On Wed, 21 Jun 1995 05:55:01 GMT, Brian Gaff Sam Dept. said:
> I wonder if the crystal is duff? This
> would certainly screw it up! If its drifted, it could still be
> working but some sets would be able to cope, while others not.
It could be that. How can one test a crystal? Or, what are the chances
that the trimmer is to blame?
BTW I had it the wrong way round yesterday (not that it matters). I have a
data sheet for the MC13077, whereas the one in the Sam says MC1377P. The
1377 seems to have different pinouts from the 13077. Would anyone know what
else is different about it?
The 13077 has four different ways of generating the subcarrier. Each
one generates a 17.734475 MHz signal, which is 4xFsc where Fsc is the
subcarrier frequency of 4.4336MHz.
/
___||______|#|_______pin 8 1. Oscillator free run with crystal
_|_ || |#| ___pin 9
- / _|_
5-25 pf xtal -
17.73MHz
/
___||______|#|_______pin 8
_|_ || |#|
- /
5-25 pf xtal 17.73MHz 2. Oscillator phase lock with crystal
to subcarrier reference
subcarrier 1000pf
reference input____||__pin 9
||
220R
17.73 MHz____||_---____pin 8
drive || --- | 3. Direct drive of oscilator with
1000pf === 150 pf 4xFsc source
|
+----pin 9
_|_
-
___________|#|_______pin 8
_|_ |#|
- 17.73MHz resonator 4. Oscillator phase lock with resonator
to subcarrier reference
subcarrier 1000pf
reference input____||__pin 9
||
I have no idea why I just typed that in. However, I suppose the Sam must
use something of the form of (1) since that's the easiest. Is it possible
that the 1377 also requires a 17.73MHz but they put in a 4.43MHz one because
they are more common? Would that kind of thing work anyway?
It occurs to me that one reason for having a 4xFsc oscillator is so that
you can generate a signal whose phase differs by 90 degrees to encode the
R-Y signal.
[conspiracy theory: could it be that the Sams that have worked perfectly
actually do have 17.73MHz crystals in them? :-) ]
imc
