Arbiter Systems 931A, Operation Manual

Page: 67 / 161

channel, press the CHANNEL 1 SELECT or CHANNEL 2 SELECT keys.
The harmonics display appears as follows:
In the example above, the display shows the harmonic content of the
AC current at the input of a switching power supply. While all harmonics
are shown, only the odd harmonics have enough amplitude to be displayed.
The first, vertical bar (on the left) represents the fundamental. An
arrow, below the second bar, represents the cursor, and lies under the third
harmonic. Move the cursor in increments of 1 harmonic with the right and
left arrow keys. Change the vertical scale to better view harmonics with
the up and down arrow keys. The harmonic magnitude factor applied from
using the up/down arrow keys appears in the upper right corner.
The second and third lines of display text describe individual harmonic
information, highlighted by the cursor. These values include the following:
ˆ
The channel being measured
ˆ
The input configuration for the channel
ˆ
The harmonic number
ˆ
The harmonic amplitude as a percentage of fundamental amplitude
ˆ
The rms amplitude of the harmonic
ˆ
The phase angle of the harmonic, relative to the fundamental
ˆ
The frequency of the harmonic
The top line of text shows THD and K-factor, which relate to the
overall input signal, and do not change when the cursor is moved.
The Model 931A analyzes harmonics by first determining the AC
fundamental and the DC components of the signal. It then removes them
algebraically from the digitized waveform. Fast Fourier Transform (FFT)
analysis determines the residual for the spectral display, K-factor, and
individual harmonic measurements.
Total Harmonic Distortion (THD)
is determined by dividing the rms of the residual by the wide-band rms
amplitude. With these techniques, harmonic measurement accuracy does
not depend upon fundamental frequency.
51