Importance of Good Terminators for RF / Pulse Applications - Page 1
Forum section: electronics / testgear

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 12, 2014, 02:13:43 am
content: From previous threads, e.g.https://www.eevblog.com/forum/testgear/50-ohm-terminators/Here's an example of bandwidth and terminator use.My terminators seem to be pretty awful: not sure if it's the effect of the tee, or the terminators themselves (who would use a wirewound resistor in a terminator..??!), but see for yourself.Setup:Avalanche pulse generator, with 20ns pulse line10ns cable from generator to scopeAt scope: tee + terminatorFirst, Tektronix 475, 200MHz bandwidth. Looks a little lumpy, doesn't it? The risetime is mostly there, though.100MHz bandwidth. Still a little lumpy, so, whatever's causing those bumps, isn't really great for general purpose (Tek 465 and many other entry-level scopes are around this 100MHz baseline -- take note).20MHz bandwidth. Not lumpy, but you can't even tell it's a pulse anymore, it's just a... pile. Note the bandwidth reduction is 2.5 times greater than the step between the first two, so this is a pretty drastic change, rather than a progression.Tektronix TDS460, 400MHz bandwidth. You can see the wave does indeed look lumpy like that, plus some finer bumps that weren't visible before. Likely, this signal source doesn't have much energy beyond 400MHz, so that a scope of 1GHz wouldn't see much different, and would finally be observing the true risetime of this source.External terminator disconnected (tee remains), internal terminator switched on. The change is pretty dramatic!Tim

author: BillWojo
author_rank: Regular Contributor
timestamp: October 12, 2014, 04:29:22 am
content: And how would that look with a high quality terminator on the "T"? Also how long is a 10ns cable? I never heard of a patch cable being referance like that. All in all it's very interesting and informative. ThanksBillWojo

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 12, 2014, 06:12:56 am
content: Not sure, none of my terminators seem to be any better. It looks about the same if I shove a 1/2W carbon film resistor (minimal lead length) in the end of the BNC tee.Probably, a good quality inline terminator, thus as close to the scope as possible (which in my case is an undefined stub length, plus 15pF at the end), would do better, but by how much over an ideal termination (in my case, the internal 50 ohm), I can't quite tell.Tim

author: David Hess
author_rank: Super Contributor
timestamp: October 12, 2014, 02:05:56 pm
content: There was a discussion about terminators here some months ago where I did some tests comparing feedthrough termination to internal termination to a good terminator attached to a BNC-T.The feedthrough terminators, even the "slow" ones, that I have perform just as well as the internal termination on my 300 MHz Tektronix 2440. Tests using a good termination on a BNC-T showed noticeable not not necessarily significant degradation below 100 MHz. I think I decided that the practical limit was about 70 MHz. I did not have any Ethernet "thinnet" style terminators to try (I think they evaporated over the years.) but I suspect they would perform more poorly.My pulse source for these tests was the less than 600 picosecond transition time from my PG506 flat level pulse generator and my 250 MHz leveled sine wave generator. I do not have anything faster yet.

author: w2aew
author_rank: Super Contributor
timestamp: October 13, 2014, 12:34:03 am
content: And of course, you have to remember that equally important (or more important) than the quality of the terminator is the location of the termination and any stub, trace, etc. that exists to the measurement point (sample and hold, preamp, etc.) You can put a perfect microwave rated termination on a Tee and have a lousy result because of the stub formed by the Tee and the input to the scope's front end. This is the primary reason why internal terminations will generally outperform most external terminations, especially those connected via a Tee.

author: David Hess
author_rank: Super Contributor
timestamp: October 13, 2014, 01:43:34 am
content: Quote from: w2aew on October 13, 2014, 12:34:03 amI was just surprised that the feedthrough terminators performed identically to the 2440 internal terminations. Maybe the internal ones are not all that good. The test using the BNC-T was just for edification and returned results consistent with my expectations.

