Physics 405

5/24/05

NIM Electronics, 50 ohm Termination, etc. (Exp. 9 and 11)

In Experiments 9 and 11, we are dealing with fast pulses (a few ns to tens of ns wide) and NIM (Nuclear Instrumentation Module) electronics for which 50-ohm input, output, and cable impedances are expected. There are NIM modules that serve as amplifiers, discriminators, coincidence modules, etc. They are inserted into a NIM bin (or crate) that supplies power to the modules. This is a flexible method of building up custom circuitry.

The Tektronix 2245A scope has an input impedance of 1 M-ohm. To observe the output pulse of the photomultiplier tube anode with an oscilloscope in the same way that the NIM modules will, the scope must also appear to have 50 ohm input impedance. To accomplish this we add a 50-ohm terminator (resistor) at the scope input using a BNC tee. The input signal goes into one branch of the tee and the 50-ohm terminator is attached to the other branch. This puts a 50-ohm resistor in parallel with the 1 M-ohm scope input impedance so the scope then looks like a 50-ohm load to the signal, just like the inputs to NIM modules.

Also use the 50-ohm terminator when observing all the fast NIM logic pulses, such as those from the discriminators or coincidence modules. To avoid partial reflection of the fast signals, impedance mismatches must be avoided. The NIM standard for a logical True signal is a current pulse of -16 mA. This translates into -800 mV when the impedance is 50 ohms, as expected. Use the scope to observe the discriminator pulses with and without the terminator to see its effect.