Is there a modern equivalent to old AC Voltmeters that measure 2MHZ+? - Page 1
Forum section: electronics / beginners

author: nsummy
author_rank: Contributor
timestamp: January 07, 2025, 09:11:42 pm
content: Restoring an old HP 4204A Oscillator and I am tearing my hair out with the "calibration" (more like troubleshooting). The main 2 measurements required to both troubleshoot & calibrate are the AC voltage (in which the frequency can hit 1mhz & around 10v) and obviously the frequency itself The manual recommends a 3400a or 400E AC Voltmeter. There are plenty of them out there for $50-$100 but it just seems silly to buy something like that for this limited use. There are some heathkits but they all seem in various states of disrepair & aren't much cheaper. Google hasn't turned up much in this regard. Thank you!

author: TimFox
author_rank: Super Contributor
timestamp: January 07, 2025, 09:27:45 pm
content: You could try a decent digital oscilloscope with RMS calculation function.You can check the voltage calibration against a battery that you measured with a good DMM.The classic 400 and 3400 voltmeters are analog readout: the 400 series are average-responding, RMS-calibrated, but the 3400 are true-RMS responding.A better choice if you want to buy a used meter would be the -hp- 3403c true-RMS with digital display.

author: bdunham7
author_rank: Super Contributor
timestamp: January 07, 2025, 10:06:42 pm
content: Quote from: nsummy on January 07, 2025, 09:11:42 pmIf you buy one that cheap you'll just have another instrument to restore before you get to work.Many decent budget DSOs will do everything you need. That instrument is +/-3% flatness, you can get better than 1% from a DSO if you have the right one and try a bit. One tip on that, it depends on both which DSO you choose and how you setup probing, but unless you can separately calibrate the scope (with a good AWG, for example) you might get more certain results by using no probe at all and just connecting directly to the scope.I just happen to have everything on the bench to demonstrate:

author: nsummy
author_rank: Contributor
timestamp: January 07, 2025, 11:18:50 pm
content: That is amazing! Thank you. and yes, buying something old and untested to perform tests on something else was my largest concern. I have a 90s era 45645A oscilloscope that I bought just to be sure I would use it. It does provide the readings I need but bounces around all over the place so I don't think its accurate for this type of work. might be time to upgrade. Thank you for the insight.

author: radiolistener
author_rank: Super Contributor
timestamp: January 08, 2025, 12:55:58 am
content: Quote from: TimFox on January 07, 2025, 09:27:45 pmOscilloscope has not a flat frequency response (about ± 3 dB within its working bandwidth, for voltage it means that 1 V can be displayed as 0.708...1.413 V or about ±35%) and very high measurement error. As I remember, the good decent digital oscilloscope has about 5% measurement error or even worse. Calibration standard needs to be at least 10 times more precise than measurement equipment which you want to calibrate. So, oscilloscope is a very poor choice for calibration of a precise measurement equipment.If you want to calibrate measurement equipment which has 5% error, you're needs to use calibration standard with precision 0.5%. So, if you use device which has up to ± 35% measurement error as a calibration standard, its not a calibration...

author: bdunham7
author_rank: Super Contributor
timestamp: January 08, 2025, 01:36:27 am
content: Quote from: radiolistener on January 08, 2025, 12:55:58 amThe problem with generalities and "rules of thumb" is that while they seem helpful in the abstract, when you try to apply them to a specific situation they're often wrong.Some DSOs might have gain errors of 5% and the bandwidth is specified by the 3dB point, so if you are using them near the BW limit and it isn't an accurate model, then you're right. However, at 1-2MHz on an oscilloscope with a 200MHz BW and a manufacturer-specified DC gain accuracy of "less than 1%, 0.5% typical" (SDS2000X HD spec) or one you can characterize yourself (my case) you won't really have much to worry about. The photo I posted was not staged. I'm working on the 510A reference standard and the meter was already there, I just added the scope (cold and no auto-cal) and took a picture.A TUR of 10 is an old school way of thinking, now you do uncertainty calculations and you also read the manual. In this case the manual for the oscillator he is working on specifies a test instrument with an accuracy of 1%. If you had one of the instruments they list and it was actually accurate to 1% you'd either be very lucky or you would have gone to a bit of trouble and expense to make it so. The DSO--as you can see--seems to be much better than required.

