The Use of Test Gear - Keith Wevill's Golborne Paper.

[Keithstv]

Well I've added my avatar in the form of our pooch which will be familiar to those who are on Pauls forum.

All the favourable responses and a suggestion from Yorkie plus the thread having a 5 star rating have encouraged me to put together a follow up talk probably covering some of the stuff I had to leave out. I'll reveal more once I've had a chance to try a few things.

And thanks for the warm welcome.

Keith

[Mark R]

Excellent!
Obliged keith

cheers Mark

[pwdrive]

Good man, Tech. talk is always welcome and of use.

Lawrence.

[Andrewausfa]

Thanks for this Keith, it's very well written and I wish I could have been there for your talk.

This sort of thing is very useful for those, like me, who've not worked in the industry and have to pick advice up from the internet, books or interpret forum posts, having no-one to stand on ones shoulder and advise on the correct way of doing things.

Andrew

[Nick]

Thanks for the paper, Keith - lots of good stuff in there.

One small question/issue, if I may... You mention about some meters having an "RMS" setting and you warn that this is typically limited by frequency. However, there is a hidden issue with RMS measurements on most meters.

By definition, the RMS value of a waveform is the DC equivalent constant voltage or current that would cause the same heating effect in a purely resistive (not reactive) load over the sample period. For a steady-state DC system this is simply P = I * V; with varying values this becomes Prms = Irms * Vrms. "RMS" stands for "Root Mean Square" - the so-called "quadratic mean" value - it turns out that the True RMS value of a signal as computed by the quadratic mean is identical to the DC equivalent voltage of the waveform - Google for the first-principles maths if you need that.

The original analogue meters that measured RMS current values did indeed use a pure resistive load with a thermocouple that measured the temperature rise and displayed the RMS power via that - as we shall see, this was a far "truer" RMS reading than most digital RMS or "True RMS" meters give.

For a pure sine wave, Vrms = Vpeak / squareroot(2) and likewise Irms = Ipeak / squareroot(2).

There are two points here - the first is that pretty much every cheapo meter does indeed assume a sine wave for AC, and thus the "RMS" I or V value is normally an estimate calculated from the above equations.

A "True RMS" meter does (or should!) not assume a pure sine wave or a periodic waveform at all - it should be waveform independent and should sample at a far higher frequency than its specified highest frequency - using the Nyquist criteria, this should be at least twice the peak frequency, but for a good meter, at least 4 times or higher (e.g. 10x). The value displayed should be the true RMS value calculated between display updates (the square root of the arithmetic mean of the squares of the sample values taken during the interval).

Herein lies the problem. Nearly EVERY so-called "True RMS" meter I've seen is AC coupled on the RMS settings. This means that any DC component (aka "offset") in the signal under test (SIT) is ignored - this flies in the face of the definition of an RMS value and I'm not just being picky and a purist here - this can be important.

You can easily check if your meter handles or ignores any DC component by setting it to measure RMS volts and applying a DC voltage. If it reads 0, then the RMS scale is AC coupled and the meter is giving incorrect results - think about it - the RMS value of a steady-state DC voltage should be the same as that DC voltage.

Some meters - e.g. I have a Tektronix DMM 916 - are genuinely "True RMS" as they are DC coupled and can accurately measure real RMS values for current & voltage - in the case of the DMM 916, up to 2MHz - what's more, for compatibility with most engineers' incorrect expectations of RMS, the DMM 916 has an AC-coupled True RMS setting too, but at least it makes that explicit !

Most meters when measuring RMS or True RMS lie to you as either they assume a pure sine wave and estimate the result and/or are AC coupled so that any DC offset is incorrectly ignored. Its a convenience & cost saving for the meter manufacturer as the resulting signal is centred around zero meaning a simple peak measurement and maths can give a sort-of RMS-ish value.

Note that the old "heat a resistor and measure the power it gives off" method is, assuming you can measure the resultant power accurately, by definition the most accurate way of doing this as the resistor's thermal lag integrates any signal nicely.... also, note that for small signals, unless the meter uses an ideal precision rectifier, small signal RMS AC values may have a large error component.

Cheers

[Mark Hennessy]

Hi Keith,

Glad you found your way here - welcome

I'm just reading through your notes, which are most excellent - just right.

You mention that you suspect that your Fluke uses reciprocal counting, and you're dead right. Indeed, many frequency counters do this - especially the ones that use a microprocessor to control the count (as opposed to using a microprocessor to just manage the user interface). Indeed, some switch between reciprocal and gated depending on frequency. Obviously, the reciprocal calculation is done by the microprocessor. Counter that do this normally have a period mode as well.

I've spotted an minor error on page 8: when discussing the -3dB point of a 'scope, you say that's when the amplitude has fallen by 50%. It would be 50% if discussing power, but when considering voltage, it's 70.7% (1 over the square root of 2). As a handy tip, when finding the -3dB points of a circuit, I adjust the amplitude in the pass-band to give 8 divisions of height, and adjust the frequency until I see 5.6 divisions (2.8 from the centre line).

I know from my day job that putting together something like that isn't easy, and it's clear that a lot of prep went into it - perhaps a lot more than most people might realise! Just building up the demo circuit boards takes time, and being sure it'll all work on the day is exciting to say the least. But I bet you enjoyed it

Thanks for making the notes available - they are an excellent resource.

All the best,

Mark

[Keithstv]

Nick

The RMS value as mention should include the dc component but there are circumstances where the RMS value of the AC component is required, I'm thinking here of ripple voltages on a switch mode supply. My DSO at work has an RMS measurement facility which will include the dc component if the input is set to dc but for these ripple voltage measurements we tend to use the ac setting.

Many of the RMS meters, like the AC ranges on both analogue and digital multimeters, will quote a maximum frequency for measurements.

Mark

You are correct in the definition of the -3db point and I did think about changing it but left it as -3db as most people will have heard of the -3db point.

I did enjoy putting this together and checking out the circuits although like most things a lot was done in the last week before the event. It's very frustrating to have it all prepared and converted to pdf or printed out only to find a small error that once you've spotted it you have to correct it. I find the best way is once I proof read it I leave it for a day or 2 then re-read it and correct all the errors that escaped in the first reading.

In many respects those of us with years of experience are duty bound to pass on our experiences and knowledge to others. I hope this hand out achieves this.

Keith

[Mark Hennessy]

Yes, it's impossible to proof-read your own work. I find it's easier if it's printed out, but yes, a bit of time definitely helps

Regarding the -3dB thing, you're definitely right to talk about finding the -3dB point(s). It's what most people think of when designing filters, and it's widely used. Few people, if anyone, would use the -6dB point for anything in this context. The only thing that needs amending in your handout is the practice of finding half the voltage - we're looking for 71% of the voltage, not 50%.

I do mostly audio engineering these days, so as you can imagine, decibels are at the front of my mind

I completely agree with your closing sentiment, and it's why I do a teaching job, and why I contribute to forums, and why I have a website (that needs a lot of updating, but the thought was there). We should always endeavour to leave the world in a slightly better state - not exactly easy!

Mark