[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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