author: marshallh
author_rank: Unknown
timestamp: October 13, 2014, 02:50:00 am
content: I compared a avanlache pulser response both thru a Tek 50ohm passthru and Lecroy internal termination. With 1ghz bandwidth the difference was pretty substantial.Internal:

author: David Hess
author_rank: Super Contributor
timestamp: October 18, 2014, 05:33:00 am
content: Quote from: marshallh on October 13, 2014, 02:50:00 amWas the input bandwidth 1 GHz when the feedthrough termination was used? The fastest high impedance inputs I am familiar with are only 500 MHz which requires a 12 picofarad input capacitance. 1 GHz implies an input capacitance of 6 picofarads.

author: Howardlong
author_rank: Super Contributor
timestamp: October 20, 2014, 09:25:33 pm
content: FWIW a couple of days ago I did some comparisons of a few probe options against an Agilent 54831D 600MHz/4GSa/s scope and anunmodifiedRigol MSO1074Z-S 70MHz/1GSa/s scope. Apart from the homebrew 21:1 probe, I used stock probes supplied with each instrument, namely the Agilent 1165A 600MHz passive probe and the Rigol RP2200 150MHz passive probe respectively. Both scopes were running a single channel only at their respective full sample rates.The source I used was a 125MHz canned oscillator that I had in stock mounted on some copper clad PCB. It is RS part number 478-5137, aka Epson Toyocom SG8002CAPCB125MHZ. It's a 3.3V CMOS output (none of your clipped sine rubbish, so it probably has an impressive signal on airband) in a 7x5mm ceramic base surface mount package.1. The first test was using the stock 'scope probe grabber clip.Agilent 54831DRigol MSO1074Z-S2. Next, on the Agilent only, I used the ~1.5" dual lead adapter(there wasn't such an adapter supplied with the Rigol probe).Agilent 54831D3. Then the spring ground connection was used.Agilent 54831DRigol MSO1074Z-S4. Finally, a test with a homebrew 21:1 probe, ie a 1k resistor on an SMA connector, RG58 to scope, internal 50 ohm termination with the Agilent, BNC T piece termination for the Rigol.Agilent 54831DRigol MSO1074Z-SThat final tests shows that your probably fighting a losing battle with the Rigol. However, with the Agilent, the rise and fall times are markedly improved compared to the spring clip thingy.Cheers, Howard

author: Co6aka
author_rank: Unknown
timestamp: October 21, 2014, 02:10:01 am
content: Quote from: Howardlong on October 20, 2014, 09:25:33 pmSure doesn't look like a non-reactive type.Other than specifically-made RF resistors, I've found that one-layer flat film SMT resistors work best. I've tried making attenuators and terminators with good-ol-fashioned Allen-Bradley carbon comp axial lead resistors but they're kinda funky above HF.Quote from: Howardlong on October 20, 2014, 09:25:33 pm"RG58" is something that's w-i-d-e-l-y open to interpretation. I got rid of most of my "RG58" RF cables and use only RG142 and RG400 or some similar sized "6 GHz" marked stuff that came out of a mil-surplus lot. Also have some ultraflexible solid-jacket SMA-SMA jumpers, but they're only about 16 inches long.Remember that steep edges are made up of lots of high-harmonics -- waaay up there -- so think until your giga hertz big time.

author: Howardlong
author_rank: Super Contributor
timestamp: October 21, 2014, 09:52:26 am
content: The idea for the 21:1 probe came from herehttp://koti.kapsi.fi/jahonen/Electronics/DIY%201k%20probe/where the author measures rise times <100ps with it.I measured the RG58 cable on my HP 8753A VNA upto 3GHz, and you're right, it's not great, but this is as nothing compare to putting BNC connectors on the end, the use of T pieces, BNC terminators and interseries adapters.Yesterday and today I did some further testing with the Rigol MSO1074Z-S only. This test uses the same 21:1 DIY probe and a specially made cable with the scope end internally terminated with an 0603 49.9 ohm resistor. This improved things, but I also noticed that reducing the sensitivity on the scope also improves the rise time significantly, I assume because it reduces the slew rate of the ADC driver opamp. It makes for a more reasonable, more "square" result. (Note that on the first screen print I accidentally had the 10x probe setting switched on the scope).

