Intermodulation distortion product TSH350
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6 Intermodulation distortion product
The non-ideal output of the amplifier can be described by the following series:
where the input is V
in
=Asinωt, C
0
is the DC component, C
1
(V
in
) is the fundamental and C
n
is
the amplitude of the harmonics of the output signal V
out
.
A one-frequency (one-tone) input signal contributes to harmonic distortion. A two-tone input
signal contributes to harmonic distortion and to the intermodulation product.
The study of the intermodulation and distortion for a two-tone input signal is the first step in
characterizing the driving capability of multi-tone input signals.
In this case:
then:
From this expression, we can extract the distortion terms, and the intermodulation terms
from a single sine wave:
● second order intermodulation terms IM2 by the frequencies (ω
1
-ω
2
) and (ω
1
+ω
2
) with an
amplitude of C2A
2
● third order intermodulation terms IM3 by the frequencies (2ω
1
-ω
2
), (2ω
1
+ω
2
), (−ω
1
+2ω
2
)
and (ω
1
+2ω
2
) with an amplitude of (3/4)C3A
3
The intermodulation product of the driver is measured by using the driver as a mixer in a
summing amplifier configuration (see Figure 33). In this way, the non-linearity problem of an
external mixing device is avoided.
Figure 33. Inverting summing amplifier (using evaluation board SO8_S_MULTI)
V
out
C
0
C
1
V
in
C
2
V
2
in
… C+
n
V
n
in
++ +=
V
in
A ω
1
tsin A ω
2
tsin+=
V
out
C
0
C
1
A ω
1
tsin A ω
2
tsin+()C
2
A ω
1
tsin A ω
2
tsin+()
2
… C
n
A ω
1
tsin A ω
2
tsin+()
n
++ +=
+
_
R
R
fb
100
V
out
R2
V
in2
V
in1
R1
+
_
R
R
fb
100
V
out
R2
V
in2
V
in1
R1
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