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262
Bandpass Signaling Principles and Circuits Chap. 4
amplitudes of K 1 A, and each of the third-order intermodulation products will have amplitudes
of 3K 3 A 3 4. The ratio of the desired output to the IMD output is then
R IMD = 4 3 a K 1
K 3 A 2 b
(4–52)
The input intercept point, defined as the input level that causes R IMD to be unity, is shown in
Fig. 4–6. The solid curves are obtained by measurement, using two sinusoidal signal generators
to generate the tones and measuring the level of the desired output (at f 1 or f 2 ) and the
IMD products (at 2f 1 - f 2 or 2f 2 - f 1 ) with a spectrum analyzer. The intercept point is a
fictitious point that is obtained by extrapolation of the linear portion (decibel plot) of the
desired output and IMD curves until they intersect. The desired output (the output at either f 1
or f 2 ) actually becomes saturated when measurements are made, since the higher-order terms
in the Taylor series have components at f 1 and f 2 that subtract from the linearly amplified
output. For example, with K 3 being negative, the leading term in Eq. (4–51) occurs at f 1 and
40
30
20
10
3 dB
Intercept
point
0
3-dB compression levels
RF output power (dBm)
10
20
30
40
50
Desired output
45 dB
Third-order
intermodulation
product
(two-tone test)
60
70
90 80 70 60 50 40 30 20 10 0 10
RF input power (dBm)
Figure 4–6 Amplifier output characteristics.