Gang,
You will remember a few years ago I had problems with my DATUM
9390-52054 GPS Time Code and Frequency Standards. For those that
were not involved at the time but might be curious about the history
of my grief with these units, I have included one of my epistles for
background purposes at the bottom of this newer epistle. That said...
About a week ago one of the two modified DATUM 9390-52054 (herin
after known as "The Unit") lost lock and went about 23 Hz
high. Checking, I discovered that the DAC went from its normal
number of about 28000 to 65775, which is all the way to the stop. In
addition, the EFC voltage went from its normal 2.08ish volts to 4.76
volts. My first test was to power cycle the unit and see what
happened. What happened was that it was still a mess. I tried a
reboot which brought it back to a good starting point and it looked
like it was going to work fine. However, once it got down somewhere
near 10E-12 it went bonkers and dove off the deep end as described above.
My next step was to take my trusty 5k POT (see below) and connect it
into the circuit as described below. I was able to bring the EFC
voltage to 2.08 volts and have the oscillator go to almost dead on 10
MHz. It sat there for a few hours with no problem. I tried tapping
on things to see if I could make it act up. I was convinced that
something had gone wrong with the McCoy VCXO, but in all aspects it
behaved like it was supposed to. A replacement VCXO was a few days
away so while I was waiting I continued to futz around with the
unit. I even replaced the components associated with the flywheel,
the 5100 Ohm series resistor and 47 ufd capacitor. By the way, in my
epistle below I said it was a 4.7 ufd capacitor, my typo, it is
indeed a 47 ufd electrolytic capacitor. The original capacitor
checks fine in my Sencore LC-102, and the 5100 Ohm resistor also
checked fine for value and noise under load.
So, what happened? It had been working fine for a little over two
years since the units last surgery. That night as I laid in bed
awake pondering this, I kept thinking that the symptoms were pretty
much the same as before I put the flywheel in two years plus ago.
The next morning when I got up I modified the flywheel. Same
circuitry, but instead of the EFC voltage feeding in the through the
5100 Ohm resistor towards the 47 ufd capacitor, which is hung across
the EFC terminal to the ground terminal of the oscillator, I replaced
the 5100 Ohm resistor with a 18k resistor - just to see what would
happen. Since making that one change the unit works flawlessly and
no matter what, I can't make it misbehave. I ran it through several
days of power cycling, resetting and other graceful abuse and it
always come back like it should. - Remember, I said this before!
Yesterday morning it was at 1E-12 so I powered him down, disconnected
all my monitoring paraphernalia, put everything all back together and
stuffed him back in the rack. Within 30 minutes he was at
~300E-12. I then did a master reset which reset everything,
including the EFC to nominal center of about 2.48 volts. After about
18 hours he's sitting at 0E-12. The DAC is now sitting at 31794 and
the EFC voltage ahead of the 18k resistor is at 2.41 volts. The EFC
voltage at the EFC pin on the oscillator is at 2.03 volts. Any noise
on the EFC line is below what my trusty TEK 453 can see with his gain
cranked wide open.
Why do I all of a sudden need a bigger flywheel? Did the VCXO
oscillator develop hiccups that I missed? Did the DATUM develop
hiccups that I haven't seen? Who knows.
Below is the main gist of the issues from about two years ago.
Burt , K6OQK
Here's from a few years ago:
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 constants 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 and 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
Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California U.S.A.
b...@att.net
www.biwa.cc
K6OQK
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