Communications Receivers, Fourth Edition by Ulrich L. Rohde & Jerry C. Whitaker

Author:Ulrich L. Rohde & Jerry C. Whitaker [Rohde, Ulrich L.]
Language: eng
Format: azw3
Publisher: McGraw-Hill Education
Published: 2017-03-10T05:00:00+00:00

Figure 6.11 shows an overdriven amplifier and its third harmonic. The dashed line represents a linear gain. It is important to notice that the output level begins to decrease at a certain drive level even when the input signal is further increased. This situation leads to an instability within the predistortion loop and must be avoided under all possible load conditions. Secure operation is maintained when the amplifier is driven slightly higher than the 1 dB compression point.

FIGURE 6.11 Gain and third harmonic with k1 = 10 and k3 = 2.

The result of a linearized SSB transmitter by means of the adaptive predistortion are presented in Figure 6.12. The third- and fifth-order intermodulation products are reduced by −64 dBc or −70 dBPEP . The marker points to the carrier frequency. It is clearly visible that the intermodulation products within the passband of the 5-kHz low-pass filter are cancelled, whereas the seventh- and ninth-order products are even higher than without predistortion (see Figure 6.13). A possible reason is that these products are further distorted by the rapidly changing phase on the filter slope of the LPF.

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