[time-nuts] questions on uncompensated crystal oscillators
SAIDJACK at aol.com
SAIDJACK at aol.com
Wed Jul 5 06:48:33 UTC 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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