BNC Cable as 1X Probe - Page 1
Forum section: electronics / beginners

author: Phaedrus
author_rank: Frequent Contributor
timestamp: October 23, 2014, 06:53:31 pm
content: Can I use a 50ohm BNC cable as a stand-in for a 1X passive oscilloscope probe in a semi-permanent test fixture? Are there any traps in terms of termination or impedance I need to look out for? This is for a low-speed application, <20MHz, for measuring PSU pk-pk ripple.

author: Simon
author_rank: Unknown
timestamp: October 23, 2014, 07:19:37 pm
content: I was asking a similar question recently, you should be ok, what you need to check is the capacitance of the cable as the frequency goes up the reactance goes down and you can overload your device under test but being a power supply you will probably be ok.

author: Rufus
author_rank: Super Contributor
timestamp: October 23, 2014, 07:34:58 pm
content: https://www.eevblog.com/forum/testgear/rigol-dg4162-amplitude-inaccuracies/Shows how a 50 ohm coax is not a 1x scope probe.

author: Simon
author_rank: Unknown
timestamp: October 23, 2014, 07:40:03 pm
content: putting a resistor in series with the "tip" helps, it depends on how accurate you want to be, you will end up with potential distortion.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: October 23, 2014, 08:05:55 pm
content: Quote from: Rufus on October 23, 2014, 07:34:58 pmI remember watching the episode Dave did in response that thread, I'll rewatch it, but I think at low frequencies it shouldn't make a difference, right? Plus, the cable capacitance should be swamped in my application, because our testing spec requires 10uF and 0.1uF caps at the point of test.

author: Simon
author_rank: Unknown
timestamp: October 23, 2014, 08:08:58 pm
content: you will probably be ok, but if you go measuring a high frequency from a high impedence source like that expect distortion and inaccuracies

author: tggzzz
author_rank: Super Contributor
timestamp: October 23, 2014, 08:41:21 pm
content: Quote from: Phaedrus on October 23, 2014, 06:53:31 pmYou can probably get away with it provided the source impedance is less than the cable capacitance's impedance at the frequency of interest.Try it and see, verifying your measurements by other means!

author: AG6QR
author_rank: Frequent Contributor
timestamp: October 23, 2014, 08:54:49 pm
content: Quote from: Phaedrus on October 23, 2014, 08:05:55 pmAt a low enough frequency, it should be OK. But what's low enough? You mentioned 20 MHz. That's a wavelength of 15 meters. A quarter wavelength is 3.75 meters. A coax with improper termination acts like an impedance transformer, and a quarter wavelength of coax will make an open circuit at one end look like a short circuit at the other end. If your cable is a significant fraction of that 3.75 meter length, the signal you're probing will see a very different (read: much lower) impedance than the scope's high impedance input. Is your cable going to be longer than a few cm? Will it be a couple of meters long? Then this is something you'll want to consider if you care about accurate measurements and/or if you care about the load you're putting on the circuit under test.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: October 23, 2014, 09:01:55 pm
content: Thanks for the advice! The cable will be about 1m long, and our main frequencies of interest are DC to ~10MHz, with most of the action happening in the 20KHz-1MHz range. We're using a Tek TDS 3014C with the 20MHz bandwidth limit on, or else the actual PSU ripple gets swamped by high frequency crap.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 24, 2014, 12:46:52 am
content: Testing power supply ripple? Sure. Terminate the cable, and don't forget ample common mode rejection (lots of ferrite beads?). The source impedance (with the traditional 10uF ceramic at the output, or whatever) should be low enough not to mind."with most of the action happening in the 20kHz-1MHz range"Yeah, sure it is...I've seen more than enough power supplies on "extended bandwidth" to know what that means...Tim

author: ConKbot
author_rank: Super Contributor
timestamp: October 24, 2014, 11:13:06 pm
content: Quote from: Simon on October 23, 2014, 08:08:58 pmIts not just high impedance, even low impedance sources can have distortions appear when using a BNC + cliplead adapter. Just saw this at work the other day. For some reason, my coworkers like to use coax + clip lead adatpers on scopes (fair enough I guess, they are laying around for on power supplies etc.. ) But with ~ 1m of coax and a 5 MHz RC filtered signal, a bunch of ripple on the rising edge showed up. Why? The source impedance from the high speed buffer was too low (way below 50 ohms) so the reflections off the 1M input impedance of the scope hit the low impedance source, and bounce back up to the scope again. But the same behavior isnt observed with a signal generator. The sig gen has a 50 ohm series termination, so any signals that bounce back get absorbed by that. With a proper 10x probe, or with a 50 ohm tee'd on the original (crap 1074Z's dont have internal 50 ohm?) Everything looked fine.50 ohm series resistor for the probe, 50 ohm termination, if you can set your scope up manually to have a 2x probe, then the readings will be correct too. Guessing a power supply wont complain about a 100 ohm load on it

author: David Hess
author_rank: Super Contributor
timestamp: October 25, 2014, 02:42:51 pm
content: Quote from: Rufus on October 23, 2014, 07:34:58 pmWhen doing power supply testing like this, I have gotten the best results by either using a coaxial connection to a x1 or x10 passive probe or doing what you are suggesting but using a terminated coaxial cable with either a 50 ohm (x2 attenuation) or 450 ohm (x10 attenuation) series resistor. The problem with the coaxial cable and 50 ohm termination is that AC coupling becomes difficult if you want to keep the low frequency response and you have to be careful not to apply large DC voltages to the 50 ohm oscilloscope termination.Using just the coaxial cable connected directly to the oscilloscope without termination has always resulted in too much pulse distortion and peaking for me. x1 passive probes solve this by using a lossy center conductor to provide critical dampening so they have a clean frequency and phase response. The tip to BNC resistance on a x1 probe is in the hundreds of ohms. The ones I have handy at the moment measure 295 ohms.One neat little trick that I like to use especially around switching power supplies because of leaking magnetic flux is to solder a short RG-316 coaxial pigtail to the test point and then use a coaxial BNC to probe tip adapter to attach a standard x1 or x10 passive probe. This adds to the input capacitance of the probe but not by a lot and the short length of the pigtail avoids serious reflections.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: November 01, 2014, 05:26:17 am
content: @David, is this about what you described?I figured I should allow about 5 ohms for connectors and cable. The coax we have measures about 0.5-0.7 DC ohms per meter, and a couple ohms for the BNCs.

