > I've been following this thread with some interest. I have no idea what a > LTE-Lite module is, but I believe the issues being discussed is essentially > the same issue that I had a year or so ago when I had to make repairs to my > two DATUM 9390-52054 GPS references. At that time I copied this list on the > various steps from discovery of the power supply noise grief to further > discovery of problems with the original factory supplied internal Vectron > VCXO oscillator module. > > After replacing the internal switching power supply with an outboard Cisco > unit, I went on to look at what I felt was instability of the 10 MHz > reference. According to the front panel display, the error would wander > anywhere from 0E-12 to 50 or 100E-12. For my use, this wasn't a major > problem, but one that bothered my instinctive curiosity and another step in > my life in searching for a way to improve things. > > The original oscillator module in the 9390 was a Vectron 716Y2690. This has > a frequency trim adjustment on the side to bring the oscillator into tracking > range for the DATUM 9390. In one of my two units the adjustment would jump, > which I attributed to a defective trimming capacitor. My friend Stu, K6YAZ > had previously given me two McCoy MC597X4 VCXO modules that do not have a > frequency adjustment other than by way of the EFC control. Looking at the > specs on these modules it looked like they might almost be electrically a > drop in replacement for the original Vectron modules, although the McCoy's > were about one-quarter the size. The McCoy's require 5 volts Vcc rather than > 12 volts that the Vectron required. Not a problem. Testing confirmed that > the EFC tuning voltage indeed went the same direction the McCoy requires. > > Since I don't have the sophisticated equipment that many of you have to > comparatively confirm stability, I decided to modify only one of my 9390's > and compare the results to the other one. The two 9390's have separate > antennas mounted about 3 feet apart and in a pretty clear view of the sky. > > I stuffed the McCoy module in place of the Vectron but instead of connecting > the EFC lead, I used a 1k pot with the top connected to 5 volts through a > small resistor, the bottom to ground, and the arm to the EFC pin on the > McCoy. Using the other 9390 for comparison, I was able to determine that in > order to have the McCoy output 10 MHz, the EFC voltage wanted to be slightly > under +4 volts, essentially the same as the original Vectron. Great, what > could go wrong? I shut everything down and connected the EFC control voltage > to the EFC terminal on the McCoy. As the McCoy came up to temperature I got > a tracking light and the 10 MHz spigot came nicely onto 10 MHz, sat there and > then wandered off frequency and after a while came back and overshot in the > other direction. I figured this would be a process that would go on for a > day or two and the "pendulum" would eventually settle in. After several days > this did not happen and the 9390 gave me a tracking error. Apparently, the > time co nstants in the loop and the sensitivity of the EFC control in the McCoy did not play well together. Pondering the situation I decided to slow down the EFC voltage change. I did this by putting a 4.7 uf capacitor across the EFC pin to the ground pin and fed the EFC voltage to the EFC pin through a 5100 Ohm resistor, essentially, in my opinion, hanging a flywheel across the EFC line to the McCoy. Since with the smaller McCoy I had additional space within the 9390 I also made a sandwich type enclosure out of foam for the smaller McCoy to help isolate it from tempreture changes. I let the unit run for about 24 hours and noted that it had settled in nicely and sat, according to its display, at 0E-12 for well over the next 24 hours. Comparing this to my stock 9390, this appeared to be correct except for some small amount of wandering - the stock unit was showing variations of 1E-12 to about 10E-12, the amount of drift they had both always shown. I watched this for about two weeks an d while the modified 9390 sat at 0E-12, the stock unit continued to show the same amount of drift it always had shown. > > I modified my second 9390 with the other McCoy VCXO and now the two units sit > within 0 to 1E10-12, and comparing the two using both a 1:1 Lissajou and > separately using one to trigger a scope that's monitoring the other, I > believe things are much improved. In the year plus since I've modified these > two units they've sat quite steady and have survived some deliberate power > interruptions just to see what would happen. I have detailed pictures if > anyone is interested. > > I don't know if the above offers any input of value, or even how scientific > it is according to "deep" Time-Nuts standards, but it's what I did. > > Burt, K6OQK > >> From: "Poul-Henning Kamp" <p...@phk.freebsd.dk> >> >> Subject: Re: [time-nuts] LTE-Lite module >> -------- >> In message <9bc23a13-646f-49c6-9ff9-d42fa5ec8...@aol.com>, Said Jackson >> writes: >> >> >Then at some point the crystal 'snaps' and jumps in frequency, overshooting >> >the desired frequency and causing the P term to start pushing in the >> >opposite >> >direction repeating the cycle. >> >> If your hardware does not respond to the output, any PI(D) loop will go >> bezerk, and there's nothing you can do about it. >> -- >> Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 >> p...@freebsd.org | TCP/IP since RFC 956 >> FreeBSD committer | BSD since 4.3-tahoe >> Never attribute to malice what can adequately be explained by incompetence. > > Burt I. Weiner Associates > Broadcast Technical Services > Glendale, California U.S.A. > b...@att.net > www.biwa.cc > K6OQK > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
