[time-nuts] questions on uncompensated crystal

[SAIDJACK at aol.com]

Hello Dr. Stellmach,

 

Great info! Thanks so much for the detailed analysis.
 
Some comments:

>   27MHz can be realized by fundamental crystal, but it's  on edge of
>   technology because the crystal is pretty thin  already. Therefore, a
>   tighter specification is critical and  costly, so use PLL design with
>   9MHz or less  instead!


Instead of a PLL, it's also possible to use 3rd overtone crystal with an LC  
tank circuit on the output side of the inverter (after the series resistor).  
This allows the crystal to be about 3x thicker.
 
You are absolutely right, fundamental crystals get very thin, brittle, and  
shock-sensitive above 24MHz. These days, there is very little price  difference 
between a fundamental 27MHz or a 3rd OT 27MHz though. Thus the LC  tank may 
actually increase the system cost by $0.05 - $0.1 versus a fundamtental  27MHz 
part; I have had different opinions from different crystal vendors in Asia  
about this (depending on their capabilities).

>   We often have discussions between the (digitally  minded) designers of
>   the inverter and the crystal supplier:  For us, an additional critical
>   aspect for series production  are oscillation start margin (which is
>   very crudely checked  by our hardware designers)


 
This is the biggest problem in my opinion. Infineon  (_www.infineon.com_ 
(http://www.infineon.com) ) has a  great application note on this on their website 
(how to guarantee  start margin in mass production etc).
 
You mentioned that you have "clock trimming by software correction":
 
How does this work in mass production? Do you measure the frequency during  
production at room temp, then add/subtract a number of seconds from the real  
time clock-time once a day or so to compensate for the offset?
 
Also, what kind of failure rates and failures do you see with such a large  
sample set in the field :)? Do you see problems with aging, mechanical  
vibration, the large temperature changes during day/night etc?
 
Thanks,
Bye,
Said