SE330915B - - Google Patents

Abstract

1,096,615. Demodulation circuits. MARCONI CO. Ltd. Jan. 29, 1965 [May 12, 1964], No. 19711/64. Headings H3Q and H3R. In a demodulating arrangement for amplitude, frequency or phase modulated signals the gain of an amplifier following the demodulator is varied to compensate the signal amplitude for non-linear distortion introduced in the demodulator. Received amplitude modulated signals are applied with a signal from source 9, having a frequency F2, different from the carrier and modulating signal frequencies, to the input of detector 3 and the modulation signals are selected by filter 4 and passed via a delay 5 and controlled amplifier 6 to output terminal 8. The second harmonic 2 F2 of the signal from source 9 at the output of the detector is a function of detector non-linearity and varies in phase by 180 degrees as the detector input rises above and falls below the level corresponding to the point of inflection of the detector characteristic. This signal is selected by filter 14 and compared in phase detector 15 with a similar frequency signal obtained from source 9 via doubler 16 and phase shift circuit 17. The output of the phase detector is amplified 13 and used to control the gain of amplifier 6. The delay circuit 5 compensates for the delay in the control loop. In a frequency modulation arrangement, Fig. 3 (not shown), a discriminator replaces the detector 3 and the signal from source 9 frequency modulates a local oscillator feeding a frequency changer stage preceding the discriminator.