Roy Lewallen wrote:
wrote:
. . .
Random noise voltage or current can ONLY be calibrated by
a TRUE Root Mean Square measuring instrument. By "true" I
mean a thermionic type such as an RF power meter (thermistor,
bolometer, etc. sensor). Few voltmeters on the market have
TRUE RMS measuring capability; those that do are specified
as such and rather on the expensive side.
. . .

Not too long ago, I got an HP3400A (10 MHz bandwidth) on eBay for the
purpose of measuring noise. Don't recall what I paid, but it was very
reasonable. Since truly random noise also has low-frequency
fluctuations, an RMS meter reading wanders quite a bit while measuring
noise. I find it easier to eyeball interpolate an average reading with
the analog meter of the HP3400A than with a digital meter.

I agree there on analog display versus digital.

Dug out my CD of old "saves" and found the Jim Williams article
in the 11 May 2000 edition of EDN (www.ednmag.com). In that
he lists the following as having zero error when compared to using
the Linear true-RMS IC: HP 3400, 3403C, 3478; Fluke 8920A.

By the bye, the 1967-era HP pseudorandom noise generator has
the following "calibration" procedure to check RMS output voltage:
Connect it to an HP digital voltmeter-recorder, set it to slow
speed and record 1023 voltage readings (corresponding to the
slow-speed 'flat' pseudorandom waveform parts), then take the
sum of the squares of all those readings and find the square root
of it. Heh heh heh...I can just see a calibration technician doing
all that work with a mechanical calculator in the 60s...with the QC
chief storming in after an hour or so asking "You STILL working
on that?!?" :-)