To terminate or not

[Phil Townsend]

DACs that have the ability to receive a word clock signal from an external clock usually have the ability to use the clock signal then send it on to another DAC.

Typically the use 75 ohm BNC connectors.

My question is do I need to terminate the clock out with a BNC with a 75 ohm load?

 

Also I seem to remember some issues about making your clock coax cable some certain length. But can't remember if it was some length slightly shorter than a meter. So it might be 37 inches rather than 39.45 inches. Or the inverse... 41 inches not 39.45.

 

Anyone...Anyone...Bueller...

 

 

 

[audioengr]

Typically, word-clocks are 5V signal levels and parallel terminated at the destination with 75 ohms. Best to use a 75 ohm cable and connectors, but 50 ohms usually works fine too. Cable length should not be critical. The time between edges (5msec for 192kHz) is orders of magnitude larger than the propagation time, which is 2-3nsec/foot. For a 3 foot cable, prop time is only 6-9 nsec.

 

If the edges are really slow, this can be an issue however. 1-1.5m length usually puts the first reflection outside of the capture window.

 

Steve N.

Empirical Audio