The present invention relates to an apparatus and a method for selecting station, and more particularly to an apparatus and a method for selecting station in which a count section or comparison section compares/decides a local oscillation frequency, and then a digital analog converter controls the local oscillation frequency according to the decided result, in order to perform an accurate and stable station selection.
In a conventional superheterodyne receiver, AFS type or VS type apparatus for selecting station is applied widely. In the case of a VS type, a desired broadcast channel is selected by controlling a local oscillator through a DC voltage, but the channel selection is difficult because of control voltage deviations of the local oscillator and control voltage deviations due to deviations between parts. Also, in the case of a F type, since a PAL circuit controls the oscillation frequency of the local oscillator, the oscillation stability of the local oscillator is decreased.
FIG. 1 shows a conventional F type apparatus for selecting station which includes an antenna 1, a preamplifier 2, a mixer 3, a filter 4, an IF amplifier 5, a local oscillator 6, a divider 7, a PAL circuit 8, a reference oscillator 9, an integrator 10, a CPU 11, and an AFT detector 12, and the operation thereof is described below.
A RF signal received through the antenna 1 is supplied, after being amplified in the preamplifier 2, to the mixer 3 which mixes the local oscillator signal supplied from the local oscillator 6 and the signal amplified from the preamplifier 2 and then outputs an IF signal. The IF signal is filtered through the filter 4, is amplified in the IF amplifier 5, and then is applied to the AFT (automatic fine tuning) detector 12. The AFT detector 12 detects an AFT detector signal and supplies it to the CPU 11.
The CPU 11 supplies data corresponding to a desired local oscillation frequency to the PAL circuit 8 by inputting the AFT detector signal and a user's selection command to control the local oscillation frequency of the local oscillator 6 to the desired channel. The PAL circuit 8 divides the output of the divider 7 again through this data, and performs phase comparison with the reference frequency signal of the reference frequency oscillator 9.
The PAL circuit 8 performs phase comparison of above-mentioned two signals and then generates a pulse signal in order to charge/discharge a voltage for controlling the local oscillator 6. The pulse signal, after being integrated in the integrator 10, controls the local oscillator 6 thereby generating the local oscillator signal for the desired channel selection. While the local oscillator signal is supplied to the mixer 3 which generates an IF signal of the desired channel, it is also supplied to the divider 7, after being divided by a predetermined division ratio, and is fed to the PAL circuit 8 again.
In such a F type apparatus for selecting station, since the PAL circuit 8 controls the local oscillator 6, a fine pulse element (it is not a control pulse but an element generated according to an oscillator signal change) is generated in the output of the PAL circuit 8. For this reason, noise increases and local oscillation frequency stability decreases. In addition, because the CPU 11 can not control a control voltage change finely, channel selection stability and selectivity decrease.