[time-nuts] questions on uncompensated crystal oscillators

SAIDJACK at aol.com SAIDJACK at aol.com
Wed Jul 5 19:25:46 UTC 2006


 
In a message dated 7/5/2006 12:00:27 Pacific Daylight Time,  
cfmd at bredband.net writes:

Recall  that AT-cut crystals have a third-degree  temperature-to-frequency
curve.

> 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.

Actually,  when running in such an application you can autotrim very cheaply.

The  naive approach would be just to lock it (through a software loop) and  
then
use holdover (i.e. just keep the last frequency correction prior to  breaking
the loo).

A little more advanced approach would be to  correlate the needed correction
with that of the temperature. You can  either do a lock-up table or better 
yeat,
figure out the 4 unknowns in the  third-degree equation

f = a*T^3 + b*T^2 + c*T + d

and then use  that as a separate correction method. Whenever you are tracking,
you also  record the required total correction for a few different 
temperatures.
When  you have recent measures for four different temperatures, you can solve
the  equation.

T may be any unit really, so it hasn't have to be scaled to  match "real"
scales. It should just be whatever the temperature measurement  cranks out.

> When free-running (eg playing back video from a hard  disk etc), the  
> temperature compensation should work great to  keep the 27MHz within specs.

Indeed. By providing the temperature  compensation in combination with 
tracking,
the necessary tracking-dance  will be lowered.

Cheers,
Magnus



Hi Magnus,
 
the trick is to get away without any compensation if possible, since the  
Varicap diodes, temp sensor etc all cost money that consumer type products can't  
afford. Then, the crystal total deviation (temperature, aging, thermal 
effects,  transport-related changes etc) has to be within NTSC or PAL spec (I think 
its  <= 50ppm). This is the tough part to do for cheap.
 
Even if you get the crystal adjusted at the factory to say 5ppm, can it be  
guaranteed that all the other effects combined stay at <45ppm over the  
lifetime of the product (these days it's about 6-12 months depending on the  warranty 
period)?
 
One would be surprised how many products ship that are way out of spec  (DVD 
players, Set Top Boxes, Game consoles etc). Only the TV's very wide range  of 
color-carrier-lock capability prevents more isses (e.g. a black an white  
picture). Typical TV's can lock up to 100ppm or so.
 
BTW: doing Audio/Video Synchronization using the MPEG time stamps is  
something that not all DVD players and Digital TV Set Top Boxes do. The ones  that 
don't have to skip/repeat a frame every so often and loose lip-sync after  some 
time. It's surprising how many name-brand products don't do the AV-sync at  
all to save BOM cost.
 
I personally know of a large Asian manufacturer that shipped hundreds of  
thousands of units that were totally out of spec (due to a component value  
problem).
 
During production they tested with TV's that would be able to lock to the  
bad color carrier without any issues, but in the field the frequency was so far  
off that almost all units failed. This cose them millions $$ to fix. A simple 
 frequency counter test at the assembly line would have prevented this.
 
bye,
Said



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