author: Howardlong
author_rank: Super Contributor
timestamp: October 21, 2014, 09:56:15 am
content: By the way, these two screen shots show the return loss of the modified cable with the integrated terminator, first with a 100-500MHz sweep, and secondly from 100MHz to 3GHz. The 3GHz sweep is particularly nasty, and it's why we shouldn't be using BNCs!

author: Howardlong
author_rank: Super Contributor
timestamp: October 21, 2014, 01:47:53 pm
content: Some more test reuslts with a Jim Williams pulse generator.Rigol MSO1074Z-S, JW pulser without coax "add on" into SMA-BNC cable with integrated 50 ohm termination at the BNC end... add a few feet of coax onto the JW 2pF cap to extend the pulse (note that although this shows 800ps[!] it was varying between 800ps and 1.2ns)...(Edit: This test has been re-done, see about four posts newer)... on the Agilent 58431D, using the scope's integral 50 ohm load and a 10dB SMA pad (the pulser signal was too high for the 50 ohm input).

author: David Hess
author_rank: Super Contributor
timestamp: October 21, 2014, 10:20:44 pm
content: Quote from: Howardlong on October 21, 2014, 01:47:53 pmWhat causes the variation on the Rigol?I assume it was not seen to vary on the Agilent 58431D.Added: Is there something very wrong with the Rigol's transient response? Shouldn't that match the Agilent results albeit within the limitations of its bandwidth?

author: tggzzz
author_rank: Super Contributor
timestamp: October 21, 2014, 10:27:50 pm
content: Quote from: Howardlong on October 21, 2014, 09:52:26 amIs there anything in the Rigol datacheet or operations manual that (a) indicates that occurs an/or (b) implies a reason?I can't imagine a Tek or HP/Agilent/Keysight scope not mentioning such a phenomenon

author: Howardlong
author_rank: Super Contributor
timestamp: October 21, 2014, 10:58:28 pm
content: Since doing the test I am wondering if it's either to do with the relatively high transient, or reflections between the inline 50 ohm and the scope's high impedance front end. The coax added a much higher voltage to just using the 2pF cap on the Jim Williams base design.. When I get a moment I will investigate further. (By the way, I forgot to take the 10x off again on the Rigol, it's not really 340Vpp!.

author: Co6aka
author_rank: Unknown
timestamp: October 21, 2014, 11:02:32 pm
content: Quote from: Howardlong on October 21, 2014, 09:56:15 amAlso, not all BNCs are created equal, and that too becomes quite apparent when your giga hertz.I like SMB and SMC connectors for GHz applications. I've hacked up a few SMBs to make probes, and I'm sure RobRenz could make up some really nice ones out of 'em.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 21, 2014, 11:22:54 pm
content: Quote from: tggzzz on October 21, 2014, 10:27:50 pmIndeed, my 475 for instance notes the rise/fall time is notably better for e.g. 2-3 div height than 6-8 div full screen. That's for an entire analog signal path, of course, but similar reasons could very well apply for the digital scope's preamp.Digital scopes these days often use 0/5V or -5/+5V supplies for the analog bits, which means you'll inevitably see some slew rate limiting, compression or bandwidth limiting for large signals near the rails. Nice thing about the old stuff is the wide supply rails meant freedom from that (unless you seriously overdrive the input, in which case saturation recovery may be good or awful depending on make).Tim

author: tggzzz
author_rank: Super Contributor
timestamp: October 21, 2014, 11:44:29 pm
content: Quote from: T3sl4co1l on October 21, 2014, 11:22:54 pmYour analysis appears sound. My HP1740, which presumably predates your 475, only has to note that the 1mV/div bandwidth is a reduced 40MHz.Back in the mid 70s HP delivered a minicomputer that didn't quite meet its published specifications. When Packard heard about it he sent a memo to the project manager asking him to ensure that in future the products did meet their specifications. The manager framed the memo, hung it on his wall, and the story and attitude became baked into HP's corporate folklore.That was on reason why HP's products were trusted implicitly, in comparason with their competiton. (Tektronix excluded, of course, since they had similar attitudes.)Not all change is for the better.