author: Marco
author_rank: Super Contributor
timestamp: November 01, 2014, 05:31:11 am
content: Why not just get a cheapy probe and cut it up and bodge it to the connector you want?

author: Phaedrus
author_rank: Frequent Contributor
timestamp: November 01, 2014, 05:36:33 am
content: Quote from: Marco on November 01, 2014, 05:31:11 amBecause modifying purchased equipment (even probes) scares the crap out of management, while building new equipment is a much easier sell.

author: alank2
author_rank: Super Contributor
timestamp: November 01, 2014, 08:17:23 pm
content: Quote from: Marco on November 01, 2014, 05:31:11 amThis is what I am going to try. I've got 4 of the cheap eBay probes coming and I'm going to cut one off and see how well I can make it work with 2" pig tail leads hanging off of it.

author: David Hess
author_rank: Super Contributor
timestamp: November 01, 2014, 08:36:52 pm
content: Quote from: Phaedrus on November 01, 2014, 05:26:17 amThose capacitors are going to low pass filter in the input which is probably not what you want. They should be in series with the input resistance.QuoteWith 450 ohms of input resistance, you could have +/- 5 ohms while staying within 1%. The 50 ohm termination resistance might be that far off anyway.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: November 01, 2014, 11:18:55 pm
content: I know the caps cause a low pass filter, but they're required to be there parallel to the measured signal by the Intel testing specs. I'm not sure why they were there in the first place though. They cause slight filtering at the test point; a 10uF medium ESR electrolytic, and a 0.1uF ceramic. The ceramic acts super-low-ESR for high frequency noise, the 10uF generally takes the edge off some of the lower frequency stuff. Maybe the Intel engineers figured they'd use them to remove EMI picked up by the cables? But that seems like a sloppy solution.Hm, how much difference would there be between putting the capacitors in front of the 450 ohm series resistor or behind it?

author: Rufus
author_rank: Super Contributor
timestamp: November 02, 2014, 12:11:40 am
content: Quote from: Phaedrus on November 01, 2014, 11:18:55 pmA huge difference and that is where they are required to be by the specification. What the thing plugs into will have capacitance like that (and more) so they are there to make the environment the power supply is tested in more realistic.

author: David Hess
author_rank: Super Contributor
timestamp: November 02, 2014, 03:41:55 pm
content: Quote from: Rufus on November 02, 2014, 12:11:40 amThe decoupling and bulk capacitance is on the other side of the input resistors across the load. As shown, the high frequency cutoff is 320 Hz. When the capacitors are on the load side, then they are driven by the much lower output impedance produced by the regulator and load raising the high frequency cutoff considerably.If you are only interested in line and load regulation, then the schematic shows a good way to measure it in a noisy environment without a x1 or x10 probe but if you want to see transient response and noise, then the capacitors need to be moved to the other side of the input resistor or placed in series with it for AC coupling.I looked through a couple of Intel documents on testing their VRM designs but did not find anything directly applicable other than their requirements for bandwidth to distinguish high frequency noise from load response.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: November 02, 2014, 04:11:24 pm
content: Quote from: alank2 on November 01, 2014, 08:17:23 pmWhat? Have you seen the center conductor of scope probes?

author: David Hess
author_rank: Super Contributor
timestamp: November 02, 2014, 04:25:35 pm
content: Quote from: Alex Eisenhut on November 02, 2014, 04:11:24 pmBesides being thin, the center conductor is some nickel alloy making it difficult to solder to. You can either crimp it or plate the end with something like copper.

author: Rufus
author_rank: Super Contributor
timestamp: November 02, 2014, 04:44:18 pm
content: Quote from: David Hess on November 02, 2014, 03:41:55 pmPage 21 of thishttp://www.formfactors.org/developer/specs/ATX12V%20PSDG2.01.pdfis probably where the OP is coming from.Says use a Tek P6046 differential probe or equivalent.

author: BravoV
author_rank: Super Contributor
timestamp: November 02, 2014, 04:57:08 pm
content: Quote from: Rufus on November 02, 2014, 04:44:18 pm+1 , its discussed while ago here ->"Recommended" method of measuring ATX PSU output

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: November 02, 2014, 05:28:38 pm
content: Quote from: David Hess on November 02, 2014, 04:25:35 pmYup, sometimes I feel like I'm missing the point of the conversation. Why would anyone buy scope probes to cut off the ends to a pigtail when you can buy BNC to alligator clips coax by the boatloads from China?http://www.ebay.com/itm/BNC-Q9-to-dual-Alligator-clip-Oscilloscope-test-probe-cable-length-1M-3FT-new-/331363634856?pt=LH_DefaultDomain_0&hash=item4d26cfcea8

author: Marco
author_rank: Super Contributor
timestamp: November 02, 2014, 05:33:58 pm
content: Quote from: Alex Eisenhut on November 02, 2014, 05:28:38 pmI doubted they used lossy coax for those.

author: David Hess
author_rank: Super Contributor
timestamp: November 02, 2014, 10:32:09 pm
content: Quote from: Rufus on November 02, 2014, 04:44:18 pmWell, I was looking in the wrong place.The capacitors as shown will filter out some of the highest frequency noise which is important for a peak to peak measurement. If they are on the coaxial side of the 450 ohm resistor, then the cutoff frequency will be an orders of magnitude lower invalidating the measurement they want.They recommend using a P6046 differential probe to remove common mode noise and prevent a ground loop through the oscilloscope AC ground which could produce misleading results. Given the magnitude of the noise and ripple they are looking for, I would use x1 or x10 passive probes or x10 low-z probes connected as shown and use two channels of the oscilloscope to make the differential measurement. The results will not be as good as using a real differential probe but should be adequate for this type of measurement and I am not sure a modern equivalent to the P6046 exists anyway. The P6046 differential probe may have been the longest running product Tektronix made for good reason.Oddly enough if using two separate probes, this is one of those places where an analog oscilloscope or old DSO with analog circuit paths would work better than many modern DSOs. At low sample rates high frequency noise can alias before being subtracted. If you use a real differential probe then this is not a problem.If you can verify that the oscilloscope probe ground connection is not interfering with the circuit, then you can use a single ended probe connection and avoid all of the messy differential measurement stuff.

