[time-nuts] questions on uncompensated crystal oscillators

[Magnus Danielson]

From: SAIDJACK at aol.com
Subject: Re: [time-nuts] questions on uncompensated crystal oscillators
Date: Wed, 5 Jul 2006 15:25:46 EDT
Message-ID: <d6.3c2e6b30.31dd6c3a at aol.com>

> Hi Magnus,

Hi Said,

> 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.

Yes, it is hard.

PAL is 4433618.75 +/- 5 Hz or +/- 1.12 ppm.
NTSC is 3579545 +/- 10 Hz or +/- 2.79 ppm.

That is straight out of ITU-R Rec. BT.470-6.

The MPEG transport stream is clocked at 27000000 +/- 810 Hz or +/- 30 ppm.

That is straight out of ITU-T Rec. H.222.0

> 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)?

The idea for such boxes was rather to use the externally supplied frequency as
reference and avoid calibration at factory.

> 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.

Shruder! As I work higher up in the foodchain, I have higher demands on my
products and thus gets a little blind to these massvolume/lowcost issues.

> 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.

:P

You know, you are really demorilizing me now. Stop it! :P

> 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.

(Sufficiently) Good counters are cheap enougth. Integrating them into the
production line and the whole checkout procedure is another thing, especially
in larger volumes.

Cheers,
Magnus