video switch

[Grumps]

Hi

I've got this circuit
http://i990.photobucket.com/albums/af29/Circuits1/VideoSwitch.jpg
Video_In is a 1V composite signal.
EN is from an HCT04 logic gate.
I've not really used FETs before.

If EN is low ( < 0.5V ) will the FET be non-conducting?
And if EN is high ( > 4V ), does the FET just act like an analog switch?

Sorry if this is such a basic question. No, it's not homework.

Ta.

 

[Grumps]

Not a good way to switch video. You have a built in 6dB signal loss if
you attempt to drive another cable. The off isolation is likely to be
poor. There are much better ways to do this using chips from Maxim,
Intersil, Analog Devices, TI and others.

Thanks.
Why is there an inherent 6dB loss?

The original circuit was intended to provide some form of video driver
protection if the output was connected to the wrong point. Some of our
installers have been known to connect (maybe accidentally) a 24V power line
to the video outputs. The output then dies. We can obviously detect an
abnormal voltage present and turn off the FET.

I am waiting for replies from the manufacturers you quoted, but do you know
offhand if any of them have a video buffer that has some sort of protection
like this?

 

[Grumps]

You expect them to reply?

<snip>

Actually they all did. Took 'em long enough though!