o-scope probes

[jrz126]

As some of you know that I have recently purchased some oscilloscopes for a great price. Only problem is, they didnt come with any probes.

I've been looking at them on ebay and I'm trying to figure out which ones to buy.
The main scope I plan on using is a TEK-2430A 150MHz. this means I need a probe that is capable of 150MHz?
Also, do I need to pay any special attention to the attenuation, capacitance, and resistance? I dont want to buy something that wont work. Thanks for the help

 

[Dean Huster]

You want your scope probes to have a bandwidth higher than the scope, yes. Although X1 probes sound attractive for their ability to use the scope at higher sensitivities, their big drawback is the horrible shunt capacitance they present to the circuit under test.

In fact, probe capacitance is the biggest loading factor in most instances. Consider that at your 150MHz bandwidth, a 10pF shunt capacitance is like putting a 106 ohm resistor across your circuit. 1pF of shunt capacitance is like putting a 1K ohm resistor across your circuit. That 1X probe could easily drop 200pF across your circuit which is around 80K ohms at higher audio frequencies. So watch out for the capacitance. Also, get used to using 10X probes as a normal course of action.

Avoid buying used probes, however wonderful the price may seem. Probes take a beating and cable replacement of Tektronix probes can cost more than you paid for the used probe. But do get quality probes and take good care of them. In all my years on the bench, I've never had a scope probe go bad on me, and that's a tech doing a LOT of scope work (I worked for Tektronix on the repair bench). But I took good care of the probes, too.

Since most scopes have a 1M ohm input resistance -- I'd even say that ALL modern scopes have that -- the resistance interface for the probe isn't a problem. Remember that probes depend upon the input resistance of the scope as the second half of the voltage divider, so it would otherwise be a very important consideration if you wanted the probe to have an accurate attenuation ratio.

Make sure that your probe will compensate to the input capacitance of your scope. I have seen low-frequency audio scopes with input capacitances as high as 47pF and there are some newer probes that won't handle input capacitances that high.

Dean