The present invention relates to a calibration circuit and a calibration method thereof, and more particularly, to a calibration circuit for calibrating an output level of a demodulator, and a calibration method thereof.
An FM demodulator demodulates FM signals into audio signals, and controls volume of the audio signals to be played to users. The volume control of the audio signals depends on the signal quality of the FM signals. When signal strength of the FM signal is weak (the signal-to-noise ratio of the FM signal is less than a certain threshold), the FM demodulator attenuates its output (i.e. the audio signals) to decrease noise effect. This operation is referred to as “soft mute”.
Traditionally, as shown in FIG. 1, an FM demodulator 110 is coupled to an amplifier 120 and a received signal strength indicator (RSSI) module 130 to receive an FM signal amplified by the amplifier 120 and signal strength of the FM signal detected by the RSSI module 130. In the RSSI module 130, a comparator 132 compares a RSSI signal SFM generated by a RSSI unit 131 with a reference signal Sref generated by a reference voltage source 133. When the signal strength of the SFM is equal to the strength of the reference signal Sref, the RSSI module 130 outputs a control signal to the FM demodulator 110 to start the soft-mute function, i.e. to start to attenuate the audio signal.
The attenuation degree of the audio signal can be represented by a chart, as shown in FIG. 2. The real line in FIG. 2 shows an ideal soft-mute curve, showing that the soft-mute function starts at a selected point S1 and attenuates the audio signals according to a specific slope. The process variations of the FM demodulator 110, the amplifier 120 and the RSSI module 130, however, affect the start point and the slope of the ideal soft-mute curve. The practical attenuation executed by the FM demodulator 110 is represented by the dotted lines shown in FIG. 2. The input voltage offsets of the FM demodulator 110, the comparator 132, the reference voltage source 133 and buffers in the RSSI unit 131 cause the start point to drift away from S1. The gain variations of the FM demodulator 110, the amplifier 120 and the comparator 132 result in slope variation. As the practical soft-mute result varies from the ideal design, the audio signal output from the FM demodulator 110 cannot be optimum.