[sdiy] crystal oscillator oscillator seeming to run at wrongfrequency

[Tim Stinchcombe]

> sorry if this question is off topic.
> I've been looking at a video generator board that has two crystal 
> oscillators on it, one is at 14.318 Mhz and 17.734 Mhz.
> ( these are for the ntsc and pal standards ).
> both use a 74lcx04 ic with 1Mohm feedback resistor and 22pf 
> caps. but the frequency of the 14.318 crystal measures 14.322 
> , the other is 
> much closer to the 14.318.

Assuming: here you really mean the PAL xtal is closer to its value of
17.734MHz than the NTSC xtal is to its 14meg value; that both oscillators
*are* running and giving something close'ish to where they should be; that
the circuit design is known to work; and ...

> the chip the clocks drive is the  Epson S1D13806

...the technical manual for this being an impressive 730 pages (= whoa,
complicated!), I would venture a guess and say that almost certainly your
problem lies elsewhere!

> I just wondered if it is possible that this might be a design 
> fault with 
> the circuit ?

However... the Pierce oscillator circuits you mention (simple inverter+large
feedback R+two caps), are pretty cheap and cheerful oscillators that will
get you close to the nominal xtal frequency, but you'd probably need
something a bit more involved to get it *exact*. Hence on the basis that the
circuit is designed to work with these, I again would be willing to bet that
as long as they are running and appear close to what they should be (as
measured say on a reasonably capable scope), they are unlikely to be the
source of your problem. Though having said that, I note the evaluation board
in the manual uses a considerably more robust clock 'regime': a little
'clock module' for the 17meg, and a more complicated programmable clock
generator/synthesizer for the 14megs, so it is possible the 'slopiness' in
the frequencies of the Pierce set-ups is just not good enough...

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk
www.timstinchcombe.co.uk