Demodulator and filter for coded telephonic pulses - uses operational amplifier with feedback components providing filter frequency response

摘要

Coded telephone pulses are amplitude modulated and transmitted with time sharing to a receiver. To demodulate the signal they are applied to a filtering and demodulating circuit which compensates for the frequency response of the lines. An input signal is applied to the inverting input (-) of an operational amplifier via a series resistor (R1) and capacitor (C3). This capacitor provides dc coupling while a capacitor (C1) across the input terminals provides integration of the input signal to eliminate parasitic signals. The amplifier output (5) is connected to one feedback path comprising a resistor (R2) in series with a resistor (R6) and capacitor (C4) in parallel which affect the low frequencies. A second feedback path comprises a series resistor (R4) and capacitor (C2) with a resistor (R3) connected to earth. These component valves are chosen to satisfy the frequency response required.