1X probe acts weird (needs compensation?) - Page 1
Forum section: electronics / beginners

author: beowulfenator
author_rank: Contributor
timestamp: September 07, 2018, 08:30:59 pm
content: Hi!I got my hands on a 1X probe (the manual says "Elditest GE1502") and hooked it up to Tektronix TAS 485. This is what I get when I probe the "probe comp" input.My 10X probes (HP 10071A) work fine, and I can adjust them to make the waveform square. But this probe (and a piece of any wire for that matter) produces this skewed waveform. Am I doing something wrong here?

author: ArthurDent
author_rank: Super Contributor
timestamp: September 07, 2018, 08:52:59 pm
content: Check what the input specs are on the two scopes. I noticed that different scopes have different capacitance loading on the probes that would cause the problem. You probably can't adjust a 1X probe to work without changing the coax length or perhaps a low value resistor that may be in series with different 1X probes.The Tek may be 1Mohm/20Pf

author: beowulfenator
author_rank: Contributor
timestamp: September 07, 2018, 09:08:22 pm
content: The scope is indeed 1MOhm/20pF. According to the datasheet, the probe's input impedance is 65 pF. But then there are two issues: first, a short piece of wire gives the same picture and second, the P6101B probe that is mentioned in the scope's manual as compatible has input impedance of 1MOhm/100pF.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 07, 2018, 09:41:11 pm
content: Dave has a good explanation of what your 1X scope probe problem is. Watch especially from minute 18 onward which will describe the difference you are seeing between the 10X probe designed to work with your scope and some other 1X probe not designed to match your scope.

author: beowulfenator
author_rank: Contributor
timestamp: September 08, 2018, 08:24:35 am
content: Oh, I've seen that video before. I've just watched it again, and he describes a switchable 1X-10X probe that's optimized for 10X and works poorly at 1X. I've measured DC resistance of my probe, it's roughly 180 Ohm. There should be no attenuation at 1 MHz.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 08, 2018, 12:27:42 pm
content: I've told you what the problem is, Dave's video clearly explains what the problem is, and you can see the problem on the scope in front of you. Eventually it will become clear.

author: StillTrying
author_rank: Super Contributor
timestamp: September 08, 2018, 01:56:37 pm
content: According to my logic, it can't be anything to do with the probes or compensation, it must be a faulty 1V/Div range.Does CH2-CH4 do the same, view the 1kHz at X1, and X10 at the same time, a bit wire will do for the X1 connection, to check it's not the 1kHz itself.

author: beowulfenator
author_rank: Contributor
timestamp: September 13, 2018, 05:32:39 pm
content: So this is where it gets strange. In Dave's video he specifically addresses the problems at higher frequencies. But this is kilohertz range! I've even tried hooking up a signal generator with a really short coax cable. The weird part is that the signal doesn't look square at lower frequencies (roughly 330 Hz in my example), but at 33 kHz it is just fine. The probe behaves similarly. How can an impedance mismatch be such a problem at 300 Hz?Oh, and the effect is more pronounced with AC coupling and less with DC coupling!

author: beowulfenator
author_rank: Contributor
timestamp: September 13, 2018, 05:39:49 pm
content: Holy cow, you're right! The other channels are fine, it's just CH1 that's weird!

author: ArthurDent
author_rank: Super Contributor
timestamp: September 13, 2018, 06:17:41 pm
content: It looks like what you were describing as compensation problems could actually be capacitive decay. If you say the trace looked o.k. with 330hz and a x10 probe but the x1 gave this bad display, I wonder if the x10 probe would look similar at 3hz input, because it would have a longer time to decay that you won't see at 330hz.Try putting a capacitor in series with the x1 probe on CH2-4 and see if you can simulate the same condition. You probably will need a fairly small cap to get the same waveform. If this can be verified it means there is a cap in series with CH1 but not CH2-4 the way you have it now set. If the 4 channels are set exactly the same but CH1 give a different display, it may be a dirty switch/relay that shorts out the cap to go from for A.C. to D.C. coupling.

author: beowulfenator
author_rank: Contributor
timestamp: September 14, 2018, 08:12:46 pm
content: Quote from: ArthurDent on September 13, 2018, 06:17:41 pmYou're right! A capacitor in series makes other CH2-4 behave like CH1. At 100 nF and above there is no effect. At 1nF the effect is greater and at 10nF it is less pronounced. Channels are configured exactly the same (DC coupling).So do you think it's because of a problem with the CH1 relay? A dielectric gap between relay contacts that should normally be shorted?

author: David Hess
author_rank: Super Contributor
timestamp: September 15, 2018, 11:42:35 pm
content: Quote from: beowulfenator on September 14, 2018, 08:12:46 pmOr a crack somewhere later in the high impedance circuit before the FET buffer. The whole thing is part of a hybrid.Compare the DC response of channel 1 with the AC response of any of the working channels. If they are identical, then the problem is probably with the relay.