author: radiolistener
author_rank: Super Contributor
timestamp: January 08, 2025, 03:01:40 am
content: Quote from: bdunham7 on January 08, 2025, 01:36:27 amYes, that's correct - under some specific favorable conditions, you may achieve a 0.5–1% error when taking readings on an oscilloscope at certain frequencies. As you know, even a broken clock can show correct time at least twice per dayHowever, the challenge lies in the oscilloscope's frequency response, which is typically not linear. Instead, it follows a curve and often exhibits both positive and negative ±3 dB variations within its operating bandwidth.While it is true that these variations are frequently more pronounced near the oscilloscope's bandwidth limit, this cannot be taken as a general rule. Many oscilloscopes also exhibit ±3 dB deviations in the middle of their operational bandwidth. And even more often at beginning of operational bandwidth (at low frequencies below 500 Hz).In most cases, the error tends to be more significant above BW/3 and below BW/1000. Operating within this frequency range generally yields accurate measurements, but exceptions can occur. For example, an amplitude error of up to 30% within this range may still fall within the oscilloscope's specified tolerances.Therefore, you can certainly use an oscilloscope for preliminary assessments, but keep in mind that its specifications allow for significant deviations, which may not always be obvious - especially to a beginner. This should be taken into account. Note that an oscilloscope exhibiting such large errors in most cases functioning correctly, as these deviations are within its specified tolerances.For example, modern Siglent oscilloscopes often can have up to 25-30% amplitude error near 30-50 Hz which can be easy detected visually as a waveform distortion on a low frequency square wave. And this is within it's specification.Here is example of a very good 50 Hz square wave 1 Vpk-pk on a modern Siglent 100 MHz oscilloscope:These waveform distortions are due to frequency response curve of oscilloscope and its 30% amplitude measurement error at 50 Hz is within ±3 dB specification for 100 MHz bandwidth.Of course, I deliberately chose conditions under which the oscilloscope produces such a large error, and for other frequencies, the precision is much better. However, as you can see, this occurs far from the oscilloscope's bandwidth limit, so when using an oscilloscope, you're not guaranteed to avoid encountering such a large error.

author: bdunham7
author_rank: Super Contributor
timestamp: January 08, 2025, 03:59:45 am
content: Quote from: radiolistener on January 08, 2025, 03:01:40 amWow, you need some better oscilloscopes because I'd call that behavior "broken". There's no logical reason that an oscilloscope would vary that much from DC to low frequencies. I don't recall seeing a scope with 3db variations mid-band, but I have seen probes that are wonky like that, which is why I recommended a direct connection.QuoteI've previously demonstrated the LF step response issues on the very scope I used earlier, the SDS1104X-E. This LF square wave distortion artifact is a step-response issue and doesn't appear to significantly affect measuring an AC RMS voltage--for a sine wave anyhow.Edit: Have you actually tested the amplitude response of your scope with a 50Hz sine wave?Your distortion does look different than mine and it's possible you have a different problem--but still not one that is common or representative of DSOs in general.Edit: Actually, since I have the thing all set up, I tried 10 to 100Hz and got errors of less than 0.6%. And even on the 100Hz square wave with the distortion visible, the RMS measurement was correct. The distortion you are showing is pretty bad. IIRC, when we were all discussing your scope much earlier (years ago by now) we more or less concluded that it was broken, even if by design. "Within specifications" is corporate speak for "we can't afford to fix that or we don't know how--but hey hopefully the next model will be better". However this is a particular scope or at most a small subset of modern DSOs, albeit a black eye for Siglent IMO. Scopes that I regard as good do not behave this way AT ALL. My comments to the OP regarding using a DSO for this purpose quite specifically avoided claiming that all or any DSO would work well, even though I suspect that the vast majority that are 10 years old or less will do just fine.

author: radiolistener
author_rank: Super Contributor
timestamp: January 08, 2025, 04:41:08 am
content: Quote from: bdunham7 on January 08, 2025, 03:59:45 amWhy broken? This is within 3 dB oscilloscope specification.Some instances may have better results, some worse. This is pretty common issue.Quote from: bdunham7 on January 08, 2025, 03:59:45 amThe picture above with 30% measurement error is taken with direct coax cable connection and enabled 50 Ω termination.Quote from: bdunham7 on January 08, 2025, 03:59:45 amYour oscilloscope is more expensive than 1000X series, so its not surprise that it can give better results.With other brands like Hantek and Chinese noname models, you may get an even worse response curve, and this can occur not only near the bandwidth borders but also in the middle of the working frequency range.

