scope probe impedance

This video should give you a better understanding od scope probes.

to be the same?

e-probes-revealed/

Mikek,

This sounds like a problem doesn't it?

Mike-

I have not looked at Mikek's video. It may explain things better than I can.

One reason for using a ten to one scope probe, is so you can have a variable capacitor in parallel with the probe's resistance. By adjusting the capacitance, you can "flatten" the probe's frequency response.

This capacitance is often adjusted while observing a square wave pattern. You would set the capacitor so the leading edge of the square wave has minimum overshoot and maximum rise time. When adjusted, the capacitive voltage division should be the same as the resistive voltage division.

Fred

The whole concept of a scope probe is to get minimal DC and AC loading on a circuit under test without making the scope insanely expensive. There are a couple ways of doing this. The simplest is to put an attenuator at the tip of the probe, and then have a network at each end of the cable to null out the effect of the cable. Ideally, there would be a 10:1 reduction of capacitance at the probe tip, but parasitic capacitances make it a bit worse. The 10:1 probe is optimized for a scope with a 1 MOhm input over some range of capacitances. It has a trim cap to give a square response from the test pulse output of the scope.

There are also cases where a source is designed for 50 Ohm loading, and you just feed it into the scope with the scope set for 50 Ohm termination. This gives the best signal fidelity at the cost of a lot of resistive loading.

You can also get FET follower probes that have under a pF of loading, but these will usually set you back $1000 each.

Jon

Bob Pease did a nice writeup in "Troubleshooting Analog Electronics" showing how he made a low-loading FET probe in his lab... a scrap of PC-board material with much of the copper removed and a few holes drilled in its, some common-to-junkbox-grade parts, and some stiff wire soldered on the end to act as a probe.

Calibrated it ain't, but it sounded as if it was good enough to be useful for many purposes.

Not necessarily, it depends on the circuit you are measuring. Mikek

Yes, the scope has the option of switching the input to

Well, it's a 10x probe.. the scope needs to be switched to 1 meg input so that you'll get a proper scale of 10:1 and there are menus to select the probe multiplier so your scaling will be correctly displayed. Using a 1:1 probe induces noise and reduces your input level capabilities. It also loads the circuit you're testing, more so than a multiplier probe would. Jamie

Absolutely not.. it's needed to balance things out at high freq.

You should go for a probe that is rated higher than the analog bw of the scope.

For example, a 300 Mhz scope should be using like a 500Mhz probe, at least that is what I do..

I have set of old active fet probes that are rated 1 Ghz when things get sticky!

Jamie

"Mike Miller"

I am looking at a Tektronix TDS3054B Digital Oscilloscope.

1 M? in parallel with 13 pF or 50 ?

These probes look to have an impedance of 10 M?

4.) Is there going to be an issue with the 1 M? verse 10 M?.

Cos they have a 9Mohm resistor inside the probe part that forms a 10:1 divider.

The connecting cable is of very low capacitance so the loading seen at the tip is only 8pF.

This is about 10 times less than experienced with a 1:1 probe.

.... Phil