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Hi all,

So a while back I had the good fortune to find a brand-new, never used P6201 probe and accessories. The probe by itself is of course rated to 900 MHz, 100 Kohm, 3 pF. It comes with a slip on 10x 1 Mohm, 1.5 pF divider as well as a slip on 100x 1 Mohm, 1.5 pF divider. Both the dividers have standard screws for compensation.

The scope I am using the probe with is a 2465B. The P6201 is powered from an external 1101 power supply and is set for internal 50 ohm termination, AC coupled, DC offset off. The calibrator signal is 5 Hz-5MHz and it says to compensate probe at 1 ms/div horizontal. The 100x divider seems to compensate nicely, however the 10x divider won't (seemingly). The photos in this album I uploaded (/g/TekScopes/album?id=89686) show what happens from 1 ms/div, 20 us/div, 2 us/div, and 100 ns/div. It seems to get after a certain point as I speed up the scope.

Before I go and blame the 10x divider for being bad, I'm thinking that I might be fundamentally misunderstanding how to compensate when using an active probe? I read the P6201 manual and it only mentions the compensation screws in the dividers. Doesn't say anything special about using them, so I assumed that it should work just like compensating a passive 10x probe. However, since it looks better at fast horizontal settings, I'm thinking this might just be how this probe behaves at low frequency (relative to it's own passband)? I hope this is a PEBKAC (problem exists between keyboard and chair) problem rather than an issue with this probe...

Thanks,

Sean

Hi Sean,
There's nothing wrong with your P6201.Two recommendations:
1. Put the 2465B input at 1 MOhm DC or 50 Ohm DC, *not AC*. Use the P6201's External Terminator setting if the latter and in both cases use the probe's setting to select AC or DC coupling.
2. Compensate the P6201 at either 20us, 2 us or 100 ns / div, *not 1 ms* / div

Raymond

Raymond,

Thanks for your reply! I set up the scope and the FET probe as you said, and compensated it at 2 us. The adjustment was a bit fiddly so I followed the manual instructions for removing the tiny PCB inside the attenuator and cleaned + exercised the trimmer, which seems to have settled that down. Now it compensates much better! Thank you!!

Sean

On Fri, Apr 19, 2019 at 02:10 AM, @0culus wrote:

Hi Sean,
Glad that's solved.

I assumed that it should work just like compensating a passive 10x probe.

You have to realize that the compensation C of Hi-Z passive probes compensates for the cable and termination C at the 'scope that the probe tip "sees".
The circuit in the head of the FET probe effectively isolates the (low-impedance) cable from the tip.
Have fun with your P6201 and don't forget it is vulnerable to damage from static and higher input voltages, see the manual. Only use it without attenuator when necessary.

Raymond

On Thu, Apr 18, 2019 at 10:00 PM, Raymond Domp Frank wrote:

Hi Sean,
Glad that's solved.

I assumed that it should work just like compensating a passive 10x probe.

You have to realize that the compensation C of Hi-Z passive probes compensates
for the cable and termination C at the 'scope that the probe tip "sees".
The circuit in the head of the FET probe effectively isolates the
(low-impedance) cable from the tip.
Have fun with your P6201 and don't forget it is vulnerable to damage from
static and higher input voltages, see the manual. Only use it without
attenuator when necessary.

Thanks for the explanation and correcting my flawed assumption. I was aware that it is very sensitive, so I've been extra careful and will continue to me. It's a great tool to have. I've noticed it is highly sensitive to RF. I have an experiment on my bench with a 100 MHz clock coming out of an FPGA into my logic analyzer for state analysis and the P6201 is picking up the 100 MHz signal on top of the calibrator signal of the 2465B.

Sean