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RF Application Reports

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.

SYSTEM A

UPPER

SIDEBAND

1 kHz

CARRIER

1 kHz

LOWER

SIDEBAND

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.

SYSTEM B

UPPER

SIDEBAND

CARRIER

1 kHz

LOWER

SIDEBAND

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.

SYSTEM C

1 kHz

MOTOROLA

SEMICONDUCTOR

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