Hi
I'm trying to drive 3V TTL and MECL signals over 4 feet of cable to my Device Under Test. Max frequency =20 MHz. I have settled on twisted pair cable because a lot of the signals -other signals- are differential. I think TTL is 100 ohms but I'm wondering if I could use 50 ohm twisted pair cable or I should use 100 ohm twisted pair cable.

I'd like someone to confirm the impedance that I should be using. I have to measure rise and fall time and delay between signals ( signal in and signal out of the device) so I will have to factor in how much prop delay over 4 feet of cable.

Thanks in advance

 

Hi
I'm trying to drive 3V TTL and MECL signals over 4 feet of cable to my Device Under Test. Max frequency =20 MHz. I have settled on twisted pair cable because a lot of the signals -other signals- are differential. I think TTL is 100 ohms but I'm wondering if I could use 50 ohm twisted pair cable or I should use 100 ohm twisted pair cable.

I'd like someone to confirm the impedance that I should be using. I have to measure rise and fall time and delay between signals ( signal in and signal out of the device) so I will have to factor in how much prop delay over 4 feet of cable.

Thanks in advance

Depends on the actual driver chip you are using.
Why don't you take the chip and measure the output impedance?

You can add a series resistance to bring the total driver output impedance to match that of the cable you plan to use.

 

TTL has no particular characteristic impedance on the input or output but there are TTL drivers and receivers that work with transmission lines. I think they are generally designed for 100 ohm twisted pair.
You should consider using those to drive the cable.