switching between digital sources

[petrusg]

I am considering making a switch box for 2 or 3 digital line sources with a line out to a DAC. I am thinking in terms of coaxial wires. I assume a regular rotary switch will do but i am unclear about handling the earth connection. Do I switch eart as well as line or do I connect to common earth in the box?
I would be very grateful for any advice.
P

 

[DF96]

I would not switch earth. Build it as though it was for RF - it is! Short connections, low inductance. If possible, measure its effect on impedance at high HF or low VHF. A local radio amateur might have an antenna analyser you can use.

 

[petrusg]

Many thanks, DF96!

 

[Ryssen]

I am using a box from ebay,it uses a remotecontrol,it works fine I think.
Just a hint.

 

[petrusg]

Hej, Åke, thanks for the hint !

I will look into this. I see there are some items there that might suit my needs, such as this: 3 WAY PHONO RCA STEREO AUDIO INPUT SELECTOR SWITCH | eBay

My digital sources already have volume control. For digital signal I would only need one channel.

 

[DF96]

Analogue selector boxes might not work too well. Depends how they are constructed. You need to maintain some rough approximation to 75 ohm impedance through the box.

 

[petrusg]

I see - do you think that could be achieved by tweaking?
Thanks,
P

 

[Frank Berry]

Video switch boxes should work just fine. They are designed for 75 ohm operation.

 

[petrusg]

Thank you Frank!
I will look into this. Perhaps one of these would do?:
Switch Boxes - Networking components for OEMs and Systems Integrators in Pittsburgh, PA

 

[DF96]

The BNC ones are almost what you need, except that they will be 50 ohm instead of 75. They might be good enough if all connections are short.

 

[cliffforrest]

I'm pretty sure video BNC is 75ohm, to match its coax.

 

[DF96]

True, but networking is 50ohm. Maybe the switch box is 62 ohm, so it sort of works with both?

 

[cliffforrest]

10base-T networking is (was) classic 50R terminated BNC / coax, but I have never seen this switched as in a "1 of n" selector! Besides, that is VERY dead technology!

Video switches, however, can use coax center-wire switches - reeds are good and make keeping the 75R impedance within bounds very easy.

 

[petrusg]

Thank you. I know very little electronics as this question may or may not confirm: Does a 25 ohm resistor fitted in series make any sense?

 

[DF96]

No. Transmission lines don't quite work like that.

 

[petrusg]

Thank you, DF96!

 

[petrusg]

Perhaps this might work?:
2-Way Push Button 75 Ohm RF A/B Switch Box-by-Connectronics | Bizrate

 

[DF96]

What connectors does it have? You might need to use adaptors.

 

[petrusg]

it looks like tv connectors to me....
on the other hand it looks like the BNC is 75 ohm as pr. here:
4 In 1 Out Composite Video Switcher