author: alank2
author_rank: Super Contributor
timestamp: November 03, 2014, 12:38:14 am
content: Quote from: Marco on November 02, 2014, 05:33:58 pmExactly, I want to see if I can make a non-bulky 10X probe with just two 2" pig tails... Something similar to this perhaps, but less bulky:http://www.probemaster.com/product_info.php?cPath=1_39&products_id=308

author: alank2
author_rank: Super Contributor
timestamp: November 03, 2014, 07:39:52 pm
content: I had a real 1X probe show up from eBay today I thought I'd test a bit. It does have a lossy cable, resistance was around 210 ohms or so measuring the bnc center pin to the probe tip. I think the probe is a M12X1 Coline probe at least that is what it is labeled.Signal is coming from a metal square 20MHz oscillator I have, 5V square wave.R1 - 10X probe, might be Rigol RP1300 or probemaster 10X fixed, I can't remember which I tried, prob probe master which has not been compensated yet.R2 - Coline 1X probe with spring clipR3 - My RG58 bnc 1X probe, just has 2 1" pigtails right from the cut coax.CH1 - My RG58 bnc 20X probe (made using the instructions here with a 1K resistor, low cap probe) with 50ohm scope term enabledR3 - Can I conclude that the bumps mid wave are because there is no lossy coax and the signal is reflecting back because it isn't terminated into 50 ohms? Would that be right?R2 - Why the more square wave point at the lower left and upper right and the softer transaction in the upper left and lower right?I do have some cheap eBay 100MHz "6100" probes coming I am going to cut one off and see if I can make a compact no bulk pig tail 10X probe that works decently...Thanks,Alan

author: tggzzz
author_rank: Super Contributor
timestamp: November 03, 2014, 07:50:06 pm
content: Quote from: alank2 on November 03, 2014, 07:39:52 pmProbably a mixture of problems caused by the high capacitance (C ~= 100pF?) of the coax: RC time constant (where R is the source impedance), and LC ringing with the inductance of the ground lead (rule of thumb L=1nH/mm)QuoteProbably RC time constant of the source output resistance and cable capacitance.Quote

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 04, 2014, 02:02:26 am
content: Quote from: alank2 on November 03, 2014, 07:39:52 pmLooks pretty good. A little rounding compared to the "no probe", but probably quite reasonable.It's unusual that a probe actually rolls off at its stated bandwidth; normally that's just where they stop specifying. There may be dips and peaks at higher frequencies, but from an instrumentation standpoint... you're probably better off filtering those frequencies, than trying to measure them.QuoteVery round, as you should expect. But also just a little snap to it, interestingly enough. More on that later...QuoteAs awful as you'd expect!The bounce is clear, and even a second return is probably visible too. This would be due to the wave bouncing between scope and source; neither is quite perfectly matched.If the scope end were unterminated but the source was terminated, you'd see a single rising edge (with some sloppiness due to loss in the cable), going up to nominal voltage, and no subsequent steps.Because: at the instant the source switches, the source impedance acts as a voltage divider with the line, and because they are matched, the source terminal voltage rises to half the switched voltage. This half height wave zips down the line, and reflects off the scope end. At the point of reflection, it doubles up on itself, so by superposition, the amplitude goes from zero to full in one step. As the wave continues back, it charges the rest of the line, from scope to source, to full voltage. When the wavefront hits the source, it gets absorbed back into the source. Which, because it was delivering half load current and a half-height wave hit it, the current cancels, the voltage doubles (up to full amplitude), and current flow ceases. The situation then remains stable until the next event.Likewise, if the source was unterminated but the scope was, the source impedance would divide arbitrarily with the line / terminator impedance, so you wouldn't be measuring the full amplitude, but you would be measuring the correct waveform (no waves reflect off the terminated scope to bounce around on the line).Which is what you're doing with the 20:1 "no-probe", by artificially increasing the source impedance -- and simultaneously setting the attenuation factor to a conveniently round number so you can still calibrate the amplitude scale.QuoteThis trace is quite noisy, was it on a very low (mV/div) scale to get this reading? (The screen says 4V/div, but that's just what it's calling the display scale, not what's actually at the input.)The tricky part about data analysis is, when you're working with a few bits worth of data (or the equivalent amount of analog noise -- or both), it's hard to draw firm conclusions from it. So, this wave does appear sharper than the 10x probe does, but it could just be the noise making it look that way.QuoteYes!QuoteNot sure what you mean, unless you're referring to the initial rise (the "toe", when the voltage begins to change, say in the first 0-1V) as compared to the final rise (or settling, or "knee", as the voltage settles to 4-5V)?If that's the case, then it's because the probe response approximates a well damped filter (only real poles* on the complex plane), and the response is a series of simple exponentials:v(t) = 1 - A_0 e^(-c_0 t) + A_1 t e^(-c_1 t) + ... + A_n t^n e^(-c_n t)*It's not necessary that it be "all real", but this is a simple case to illustrate.And, quite simply, as you'll see from plotting curves in this family, that's just how the exponent behaves: it goes fast, then slow. Or, if you prefer a dynamic perspective: the voltage difference drops over time, so the current (force causing the voltage to equalize) drops over time. So, the rate constantly slows down, and the voltage never quite reaches exactly its full value. (In reality, once that difference is down in the noise floor or offset error of your instrument, you can't care less.)Now, about the lossy 1x probe: it's interesting, because it's still a transmission line. It has finite resistance, and therefore, it neither fully attenuates the incident wave (= what you see on the screen), nor the reflected wave (which goes back to the source, and potentially reflects again, and so on). I suspect... there is a characteristic of this probe, where, after the wave goes down, it's partway charged up, but just a little bit reflects off the scope, goes back (getting attenuated even more), bounces off the source, and returns in a third trip, just barely enough to make the amplitude settle much more suddenly than, say, a single pole (RC) filter would.I marked this point in the picture below: the time from initial "toe" to that apparent, not really so much a peak, but almost more like an inflection point, with the time C.The other points are:A is apparently the electrical length of your 1x "not even trying to be a" probe,B is approximately the rise time of the 1x (lossy) probe.What's interesting, is C looks a lot like A, which is probably quite reasonable, because I bet the two cables are similar length, and that this says something about the proper matching of probe and the choice of cable.Tim

