Increasing the BW of my 1960s Tektronix P6042 current probe - Page 1
Forum section: electronics / testgear

author: joeqsmith
author_rank: Super Contributor
timestamp: March 10, 2015, 01:14:16 am
content: I had hijacked a repair thread on the P6042 current probe and decided to move my posts into the test equipment area. My original posts my be found here:https://www.eevblog.com/forum/repair/tektronix-p6240-current-probe-repair/A manual for the probe may be found here:http://www.atecorp.com/ATECorp/media/pdfs/data-sheets/Tektronix-P6042_Manual.pdfSee attached graph below for power limit.When I rebuilt mine (hard to say repair because about the only thing that was good on mine was the sensor and diff amp) through mine I remember thinking that the BW could be improved on. Once I posted about my repairs and had seen that all those years ago I thought about improving it, I decided to put up. My previous posts show my test jigs and the BW of the modified probe (almost double).So far I have been unable to prove how much current the probe can handle at the higher BW. I ran some tests up to 80MHz at almost 400mAp-p and it appears linear.This video shows the probe connected to my network analyzer.https://www.youtube.com/watch?v=6f8zoyBxizs&feature=youtu.beThis short video shows me changing the probes input current while measuring it's output, up to 380mA.https://www.youtube.com/watch?v=RVY9q5JhHHU&feature=youtu.beMy next step is to get an amplifier together with enough power to see where the probe falls off. Inductors are on order and I hope to have something in the next week or so.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 11, 2015, 03:24:12 am
content: The transformers made it in. I gave them a try and it helped a lot but looks like the losses are too high. Coax next.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 13, 2015, 12:14:05 am
content: If we assume the absolute operating current in after 50MHz is linear, the probe should handle 1Ap-p. Obviously I will be testing way outside the original specs, so there is some risk the probe could become damaged. A friend has suggested I pulse it to make sure I don't over heat the attenuator or probe head.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 13, 2015, 01:41:35 am
content: It took some effort but was able to strip off some RG174 braid just fit it into the beads. I tried it out and it can easily exceed 1Ap-p now into a 50 ohm load at the higher frequencies. PC parts are all over the place right now. I plan to try it out in a few days.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 15, 2015, 05:22:56 am
content: Just the basic math,antilog(-30.46/20)=0.0299916If the modified P6042 will handle 1Ap-p as suspected, that's 50Vp-p * 0.0299916 or 1.5Vp-p at the DSO.Shown with probe attached. Signal generator driving the amplifier set to 80MHz. The P6042 is set to 0.2 Amps scale.The output from the attenuator (trace 1 green) is 1.45Vp-p.1.45/0.0299916=48.347Vp-p48.347Vp-p/50ohms=0.9669Ap-pThe output from the P6042 (trace 2 blue) reads 249mV. 0.2A*249mV/50mV = 0.996Ap-p or 29.1mA of error. Close enough.I let the probe run like this for about 5 minutes and saw no problems.

author: edavid
author_rank: Super Contributor
timestamp: March 15, 2015, 04:35:53 pm
content: I'm confused, how did you increase the bandwidth? Did you modify the amp circuit?

author: edavid
author_rank: Super Contributor
timestamp: March 15, 2015, 05:03:42 pm
content: Schematic please?

author: edavid
author_rank: Super Contributor
timestamp: March 15, 2015, 08:00:14 pm
content: I meant the schematic of your modified amp

author: joeqsmith
author_rank: Super Contributor
timestamp: March 16, 2015, 12:44:18 am
content: In the other thread I had posted about the differences between the two schematics and how the one manual talks about adjusting this stage as part of the high frequency alignment. In that same manual, they do not actually mention it as part of the alignment except in the heading. I am not sure why they went to the fixed parts shown in the on-line manual. Maybe to reduce cost, make it easier to align, or there was a stability problem.Changing this section is how I was able to get the gain (shown in the previous thread). I then just added some compensation to flatten it back out.If you wanted to try and replicate it, make sure you have a way to inject known currents into the probe first. Don't try those first two loops I made. Those were crap.

author: edavid
author_rank: Super Contributor
timestamp: March 16, 2015, 03:35:48 pm
content: I must be blind, because I looked at the other thread again, and I can't find anywhere where you say exactly what you changed

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: March 16, 2015, 03:40:27 pm
content: Nice work. Glad it wasn't the sensor after all, I'd have bet money it was dead. The old ones are so easy to destroy. A scowl can damage them...

author: joeqsmith
author_rank: Super Contributor
timestamp: March 19, 2015, 04:24:35 am
content: Quote from: Alex Eisenhut on March 16, 2015, 03:40:27 pmI think you confused me with the other person who was trying to repair theirs.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: March 19, 2015, 06:02:08 pm
content: Quote from: joeqsmith on March 19, 2015, 04:24:35 amI think I did...