author: Howardlong
author_rank: Super Contributor
timestamp: October 22, 2014, 06:10:43 pm
content: I re-did the Jim Williams test. Firstly, I noticed today that the SMA-BNC with integral 50 ohm resistor wasn't 50 ohms. I don't know if it was physical or electrical stress (I was pumping quite a few volts into that 0603 49.9 ohm resistor albeit for only a few ns) but it was open anyway. I think we should forget that waveform as operator error on my part for now. Anyway, I fixed he internal 50 ohm termination and have re-run the tests, this time with an inline SMA 20dB pad at all times and with a reduced coax length on the 2pF cap too of about 50cm.Rigol MSO1074Z-S, 500mV/div, 1X probe, 5ns/divRigol MSO1074Z-S, 200mV/div, 1X probe, 5ns/div. I was careful to ensure that the signal wasn't (apparently!) saturating. Note the very significant increase in rise time. You also get a relay click at the attenuator change form 500mV/div.Agilent 54831D, 500mV/div, 1x probe, 10ns/divAgilent 54831D, 500mV/div, 1x probe, 1ns/divAgilent 54831D, 500mV/div, 1x probe, 1ns/div, direct connection to scope, internal 50 ohm loadSomething else I've noticed on the Rigol is that even on a single shot stopped waveform, the measured Vpp value also changes as you change the vertical scale. I tried to reproduce the anomaly regarding the 200mV to 500mV rise time measurement above with an RF signal generator terminated to 50 ohm at the scope, and couldn't, so I'm not sure what's happening with the JW as a pulse source on the Rigol in that scenario.

author: MarkL
author_rank: Unknown
timestamp: October 23, 2014, 01:59:34 am
content: I've noticed that you only have 5 sample points per division on the Rigol. So, it's trying to make a rise/fall time calculation with only a point or two in the transition area.It could be doing the sin(x)/s reconstruction poorly with this few points. If possible, perhaps try turning off interpolation or switch to dots mode to see what you're actually getting. You could also download the raw data and examine it that way.Or maybe it's just doing a bad job doing the rise/fall time calculation. If it has measurement cursors, try to see where it's anchoring its measurements. Or do the measurement manually if you can zoom the horizontal any further. You've chopped off the top of the 200mV/div waveform so I can't really tell if 3.5ns is right.Are you doing this single shot? Do an average, or look at the rise/fall statistics (std deviation) to see if it's all over the place. It may also be dependent on where trigger point is occurring and where the samples land.

author: tggzzz
author_rank: Super Contributor
timestamp: October 23, 2014, 08:38:52 am
content: Quote from: MarkL on October 23, 2014, 01:59:34 amAlways worth playing around with that, and it is usually my preferred display mode. It is easy for the eye/brain to "fill in the gaps".I've seen cases where turning off interpolation revealed the waveform to be more repeatable than implied by the interpolation.

author: tautech
author_rank: Super Contributor
timestamp: October 23, 2014, 08:48:51 am
content: Quote from: MarkL on October 23, 2014, 01:59:34 amMaybe it's just under the OSD?