author: MrAl
author_rank: Super Contributor
timestamp: January 08, 2025, 04:57:54 am
content: Quote from: nsummy on January 07, 2025, 09:11:42 pmHi,Wow, I havent seen one of these meter for years and years and years.Used one way back a long time ago in school for various experiments, always meant to get one myself but never got around to it, then they became obsolete or something.Good luck finding one that works right now.

author: bdunham7
author_rank: Super Contributor
timestamp: January 08, 2025, 05:10:27 am
content: Quote from: radiolistener on January 08, 2025, 04:41:08 amNot expected behavior and as I already said, applying the 3dB spec to this issue is corporate weaseling or plain ignorance. Not acceptable and certainly not a feature of any A-brand scopes I've ever seen. Thus the term "broken by design".I don't want to go off in the weeds debating your setup there, but IDK why you'd have that set up with 50R inputs (not how you'd typically measure voltage) or why you would overdrive the inputs (1Vp-pat 100mV/div is a 25% overload...) but I'll assume the issue persisted even under other conditions. Um, right?

author: Aldo22
author_rank: Super Contributor
timestamp: January 08, 2025, 10:18:50 am
content: Quote from: radiolistener on January 08, 2025, 04:41:08 amOh, it's Hantek bashing day again?The good thing about measuring devices is, that you don't need a crystal ball, you can just check it for the range of interest.The $130 Hantek has a signal generator built in and is SCPI controllable. So it's no work to check that.Here 1kHz - 5MHz at 5V (Raw data also attached, see last column).As far as I can see, all measurements are under 2% deviation (percent, not dB).Everyone can decide for themselves whether this meets the requirements, but I don't think the DSO2000 is significantly less accurate in this measurement range than other 8-bit scopes.

author: bte
author_rank: Regular Contributor
timestamp: January 08, 2025, 10:48:19 am
content: Quote from: radiolistener on January 08, 2025, 12:55:58 amIf there are 3 dB variations within working bandwidth, then how is working bandwidth defined? Is there another definition of working bandwidth or is this just talking about what's written on the device as working bandwidth?

author: radiolistener
author_rank: Super Contributor
timestamp: January 08, 2025, 12:19:14 pm
content: Quote from: bte on January 08, 2025, 10:48:19 amThis question should be addressed to Siglent and other oscilloscope manufacturers.

author: mawyatt
author_rank: Super Contributor
timestamp: January 08, 2025, 03:49:10 pm
content: "In most cases, the error tends to be more significant above BW/3 and below BW/1000. Operating within this frequency range generally yields accurate measurements, but exceptions can occur. For example, an amplitude error of up to 30% within this range may still fall within the oscilloscope's specified tolerances.""For example, modern Siglent oscilloscopes often can have up to25-30% amplitude error near 30-50 Hzwhich can be easy detected visually as a waveform distortion on a low frequency square wave. And this is within it's specification."Just did a quick test with our SDS2504X Plus and SDS824X HD. Direct coupled and both show almost no variation in amplitude from 1Hz to 10MHz with signal from our SDG6022X AWG which we know has a flat response.Suspect the "variations" discussed has more to do with the DSO Probe than the actual DSO itself, as this was evidenced in the previous DSO Probe discussions and how they effect the measurements at hand as well as how they effect the DUT!!This brings up the discussion of DSO probing which do you want results "what-was vs. what-is controversy" as coined by Wilson, however that's for another discussion.BTW we see no discernible change in amplitude between30-50Hz mentionedfor both of our Siglent DSOs!!Edit: Just checked the Rigol DHO814 with the same results!!Best

author: mawyatt
author_rank: Super Contributor
timestamp: January 08, 2025, 04:38:32 pm
content: Quote from: bdunham7 on January 07, 2025, 10:06:42 pmThat's what we did as mentioned above.Your Fluke Thermal RMS Meter, is this the one with the differential transistors with integral heat producing resistors where the die inside the case (recall TO-99) was suspended with wire-bonds for thermal isolation?BTW hope you are not near the fires in CA!!!Best

author: Aldo22
author_rank: Super Contributor
timestamp: January 08, 2025, 05:39:25 pm
content: Quote from: mawyatt on January 08, 2025, 03:49:10 pmHantek DSO2000, 10Hz-1kHz sine at 5 Volt.Maximum deviation: 4.88V (from 5V): <3%.