author: tld
author_rank: Regular Contributor
timestamp: November 04, 2014, 07:46:37 am
content: Quote from: Phaedrus on November 01, 2014, 05:36:33 amIf this is for work, and management is sceptical of modifying, I'm tempted to suggest simply having them throw money at the problem to make it go away. Figured I should mention these two real quickly:BNC to BNC 10x probe:http://www.probemaster.com/product_info.php?cPath=1_39&products_id=309BNC to dual lead 10x probe:http://www.probemaster.com/product_info.php?cPath=1_39&products_id=308In this case, a DIY-solution to save them money only makes sense if they're for it. If they're going to fight you on saving them money, just don't. Or give them the choice. "We could grab these expensive ones, or I can whack something together real quick to save us $100.".Personally I'd be tempted to go DIY for the learning if it was hobby/educational, but opt for expensive if it was for work, even if just to save myself from the potential "Anything I didn't think of?"-things. There are always enough problems that needs fixing, solutions that need to be found, things that needs to be done, or potentials that want to be realized anyway.tld

author: alank2
author_rank: Super Contributor
timestamp: November 07, 2014, 07:40:54 pm
content: It is for fun and learning.I got the cheap chinese probes in today. I cut one off and measured about 100 ohms in the 1M length.I soldered on a smd 0603 9M resistor without too much difficulty along with a metal pin. This on its own will not calibrate into a square wave. I added a 10pF cap I had (one with leads I don't have any smd ones) and it has edges now, but still not square. Do you guys think I need to add more capacitance? Other ideas?Thanks,Alan

author: w2aew
author_rank: Super Contributor
timestamp: November 07, 2014, 09:14:58 pm
content: Quote from: alank2 on November 07, 2014, 07:40:54 pmWatch this tutorial to learn how 10x probe compensation works.... (hint, the compensation capacitor value depends on the scope input capacitance which varies by scope manufacturer/model/bandwidth, that's why it is adjustable)

author: alank2
author_rank: Super Contributor
timestamp: November 07, 2014, 09:45:55 pm
content: How does that related to one that adjusts at the BNC connector? I tried both a 15pF and 13.3pF across the 9M resistor and while I could calibrate both to a square wave using the scope calibration, the test 10 MHz signal I was looking at still looked pretty bad...

author: w2aew
author_rank: Super Contributor
timestamp: November 07, 2014, 09:58:41 pm
content: Quote from: alank2 on November 07, 2014, 09:45:55 pmFor probes that have the adjustment at the BNC connector end: These typically have a fixed capacitor across the 9M resistor at the probe tip, and then a variable cap from the input to ground at the BNC end (in parallel with the scope input capacitance). In both cases, you adjust them to get a good response on the 1kHz compensation signal (which happens when the capacitive divider ratio equals the resistive divide ratio).At 10MHz and above, the pulse response is going to be dominated by parasitics (ground lead inductance, etc.), not the compensation adjustment. If you're trying to build your own 10x probe, you'll quickly discover why high performance passive 10x probes get pricey - they're not simple to do.

author: tggzzz
author_rank: Super Contributor
timestamp: November 07, 2014, 10:29:05 pm
content: Quote from: w2aew on November 07, 2014, 09:58:41 pmTrue for passive nominally-high-impedance *10 probes, but... at a few hundred MHz they aren't high impedance any more. Better to use a passive low impedance "Z0" *10 probe, which actually has equivalent or higher impedance, and they are relatively easy to make yourself.There are a few specialised passive high impedance *10 probes which are usablewith great care and attentionat 350-500MHz. Principal features: 6.5pF tip capacitance and a 10pF scope capacitance.

author: alank2
author_rank: Super Contributor
timestamp: November 07, 2014, 11:52:39 pm
content: Quote from: w2aew on November 07, 2014, 09:58:41 pmYep, I'm getting that. I can calibrate it with a 15pF or 13.3pF cap across the 9M, but it still looks terrible. The chinese P6100 probes are not very good (as expected).P6100 with 20 MHz oscillatorMy cut and reworked probe, 9M R with 15pF across it:All this really makes me appreciate the Rigol RP1300's I usually use:Honestly my 1X RG174 bnc cable to pigtails didn't look any worse than the cut and reworked 10x probe, the signal just bent in a different direction. Maybe my *dream* of having a simple no bulk probe 1x or 10x for slower signals <20MHz is more trouble than it is worth!

author: w2aew
author_rank: Super Contributor
timestamp: November 08, 2014, 02:08:54 am
content: Quote from: tggzzz on November 07, 2014, 10:29:05 pmAbsolutely right, but you've got to have to 50ohm termination at the scope for them. Very easy to get several GHz BW with a well designed 10x Z0 probe.QuoteTrue. The new 10x passive probes for the 3000/4000 series Tek scope have 3.9pF load capacitance, resulting in usable 1GHz BW.

author: Rufus
author_rank: Super Contributor
timestamp: November 08, 2014, 02:44:13 am
content: Quote from: w2aew on November 08, 2014, 02:08:54 amStill going to look like about 40 ohms at a gig though.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 08, 2014, 02:57:01 am
content: This is an inverse filter problem:Every time the waveform rises or dips, it's because there's a zero or pole in the frequency response (respectively, assuming all real). What you need is not an R||C, but a network which is complementary to the response of the cable itself.As you've discovered, the cable alone is not simply a resistance and capacitance, so it isn't sufficient to compensate it with the same at the tip!This is what you pay for in a probe: the correct values, already optimized, and embedded in a robust plastic handle.To design this yourself, you're best off iterating a procedure like this:- Measure the fully uncompensated response. There will be a dominant RC time constant. Calculate your first guess C in proportion to this (Ccomp = tau / Rser).- Measure again; first, verify that the peak amplitude is where you want it (use a variable cap at first?). Also, observe how different points of the waveform vary as you vary that cap.- Try to adjust it so that it's flat further down the wave, without dipping at all. Don't get greedy and attempt to find the fastest risetime first.- Repeat. Measure the rise time, pick another RC, and put that in parallel with the R-C you already have. Etc.Tim

author: Marco
author_rank: Super Contributor
timestamp: November 08, 2014, 04:01:31 am
content: Quote from: alank2 on November 07, 2014, 11:52:39 pmThe only type of passive probe with massive ground leads like this which can work well which I am aware of is theBalanced coaxial probefrom Douglas Smith. Does not work to DC and the combiner looks expensive though.You might be able to do a cheap 180 degree combiner by using multiple ferrite chokes of decreasing frequency coverage, ground the shields and one of the coax conductors (with a DC blocking capacitor) at the scope. Put the chokes across both cables with the lowest frequency coverage closest to the scope. I'm not entirely sure how this should be terminated, but I think with 100 Ohm.

