Circuit and method for producing a filtered signal

摘要

The apparatus produces a filtered frequency-modulated signal (Smod) at a given carrier frequency (Fp), and comprises a voltage-controlled oscillator (20) regulated by a signal (Vref) in a programmable frequency generation block (10). A band-pass filter (60) is controlled by the same signal (Vref) to adjust to the given frequency delivered to a power amplifier (50) connect to an antenna (18). The filter (60) is an active-type filter and the response curve is symmetrical with respect to the central frequency of the pass-band. The Q-factor is high at the controllable central frequency. The oscillator (20) is connected in a phase-locked loop (PLL) and the control signal (Vref) is generated by a phase compara (24) connected to a low-pass filter (26), where the comparator is with the first input of reference signal (CLK ref) and the sec input from a frequency divider (22). The oscillator (20) and the filter (60) can be implemented in the form of integrated circui of the same substrate. The oscillator (20) and the filter (60) each contain a resonant circuit (70) for the conversion of the control signal (Vref) to a parameter of frequency characteristic. The resonant circuit (70) contains a component having a characteristic variable as a function of the control signal, as is a varactor diode with capacitance variable as a function of biasing voltage. The filter (60) receives the modulated signal (Smod) from the output of modulation block (12) with an input of audio signal (SA), and is connected via a preamplifier (46).