[time-nuts] questions on uncompensated crystal oscillators

SAIDJACK at aol.com SAIDJACK at aol.com
Wed Jul 5 02:48:33 EDT 2006


 
In a message dated 7/4/2006 09:52:33 Pacific Daylight Time,  stans at sycard.com 
writes:

Hello,
Please excuse the fact that this is not what we all  consider precise
frequency.  I am selecting crystals to use for a  medium to high volume
application. 
The basic criterion are: 
- used  with a VLSI inverter based oscillator with
3.3 V supply, but  somewhat high impedance output. 
- frequency: 27.000 MHz fundamental within  40 (or so) ppm over
temperature including 7 to 10 years of  aging. 
- HC49S case 
- no production line trimming 
- low cost  

After looking at the design issues, I wonder if some of you
haven't  faced similar designs and have some suggestions
regarding the following  issues: 




Hi Stan,
 
I have designed a circuit that does exactly this, and published it in EDN.  
See page 92 for the article:
 
_http://www.edn.com/contents/images/112703di.pdf_ 
(http://www.edn.com/contents/images/112703di.pdf) 
 
It's a VCXO 27/32MHz reference for a Video Decoder. You can delete the  
Varicaps if you don't need it to be adjustable.
 
BTW: The crystals and inverter used in the design are quite good, the  
circuit generates less than 3ps jitter (measured on a Wavecrest  SIA-3000) - jitter 
is really what counts in digital timing for video  etc.
 
Some caveats to watch out for:
 
   * for NTSC you can have up to about +-840Hz deviation and  still be within 
limits.
 
   * Don't overdrive your crystal (see manufacturers spec)  otherwise it can 
age too fast, and get damaged. Typically, use about 100uW or  so. Measure the 
current into the crystal using a fast AC current probe
 
   * Make sure you have enough gain to guarantee startup (the  circuit should 
work with at least 3x series resistance as will be used  in production). This 
will mostly depend on the crystal ESR (should be as low as  possible), while 
still not overstressing the crystal
 
   * 10ppm crystals are very(!) expensive, rather use trimming  during 
production testing by removing/adding load cap options. This allows you  to use a 
cheaper crystal (with a higher ppm rating),  and spend more money on lower 
thermal susceptability specs.
 
Hope this helps,
bye,
Said
 
 
 
 
 
 


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