author: tggzzz
author_rank: Super Contributor
timestamp: November 08, 2014, 10:18:51 am
content: Quote from: alank2 on November 07, 2014, 11:52:39 pmThe ground lead ismuchtoo long and has too much inductance.Have a look at the pictures of the 6GHz probe inhttps://www.eevblog.com/forum/testgear/teardown-hp-54006a-6-ghz-passive-probe/or the HP10020A/HP54006A inhttp://www.metrictest.com/catalog/pdfs/product_pdfs/hp_1143a.pdf

author: tggzzz
author_rank: Super Contributor
timestamp: November 08, 2014, 10:27:23 am
content: Quote from: Rufus on November 08, 2014, 02:44:13 amPrecisely. Whereas the passive Z0 probes have "<0.7pF" (HP10020A) and 0.25pF (HP54006A), and so the "low impedance" probes are actually much higher impedance at those frequencies.The 1.5GHz HP10020A dates from 1972, so it is scarcely new technology.The 6GHz HP54006A is newer.Seehttp://www.metrictest.com/catalog/pdfs/product_pdfs/hp_1143a.pdffor images

author: David Hess
author_rank: Super Contributor
timestamp: November 08, 2014, 10:49:17 am
content: Quote from: Rufus on November 08, 2014, 02:44:13 amProbe bandwidth is usually specified assuming a 25 ohm source resistance from a terminated 50 ohm source which is unrealistic in many cases but at least it is objective. They are expensive and difficult to find but probe tip adapters with a built in termination exist for making this test.

author: David Hess
author_rank: Super Contributor
timestamp: November 08, 2014, 11:00:43 am
content: Quote from: alank2 on November 07, 2014, 11:52:39 pmSome Chinese P6100 probes are better than others so I wonder if this is a case of a Chinese knockoff of a Chinese knockoff or poor quality control. When tested with a reference flat top pulse pulse generator, my 100 MHz P6100 probes work fine within their specifications and produce results which are indistinguishable from my better probes or a 50 ohm connection.I would be leery of relying on the pulse fidelity of a standard oscillator without checking it against a known standard. I use an old style sampling oscilloscope for this sort of thing.I do not remember if this was mentioned earlier in the discussion but Tektronix wrote a great guide to probe design and compensation which is available online:062-1146-00 - Oscilloscope Probe Circuits - November 1969http://www.ece.vt.edu/cel/docs/TekProbeCircuits.pdf

author: w2aew
author_rank: Super Contributor
timestamp: November 09, 2014, 01:36:16 am
content: Quote from: tggzzz on November 08, 2014, 10:27:23 amYes, of course. This is why I use an old Tek P6150 when probing UHF and above circuits. 9GHz of bandwidth, 500ohm resistive input impedance, shunted by about 0.15pF.

author: David Hess
author_rank: Super Contributor
timestamp: November 09, 2014, 07:16:35 am
content: Quote from: w2aew on November 09, 2014, 01:36:16 amIt has always puzzled me that there are no Chinese knockoffs of x10 and x100 low-Z probes. Even if they were only good to 500 MHz or 1 GHz, that would be an improvement over high impedance passive probes.

author: Marco
author_rank: Super Contributor
timestamp: November 09, 2014, 12:23:52 pm
content: I'd rather have an active 1x/10x probe (1x for low amplitude AC coupled signals) for a buck more in BOM cost, getting to a GHz has been done discrete.That said, not enough market for either.

author: alank2
author_rank: Super Contributor
timestamp: November 10, 2014, 01:32:00 pm
content: Quote from: David Hess on November 08, 2014, 11:00:43 amAll 4 of the P6100's I have look the same. Well, the 3 that are not chopped that is.Quote from: David Hess on November 08, 2014, 11:00:43 amI tested a few probes that I had yesterday for fun on the 20 MHz oscillator.RP1300 - the top and bottom of the square wave looks just like my picture above.RP3300 - still looks very much like the RP1300, but maybe not as square on the top rising edge and bottom falling edge.RP2200 - I was surprised at how bad these looked. Considerable overshoot such that it doesn't look like a square wave. I tried two of them, same results. I made sure they were compensated, same results.Probe Master 150MHz 4904-2 (150MHz,15pF,1.5M length) - Looked like the RP3300 but a bit more sloppy on the top rising edge/bottom falling edge. Not bad though, certainly much better than the RP2200. Makes me wonder how nice the 250MHz 4905 model would look.Quote from: David Hess on November 08, 2014, 11:00:43 amI read through some of this and skimmed some of this, it looks like a great resource. It is funny to see typewritten pages these days!David - if you were trying to make a low bandwidth low bulk probe as I have been trying and not doing well at, what would you do? Someone mentioned that my ground lead was too long above, but I think it is less than 2". The signal looked the same even when I had it 1cm long and just pressed it against the top of the oscillator metal can. I will say that my 10X chop off test with a 9M resistor and 15pF cap across it did begin to resemble what the probe looked like before I cut it, which is to say not as clean as I wanted. I wonder if I had cut a nicer probe off (one that looked good in the first place) if I would have gotten better results. I wonder if that would be cutting off the good part or if the good part is the lossy cable and adjustment box at the bnc end...

