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Hi maybe someone can help me here, basically I am trying to build a computer controlled coaxial switch. I am using a regular Integrated circuit multiplexer to handle which channel is patched to output. So basically I have:

              (8 X Coaxial Inputs)
              I I I I I I I I

microcontroller ----> | Multiplexer | --------------- I (1 X Coaxial Output)

The idea is so that I can use a computer to control which of my 8 video feeds that I can watch. I thought that because the connections are straight through I wouldn't have to worry about attenuation as much, but I tried a sample setup using a breadboard and I can barely see the picture. Any ideas as to how I can make it work?

I am currently using a breadboard with all the components I listed Plus some small gauge cables (so that they fit on the bread board)

UPDATE: This is what My setup Looks like http://img6.imageshack.us/img6/6320/49195403.png the MUX is a UTC 4051 Data Sheet is Here http://alldatasheet.com/datasheet-pdf/pdf/173652/UTC/4051.html

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Can you give us more detail on how exactly is everything connected? A picture of the breadboard maybe? Are you sure that you connected shields properly? – AndrejaKo Apr 16 '12 at 18:25
This is close to how it should look, How am I supposed to shield the cables? I am new to dealing with hardware... I am literally soldering a cable from the coxial inputs straight into an input on the multiplexer.... img6.imageshack.us/img6/6320/49195403.png – Commander147 Apr 16 '12 at 18:42
I meant the actual picture of the hardware you have. Anyway, what I meant to say is that the shields of the coaxial cables should be connected together. Also there is no such thing as regular Integrated circuit multiplexer. Give us the exact model number. Also I believe that the multiplexer could be attenuating the signal, so that's why the exact model is important. – AndrejaKo Apr 16 '12 at 18:51
With projects such as this one, the problem is that in order to maintain the signal quality, you need to have matched impedance along the whole cable and the multiplexer is going to cause problems there. Also the breadboard could be a problem. The frequencies used by TV systems are an order of magnitude larger than frequencies for which breadboards are designed, so you could have all sorts of problems from that. – AndrejaKo Apr 16 '12 at 18:54
lol Sorry I know I sound like a total NUB the MUX is a UTC 4051 Data Sheet is Here alldatasheet.com/datasheet-pdf/pdf/173652/UTC/4051.html – Commander147 Apr 16 '12 at 18:55
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1 Answer

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I bet you didn't buffer the output.

If you tried to direct drive your 75ohm load, you would discover a lot of attenuation from the 250ohm switch resistance.

Use 10K load or so and buffer the output with a 75ohm line driver or whatever you can buffer with 10MHz BW or so. Consider how critical your levels need to be and matched source impedance to 75 ohm. Ideally everything sourced to your monitor is 75ohm for NTSC composite video.

I hope that works and my intuition is correct.

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OK Thanks So much for the feedback guys.. so I found out that my problem was that I wasn't grounding my cables correctly.. I have also moved on to using Component cables instead of RCA (I just happen to have RCA terminators that I can easily attach cables to)... Anyways Regrading how to create a buffer, I purchased NTE947D OP AMP data sheet is here -> datasheetcatalog.org/datasheet/nte/NTE947D.pdf HOW DO I Connect this up to my current setup? I see that it has 2 channels but I will only be using one channel to do this right? I am a little lost as to how I am supposed to hook this up – Commander147 Apr 18 '12 at 4:44
(Is my output Video Signal from the MUX going to be the Invert Input??? and my final OUTPUT RCA going to be the Output A???) – Commander147 Apr 18 '12 at 4:44

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