[amsat-bb] AIDC 3722 Downconverter Crystals and Stability

[BobsImsai8800@xxxxxxx]

>From K5GNA:

I have experienced the frequency jumping that Joe is seeing.  I have measured 
the frequency with a synthesizer into the downconverter and back out to a 
counter, all locked to the same Rubidium standard.  I have also measured the 
frequency directly into the counter from the LO.  Both show the short term 
jumps that you are seeing.  

What I am not observing is a change in the beat note when I receive a signal 
from the synthesizer.  I use the synthesizer with a rubber duckie antenna to 
generate a signal so I can find my frequency on 2401 MHZ, then I will know 
where the satellite is.  The beat note does not seem to jump around in the 
same manner as the counter would indicate. Possibly we are observing noise or 
FM that the counter is seeing, causing a miscount in the frequency counter.  
Now that my synthesizer needs alignment, I can't perform this test again 
immediately.  I am hoping someone else can observe the fairly stable beat 
note -- and not measured against the satellite.

The temperature stability has always been my question, but when the 
downconverter is run sealed, it doesn't change that fast. It does change with 
temperature, as would be expected.  The sealed environment with the built in 
heater of the downconverter's power supply means that it is fairly stable in 
frequency with temperature.  Leaving it continuously on is normal for these 
downconverters.

In order to compensate for temperature changes, like DEM and SSB do,  I have 
placed some Keystone 60 degree Centigrade PTC thermistors on the original 
crystals and run them.  I have soldered them to the crystal and also used 
heatshrink with thermal compound.  The thermistors stabilize at 60 degrees C 
quickly and the current is only 30 - 40 MA at 12 VDC.  The units are 
available from DEM and Mouser Electronics. If you want to learn more about 
the PTC thermistors, go to DEM's web site, look up the thermistors under 
their components and click on the device to get a nice spec sheet from 
Keystone. 

With temperature stabilization, I am looking at the same small frequency 
excursions as without any temperature stabilization, so changes in crystal 
temperature can be ruled out as a cause of the rapid frequency jumping.

As the temperature of the crystal and thermistor go up, the frequency goes up 
a little, then it goes down a little, then it goes way up.  This would 
indicate that the crystal has a temperature curve with a knee near ambient. 
An ideal knee would be 50 - 60 degrees for the thermistor.  The frequency 
goes up about 45 KHZ when the crystal temperature is raised to 60 degree C by 
the thermistor. I have also observed that the total range of the trimmer 
capacitor is about 200 KHZ.  With 1/2 mesh on the trimmer there is about 80 
KHZ difference (or 120 KHZ depending on where you start).

I talked to Jan Crystals today and they have evaluated the crystal I sent and 
have the real spec's on the crystal, so they can be ordered from them for 
$20.00 each plus postage.  I am looking into ordering 100 of the crystals, at 
10 parts per million tolerance, cut for a 60 degree PTC thermistor.  If you 
want one immediately from Jan (800- 526-9825), you might want to order them 
for ambient instead of 60 degrees C, but tell them it is a TSI AIDC 3733 and 
what temperature you want. It will take 6 weeks to get my order of 100, a lot 
less if you order from them.  As soon as I decide, I will be taking 
reservations for ordering the right crystals, probably with PTC thermistors 
at a little less than the single quantity price. 

The Engineer left with the notes today before I called Jan, so it will be 
tomorrow before I can post the crystal's real specifications.


TNX & 73,

Bob Seydler
K5GNA
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