4
\$\begingroup\$

I building an interconnect board which will hold one side of the video balun pair (plus power and rs485). The other side will be a commercially available video balun which connects to the camera.

Right now I am having trouble matching the two sides.

I ordered three coils from coil craft - PWB3010LB, PWB-1.5-ALB, and WB1010-SMLB

All of them transfer the video, but the vertical sync is off by ~40%. the colour is slightly off (a little bit pink), but otherwise the video is fine! :)

What does the vertical sync problem indicate?

Thanks!

enter image description here

\$\endgroup\$
3
  • \$\begingroup\$ What do you mean by 'off by 40%'? What video format are you using? Can you post a schematic of how you have connected the transformer? \$\endgroup\$
    – MikeJ-UK
    Mar 27, 2012 at 9:25
  • \$\begingroup\$ Sorry, I mean the top of the video is located at just below the middle of the screen. I am transmitting NTSC. \$\endgroup\$
    – michael
    Mar 28, 2012 at 4:53
  • \$\begingroup\$ OK, I missed the NTSC tag! \$\endgroup\$
    – MikeJ-UK
    Mar 28, 2012 at 10:00

1 Answer 1

3
\$\begingroup\$

It looks like you are transformer-coupling the video. The problem is that in order to preserve the vertical sync pulse, you need a bandwidth extending down to well below 60Hz (the field rate), otherwise you will see excessive 'tilt'. The PWB3010-1LB is specified with a bandwidth down to 3.5kHz (presumably in a 50\$\Omega\$ system) and this is nowhere near low enough. The way commercial video baluns are connected is like this :-

Video balun schematic

This arrangement ensures that both sides of the twisted-pair have an approximately equal impedance to ground and has a bandwidth down to dc (so there is no galvanic isolation - which may be an issue for you). The symmetrical footprint of the Coilcraft transformer means that it can be rotated through 90° to configure it like this.

\$\endgroup\$
2
  • \$\begingroup\$ hmmm, do you know any sources for coils appropriate for this application? \$\endgroup\$
    – michael
    Mar 28, 2012 at 19:06
  • \$\begingroup\$ @michael - It looks from your layout that you could just rotate the transformer by 90 degrees and try it out - it might be OK. \$\endgroup\$
    – MikeJ-UK
    Mar 28, 2012 at 19:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.