author: David Hess
author_rank: Super Contributor
timestamp: November 10, 2014, 04:35:18 pm
content: Quote from: alank2 on November 10, 2014, 01:32:00 pmI have seen tests of several different "P6100" probes which were rated for different bandwidths and the higher bandwidth ones, above 100 MHz, were terrible. If you bought yours all at the same time from the same source then I would expect them all to perform the same.QuoteThe faster probes should show more overshoot if it exists in the original signal. The slower ones should show a bandwidth limited version of what the faster ones show. If the slower probes are showing more overshoot, then either their increased loading is affecting the source or something is very wrong with the probe. The RP3300 and RP2200 have about the same input capacitance so I doubt it is a problem with the signal source.Naturally in Rigol's thorough way of leaving out what is important, the table on their web site describing their probes leaves out tip capacitance.While it does not represent real world usage very well, the objective way to test probes and the way they are specified is with with a feedthrough terminator on a 50 ohm source and a coaxial probe tip adapter. This removes the effects of the ground lead and the probe sees a 25 ohm source impedance.QuoteThe easiest way is just to use a x1 or x10 high impedance passive probe which can be semi-permanently installed using a coaxial probe tip adapter.If the probe itself is too bulky, and this is what I have done in the past, solder a short coaxial pigtail directly to the test point and ground and connect the other end of the pigtail to the x1 or x10 probe using a coaxial to probe tip adapter. I like using RG-316 cable for this because soldering will not melt it but RG-174 works just as well. The cable impedance does not matter because the length is short and the frequencies of interest are low so you could use 75 ohm cable like RG-179 which you might find in an old VGA adapter cable with BNC connectors already installed. The capacitance of the cable adds to that of the probe tip but the output impedance of the power supply is very low so this does not matter.If you do not have a coaxial to probe tip adapter, as a test try connecting the probe tip and ground to the bare wires on the end of the short coaxial cable. Since the frequencies you are interested in are low, the lack of a coaxial adapter probably will not matter.A direct coaxial connection is trickier because you want to protect the 50 ohm termination at the oscilloscope from high DC voltages. To do this, I would use a 50 ohm cable with a 50 ohm termination on the oscilloscope side and a 450 ohm resistor in series with an AC coupling capacitor made up of several capacitors in parallel on the probe side. Bandwidth limiting depends on the oscilloscope in this case. A 10 microfarad solid tantalum capacitor in parallel with a 0.1 microfarad ceramic capacitor would yield a low frequency cutoff of about 30 Hz.

author: w2aew
author_rank: Super Contributor
timestamp: November 10, 2014, 06:36:12 pm
content: Quote from: David Hess on November 10, 2014, 04:35:18 pmYou can always make yourself a high performance probe adapter/socket, with some bare tinned wire and some patience...

author: alank2
author_rank: Super Contributor
timestamp: November 10, 2014, 10:05:18 pm
content: Another try with more patient soldering. 9M resistor 15pF capacitor:Top trace is the cut probe pictured above, the bottom probe is a stock P6100 probe:Any ideas what I can do to the cut one to make it look more like the stock one?Thanks,Alan

author: David Hess
author_rank: Super Contributor
timestamp: November 10, 2014, 11:34:37 pm
content: Quote from: alank2 on November 10, 2014, 10:05:18 pmUse less capacitance. I would have started off using a small trimmer capacitor.

author: alank2
author_rank: Super Contributor
timestamp: November 10, 2014, 11:37:09 pm
content: Quote from: David Hess on November 10, 2014, 11:34:37 pmMy first attempt with 10pF wouldn't compensate. I can try to use multiple caps to go to 11 or 12 or 13.3 perhaps...

author: alank2
author_rank: Super Contributor
timestamp: November 11, 2014, 01:00:27 am
content: I found a 2-27pF trimmer and soldered it up. I tried both ends of the trimmer that would compensate and neither looks any better. That initial spike is always there.

author: alank2
author_rank: Super Contributor
timestamp: November 11, 2014, 01:11:51 am
content: I found a 2-27pF trimmer and soldered it up. I tried both ends of the trimmer that would compensate and neither looks any better. That initial spike is always there.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 11, 2014, 03:38:09 am
content: Try a series resistor in addition to that.

author: David Hess
author_rank: Super Contributor
timestamp: November 11, 2014, 05:11:01 am
content: If you are going to try and duplicate a x10 high impedance probe using a 9 Mohm resistor and standard coaxial cable then you are going to have problems. Page 14 of the Tektronix document I linked describes the issue; you need an unbypassed resistance in series with either or both ends of the cable to improve the high frequency compensation and even that will only get you to 30 MHz or so before ringing becomes a problem.A x10 50 ohm probe with a 450 ohm series resistor or a x100 50 ohm probe with a 4950 ohm resistor will be much easier.

author: alank2
author_rank: Super Contributor
timestamp: November 11, 2014, 01:08:42 pm
content: Quote from: David Hess on November 11, 2014, 05:11:01 amI was attempting it with a lossy coax - that is why I bought the P6100 probe to cut it.T3sl4co1l I will try adding the resistance - I got tired of it last night and made a 1X probe out of the one I was playing with. It isn't too bad up to around 10 MHz or so.

author: David Hess
author_rank: Super Contributor
timestamp: November 11, 2014, 02:42:37 pm
content: Quote from: alank2 on November 11, 2014, 01:08:42 pmSorry, that was my mistake. The coaxial cable shown in the photo does not look like what I would expect from a P6100 probe.

author: alank2
author_rank: Super Contributor
timestamp: November 11, 2014, 02:44:35 pm
content: Quote from: David Hess on November 11, 2014, 02:42:37 pmNo worries, I do wonder if I got some knockoff knockoff P6100's given their performance... They were qty 4 for $23 bucks or something.

author: David Hess
author_rank: Super Contributor
timestamp: November 11, 2014, 03:33:42 pm
content: Quote from: alank2 on November 11, 2014, 02:44:35 pmThat is what I suspect happened although I am not going to cut one of mine apart to find out. It would be easy enough for an unscrupulous manufacturer to substitute standard coaxial cable for the lossy cable used in good oscilloscopes probes and the fact that you can solder to the center conductor indicates that this happened.My P6100s display nearly ideal 100 MHz performance with a 3.5 nanosecond rise time and aberrations which are no larger than those from my oscilloscopes themselves.

author: alank2
author_rank: Super Contributor
timestamp: November 11, 2014, 03:42:22 pm
content: The center conductor in the 1M P6100 I cut measures 100 ohms.So your P6100 do not display the mid cycle bump seen here?

author: David Hess
author_rank: Super Contributor
timestamp: November 11, 2014, 07:10:24 pm
content: Quote from: alank2 on November 11, 2014, 03:42:22 pmSo much for that idea then. My P6100 probes measure 300 ohms from the tip of the probe to the center pin of the BNC but that would include any lumped resistances in series with the center conductor. If yours is 100 ohms, then you could try adding a 200 ohm resistor to the probe end of the cable.QuoteNo, they do not or at least not to an extent that I can measure accurately.Using a ~600 picosecond reference flat top pulse generator intended for oscilloscope calibration (Tektronix PG506), my P6100 probes display a transition time of exactly 3.5 nanoseconds on a 100 MHz Tektronix 2232 oscilloscope. The transient response is a little uneven out to 15 nanoseconds but matches the specifications of the oscilloscope with a variation of about 1 minor division out of 5 major divisions which is 4%. Your photos show a variation more than 4 times greater than that while your earlier tests show that your signal source and oscilloscope are pretty good and much better than that.Interestingly enough in DSO mode on the 2232, the variation is significantly less and settles within 5 nanoseconds but I recalibrated the digital mode transient response not long ago and have never fiddled with the analog transient response. Maybe I need to think about recalibrating the analog transient response.

