Measuring RMS noise...

[pwdrive]

....with an oscilloscope:

https://www.youtube.com/watch?v=zHU9xdU51FY

Also posted on t'other forum.

Lawrence.

[AlanBeckett]

Well I'm damned.
I've got the 'scope.
I can't see any particular use for the technique at the moment but you never know.

Alan

[peter scott]

I like it!

Peter

[ppppenguin]

I've known this method for many years. Pretty much standard for analogue video measurement. It's certainly described by Weaver in 1971 "Television Video Transmission Measurements". He calls it "Tangential noise measurement" and his technique involves adding a calibrated square wave to the signal under test but the principle is the same. Likely to pre-date Weaver's book - though he does not refer to it in BBC monograph 24 from 1959
http://downloads.bbc.co.uk/rd/pubs/archi...aph_24.pdf

Usual caveat that you need to define the bandwidth over which you're measuring the noise. 100kHz noise is irrelevant for audio.

Won't work on a digital scope. Even the best "digital phosphor" ones probably won't be much good.

[pwdrive]

The original article that's referred to in the video link...Tekscope April 1969:

http://w140.com/tekwiki/images/e/e3/Teks...r_1969.pdf

Lawrence.

[valves4amps]

Absolutely brilliant. I suspect there are many other simple techniques using scopes.

Dave.

[VintageTest]

I remember the 70s - just, and true RMS meters were expensive then - Levell TM3B and HP3400A to name but two. Not like today, where just about every multimeter worth it's salt has an RMS measuring capability . . .

[Nick]

At lot of DMMs aren't really true RMS as most are AC coupled and thus ignore any DC component. Some give you the option of including DC, most don't.

[Craig]

The problem with AC measurement is that there is no fundamental definition of an AC waveform. "Say What??" I hear you say.

But in terms of measurement standards, the DC volt is closely defined in terms of fundamental constants and is measured using an array of Josephson Junctions.
Resistance is likewise defined via the Quantum Hall Effect Resistance Standard
DC current is still a work in progress, the aim being to count the number of electrons per second to define a fundamental comparison standard.

But - you see that they are all DC quantities. There is no such definition of AC quantities in terms of fundamental constants.

So given, say an AC calibration instrument with which you are going to calibrate your Fluke DMM - how can you be certain that the AC calibrator is in calibration?

The only way that this can be done is to compare the heating effect of the AC quantity with a DC quantity. That is usually done using a highly refined thermocouple. Fluke themselves made one called the 540B Thermal Transfer Standard, although this is now obsolete. One of several papers from NIST on this topic is here http://nist.gov/calibrations/upload/sp250-27.pdf

[Alistair D]

I remember the 70s - just, and true RMS meters were expensive then - Levell TM3B and HP3400A to name but two. Not like today, where just about every multimeter worth it's salt has an RMS measuring capability . . .

Just to clarify, the TM3B manual states that it is true RMS on the H.F. ranges. On the L.F. ranges it is mean reading and is calibrated for the RMS value of a sine wave. Still great meters though.

Al