Question about differential probes  - Page 1
Forum section: electronics / beginners

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: November 12, 2021, 11:55:15 pm
content: Looking at adding one of these to my tool box .Reasons are for check quality of power going into equipment 120vac , 240vac US - to avoid equipment giving bad resultsAlso producing power alternative energy inverters , generators etc.Next ham radio lot of the same , but looking at transformers - like transformers in HF amps - keep the noise at home - instead of one airAnd to learn more about electronics in general .I do have a Rigol oscilloscope - to use one with , let alone lots of other toys in the tool-box .So to the question , I see so many with similar & differering spec.s , or close like the MICSIG has 3 that I found with different voltage ranges 700 , 1300 & 5600 , so why not get the 5600 volt unit , trying to find limitations of uses / or most useful , the higher voltage to check high voltage transformers of amps .Thanks

author: David Hess
author_rank: Super Contributor
timestamp: November 13, 2021, 12:45:15 am
content: Check the attenuation settings. Higher input ranges normally go with higher attenuation which increases noise.

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: November 13, 2021, 12:48:33 am
content: Thanks , did more searching since posting question , and found that to be the biggest difference ?Dang , I want to have my cake & eat it too , low noise & high voltage --- like always have to a low voltage unit & then latter get the high voltage unit

author: Caliaxy
author_rank: Frequent Contributor
timestamp: November 13, 2021, 01:17:25 am
content: Well, the answer is simple in this case: get the 700V Micsig probe, as it has 10x/100x attenuation - good for both worlds (if the highest voltage you plan to probe is 240V anyways). I have the 1300V version, which is awesome for my application (750V pulses) but not so great for small signals (it’s 50x/500x…)But one thing I learned: don’t expect to use one tool for all the measurements. Implying “buy only what you need, when you need it”…Other than the Micsig differential probes, an interesting option is Picoscope 4444, if you are ok with USB scopes, lower bandwidth (20MHz) and lower voltages (but 12-14 bit resolution).

author: CaptDon
author_rank: Super Contributor
timestamp: November 13, 2021, 01:34:26 pm
content: Be aware, some of these diff probes have frequency ratings such as 15mhz, but have 1.5 foot or longer leads. Those lead lengths are a disaster at those multi-megahertz frequencies. I cut the leads on my LeCroy to about 4 inches. Expect several millivolts of noise coming from the probe at all times which means you want your input signal (after the scaling factor X10 X100 etc) to be probably no less than 10 times the amplitude of the noise. Example, probe noise 5 millivolts peak to peak, so with a minimum signal to noise on the screen of 10:1 that would be 50 millivolts pk/pk and with a probe scaling of 10:1 your minimum useful input signal would be .5v pk/pk and with a probe scaling of 100:1 your input signal should be 5v pk/pk or higher for a good visual display on the scope. Some of the cheap diff probes have a huge D.C. offset that drifts like crazy!!

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: November 13, 2021, 03:32:11 pm
content: Thanks , at this time I haven't considered lower voltages so much , I'll keep in mind when I get to those .Some more reasons for interest in these is using scope to check AC sources grid & generated .I've watched some use & show some of the noise issues .

author: CaptDon
author_rank: Super Contributor
timestamp: November 13, 2021, 03:50:23 pm
content: Depending on what you have connected to your 'grid' distortion is becoming almost meaningless. The input of a well designed SMPS is usually a bridge rectifier and capacitor bank. It would hardly even care about 6 volts of interfering signal on top of 120vac. If the capacitors were large enough on the input side it wouldn't even care if it was powered from 25hz railroad frequency (mostly gone these days although once common to the GG1 locomotive power grids) or from 400hz aircraft power. Items powered by transformers may get upset over an unsymmetrical sinewave and possibly saturate. Super anally retentive Audiophools may get upset about .1% distortion on their power grid. These are the same chuckleheads who buy $100.00 wall outlets to power their amplifier that uses #18 wire internally for the line fuse and power switch wiring. Oh, and those wonderful wooden pucks to raise the speaker wires up off the floor to avoid 'stray capacitance' loading down their 'highs'.