author: TimFox
author_rank: Super Contributor
timestamp: January 08, 2025, 05:41:37 pm
content: 0.2 dB

author: bdunham7
author_rank: Super Contributor
timestamp: January 08, 2025, 05:57:28 pm
content: Quote from: Aldo22 on January 08, 2025, 05:39:25 pmIDK how you have your scope set up, but you do have to be careful and know how your RMS measurements are gated. On my SDS1104X-E it makes the calculation according to what is on the screen so you need either make sure you have full cycles or else slow the sweep down so that you have many more periods on the screen. On scopes with long memories and high sample rates this isn't an issue, the screen can be a blur and you'll still get a reading just fine.

author: bdunham7
author_rank: Super Contributor
timestamp: January 08, 2025, 06:06:23 pm
content: Quote from: mawyatt on January 08, 2025, 04:38:32 pmThe thermal converter on the 8506A is sparsely documented (at least as far as I can find) and the only manual I have with an actual schematic of the TRMS board appears to have something wrong with the layout picture. However, I assume you're talking about U6, which is a transistor pair and two 400R resistors in what appears to be a TO-99-10 can. I know this device was supposed to be some sort of breakthrough because it could achieve high accuracy very quickly with "only" a 6-second reading interval but I didn't know about the suspended-by-bond-wires trick. The HP 3400/A actually is more interesting looking although I believe it is far more primitive albeit with better BW.We're not close enough to break out the marshmallows but we have the 90MPH winds and are still watching anxiously. Theres some really expensive real estate going up in flames right now.

author: Aldo22
author_rank: Super Contributor
timestamp: January 08, 2025, 06:07:06 pm
content: Quote from: bdunham7 on January 08, 2025, 05:57:28 pmThank you.These are PkPk measurements.But the script also adapts sec/div to the respective frequency (for phase).

author: mawyatt
author_rank: Super Contributor
timestamp: January 08, 2025, 06:28:37 pm
content: Quote from: bdunham7 on January 08, 2025, 06:06:23 pmWe saw an early version of this probably 40~50 years ago, they had thinned the die to reduce thermal mass and suspended it by wire bonds inside the TO-99 case. Don't think Fluke actually fabricated the die tho, recall this was Motorola and not sure if this was what went into their instruments.QuoteTwo folks we know at Keysight (CTO and Technical Fellow) lost their homes to the Santa Rosa fires in past, they didn't have much time to save things and get out safely!!West coast has fires and mud slides, we get hurricanes here on east coast. We got flooded by hurricanes and rebuilding now. Dealing with FEMA, Insurance and local regulations is PITABest

author: bdunham7
author_rank: Super Contributor
timestamp: January 08, 2025, 07:18:17 pm
content: Quote from: mawyatt on January 08, 2025, 06:28:37 pmAccording to the date on my oldest copy of the manual the 8506A came out in 1983, so 42 years ago already!

author: David Hess
author_rank: Super Contributor
timestamp: January 08, 2025, 10:13:03 pm
content: Old style high frequency AC voltmeters used either thermal or sampling methods to achieve high bandwidth. I do not know of any modern instruments suitable to replace them, and the best RMS converter currently made by Analog Devices (Linear Technology) only achieves 1% up to 500 kHz with 15 MHz of bandwidth, which give an idea of how difficult this is to do without the proper technology. This suggests that an oscilloscope with a bandwidth of 60 MHz *might* be able to do it, but how do you verify that?What I might do now is calibrate an oscilloscope using a reference level sine source at the frequency of interest, but reference level sine sources are not common either. If I had to make the measurement right now, I could do that to better than 1% accuracy (better than 0.1 dB) up to 50 MHz with my reference source, or use my sampling oscilloscope which has flat response from DC to microwaves.You might be able to find a sampling voltmeter on Ebay, but that will likely lead you down the path of instrument repair.Quote from: mawyatt on January 08, 2025, 03:49:10 pmHow do you know that the SDG6022X AWG has a flat response?

author: mawyatt
author_rank: Super Contributor
timestamp: January 09, 2025, 02:34:13 am
content: Quote from: David Hess on January 08, 2025, 10:13:03 pmMeasured with a SA over that rangeChecked our SDG2042X it was flat also.For lower frequencies we checked our three KS34465As, AG34401A, HP34401A, DMM6500 and SDM3065X with both AWGs and SA to see if there was any AC discrepancies over various frequency ranges with these various instruments.As was shown elsewhere with the DSO Probes, there was significant variation introduced wrt frequency with the probes.Best