author: David Hess
author_rank: Super Contributor
timestamp: October 23, 2014, 01:49:36 pm
content: Quote from: T3sl4co1l on October 21, 2014, 11:22:54 pmWhere is this noted? I looked through the 475 documentation and did not find it. I have seen this sort of specification before but it is insignificant on all of the oscilloscopes I have used. Almost all analog oscilloscopes would be using 5 divisions anyway for a rise/fall time measurement.Quote0/+5.0 to 0/+1.8 volts is typical now depending on the stage for the analog supply but the signal path is limited to below 2 volts differential peak-to-peak. I do not know what preamplifier the Rigol 1000z series uses but the ADC is a Hittite HMCAD1511 (1.8 volts) which supports a 2 volt peak-to-peak differential input. The 2000A series uses the National/TI LMH6518 amplifier (5 volts) and ADC08D500 ADC (1.8 volts) which supports a peak-to-peak input below 1 volt.None of these parts specify a slew rate limit as such although it could be estimated from their full power bandwidth but that yields numbers in excess of what is required. The ADCs naturally do not specify overdrive recovery but the LMH6518 preamplifier has a specified faster overdrive recovery than its settling time which is pretty neat. Overdrive recovery would almost certainly be limited by the input buffer but some DSOs have horrendous overdrive recovery in later stages.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 23, 2014, 02:01:27 pm
content: Quote from: David Hess on October 23, 2014, 01:49:36 pmHmm, looking through my documents, I don't see it either...Maybe I conflated that with something I read elsewhere, perhaps a lesser quality scope.Tim

author: David Hess
author_rank: Super Contributor
timestamp: October 23, 2014, 02:56:37 pm
content: Quote from: MarkL on October 23, 2014, 01:59:34 amIf I run a similar test on my 2440, then equivalent time sampling returns a 1.30 nanosecond rise time which almost exactly matches the specified 300 MHz bandwidth of the oscilloscope given the 600 picosecond rise time of my calibration source. Without ETS and just using sin(x)/x reconstruction, the rise time drops to 2.5 nanoseconds with a variation between about 2.1 and 2.9 from aliasing and intermodulation if averaging is not used. That matches up fairly closely with the typical DSO specification of sampling frequency/4 for real time bandwidth when sin(x)/x reconstruction is used.The Rigol has twice the real time sample rate so with sin(x)/x reconstruction and averaging, I would expect a rise time of about 1.25 nanoseconds which is not far from the measured 1.7 nanoseconds but how can 1.7 nanoseconds make any sense with a 70 or even 100 MHz analog bandwidth? If I limit the vertical bandwidth of my 2440 to 100 MHz and use sin(x)/x reconstruction without ETS, then the rise time is almost exactly 3.5 nanoseconds with half the sample rate of the Rigol.I have read speculation before that Rigol's sin(x)/x reconstruction has a problem. Apparently at high frequencies, the sample points do not match up with the reconstructed waveform. On my 2440, I can have it display the real sample points within the sin(x)/x reconstruction and they always line up exactly despite a lower sample rate and higher bandwidth.The discussion here gets interesting past the first page:https://www.eevblog.com/forum/chat/rigol-ds1000e-series-possible-errorfail-in-sin%28x%29x-interpolation/15/QuoteThe automatic measurement may be getting confused when the histogram cannot find the top of the waveform. A cursor measurement may be needed but I have my doubts because I think the pulse should look much cleaner.QuoteIt is not completely clear how these Rigol's generate their trigger timing without dedicated hardware to support equivalent time sampling. I assume their digital trigger operates after reconstruction.

author: David Hess
author_rank: Super Contributor
timestamp: October 23, 2014, 02:57:08 pm
content: Quote from: Howardlong on October 22, 2014, 06:10:43 pmDoh! That explains the earlier waveform then. It does look like a TDR result.QuoteI am concerned about the shape which does not match what I would expect simply do to lower bandwidth.

author: David Hess
author_rank: Super Contributor
timestamp: October 23, 2014, 03:07:19 pm
content: Quote from: T3sl4co1l on October 23, 2014, 02:01:27 pmI know some very old (or cheap?) oscilloscopes suffered from this. I suspect the difference is that they used a vertical amplifier design which did indeed have low full power bandwidth do to slew rate limitations. I think I have run across it in Tektronix oscilloscopes during calibration where the effect is suppose to be less than 1 minor division at the extremes of the CRT graticule and it was included as part of the linearity error.

