Oscope measurement and probe grounding

At best you are measuring the circuit you are probing with a ground that is referenced to your scope ~ So you measurement is off by the voltage drop induced by the probe cable. At worst your probe is just an antenna and your picking up a lot of additional noise.

How ever because you probe does not even cover the frequency being measured it may look better then it really is. You need a probe that works over the frequency range being measured, and a probe which is grounded as near the probe point as possible.

Reply to
Loading thread data ...


I have just set up my ERA-1SM amplifier, like it says to do here (P. 2) :

formatting link

You can clearly see the output point.

My question involves my Oscope measurement technique.

What am I measuring if I just connect the probe to the output point without grounding it? That is, just attach the probe point to the output and do not clip the probe ground to circuit ground. I am getting values, but I'm not sure what it represents?

I am using a Pico 212 scope that is attached to my laptop using the RS-232 port.

Further I am using probe that is rated to 100MHz but I believe the circuit is operating more toward 1 GHz - so do I need to get a "faster" probe to meaure this circuit? I don't need any very precise results, I just want to see if I have the amplifier set up correctly to work.

--TIA, jw

Reply to




It is no real way of knowing what you are measuring without the ground connected. Many times the scope will just pick up 60 hz powerline noise (assuming you are in an area near powerlines and they are 60 hz.

Also you need to provide an input to the amp. Without that you should not see much at the output.

Reply to
Ralph Mowery

Probe ground has to be connected. Whether you will see anything worthwhile at 1 GHz with a 100 MHz probe and a 25 - 50 MHz scope I can't say.


Reply to

Probe ground is essential. Furthermore, the ground wire needs to be WAY shorter than you probably think. Zero is a good value.

One way to get a feel for the problem is to consider the probe input capacitance and the inductance of the ground lead as a series resonant circuit. At the resonant frequency, the probe puts a dead short across your circuit. The measurement becomes useless at much lower frequencies.

So, what's the inductance of the probe wire, the scope, the line cord, the wall plug another line cord another power supply and the wires to your test circuit?

Hook up the ground with as close to zero length as you can get. And all this assumes the bandwidth of the probe and scope are up to the task. mike

Return address is VALID but some sites block emails
with links.  Delete this sig when replying.
 Click to see the full signature
Reply to

Looks like I have to rent a fast scope. --jw

Reply to

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.