Is there a modern equivalent to old AC Voltmeters that measure 2MHZ+?

[nsummy]

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!

[TimFox]

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.

[bdunham7]

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. 

If 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:

[nsummy]

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.

[radiolistener]

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.

Oscilloscope 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...

[bdunham7]

Oscilloscope 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.

The 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.

[radiolistener]

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)

Yes, 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 day

However, 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.

[bdunham7]

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).

Wow, 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.

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.

I'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.

[radiolistener]

Wow, 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. 

Why broken? This is within 3 dB oscilloscope specification.
Some instances may have better results, some worse. This is pretty common issue.

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.

The picture above with 30% measurement error is taken with direct coax cable connection and enabled 50 Ω termination.

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.

Your 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.

[MrAl]

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!

Hi,

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.

[bdunham7]

Why broken? This is within 3 dB oscilloscope specification.
Some instances may have better results, some worse. This is pretty common issue.

The picture above with 30% measurement error is taken with direct coax cable connection and enabled 50 Ω termination.

Not 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 (1V_p-p at 100mV/div is a 25% overload...) but I'll assume the issue persisted even under other conditions.  Um, right?

[Aldo22]

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.

Oh, 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.

[bte]

Oscilloscope 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.

If 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?

[radiolistener]

If 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?

This question should be addressed to Siglent and other oscilloscope manufacturers.

[mawyatt]

"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 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."

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 between 30-50Hz mentioned for both of our Siglent DSOs!!

Edit: Just checked the Rigol DHO814 with the same results!!

Best

 

[mawyatt]

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. 

If 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:

That'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

[Aldo22]

BTW we see no discernible change in amplitude between 30-50Hz mentioned for both of our Siglent DSOs!!

Edit: Just checked the Rigol DHO814 with the same results!!

Hantek DSO2000, 10Hz-1kHz sine at 5 Volt.
Maximum deviation: 4.88V (from 5V): <3%.

[TimFox]

0.2 dB

[bdunham7]

Hantek DSO2000, 10Hz-1kHz sine at 5 Volt.
Maximum deviation: 4.88V (from 5V): <3%.

IDK 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.

[bdunham7]

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!!!

The 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.

[Aldo22]

IDK 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.

Thank you.
These are PkPk measurements.
But the script also adapts sec/div to the respective frequency (for phase).

[mawyatt]

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.

We 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.

 
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.

Two 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 PITA

Best

[bdunham7]

We 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.

According to the date on my oldest copy of the manual the 8506A came out in 1983, so 42 years ago already!

[David Hess]

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.

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.

How do you know that the SDG6022X AWG has a flat response?

[mawyatt]

How do you know that the SDG6022X AWG has a flat response?

Measured with a SA over that range

Checked 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