SFRA45 User Manual
11-1
11
AC RMS Mode
The RMS voltmeter measures the AC rms of the signal
present at the input terminals to the bandwidth of the
instrument (>1MHz). Care must be taken when measuring
low signal levels to minimise noise pick on the input leads.
The RMS voltmeter measures the elementary values:
rms
peak
surge
and derives the values: ac, dBm and crest factor.
The rms value of a periodic waveform, v(
), is given by:
2
rms =
[ 1/2
v2(
) d
]
0
For a sampled signal, the formula becomes:
i = n-1
rms =
[ 1/n
v2[i] ]
i = 0
where n is the number of samples for an integral number
of complete cycles of the input waveform.
These are fundamental definitions that are valid for all
waveshapes. For a pure sinewave, the formulae equate to
peak/
2, but this cannot be applied to other waveshapes.
The SFRA45 computes the true rms value from the
fundamental definition for sampled data.