author: MarkL
author_rank: Unknown
timestamp: October 24, 2014, 01:51:15 am
content: Quote from: David Hess on October 23, 2014, 02:56:37 pmThat's an interesting read; thanks for the pointer. Not sure there were any solid conclusions in that thread, though.How the Rigol could come up with 1.7ns is exactly what I was wondering as well, hence the barrage of questions to Howardlong. To even start to figure it out, it has to be known where the real sample points sit, and if it's consistent capture to capture.It could be an artifact of sloppy ADC interleave clocking, but that's only a guess at this point. It's possible the designers did not concern themselves with this level of waveform accuracy that's already well beyond the scope's BW.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 24, 2014, 03:33:22 am
content: Quote from: Howardlong on October 22, 2014, 06:10:43 pmIs it just the readout of Vpp,or the displayed signal as well?If you count the graticule divisions "Greybeard style" you can determine this.

author: David Hess
author_rank: Super Contributor
timestamp: October 24, 2014, 05:45:22 am
content: Quote from: MarkL on October 24, 2014, 01:51:15 amI concluded some things based on that discussion thread and another but nothing about the sin(x)/x reconstruction. Some of his test results in that discussion thread may be explained by aliasing during reconstruction but I have yet to see a real time DSO where that was not a limitation.QuoteIt is not beyond the oscilloscope's bandwidth. With averaging it should be able to make measurements up to its bandwidth limit. Intermodulation distortion from the ADC *can* cause artifacts which make the rise time look faster than it should be but the effect is relatively small if annoying and averaging should take it out.Looking carefully at the two photos again and reading Howardlong's description, they are of the same saved data but with the vertical scale changed. To me that implies that the measurements are made on the display record and not the waveform record because the former changes when you alter the scale but the later does not. Maybe Rigol does not even maintain a separate waveform record. That explains why the various volt measurements change a little bit. The rise time measurement is just broken but it is not clear how because the top and base measurements are not out of line.Wasn't there an issue with protocol decoding where only what is on the display can be decoded? That might fit with this.So it is not really displaying or making a 1.7 nanosecond rise time signal. It is just measuring it wrong somehow. It seems like it would be significant that the false rise time measurement is exactly 1/2 of the real measurement but then why do the fall time measurements vary by a different factor?Maybe the measurement algorithm Rigol uses is just poor. My 2440 makes exactly the same measurement without problems using half as many sample points, no ETS, with or without averaging, and worse digitizer performance.

author: Howardlong
author_rank: Super Contributor
timestamp: October 24, 2014, 10:36:15 am
content: A couple of clarifications...QuoteIf you're referring to the two following two screenshots, then they were two different single shot acquisitions, each taken at the displayed vertical resolution. There is a relay click between the 200mV and 500mV setting (with 1X probe), I was suggesting that a different signal path at the front end has different filtering characteristics. I tried other vertical resolutions that didn't click any relays and the rise time wasn't significantly affected.It is true, however, that on captured acquisitions, changing the vertical resolutionafterthe capture also shows aberrations, although the difference is nowhere near as much.QuoteI think that is probably a red herring: the problem with serial decoding is that (a) the scope will only decode what's on the screen (not in memory) and (b) as you slow timebase in real time, or equally zoom out on a capture, the underlying sample rate used to decode also decreases to such an extent that it will no longer properly decode, and although the decoder's sample rate is shown, it doesn't appear to be changeable by the user.There appears to be a "feature" on the firmware I'm using: I cannot turn off sin(x)/x unless I have at least three channels running (weird!).FWIW, with sin(x)/x on, with the 3dB points with a 1Vpp signal when using an RF signal generator, terminated at the scope:1Ch 109MHz (1GSa/s)2Ch 105MHz (500MSa/s)3Ch 106MHz (250MSa/s, using averaging  due to aliasing artifacts)4Ch 106MHz (250MSa/s, using averaging  due to aliasing artifacts)With sin(x)/x on:3Ch 64MHz (250MSa/s)4Ch 64MHz (250MSa/s)

