[time-nuts] LTE-Lite module and the pendulum...

Burt I. Weiner biwa at att.net
Tue Oct 21 11:54:00 EDT 2014

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

>From: "Poul-Henning Kamp" <phk at phk.freebsd.dk>
>Subject: Re: [time-nuts] LTE-Lite module
>In message <9BC23A13-646F-49C6-9FF9-D42FA5EC835D at 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
>phk at 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.
biwa at att.net

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