[time-nuts] UPDATE: DATUM 9390-52054 Grief again...

Burt I. Weiner biwa at att.net
Mon Apr 15 12:35:27 EDT 2013


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.
biwa at att.net
www.biwa.cc
K6OQK  



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