[time-nuts] questions on uncompensated crystal oscillators

Stan Searing stans at sycard.com
Tue Jul 4 16:52:35 UTC 2006


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: 

 - Is there a free (or inexpensive) spice that works well
simulating crystal oscillators? 
(I did see the submissions to this email list last September on LTSpice.) 
(I have tried using the SIMetrix Intro package,
and it generally seems to produce good results,
but I found it's awkward or impossible to measure
frequency to lots of precision (taking an FFT helps),
and some results regarding oscillator frequency
shifts as a result of small delay changes in the
inverter seemed questionable to me.)

 - Are there some good papers dealing with the practical aspects of:
    - transforming the Q limiting element of the design from
      a series R to a parallel R
    - Rs vs. ESR
    - exceeding the correlation drive level of a crystal
    - measuring the drive level & calculating the maximum drive level
    - negative R and oscillator startup margin
    - consumer video color subcarrier tolerance

 - Do you have any suggested suppliers for crystals with the
   following specifications: 
   - HC49S
   - 27.000 MHz
   - 10 ppm tolerance
   - 10 ppm stability
   - 18 pF load
   - trim sensitivity less than 15 ppm/pF
   - 7 (or better yet 10) year aging less than 10 ppm
   - drive level up to 1 mW (may not need to be this high if
     the ESR is < 30 ohms or I better understand our maximum
     inverter output voltage)

If someone is interested in providing comments on the app note
I'm working on, that would also be appreciated.

Thanks 
Stan Searing






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