author: alank2
author_rank: Super Contributor
timestamp: November 11, 2014, 07:35:01 pm
content: Quote from: David Hess on November 11, 2014, 07:10:24 pmAll 3 of the uncut ones I have set to 1X measure 100 ohms - so I guess all P6100 probes are not the same... Is it likely they switched the coax type causing a change from 300ohm to 100ohm and this is the difference? Will adding a 200 ohm resistor at the end outside of the 9M/13.3pF in parallel be similar?

author: David Hess
author_rank: Super Contributor
timestamp: November 11, 2014, 07:51:56 pm
content: Quote from: alank2 on November 11, 2014, 07:35:01 pmSwapping 200 ohms of cable for a 200 ohm resistor is not equivalent but I was thinking that the P6100 probe head might include a 200 ohm resistor as part of the compensation network so you should add that as well when trying to duplicate its performance.The measurement I made does not distinguish between 300 ohms of cable and 100 ohms of cable plus a 200 ohm resistor.

author: alank2
author_rank: Super Contributor
timestamp: November 12, 2014, 12:49:30 am
content: I measured the cut off probe and it has very little resistance in it when in 1x mode (<0.2ohms)

author: David Hess
author_rank: Super Contributor
timestamp: November 12, 2014, 03:19:56 am
content: Quote from: alank2 on November 12, 2014, 12:49:30 amThe extra 200 ohms would be added in x10 mode and not x1 mode and it would not be apparent with the 9 Mohm resistor in series with it.

author: alank2
author_rank: Super Contributor
timestamp: November 12, 2014, 10:18:11 pm
content: So I don't know when to quit... I decided to cut up a poor Probe Master 150MHz probe. Instead of multiple strands in the center (the fake P6100's), it only has a very small single strand. My initial tests with a 40 MHz square wave aren't too bad with my 15pF/9M and the trimmer/9M that I have, but I decided to take the probe apart even more:T3sl4co1l's idea of adding some more resistance in series with the 9M/cap in parallel is being done here. The small resistor is 101.8 ohms.The large one is 9.1M.My peak LCR shows the cap (goofy white looking thing with a hollow center at somewhere between 5.7 and 7.4 with 6.6 being the most common reading.Can anyone read the lines on the cap?

author: alank2
author_rank: Super Contributor
timestamp: November 12, 2014, 11:02:06 pm
content: This is much better. I am seeing a 40MHz square wave that looks pretty good with my 9M/trimmer+100ohm soldered mess.One question I have is does the capacitance at the probe end only balance the capacitance at the bnc adjustment? In order words, there is a range I can adjust my trimmer at the probe end and still get the bnc end adjustment to compensate properly. Is that entire range ok, or is one end of it better or the center of it better?

author: David Hess
author_rank: Super Contributor
timestamp: November 12, 2014, 11:34:39 pm
content: Quote from: alank2 on November 12, 2014, 10:18:11 pmThat looks like a tubular ceramic capacitor or "dogbone" capacitor but the color code seems to be missing a stripe. That must be a pretty old probe.Quote from: alank2 on November 12, 2014, 11:02:06 pmThat seems unlikely but it is possible if there is not enough high frequency content because either the source lacks or or the probe is filtering it out. The capacitor which is across the 9 Mohm resistor should adjust the low frequency compensation and the 100 ohm resistor should change the high frequency compensation.

author: alank2
author_rank: Super Contributor
timestamp: November 13, 2014, 01:50:08 pm
content: I don't have the right parts to try this yet, but I did have some 0805 resistors that could be used to put together two of them in parallel and then one of them in series to that. I soldered the lossy coax wire to the parallel side and a wire to the other side pretty easily. My method is to tape them down with masking tape until they are soldered! My one concern with trying this is the strength of that center conductor being the only thing holding on the signal 2" pigtail, but I did a test with some heat shrink and that stuff has some amazing holding strength clamped down on the white insulator of the coax, so I think it is going to be fine.The dogbone capacitor measures 7.5pF now that I've cut it off the tiny pcb, so that is what I am going to try.The cable is 1.45M right now and I would like to cut it down to 1M. It measures about 320 ohm, but my peak meter (lcr40) sees it as resistance and won't measure its capacitance which is what I was trying to do. If I cut it down, do I need to change the 7.5pF cap? I read through the Tek document you posted and it would suggest yes, but I don't know how much.Thanks,Alan

author: alank2
author_rank: Super Contributor
timestamp: November 15, 2014, 10:59:47 pm
content: I got some parts in and made a probe with a 5-20pF trimmer, 9M smd 0805, and 100R 0805. Have about 2" pigtail.I can get the 40MHz signal to look pretty nice by tweaking the trimmer in the bnc jack side, but then if I connect it to the much slower scope compensation test point, it is way off. If I adjust it to look perfect there, it doesn't look as good at 40Mhz. How does this relate to the capacitance across the 9M at the tip side of the cable? I've tried playing with it a bit, but smd resistors are unforgiving and I snapped off the end of the 100R and my temporary probe tip...edit: I made up one with the 7.5pF cap - it is a bit challenging to solder the cap and 9.1M on top of each other with the coax center conductor between them, then add the 100R to the end of that. Not a lot of strength. I ended up using a toothpick to give it some body and shrink wrapped it all together. Seems pretty solid now. Actually looks pretty nice even up to my 80MHz square wave. The only issue is that the amplitude is 6.309V when it was 5.515V before with the probe.Any idea about the amplitude difference? I am using the same size components...

author: David Hess
author_rank: Super Contributor
timestamp: November 16, 2014, 02:59:35 am
content: Quote from: alank2 on November 15, 2014, 10:59:47 pmThe low frequency compensation is wrong. This is how 5 volt logic circuits appear to put out 10 volts.

author: alank2
author_rank: Super Contributor
timestamp: November 16, 2014, 03:12:35 am
content: Quote from: David Hess on November 16, 2014, 02:59:35 amDoes this mean that the 7.5pF cap I am using is too big? A smaller cap would yield a larger reactance which would yield more attenuation, do I have that right?

