Abstract: A gain controlled radio receiver for a selective call system, includes a multi-step gain controlled RF amplifier (44) coupled to receive an input signal and a measuring circuit (42) for measuring the level of a wanted frequency down converted signal. The multi-step gain controlled amplifier (44) has an input terminal (RIP, RIN), N gain stages (76A to 76E), where N is an integer of at least 2, having outputs coupled to a common output stage (82). A first of the N gain stages (76A) is coupled to the input terminal (RIP, RIN), and an (N−1) stage capacitive ladder attenuator (86) has an input coupled to the input terminal and outputs coupled respectively to an input of each of the second to Nth gain stages (76B to 76E). The measuring circuit (42) has a level control output (45) for switching-on a selected one of the gain stages which provides an amplified signal to the common output stage for frequency down conversion and demodulation.

Abstract: A receiver circuit, particularly an integrated, zero IF receiver circuit in which a direct conversion IF stage is separated from an antenna by a further fequency conversion stage which is AC coupled to the direct conversion IF stage. A single crystal oscillator generates a frequency which is multiplied prior to being applied as a local oscillator signal to the further frequency conversion stage and is used to produce another frequency, which is divided to block harmonics, prior to being applied as quadrature related local oscillator signals to the direct conversion IF stage. The direct coversion IF stage includes several dc nulling for compensating for dc offsets.

Abstract: A bandpass filter circuit arrangement in which a pair of parallel-resonant circuits (1, 2) are reactively intercoupled has a pair of signal input terminals (16, 17) connected to one of the resonant circuits (1) and a pair of output terminals (18, 19) connected to the other resonant circuit (2). Tuning of the filter is accomplished by varying the capacitive component of each circuit by varying the output of a tuning voltage source (20). In order to compensate for the deviation from the optimum intercoupling which would otherwise occur with change of tuning frequency the capacitive component of each resonant circuit is formed by two pairs (5, 7 and 6,8, or 9,11 and 10, 12) of capacitors in series, each series-connected pair effectively forming a varaible capacitive voltage divider to the tap on which the reactive intercoupling means (15) is connected. One capacitor (5, 6, 9 or 10) of each pair is variable, and all these are adjusted at the same rate.

Abstract: A bandpass filter comprises a reactively intercoupled pair of parallel-resonant circuits (1, 2) to one of which a pair of signal input terminals (15, 16) are connected and to the other of which a pair of output terminals (17, 18) are connected. Tuning of the filter is accomplished by varying the capacitive component of each circuit by means of a tuning voltage applied to a further input (19). In order to compensate for the deviation from the optimum intercoupling which would otherwise occur with change of tuning frequency the capacitive component of each resonant circuit comprises at least three capacitors (5, 6, 7 and 8, 9, 10) in series, each set of three capacitors effectively forming a variable capacitive voltage divider. The two junction points of the three capacitors of each set are each connected to the corresponding junction point in the other set by means of a respective bidirectional reactive coupling (13 and 14 respectively).