QUESTION: What does this Thermistor do??

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K1JJ:
Ola,

I have a solid state 6M transverter that has some drift in the 22 Mhz crystal oscillator. The manufacturer sent me a thermistor to correct this problem.  

As per their directions, one side of the thermistor body is soldered to the metal outside shell of the xtal and the other lead goes to +12Vdc.   The metal shield of the xtal is floating.  I measured infinite resistance from the xtal shell to ground, so there is no DC path through the thermistor to ground.  

So what is it actually doing here?  All I can figure is that the thermistor is like a variable resistor from the xtal shell to B+.  What does that do to stabilize the xtal circuit???

I actually had my own homebrew heat oven in there, but figured this was a better solution when I was told about it...

T

WA1GFZ:
Tom, they saw your get up and decided to see if you would actually try it.
actually you hace the thermister in series with the case C

K1JJ:
Quote from: WA1GFZ

Tom, they saw your get up and decided to see if you would actually try it.
actually you hace the thermister in series with the case C



It must work - cuz they wouldn't screw with Elvis!

So, what is actually happening in the circuit to null out drift?  As the thermistor gets warmer it shunts the case so ground and pulls the osc lower in freq, or something like that?

The thermistor is a 60 degree part according to the sheet.

I've been watching the osc freq and so far I'm not impressed. It starts out cold at 19.99989  and works its way up to 20.00013 within 5 minutes.  Then hovers around 20.00016.  Does not seem much better than before and worse than when I used an oven.  I can adj the trimmer to put it on 20.00000, but in the warmup mode, 200hz drift is noticable on ssb.

This is for long term stability AFTER warmup, right?

T

Pete, WA2CWA:
Quote from: K1JJ

Ola,

I have a solid state 6M transverter that has some drift in the 22 Mhz crystal oscillator. The manufacturer sent me a thermistor to correct this problem.  

As per their directions, one side of the thermistor body is soldered to the metal outside shell of the xtal and the other lead goes to +12Vdc.   The metal shield of the xtal is floating.  I measured infinite resistance from the xtal shell to ground, so there is no DC path through the thermistor to ground.  

So what is it actually doing here?  All I can figure is that the thermistor is like a variable resistor from the xtal shell to B+.  What does that do to stabilize the xtal circuit???

I actually had my own homebrew heat oven in there, but figured this was a better solution when I was told about it...

T

Sounds like Jersey Black Magic to me. But now you have a do-dad hanging on your crystal shell. Thermistor is floating one lead(case) in air. Can't see how anything would change the stability of the crystal. I think you need more input from them.

WA1GFZ:
Tom,
I've seen a lot of compensation tricks but that is a new one on me.
I have seen how the case termination effects frequency.
I would use the thermistor and a varactor diode to compensate the circuit.
Every crystal has a temperature where it oscillates at the highest frequency. above and below that temperature the frequency drops.
This temperature is usually 25 to 35 degrees C. First you need to find out which way you need to go after the circuit has warmed up. Changing the bias current or voltage to the case sounds like it is missing something.
fc

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