author: David Hess
author_rank: Super Contributor
timestamp: November 16, 2014, 11:03:35 am
content: Quote from: alank2 on November 16, 2014, 03:12:35 amYes. The low frequency compensation is done with a low frequency square wave, like 1 kHz, and if it is off, then high frequency square or sine or other waves will have the wrong amplitude but otherwise could look good.

author: alank2
author_rank: Super Contributor
timestamp: November 16, 2014, 01:03:57 pm
content: Quote from: David Hess on November 16, 2014, 11:03:35 amRigol RP1300 compensation voltage (amp) - 2.997VMy probe compensation voltage (amp) - 2.974VIt is a bit too small at 1kHz, but too large at 40MHz...

author: alank2
author_rank: Super Contributor
timestamp: November 16, 2014, 01:13:17 pm
content: So, for a probe that has an adjustment in the BNC connector, is there ANY benefit to having an adjustment at the probe tip as well? Will having both allow for more compensation/calibration that just using a fixed one at the probe tip?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 16, 2014, 01:13:30 pm
content: If it's too large (or small) past a certain frequency, simply add another R+C or R||C network to adjust it.Don't be surprised if, for three digits accuracy, you need a huge pile of them...Tim

author: David Hess
author_rank: Super Contributor
timestamp: November 16, 2014, 05:49:37 pm
content: Quote from: alank2 on November 16, 2014, 01:13:17 pmIn practice it is better to minimize the probe tip capacitance and move the compensation to the BNC connector but both designs work. I have never seen one with low frequency compensation at both ends but it is common to have the low frequency compensation at the probe and the high frequency compensation at the BNC.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: November 24, 2014, 06:05:41 pm
content: So we got our new cables with 50 ohm terminators. Just some preliminary testing without the 450 ohm series resistor gave us this result (Ch1 is old 75ohm cable, Ch2 is new cable)So we've gotten rid of a lot of the high frequency crap that was throwing off our measurements before, probably just from eliminating reflection. The result overall looks very good! The only thing strange is I was expecting more DC attenuation from the 50 ohm terminator; but aside from the high frequency noise the signal amplitude looks pretty much the same. Is the 450 ohm series resistor strictly necessary? I'm going to have to attenuate the +12V signal regardless, due to the terminator's 2W power rating, but if I could skip it on our +5V and +3.3V signals that would be great.EDIT: Actually I'm going to have to attenuate all of them anyway or the power consumption will throw off our efficiency measurements. Still would like to know for the record anyway.EDIT 2: Also, if it isn't clear we're using a 50 ohm pass thru terminator, then 1M termination on the scope. The reasoning being that if the technician accidentally applies a large DC voltage, or if DUT isolation breaks down, we'd rather lose a $25 terminator than a channel on our $3000 scope.EDIT 3:Whipped up a perfboard prototype of the test point with 450 ohm resistor. Results exceed expectations, though I'm not sure if my amplitude is strictly accurate.Ch1 = 50 ohm terminated cable with 450 ohm series resistor, X10 attenuationCh2 = 50 ohm terminated cable, X1 attenuationCh3 = 75 ohm unterminated cable, X1 attenuation

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 25, 2014, 03:04:32 am
content: Quote from: Phaedrus on November 24, 2014, 06:05:41 pmA few weeks ago, I hooked up, what was it, I think it was a current limited 150V supply, essentially -- the probed signal was at supply voltage -- to my terminated scope input. , popup on the screen reads: the terminator was disconnected because it was overheating.Well, sometimes I'm dumb, but at least Tek still had some good ideas in that age (TDS460 -- one of the first series scopes, during their "good instrument / bad interface / no service" early decline).Did they drop that feature? Does anyone know if Rigols monitor this?Tim

author: Phaedrus
author_rank: Frequent Contributor
timestamp: November 25, 2014, 04:56:59 pm
content: There's two things I never want to stake the safety of my life or property on: firearm mechanical safeties, and equipment thermal overload protection.

author: alank2
author_rank: Super Contributor
timestamp: December 09, 2014, 01:37:04 pm
content: So my tiny pcb came in, it is a 4mm x 20mm little adapter that has the coax at one side and pigtails at the other. It has the 9M/7.5pF cap in parallel and the 100R in series with them.I could not compensate properly with the scope compensation with only the 7.5pF so I took it off and put an adjustable 5-20pF cap on it. I can compensate at the 3kHz, but at higher frequencies it is not attenuated enough. I am looking at a 50MHz square wave at 5V and my Rigol probe shows an amplitude of about 4.6V. My home built probe with adapter is about 5.7V. I can adjust the 5-20pF cap on it and the cap in the bnc connector, but they do not adjust for the voltage being high. Do I need to adjust the 100R?Thanks,Alan

author: David Hess
author_rank: Super Contributor
timestamp: December 13, 2014, 01:56:31 pm
content: Quote from: T3sl4co1l on November 25, 2014, 03:04:32 amThis feature exists in many of their oscilloscopes predating the TDS460 and as far as I know, they have kept it in every oscilloscope made since then. It is trivial to implement in a DSO.There is also a way to implement instantaneous high bandwidth protection against overload using a current driven diode bridge which Tektronix used in the 7A29 vertical amplifier.I do not see anything in the Rigol documentation about overload protection of the 50 ohm termination.

author: c4757p
author_rank: Super Contributor
timestamp: December 13, 2014, 02:02:58 pm
content: Wait, some scopesdon'thave overload protection on the termination?What the fuck?My only scopes with internal termination were Tek 2400-series... Not only do they have it, they make you test it as part of the calibration procedure - something like "at W voltage, must not trip for X seconds, at Y voltage, must trip within Z seconds".

author: David Hess
author_rank: Super Contributor
timestamp: December 13, 2014, 09:04:31 pm
content: Quote from: c4757p on December 13, 2014, 02:02:58 pmMaybe it is considered a feature by the manufacturer in the sense of forcing payed replacements or repairs. Occam's Razor implies that they are just being cheap although it only requires firmware on a DSO to implement it. I would be happy enough if they just documented which part to replace and made it easy to do.QuoteOn old Tektronix analog oscilloscopes and DSOs which support 50 ohm internal terminations, I have always been surprised at how much extra effort they went to in protecting them from overload. They used several different custom ASICs for just this purpose.