Leslie, This is fairly simple using an RF signal generator and an oscilloscope terminated at 50 ohms.
The RF generator must have an output impedance of 50 ohms. The trick is to first measure the amplitude delivered at the oscilloscope from a known 50 ohm cable at 5 MHz or greater. Then, without changing any settings, connect the unknown cable and observe the amplitude. The 75 ohm cable should exhibit a higher amplitude, and will probably vary with cable length. I once used a method similar to this to determine which end of a cable was shorted due to an improperly installed crimp connector. Before I set this up for him, the unfortunate technician had to cut one connector off and then check with an ohmmeter. This gave him a 50% chance of removing the good connector. By the way, I generally mark cables with colored tape - green for 50 ohm and violet for 75 ohm. Prevents a mix-up when working in a busy lab. Hope this helps. Scott Lacey ************************************ Leslie Bai wrote: Dear members, Anyone there can share the experience to measure cables' impedance thus to identify whether a BNC is a 50 ohm or 75 ohm cable. Thanks, Leslie --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
