oscilloscope accessory capacitance compensation

[bumperbozo]

I got a Hantek IDSO1070A and the probes that come with it have HF probe compensation dials you adjust to get an accurate waveform. But the BNC to banana plug adapter or cable you can get to connect to breakout leads or back probe pins, etc, don't seem to have the compensation dials. Nor do the amp clamps. Does that not prevent accurate waveform capture?

 

[2ndGearRubber]

https://www.amazon.com/dp/B07GWMCHS8/?tag=atomicindus08-20

^ You're talking about this guy, for automotive use?


There's no need to calibrate or compensate leads, in the same way leads for a multimeter are functional as is out of the box. They are simply shielded wires. Amp clamps do "require" pressing the ZERO button or adjusting the wheel to zero if one wants zero current flow to be expressed as true zero on the scope screen. All leads and clamps pick up noise. Apply filtering after the fact if needed, and keeping leads away from electrically noisy components in the engine bay is preferred. One can also tweak the sample rate to adjust the noise in the capture, but you only get one capture. So if you under sample, over filter, or have a beyond screwed up time base - you may have lost the detail you need.

Being that there is no "universal known good" there's nothing to compensate against. I suppose one could use a signal generator. What sort of signals are you concerned about in relation to compensation? Reading true zero? Current clamps do drift over time when used for extended periods on very low current signals, but that's a function of how they work, not a scope or adjustment issue.

 

[bumperbozo]

https://www.amazon.com/dp/B07GWMCHS8/?tag=atomicindus08-20

^ You're talking about this guy, for automotive use?


There's no need to calibrate or compensate leads, in the same way leads for a multimeter are functional as is out of the box. They are simply shielded wires. Amp clamps do "require" pressing the ZERO button or adjusting the wheel to zero if one wants zero current flow to be expressed as true zero on the scope screen. All leads and clamps pick up noise. Apply filtering after the fact if needed, and keeping leads away from electrically noisy components in the engine bay is preferred. One can also tweak the sample rate to adjust the noise in the capture, but you only get one capture. So if you under sample, over filter, or have a beyond screwed up time base - you may have lost the detail you need.

Being that there is no "universal known good" there's nothing to compensate against. I suppose one could use a signal generator. What sort of signals are you concerned about in relation to compensation? Reading true zero? Current clamps do drift over time when used for extended periods on very low current signals, but that's a function of how they work, not a scope or adjustment issue.

yes, that's the one.

I'm trying to diagnose a multiple misfire, so planning to scope injector voltage, fuel pump current, primary ignition coils voltage and current, and starter current for a relative compression test. I don't really have anything other than the Picoscope example waveforms for reference because i don't think Hantek has a library of known goods like Pico has.

Pico says their probes are compensated at the factory, but I don't know if they are just talking about the TA131 or if they mean the TA125 (which you use to connect to breakout leads and back probe needles via banana plug). The TA125 doesn't seem to have the little compensating capacitor box next to the BNC connection like the TA131 has.

when i went to compensate my own probes using the square wave output on the device it started out pretty misshapen, but i was able to tune them to get a pretty square edge.

so if i use the included probes with some "T" style back probe pins, where I have compensated the HF, i would expect to get a different waveform than if i get the Hantek banana plug adapter and lead breakouts or back probe needles. which way is proper?

 

[2ndGearRubber]

Gotcha. You won't be looking so much for known good, as odd man out in such an example. Relative compression for example, the rough rule of thumb I use is 35amps peak to peak minimum. But frankly I don't care all that much. If one has two cylinders, with identical current required to take the piston to TDC compression, and only one is misfiring, one could surmise that compression is not the contributing factor to the reason cylinder A is misfiring but cylinder B is not. If both have "identical" compression "numbers", one would believe that compression that would serve as sufficent for combustion on cylinder B would certainly support combustion on cylinder A. Thus the problem lies elsewhere. Now, you have a 4 cylinder engine, and the pattern looks like " ^^^-^^^-^^^-^^^-^^^-^^^- " well you have a dead hole.


Connect the scope into the circuit with either a standard backprobe, or T-pin and alligator clip, and so long as your connections are properly secure, the patterns will be functionally identical. As a side note, compare the speed at which your compensation signal was produced at, vs a secondary ignition pattern which has a 2ms max burn time, usually less than 1.5ms. You may be running at a super high frequency above and beyond what you will be sampling. We don't care about impedance, it's DC anyways and I actually had to look up the impedance formula to confirm before writing this. We don't care about probe resistance, we're not measuring resistance and so long as our scope has proper voltage limits (or we are attenuating to said limits) the lead is simply a dead branch off of the circuit. Attenuation factors are based on the scope, for instance the scope linked above has a 35v max input. So for primary ignition or injector voltage, you'll need to attenuate. A quality attenuator will take 12.5 volts, and at 10:1 show 1.25v. Now on an 80v inductive kick, yes, we might be stuck with 7.9v on our scope. But again we're comparing the shapes and consistences, as well as mainly looking for a pintle hump in that example.


I guess long story short is - hook the lead to the circuit however you like, for what you will be measuring it's not relevant. Now, if the input voltage of the scope is 35v max, you better have an attenuator on there when you plug into the ground side of an injector or coil.