Agilent Technologies 33120A, Руководство пользователя

Страница: 287 / 312

RMS The
RMS
value is the only measured amplitude characteristic of a
waveform that does not depend on waveshape. Therefore, the
RMS
value
is the most useful way to specify ac signal amplitudes. The
RMS
value
(or equivalent heating value) specifies the ability of the ac signal to
deliver power to a resistive load (heat). The
RMS
value is equal to the
dc value which produces the same amount of heat as the ac waveform
when connected to the same resistive load.
For a dc voltage, this heat is directly proportional to the amount of
power dissipated in the resistance. For an ac voltage, the heat in a
resistive load is proportional to the average of the instantaneous power
dissipated in the resistance. This has meaning only for periodic signals.
The
RMS
value of a periodic waveform can be obtained by taking the
dc values at each point along one complete cycle, squaring the values at
each point, finding the average value of the squared terms, and taking
the square-root of the average value. This method leads to the
RMS
value of the waveform regardless of the signal shape.
Peak-to-Peak and Peak Value The zero-to-peak value is the
maximum positive voltage of a waveform. Likewise, the peak-to-peak
value is the magnitude of the voltage from the maximum positive
voltage to the most negative voltage peak. The peak or peak-to-peak
amplitude of a complex ac waveform does not necessarily correlate to
the
RMS
heating value of the signal. When the specific waveform is
known, you can apply a correction factor to convert peak or peak-to-
peak values to the correct
RMS
value for the waveform.
Average Value The average value of an ac waveform is the average of
the instantaneous values measured over one complete cycle. For sine
waves, the average amplitude is zero since the waveform has equal
positive and negative half cycles. Since the quantity of interest is the
heating value of the signal, the average value of a sine wave is taken to
mean the average of the full-wave rectified waveform. The
RMS
value of
a sine wave is equal to 1.11 times the sine wave average amplitude.
This relationship does not hold true for other waveshapes.
7
Chapter 7 Tutorial
Attributes of AC Signals
285