author: bdunham7
author_rank: Super Contributor
timestamp: January 09, 2025, 03:31:25 am
content: Quote from: David Hess on January 08, 2025, 10:13:03 pmA number of the current higher-end DMMs use sampling methods for AC, including the not-so-new 3458A. The best of them can measure 10VAC to 1MHz with less than 1% error and I've always assumed that the BW limitations were due to input scaling and protection not the sampling system itself.For higher BW measurements, I think the DSO can probably step in in most cases and is mostly limited by the same issue--flatness of the input circuitry. After all, the DSO is just a really fast sampling voltmeter and if it is taking 2 billion samples per second and the samples are 200ps wide, I don't see how it matters that the input voltage is varying at 10MHz instead of 10Hz. In my own experimentation doing this the primary sources of significant error seem to be gating issues and probes where they are used. The same applies in reverse to an AWG--unless it is designed oddly or has a defect, flatness is sort of designed in as long as you don't approach the BW limit of the output circuitry.

author: David Hess
author_rank: Super Contributor
timestamp: January 09, 2025, 06:24:45 am
content: Quote from: bdunham7 on January 09, 2025, 03:31:25 am"Sampling" in this case refers to a specific method of operation which modern oscilloscopes and the 3458A do not qualify for. It includes "obsolete" instruments like the HP 3406A and Racal Dana 9301 which use a sampling front end where bandwidth is only determined by the sampling strobe width and not the RC time constant of the input circuits. This also applies to true sampling oscilloscopes. These instruments have a very predictable non-linear frequency response in contrast to the linear single or multiple pole response of a normal instrument.So for instance a Racal Dana 9301 has a flat bandwidth to 500 MHz, where "flat" means that no amplitude correction is required because it is within the 1% measurement error of the instrument. The HP 3406A seems to be a little less than this, but the manual is difficult to read on this point. A digital storage oscilloscope would require a bandwidth of roughly 5 GHz to achieve this with single pole response and nobody has proposed using such an instrument.This is what makes the performance of instruments like the Racal Dana 9301 and HP 3406A so difficult to duplicate. Another point is that these instruments do *not* require a leveled source for high frequency calibration. Calibration can be done with an unleveled source by measuring the first null in their non-linear frequency response.

author: TimFox
author_rank: Super Contributor
timestamp: January 09, 2025, 03:27:07 pm
content: In the original post, the calibration procedure from -hp- could use an old-fashioned 400E voltmeter, which is an average-responding unit with the meter movement within a rectifier in the feedback loop.Done carefully, this works up to a few MHz, which is required to calibrate the 4204A generator.See page 1-1 of the 400E manual for specifications:https://pearl-hifi.com/06_Lit_Archive/15_Mfrs_Publications/20_HP_Agilent/HP_400E_EL/HP_400E_&_EL_AC_Voltmeter.pdf

author: joeqsmith
author_rank: Super Contributor
timestamp: January 09, 2025, 04:43:25 pm
content: +/-3dB on a scope would be bad. Mine are all 8-bits so there are other problems. It is possible to characterize the scopes ADC and correct for non-linearity errors and over sample for these lower frequencies and get some pretty good results. Some of these new high resolution scopes are really impressive though.Quote from: yalect on January 25, 2021, 09:29:09 pmhttps://www.eevblog.com/forum/rf-microwave/measuring-frequency/

author: pdenisowski
author_rank: Super Contributor
timestamp: January 10, 2025, 10:55:56 am
content: Quote from: radiolistener on January 08, 2025, 12:19:14 pmAs someone who works for an oscilloscope manufacturer (and produced a video explaining oscilloscope bandwidth): scope bandwidth is generally defined as the frequency at which the measured amplitude of a sinusoidal signal is 3 dB down from the actual signal amplitude. (see video below at 1:14)Quote from: bdunham7 on January 08, 2025, 03:59:45 amAll scopes have some amplitude variation or ripple in their "passband" (see video at 8:52), but this should be on the order of a fraction of a dB. 3dB variation in the passband would be completely unacceptable (even to me as a hobbyist) at any price point.

author: tatel
author_rank: Frequent Contributor
timestamp: January 10, 2025, 01:22:21 pm
content: Quote from: nsummy on January 07, 2025, 09:11:42 pmMaybe a Fluke 8920-21-22?However you would be in the same case that with the devices suggested to you in 4024A manual

author: David Hess
author_rank: Super Contributor
timestamp: January 10, 2025, 04:35:22 pm
content: Quote from: bdunham7 on January 08, 2025, 03:59:45 amThere is a reason that has to do with modern DSO design.Starting in the 1980s, analog oscilloscopes and DSOs changed to using some variation of a two path high impedance buffer, doing away with the temperature compensated dual JFET design. One consequence of this is that the cutoff frequencies and gain of the two paths must be matched to prevent a mid-band discontinuity in the frequency response. Early implementation had a calibration step to adjust this, but modern DSOs usually have a fixed calibration which cannot be adjusted. In practice the discontinuity is always present to some degree, but I have seen examples of modern inexpensive DSOs where it is particularly bad.The second attachment shows the early two path design from a Tektronix 2232 where the amplitude and frequency calibration adjustments are present.

