EB38 Datasheet, PDF (1/4 Pages) Motorola, Inc – MEASURING THE INTERMODULATION DISTORTION OF LINEAR AMPLIFIERS

MOTOROLA
Freescale Semiconductor, Inc.
SEMICONDUCTOR
ENGINEERING BULLETIN
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by EB38/D
EB38
MEASURING THE INTERMODULATION DISTORTION
OF LINEAR AMPLIFIERS
Prepared by: Helge Granberg
Circuits Engineer, SSB
The measured distortion of a linear amplifier, normally
called Intermodulation Distortion (IMD), is expressed as the
power in decibels below the amplifier’s peak power or below
that of one of the tones employed to produce the complex
test signal.
A signal of three or more tones is used in certain video
IMD tests, but two tones are common for HF SSB. The
two-tone test signal provides a standard, controlled test
method, whereas the human voice contains an unknown
number of frequencies of various amplitudes and couldn’t
be used for accurate power and linearity measurements.
Separation of the two tones, for voice operation equipment,
may be from 300 Hz to 3 kHz, 1 kHz being a standard
adopted by the industry.
Generation of the Test Signal
The two-tone IMD test signal can be generated by a
number of means of which the following three are the most
common:
System A — A two-tone audio signal is formed by
algebraically adding two sine wave voltages of equal
amplitude which are not harmonically related, e.g., 800 Hz
and 1.8 kHz. This two-tone audio signal is fed into a balanced
modulator together with an RF carrier, one sideband filtered
out, and the resultant further mixed to the desired frequency
and then amplified. The system is useful in testing complete
SSB transmitters. A commercial transmitter can also be used
as a signal source for testing linear amplifiers.
LOWER
SIDEBAND
UPPER
SIDEBAND
1 kHz
CARRIER
1 kHz
f₀ - f₂
f₀ - f₁ f₀ f₀ + f₁
SYSTEM A
f₀ + f₂
System B — In this method, a signal of approximately
500 Hz is fed into a balanced modulator together with an
RF carrier and amplified to the required power level.
The resultant is a double-sideband signal that resembles
a single-sideband signal generated under two-tone sine
wave conditions. Viewed on a scope screen, the envelope
produced by this method appears the same as a SSB
twotone pattern. However, unlike the System A test signal,
there is a controlled and fixed phase relationship between
the two output tones. This system is widely employed to
generate the test signal for linearity measurements.
LOWER
SIDEBAND
1 kHz
UPPER
SIDEBAND
CARRIER
f₀ - f₁ f₀ f₀ + f₁
SYSTEM B
System C — Two equal amplitude RF signals, separated
in frequency by 1 kHz, are algebraically added in a hybrid
coupler. The isolation between input ports must be high
enough to avoid interaction between the two RF signal
generators. Short-term stability (jitter) should be less than
one part per million at 30 MHz. The carrier is nonexistent
as compared to A and B, and the two-tone signal is
generated as the RF voltages cancel or add at the rate of
their difference frequency according to their instantaneous
phase angles. Because no active components are involved,
very low IM distortion is achievable. This system is useful
in applications where low distortion and low power levels are
required.
1 kHz
f₁
f₂
SYSTEM C
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