| 摘要 |
An analogue signal, such as a broadband TV signal from a CATV system is input at 1. The signal is mixed with a pilot tone which has frequency well below the e lowest frequency in the signal. The signal is fed to an electro-optical modulator 7 which has two complementary outputs 11, 12. A portion of each of the two outputs is sampled at 10, 13. The second harmonic component of the pilot tone are extracted from the two samples and the two components are fed to algebraic adder 23 to produce a difference signal. The difference signal leaving the adder 23 is fed to analog multiplier 28, where the difference signal is compared with a signal from doubler 31 that is double the frequency of the pilot tone. The output of the multiplier is fed to differential amplifier 25 which compares the signal with the output of a bias reference signal 26. The output of amplifier 25 is fed to adder 24 where it is mixed with the pilot tone. The resultant signal is used to control the modulator 7. Effectively, the distortion of the pilot tone in the output is used to provide a pre-distortion in the modulator.
|
