Impedance Matching vs Bridging in Word Clock Distribution

Thread Starter

Joined Aug 9, 2007
I have a circuit board from a Digital Audio Device.
It is a Word Clock Distribution Amplifier.
I power it up (5V) and feed it a stable 48kHz Word Clock.
The input shows up on the Oscilliscope as a nice 5 V Square wave.

! But the 4 Outputs show a 0.2 V square wave on the Oscilliscope.!

I dont have a schematic. But it's not a very complex circuit.

A BNC input (terminiated with 75 Ohm)
delivers signal to one input of a SN75124 Triple Line reciever.
The output of this feeds two SN75123 Dual Line Drivers.
These outputs feed the BNC outputs.

I can see some resistors terminating the BNC out:
! but I can't measure their resistance !
It show up as open circuit.
My cheapo multimeter goes from 200 Ohms to 2M Ohms so the resistance COULD be very high.

For Digital Audio Word Clock distribution, the impedance at the reciever is 75 Ohms.
The signal is a 5 Volt square wave. (48kHz or 44.1kHz)

My Question:
Should I expect the impedance at the Word Clock source to be a bridging high impedance (how high?) or a matching 75 Ohms?


Joined Apr 26, 2005
From the google search on "Word Clock Distribution", alot have in and out impedances of 75 ohm.

Have you reviewed the SN74124 and SN75124 datasheets for the pinouts to aid you in troubleshooting the inputs, outputs, and Vcc?