author: David Hess
author_rank: Super Contributor
timestamp: October 24, 2014, 03:25:27 pm
content: Quote from: Howardlong on October 24, 2014, 10:36:15 amThe images did not come through but I get the idea. That explains the voltage measurement changes but not the rise and fall time measurement changes. Changing the input attenuation might slow down the input but there is no way it should speed it up that much. If it did, then the 200mV setting should be the fast one and the 500mV setting the slow one and your results show the opposite.Dave's reverse engineering shows that the front end includes a switchable x50 high impedance attenuator. The only odd thing about it is that it only has an adjustment for compensation and none for input capacitance but maybe it is there and got missed or the design is so consistent that it is not required. He has a link to the schematics he made here:https://www.eevblog.com/forum/blog/eevblog-675-how-to-reverse-engineer-a-rigol-ds1054z/Looking at I now, I also do not see any adjustments for medium frequency compensation on the DC path. Maybe Rigol handled some of the front end frequency response calibration through DSP.The odd waveform could be caused by the very fast high amplitude edge coupling through to the high impedance buffer causing it to overload. If that is the case, then the shape of the waveform will change between 200mV/div and 500mV/div where the input attenuation changes. The shape will also change if the signal level changes via external attenuation or DC level shift which would be a better test to avoid differences caused by switching the x50 input attenuator in and out. Adjusting the vertical position control might change it as well.Since only one input attenuator stage is used, the input circuits have to operate over a very wide input signal range which makes effects of overload at 200mV/div even more likely. Oscilloscope input buffers usually operate over an input range considerably smaller than the 1.6 volts the Rigol design has to handle. I checked a couple of older oscilloscope designs and found input ranges from 40 to 400 millivolts.QuoteI do not know what to make of this if the changes are significant.QuoteGiven the above I agree. I was thinking they could be related if measurements are only made on the display record.QuoteThis behavior was discussed in another EEVBlog thread. I do not think we reached any consensus about why sin(x)/x is forced on at higher sample rates.QuoteI actually meant bandwidth as measured by rise time. Your last two results however are consistent with sin(x)/x reconstruction limiting bandwidth to the sample rate divided by 4 which is common in other DSOs when real time sampling is used. 250MSa/s divided by 4 equals 62.5 MHz.

author: miguelvp
author_rank: Super Contributor
timestamp: October 25, 2014, 02:16:43 am
content: Quote from: Howardlong on October 20, 2014, 09:25:33 pmMeasuring 125 MHz, why are you surprised by the results?4 times the sampling rate, 8 and a half higher bandwidth specs and 4.8 times probe bandwidth vs less than half the probe bandwidth.Am I missing something obvious that the Rigol claims?

author: David Hess
author_rank: Super Contributor
timestamp: October 25, 2014, 03:07:23 am
content: Quote from: miguelvp on October 25, 2014, 02:16:43 amThe Rigol results are not consistent with an oscilloscope which only has a lower bandwidth and/or sample rate. The pulse shape is distorted beyond what can be explained by those two things. Nothing explains the Rigol returning a 1.7 nanosecond rise time measurement.Measuring a similar pulse edge, my generator is about half as fast but much cleaner, with half of the sample rate the Rigol is using and the same bandwidth, my own DSO has no problem. With a much higher bandwidth it still has no problem.It is possible that the pulse is getting distorted by the test setup however. As much as possible need to be kept the same when making the measurement with two different DSOs. The same external 50 ohm feedthrough termination and cable need to be used. That might explain everything except the 1.7 nanosecond measurement.