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HEWLETT-PACKARD
JOURNAL
T E C H N I C A L I N F O R M A T I O N F R O M T H E - h p - L A B O R A T O R I E S
VOL. 2 No. 12
BLISHED BY THE HEWLETT-PACKARD COMPANY, 395 PAGE MILL ROAD, PALO ALTO, CALIFORNIA AUGUST, 1951
-hp- Distortion Measuring Equipment
simple, straightforward measure- The measurement of harmonic distortion
JL ment of harmonic distortion has always is based on the definition % distortion =
been valuable in design, test, and production (Af + A' + A> + . . .) «/A, X 100, A!
work. This basic measurement undoubtedly being the amplitude of the fundamental, A2
gives as much if not more information about the second harmonic, etc. Two methods for
the operation of an amplifier as any other measuring distortion have evolved from this
single measurement that can be made. In definition. In one method, the amplitude of
spite of the development of more complex each frequency component appearing in the
methods and equipment for measuring vari output of a device fed with a single pure fre
ous effects of non-linear transfer characteris quency is measured with a special frequency-
tics, the harmonic distortion measurement, selective voltmeter or wave analyzer. The
because of its ease and simplicity, is still the measured values are then substituted in the
predominant method in use; and the demand above expression. In the second method, the
for equipment for such measurements is amplitude of a voltage containing harmonics
greater than it has ever been. In electronic is first measured; then the fundamental is fil
equipment, where the transfer characteristic tered out and the rms value of the combined
of devices such as amplifiers is relatively harmonics is measured. The ratio of the two
constant over the frequency range of inter values, expressed in per cent, is the distor
est, measurements of harmonic distortion tion. This second method has come to be
have a logical engineering basis, since the known as the "total" distortion-measuring
results of these measurements1 can be re method.
lated to those given by other methods. -hp- instruments for measuring distortion
include both the frequency-selec
tive voltmeter type of instrument
as well as several different instru
ments of the "total" distortion-
measuring type. These vary from
the complete distortion meter to
simple filter attenuator sets.
DISTORTION METERS
The usual test set-up necessary
m Figure 1. -hp- Model