AGC CIRCUIT OF ZERO IF TUNER FOR DBS

摘要

PURPOSE: An AGC(Auto Gain Control) circuit of a zero-IF tuner for DBS(Direct Broadcast Satellite Systems) is provided to use AGC characteristics of pin diodes when the strength of an RF signal inputted to a QPSK IC is strong, and to use AGC characteristics of a zero-IF IC when the strength is weak, to separate an AGC range, so as to have a wider AGC range. CONSTITUTION: A QPSK IC(110) outputs 0 to 3.0 voltage according to the strength of an RF signal. A zero-IF IC(120) divides entire driving voltages, and uses voltages only under a predetermined level, then controls the RF signal when a voltage under the predetermined level is applied from the QPSK IC(110). A transistor(Q10) is driven only by a voltage over the predetermined level, and amplifies a current. Many pin diodes(D10-D30) control the RF signal according to the strength of the current. If the strength of the signal in accordance with the strength of the RF signal is weak, the zero-IF IC(120) is driven. If the strength of the signal is big, the pin diodes(D10-D30) are driven.