Active scope probes; do they require compensation? - Page 1
Forum section: electronics / testgear

author: 0culus
author_rank: Super Contributor
timestamp: November 12, 2018, 07:37:36 pm
content: So I finally got a Tek 1101 accessory supply so I can power my P6201 FET probes. The FET probe by itself doesn't seem to have any means for compensation. The 10x and 100x voltage divider tips that the 6201 comes with (if it's still complete) do have a compensation adjustment. However, I can't get the 10x one to quite settle out on the scope cal signal. Is there a better way to compensate when you are using active probes that have a voltage divider?

author: alm
author_rank: Super Contributor
timestamp: November 12, 2018, 07:48:39 pm
content: Active probes should be connected to a resistive load, like a 50 Ohm scope input. The 50 Ohm source should not require any compensation for this. The tip will be a fairly reactive high-impedance load, so any divider in front of it will require compensation.

author: 0culus
author_rank: Super Contributor
timestamp: November 12, 2018, 08:11:02 pm
content: Quote from: alm on November 12, 2018, 07:48:39 pmThanks. So the 6201 has a switchable internal 50 ohm termination on the amplifier which I definitely have on.So on the calibration signal with sec/div set at 1ms, with the 10x divider on, the square wave has a small angle (i.e. the peaks and valleys aren't flat) and this won't compensate out. My normal P6137 passive 10x probes show a perfectly flat square wave at the same settings. Should active probes be compensated on a different horizontal timebase?

author: 0culus
author_rank: Super Contributor
timestamp: November 13, 2018, 02:49:50 am
content: So the attached pic shows just about the best I can do. I cannot get rid of that tiny peak and valley.First pic is at a faster sec/div to show the detail and the second is at the recommended 1ms. This is with the 10x slip on voltage divider.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 13, 2018, 01:24:13 pm
content: I assume you're not just randomly trying things and that you downloaded the manual and followed the alignment procedure in section 5.If that is what you are doing, what equipment was substituted? Could you post a picture of your test setup?

author: 0culus
author_rank: Super Contributor
timestamp: November 13, 2018, 02:35:06 pm
content: Quote from: joeqsmith on November 13, 2018, 01:24:13 pmI do have the manual, but this is just the compensation screw on the 10x divider the probe came with. I unfortunately don't yet have all the equipment needed for calibration (I'm gonna become a permanent patient in the TEA thread). The particular probe in question was purchased new in the box. Still has the original inspection tag and some orange sealant stuff on the screws.

author: bson
author_rank: Unknown
timestamp: November 14, 2018, 12:42:13 am
content: What kind of tip is it? Do you require compensation for other tips? None of my active FET probes, although they're not Tek, have required any adjustment or compensation, and auto zero when used with the scope.

author: 0culus
author_rank: Super Contributor
timestamp: November 14, 2018, 01:38:33 am
content: Quote from: bson on November 14, 2018, 12:42:13 amHere's a pic attached. The voltage divider slips onto the probe body.  And there's a clip tip that then slips onto the final probe tip. It also comes with a 100x divider but I haven't tried out that one. I ended up setting the compensation screw on the 10x as close as I could get it then compared it on a signal generator with one of my passive probes in another channel and it seems to be fairly close in my less-than-scientific quick and dirty testing. The amplitude it reported was slightly lower and ever so slightly out of phase with the passive probe. I have no idea which one is right though, so for all I know the FET probe is the accurate one and the passive one isn't.

author: 0culus
author_rank: Super Contributor
timestamp: November 14, 2018, 02:10:13 am
content: Honestly, though, the two were close enough that any difference is likely due to different delays in the different coax on each probe.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 15, 2018, 01:40:42 am
content: :popcorn:If they are normally supposed to be used with some other Oscilloscope, which perhaps has auto or manual probe compensation adjustment on the 'scope itself, perhaps they might not be a good choice for, what from the pix, looks to be a 7000 series analog Tek.P.S.I take it all back!Reading the manual, it specifies using Tek 7000 Oscilloscopes.That said, the manual has quite a number of "tweaks" included in the calibration section. ( pages 5-1 onward)http://w140.com/tek_p6201.pdf

author: 0culus
author_rank: Super Contributor
timestamp: November 15, 2018, 01:44:06 am
content: Quote from: vk6zgo on November 15, 2018, 01:40:42 am2400 series, 2465B to be exact. The P6201 is the recommended active probe for the 2400 series scope according to Tek literature of the day. In fact, the 2465B had a probe power option which mine doesn't have (hence the power supply).

author: David Hess
author_rank: Super Contributor
timestamp: November 21, 2018, 02:43:37 am
content: The compensation output is intended for low frequency compensation which is all that is required for an attenuator. The peak and valley shown are a higher frequency component which is not meant to be used. Do they show up with other probes?

author: 0culus
author_rank: Super Contributor
timestamp: November 21, 2018, 02:52:34 am
content: Quote from: David Hess on November 21, 2018, 02:43:37 amNo, just that probe. Also I got a chance to test it on some other signals in direct comparison to one of my passive 10x probes and it performs perfectly fine.