Electronic – Practical formula/method to predict the necessity of termination resistor

characteristic-impedancemeasurementoscilloscope

I am not into high frequency much and just know about ringing caused by reflections due to impedance mismatching in transmission lines.

So I recently saw some non-engineer researchers in a lab that they were measuring 3MHz signal with 1 meter 50 OhM coax cable and the cable was directly coupled to the scope input.

In such electronic instrumentation I am wondering when would a 50 Ohm termination required. Is there a practical way to roughly tell whether 50 Ohm termination is necessary? (Assuming source impedance is negligible, 1 meter 50 Ohm coax cable, and a 3MHz signal, scope input impedance is 100Meg)

Best Answer

In such electronic instrumentation I am wondering when would a 50 Ohm termination required. Is there a practical way to roughly tell whether 50 Ohm termination is necessary?

If you are transmitting a clock signal i.e. a square wave to a high impedance load at the end of a cable (aka transmission line) AND all you are interested in is receiving a "decent enough" waveform at the end of the cable then we have a rule of thumb: -

Terminations are only required if the highest frequency of interest (that we need to keep a decent-looking waveform) has a wavelength that is at least 10x longer than the electrical length of the cable.

1 meter 50 Ohm coax cable, and a 3MHz signal

So, with a 3 MHz square wave, we might consider that the 7th harmonic of 3 MHz (21 MHz) is the highest frequency we need to be interested in maintaining. That has a wavelength of 14.28 metres and that is 10x more than the electrical length of the cable. Under these circumstances (clock transmission) we would be OK.

However, we can look at it from the perspective of the 1 metre cable and say that the we should not try and transmit a signal (without a proper termination) that has a wavelength of greater than 10 metres. That would be a frequency of 30 MHz.

Of course, different applications have different requirements and this rule of thumb won't perfectly suit every application.