author: bte
author_rank: Regular Contributor
timestamp: January 11, 2025, 04:28:00 am
content: Quote from: pdenisowski on January 10, 2025, 10:55:56 amI know that already. My question was rhetorical to get a clear explanation for the claim of "Many oscilloscopes also exhibit ±3 dB deviations in the middle of their operational bandwidth" or maybe a few examples of such oscilloscopes. With the usual definition of scope bandwidth ("the frequency at which the measured amplitude of a sinusoidal signal is 3 dB down from the actual signal amplitude"), it is not possible to have 3 dB variations in the operational bandwidth to start with. The first 3 dB deviation would define the bandwidth. Maybe instead of the usual definition, a revised definition could be something along the lines of "thehighestfrequency at which the measured amplitude of a sinusoidal signal is 3 dB down from the actual signal amplitude". That sort of definition would make 3 dB deviations within the operational bandwidth possible.And yes, I have seen the post from David Hess. It supports such a revised definition of working bandwidth and nicely provides the reasoning for it. BTW, if anyone is interested: the book quote in his post is from "The Art and Science of Analog Circuit Design", edited by Jim Williams.

author: bdunham7
author_rank: Super Contributor
timestamp: January 11, 2025, 05:14:45 am
content: Quote from: David Hess on January 10, 2025, 04:35:22 pmI probably shouldn't have said DC, but even there the issue with the DC/LF amp section on modern scopes may not lead to as serious a problem measuring RMS voltage as it might first appear because the step response distortion can be caused by phase shift just as well as amplititude variation. I gave it a try using an AWG set to 5.000V and my 8506A in parallel with two Siglent DSOs, an SDS1104X-E (with the problematic LF distortion) and an SDS2354X+. I tested at 10, 100, 1k, 10k, 100k, 200k, 500k an 1M Hz. Results:FREQ 10 100 1000 10k 100k 200k 500k 1M8506A 4.9939 4.9958 4.9955 4.9982 4.9957 4.9866 4.9599 4.90891104X-E 5.02 4.98 5.03 5.02 5.04 5.03 5.03 4.972354X+ 5.015 5.008 5.027 5.041 5.050 5.051 5.035 4.967I think the input capacitance of the 8506A was loading the AWG down at 1MHz, removing it brought the values on the scopes back up. As you can see, either would be adequate for the OP's purpose--and more generally, they exceed any other method I have of measuring AC voltage once you get over 500kHz. Unfortunately I can't push much further because my AWG is not quite as good when it comes to flatness. 1% is 0.09dB and that's a lot to ask for from sig-gens or spectrum analyzers. I also did sweep tests on both scopes 0.1 to 100Hz . The "problem" scope has just-noticeable uneven amplitude on the low frequency sweep and the other scope is as flat as you could possibly hope for. Pictures attached.QuoteI had a look at the manuals for those instruments because I haven't seen them before. Yes, the "sampling front end" is a specific subset of sampling systems and it does have the advantages you state, but my larger point was that the sampling system itself in a DSO or the 3458A has the same inherent linearity for the same reasons. It's the input and scaling circuitry that limits BW. The thermal converter on my 8506A, for example, would have nearly infinite BW and could be used like an HP thermal RF power meter if there were a way to accurately get a scaled signal to heat up the AC resistor heater in the sensor. The sampling front end avoids any input circuit issues because it puts the sampling up front. Of course the trade-off is the limitations on input voltage and input impedance. Still, even with all that the HP has a best-case accuracy of 3% and the Racal Dana is 1.5% of reading plus 1% FS, so 2.5% at best--0.125dB for the HP and 0.09dB for the Racal Dana. Now given the 500MHz+(but not LF) that those specs apply to, I have to concede I don't know anything that can do that to 0.125dB. IDK how flat a 5GHz scope would be to 500MHz, but I'm suspecting it won't be a under 0.5dB. But for 0 to 1MHz, I think the cheap DSO has them beat.