What type of cables to buy for injecting/measuring voltage?

Question

I've been designing several circuits in an electronic lab where I'd use alligator clip leads for injecting a signal in a circuit and oscilloscope probes to measure voltage. Now I'm trying to buy my own equipment and I'm looking at a signal generators with BNC outputs but it doesn't have a lead. What online it seems alligator clip leads are used also for oscilloscope measurements which make me confused. Do I need a specific type of alligator clip lead to pass the signal from the generator to the board? My understanding is they have to be high impedance but is it generally true alligator clips have higher impedance?

Answer 1

Based on your comment that you are only interested in 10 Hz to a few kHz, you have a bit more freedom in the type of test leads you use with your oscilloscope and/or signal generator.

There are many test leads made with BNC on one end (to connect to your test equipment) and various other ends such as alligator clips, grabbers, or banana plugs, etc.

Sidebar on impedance...

I think you may be confused about test leads needing to be high impedance, rather than the test equipment having a high input impedance. When measuring a circuit with an oscilloscope, you generally don't want the scope to load down the circuit, which would affect its operation. Having a high input impedance means that the scope will measure with minimal impact to the circuit in question. For example, 50 Ω and 1 MΩ are commonly selectable input impedances, and should match the probe being used. It's common for some passive oscilloscope probes to have a 1X and a 10X selectable attenuation, which you would then match on your oscilloscope to have the correct scaling*. The probe has a selectable 9 MΩ series resistor that forms a 10:1 resistor divider with the oscilloscope's 1 MΩ resistor to allow more accurate measurement of high-frequency signals.

For more information on oscilloscope probes, check out An Introduction to Oscilloscope Probes at All About Circuits.

Another case where impedance is important is in maximum power transfer from source to destination, commonly referred to as impedance matching. For example, if you have an RF signal where impedance matching is important (e.g. radio transmitter to receiver) you want to have a specific characteristic impedance in the coaxial cable being used to connect them. Thus, something like a 50 Ω coaxial test lead with BNC connectors on both ends is common when measuring power accurately is important (usually in combination with a calibrated attenuator in such a case).

*_{Depending on the vintage and design of the probe and oscilloscope, the probe attenuation may be auto-detected, or require you to specify the probe setting manually, or may even require you to remember to apply the multiplication or division to the displayed measurement yourself.}

Back to your question...

Alligator clips and grabber clips won't have controlled impedance, so they'll introduce weird issues like interference and reflections when trying to work with higher frequencies such as 1 MHz and above. But they should be just fine for you if you're working in the range you mentioned (say DC to 100 kHz).