"Owen Duffy" wrote in message
...
On Mon, 17 Oct 2005 00:05:18 GMT, Owen Duffy wrote:

This is seriously bad, replying to one's own post... but.

It occurs to me a quick test to reveal whether a SINAD meter is RMS
responding or average responding is to test it with a 1KHz square
wave. I am not suggesting this as a cal procedure, just a test that is
more sensitive to the meter response than noise testing.

IIRC, the Taylor series coefficients for a square wave a all even
harmonics are 0, the others are 4/pi/n.

So, theoretically:
- an ideal average responding meter should read (1-2/pi)% which is
36.3% or 8.8dB on an perfect square wave;
- an ideal RMS responding meter should read
(1-(2^-0.5*4/PI())^2)^0.5*100% which is 43.5% or 7.23dB.

Does the maths make sense?



Wait a minute here. You're percents and dB is confusing.

I don't know about the 1-2/pi. It's been about a year or so since I went
through all this for that QST article using a serise resistor in the power
line to figure out power supply (and rig) power consumption - unfortunately
ignoring the pulsed nature of capacitor input power supply current,
BUT...

I don't remember the analytical expressions for these quantities. I'll use
the common numbers...

For the meter that responds to average (63% peak - I think this is 2/pi) ,
but shows RMS which is .707 of peak (1/root2), the ratio for average input
to reading = 0.707/.63 . For this I get 2/(2* root2)

Average of a square wave is equal to the peak.

So a 1 volt (pk) square wave should measure 1.11 Volts on one of these (sine
average responding, RMS displaying) meters and 1V on an rms meter.

I think I did that right?

73, Steve, K,9.D;C'I