[time-nuts] questions on uncompensated crystal oscillators

SAIDJACK at aol.com SAIDJACK at aol.com
Wed Jul 5 14:12:48 EDT 2006


 
In a message dated 7/5/2006 08:30:44 Pacific Daylight Time,  
johnday at wordsnimages.com writes:


I  would agree with you Said, 10ppm is a lot to ask and let's be 
honest,  production trimming is not so difficult these days. Without 
going to 10ppm  you can get your crystal manufacturer to group your 
crystals by cut angle.  This will give you a basic temperature 
coefficient curve - for a given  angle the tempco curves are quite 
similar. We used to do this with  fundamental mode crystals at between 
12 and 20MHz.

You can then  have a set of compensation tables and drive the varicaps 
from a DAC to  keep the crystal under control. I have done this using 
small micro's, like  the Zilog Z8, or using a bit of really crude 
ADC-Table-DAC circuitry in an  FPGA and it works quite well. This was 
done to avoid having to use TCXO's  in SCADA/DATA type radios.

For higher performance equipment we used to  have the compensation 
table built by the test system during temperature  cycling.

John 





Hi John,
 
great idea to compensate for temperature using the varicaps in a  
free-running system.
 
The basic design was for locking an MPEG video stream to a  broadcaster's 
27MHz master clock using the MPEG time stamps in a digital PLL  loop.
 
When free-running (eg playing back video from a hard disk etc), the  
temperature compensation should work great to keep the 27MHz within specs.
 
bye,
Said
 
 


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