Re: Defeating the mantis part 3

On Wed, 23 Sep 2009 14:21:25 -0700, Joerg <invalid@xxxxxxxxxxxxxxx>
wrote:

Jon Kirwan wrote:
On Wed, 23 Sep 2009 11:47:03 -0700, Joerg <invalid@xxxxxxxxxxxxxxx>
wrote:

Jan Panteltje wrote:
I was testing with those cheap PAL camera modules.
ftp://panteltje.com/pub/PAL_camara_module_box_img_1585.jpg
Apart from the possibility of 2 different cameras with
2 different video channels and 2 LCDs hanging in front of your eyes,
was investigating if those cheap modules can be synced (H and V).

Syncing would make it possible to display alternate frames for left and right.

Fed 2 into the same video channel,
ftp://panteltje.com/pub/PAL_camara_module_in_circuit_img_1584.jpg
very slight difference in H,
H bar moving across the screen, takes from 8 seconds to 18 seconds,
depending on the module under test.

Tried to change supply voltage (within specs) to see if the 17 MHz (4x 4.43 Mhz
because 4 x is so nice to make 90 degrees phase shift in PAL) crystal
could be controlled that way.
The supply voltage has very little (read not enough) effect on the crystal oscillator frequency.
So had to open one up (last resort), was lucky, they used screws, not glue...
looking for a frequency sensitive spot, found one, it is marked C18:
ftp://panteltje.com/pub/PAL_camera_module_sensitive_spot_img_1590.jpg
that is where the wire is connected too, the other wires on the golden pins are ground, +5V, and video out.

So now you are trimming it out by hand?

If you need to get the two 17MHz clocks to lock WRT each other, try a
weak connection between them. Even crystal oscillators can do injection
lock provided their frequencies aren't grossly different in the first place.

[...]

Hi, Joerg. I'm very curious about this comment in regards to
microcontrollers. I'd like to experiment with that later this year.

Before answering, I already know it's very easy to just use a separate
oscillator and drive two cpus through the inputs to their inverters
from that... that's not why I ask. What I'm curious about is when I
have two duplicate boards from the same design and where, for some
experiment in mind, I'd like to closely sync up the oscillators
directly like this to remove beat frequencies between their behaviors
for a one-off testing case. (If it were a design matter, I'd probably
take a different tack.)

I don't know about these particular oscillators Jan is talking about,
but I'm imagining the usual case I experience with micros, which is a
class-A inverter. Some of these have outputs that are (over)driven
pretty hard to cope with higher MHz crystals from a wide variety of
manufacturers. Are you suggesting by "weak connection" a high
resistance (or low valued cap?) between the two input sides of the
inverters?

No, that wouldn't give you any lock range to write home about. I'd
declare one the master and then feed from its output side to the input
gate terminal of the other, with an RC series combination (several k and
very few pF).

I was first imagining the output to input path as obvious, but then
figured I might as well just remove the other crystal then and drive
two inputs instead of just one. Why not?

(I probably won't get to test this, in the coming month. But perhaps
a month later. But I'd like to make sure I follow you while the idea
is fresh in mind.)

This isn't about crystals but may help:

http://www.oldcitypublishing.com/FullText/JAPEDfulltext/JAPED2.1fulltext/11-24pp%20GC05-06%20%28Rajput%29.pdf
http://www.imec.be/esscirc/esscirc2001/C01_Presentations/160.pdf
http://www.ece.rochester.edu/research/laics/publication.files/laics_ilc_tr.pdf

With crystal oscillators this is iffy and requires experiments, won't
work with cheap and inaccurate crystals.

In Jan's case he could also just rip one crystal and directly feed in
the clock from the other camera. However, injection looking offers some
advantages:

a. You can use low amplitudes for the link which can seriously reduce
EMC issues if any length of cable is needed to bridge a distance.

b. Chances that serious modulation gets is is reduced.

Thanks for the thoughts. I'll keep these for later reading.

Jon
.

Relevant Pages

  • Re: Defeating the mantis part 3
    ... Apart from the possibility of 2 different cameras with ... different video channels and 2 LCDs hanging in front of your eyes, ... Even crystal oscillators can do injection lock provided their frequencies aren't grossly different in the first place. ... With crystal oscillators this is iffy and requires experiments, won't work with cheap and inaccurate crystals. ...
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  • Re: Defeating the mantis part 3
    ... different video channels and 2 LCDs hanging in front of your eyes, ... Even crystal oscillators can do injection lock provided their frequencies aren't grossly different in the first place. ... oscillator and drive two cpus through the inputs to their inverters ... pretty hard to cope with higher MHz crystals from a wide variety of ...
    (sci.electronics.design)