Another update... I wanted to post more in the hopes that it may
help someone else with similar problems.
I told you about adding the simple voltage divider to reduce the
drive to the circuits following the replacement of the original
oscillator module. When I thought all was well I put the DATUM
9390-52054 back in the rack and it locked up quickly. I soon
discovered that I didn't have enough output to lock some of my
externally referenced equipment. A comparison between my two 9390's
showed that the repaired one was about 15 dB low in output! Just for
fun I removed the voltage divider and ran the replacement
oscillator's output directly to the following circuitry, just as the
original oscillator did. Testing, which I fully expected to give me
the problem of driving the DAC and EFC voltage out of range, showed
that all was well and the 9390's output levels were within a dB of
DATUM's specs. So, what happened earlier that caused me to think it
was being over-driven. I haven't any idea! I've run it through
several power cycles, some overnight, and it always comes back up
with such happiness, and with the DAC around 28400 and the EFC at
2.15 volts - right where it's supposed to be. The only thing I can
think of is that I did something foolish in my initial testing with
the replacement oscillator. The unit has been sitting in the rack
for over 48 hours with no issues. According to it's screen the
frequency error is 1E-12 and the output level is where it should be.
Thanks to everyone for your input, you've given me a good
understanding of how this particular GPS module works. I hope my
experience will be of some value down the road. I'll keep you posted
on any further 9390 adventures.
Thanks,
Burt, K6OQK
I had earlier written:
When I powered the 9390 up everything seemed to work and the
oscillator was within 1-Hertz, except that the output was about 20 dB
higher than with the original oscillator was when it was working
properly. Once the receiver locked up to 4 satellites and started
"OSC Stabilizing", the oscillator suddenly jumped about 18 Hz low and
shortly afterwards the DAC went to all zeros. I let it sit all night
figuring that it would eventually find itself. Well, it didn't, it
just sat at 18 Hz low and the DAC at all zeros, and the EFC
(Electronic Frequency Control) voltage at 0.19 Volts. (What a
revolting development!)
Very early this AM I got up and put a simple 10:1 voltage divider
using a series 680 Ohm and 68 Ohm resistor from the oscillator's RF
out to ground This brought the levels pretty much back to the same as
my good 9390. I also substituted a pot for the EFC voltage. I used
a 1 K pot in series with a 300 Ohm resistor to the 5 Volt rail. The
resistor was on the high side, the bottom end to ground, and the
wiper to the EFC terminal. I was able to tune the oscillator to
precisely 10 MHz long enough to measure the substitute EFC at +2.15
Volts. Leaving it hooked up this way I let the receiver do it's
thing just to see what would happen. It went through it's normal
routine and finally the green lock light came on. Although the
oscillator wasn't being controlled by the 9390, the 9390 thought it
was happy and the DAC went to about 28,000 - very close to ideal. I
tweaked the pot hither and yon to move the frequency and to see which
way the EFC voltage went. It went the right direction to properly
steer the oscillator and should have worked.
Pondering the situation, I powered the 9390 down and reconnected the
9390's EFC voltage back up to see what would happen this time. This
time it locked up quickly and the oscillator homed in on 10 MHz. The
EFC voltage is 2.15 Volts and the DAC settled in nicely at 28302 -
very close to the recommended DAC numbers. It's been cooking for
about an hour now and is presently at 112E-12 and moving closer all
the time. When comparing the two GPS receivers I am not seeing the
random drifts that I had seen in the past.
I suspect that the problem with the frequency jumping to -18 Hz and
the DAC going to all zeros was the result of the new oscillator
over-driving the following circuitry. In hindsight, maybe I should
have probed a bit with the scope to see if that is a correct
assumption, but I didn't do that.
By the way, Stu sells the oscillators fully tested and with a note
giving the EFC voltage for each unit to tune it to 10 MHz. If this
turns out to be as good a modification as it now appears, I'm going
to get more of the oscillators from him. I may replace the one in my
other 9390 and for sure will replace internal reference oscillator in
my IFR-1500 Service Monitor.
Burt, K6OQK
Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California U.S.A.
[email protected]
www.biwa.cc
K6OQK
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