Wittig W202 scope probe adjustment vrs? - Page 1
Forum section: electronics / testgear

author: rigrunner
author_rank: Frequent Contributor
timestamp: December 12, 2013, 01:24:16 am
content: Hello everyone,I have just been trying to take some rf voltage measurements across a matched load using my Hantek DSO and a Wittig W202 probe.I've compensated the probe using the cal on the DSO and the square wave looks ok.When i try to measure the voltage across a matched load the readings get increasingly incorrect as frequency increases.When the sig gen output (224mV) is checked on a power meter or SA the level is correct.I've got two other brand new, never been out of the wrapper W202 probes and they perform similarly.There are two vrs in the box at the bnc end of the probes. I can adjust these to correct the readings but the instructions with the probes are vague:"Within the box cover located near the BNC (W202 probe) or on the probe body near the tip (W201 probe) trimmers can be adjusted to fit the low frequencies"There is no other mention of the vrs in the instructions.Anyone know how to adjust these correctly?Thanks,Stephen

author: nctnico
author_rank: Super Contributor
timestamp: December 12, 2013, 01:49:46 am
content: What kind of frequencies? A standard 1:10 probe has a capacitance around 10pf. Now calculate the impedance and see if your probe isn't loading the circuit too much.

author: rigrunner
author_rank: Frequent Contributor
timestamp: December 12, 2013, 02:02:40 am
content: Testing 1MHz up to 100MHz.This probe has 15pf at x10 setting.50r + 15pf1MHz = 49.999r10MHz = 49.994r100Mhz = 45.229rVoltage at 100Mhz should be around 212mV with the slight mismatch. I'm reading upwards of 400mV.

author: trackman44
author_rank: Regular Contributor
timestamp: December 12, 2013, 02:18:26 am
content: Why don't you use the FFT function on your scope? Maybe there's nothing wrong with the probes. Try self calibrating your scope first, then take a measurement and see if there's a difference.Will

author: rigrunner
author_rank: Frequent Contributor
timestamp: December 12, 2013, 02:25:16 am
content: My first port of call was the self cal, and when that didn't affect the readings i used the power meter and SA to check the sig gen output.Thanks for the fft suggestion. I'll run that tomorrow and see if it sheds any light.

author: nctnico
author_rank: Super Contributor
timestamp: December 12, 2013, 02:28:52 am
content: 15pf at 100MHz is 100Ohms. Parallel to 50 that gives a Z of 33 Ohm. Maybe the source isn't happy. I'd create a passive divider probe with the source terminated in 50 Ohms. 2k in series, 50 Ohm to ground and measure from there. This way the source isn't loaded with a weird capacitive load. Either way I'd use a scope which has 50 Ohm inputs and a HF (active or passive divider) for accurate readings at 100MHz.

author: rigrunner
author_rank: Frequent Contributor
timestamp: December 12, 2013, 02:41:06 am
content: >15pf at 100MHz is 100Ohms. Parallel to 50 that gives a Z of 33 Ohm.How are you calculating that?(Your calculating capacitive reactance )When i calculated for 50r+15pf as an rc the impedance is as my prior post.45r or 33r, either way i'd expect the reading to be lower and not higher.Thanks for the divider suggestion

author: rigrunner
author_rank: Frequent Contributor
timestamp: December 15, 2013, 09:02:32 pm
content: I'm measuring now with coax from the DUT to a feed through terminator at the scope bnc and i'm seeing sensible values.I'd still be interested to know how to adjust the scope vrs, just out of interest.Thanks for the replies.