The present invention relates to an automatic frequency tuning (AFT) circuit to be used in a television receiver.
Generally, a television signal is tuned to a desired channel in a tuner of a television receiver, and is then converted into a video intermediate frequency signal based on an oscillation frequency of a local oscillator circuit so that the selected television signal becomes a carrier wave having a prescribed video intermediate frequency (for example 58.75 MHz in Japan). In the television receiver, in order to ensure that the oscillation frequency obtained from the local oscillator circuit becomes a real video intermediate frequency, the following feedback control is carried out. An AFT circuit detects a size of a difference between the frequency of the video intermediate frequency signal output from the tuner and the video intermediate frequency, and feeds back the detected result to the local oscillator circuit of the tuner, thereby feedback controlling the frequency of the video intermediate signal output from the tuner so that it becomes the video intermediate frequency.
The structure and operation of a conventional AFT circuit will be explained next. FIG. 7 is a block diagram showing a schematic structure of the conventional AFT circuit. The AFT circuit is constructed of a voltage-controlled oscillator circuit 1 capable of controlling an oscillation frequency based on a voltage, a phase detector circuit 2 for phase detecting a video intermediate frequency signal (hereinafter to be referred to as a VIF signal) output from a tuner (not shown) of a television receiver and an output signal of the voltage-controlled oscillator circuit 1, a low-pass filter 3 for integrating an output signal of the phase detector circuit 2, a phase shifter circuit 4 for shifting the phase of the output signal of the phase detector circuit 2, a phase detector circuit 5 for detecting phase of an output of the VIF signal and the phase shifter circuit 4, and a low-pass filter 6 for removing an unnecessary component of a video signal detected by the phase detector circuit 5.
The AFT circuit also includes a reference voltage circuit 21 for generating a reference voltage, a voltage comparator circuit 22 for comparing the reference voltage generated by the reference voltage circuit 21 with a voltage of the output signal of the low-pass filter 3, and a low-pass filter 23 for integrating an output signal of the voltage comparator circuit 22. The oscillation frequency of the voltage-controlled oscillator circuit 1 is expressed as fVCO and a predetermined video intermediate frequency is expressed as fIF.
The voltage-controlled oscillator circuit 1, the phase detector circuit 2 and the low-pass filter 3 constitute an APC (automatic phase-controlled) loop, and the output signal-of the voltage-controlled oscillator circuit 1 is fed back to be phase-synchronous with the VIF signal. On the other hand, the phase shifter circuit 4, the phase detector circuit 5 and the low-pass filter 6 constitute a synchronous detector circuit. The synchronous detector circuit detects the output signal of the voltage-controlled oscillator circuit 1 and the VIF signal to take out a video signal.
The oscillation frequency fVCO of the voltage-controlled oscillator circuit 1 is fed back and is phase-synchronized with the VIF signal so that the VIF signal frequency becomes equal to fVCO. Therefore, error voltage Verr output from the low-pass filter 3 also changes following this feedback control. When a voltage Vref of the reference voltage circuit 21 is set equal to the error voltage Verr that is obtained when the oscillation frequency fVCO is equal to the video intermediate frequency fIF the output signal of the voltage comparator circuit 22 becomes zero when fVCO=fIF. As fVCO gradually deviates from fIF the output voltage of the voltage comparator circuit 22 becomes larger.
As the frequency of the VIF signal is equal to fVCO, the output of the voltage comparator circuit 22 changes according to the size of the difference between the VIF signal frequency and fIF starting from when the frequency of the VIF signal=fIF as the center. An unnecessary component of this output signal is removed by the low-pass filter 23, and a result of this filtering is used as an automatic frequency tuning control voltage (AFT output) for feedback controlling the oscillation frequency that is output by the local oscillator of the tuner.
Thus, in the conventional AFT circuit, the error voltage Verr is compared with the reference voltage Vref, and the AFT output is obtained based on this comparison. Therefore, there has been a problem that the error voltage Verr is greatly affected by variations in circuits within the APC loop such as the offset in the phase detector circuit 2 and temperature characteristics of the voltage-controlled oscillator circuit 1, and the AFT output varies due to these variations.
It is an object of the present invention to provide an AFT circuit that is not dependent on variations such as offset and temperature characteristics.
According to the AFT circuit of one aspect of the present invention, a first output signal generated by a conventional automatic phase-controlled loop circuit and a second output signal generated by a phase-locked loop circuit structured based on a predetermined reference signal are multiplied together. A phase difference detector circuit and an edge detector circuit input a result of this multiplication, and output a result of an edge detection such as a signal of a pulse density corresponding to a frequency difference between the video intermediate frequency and the first output signal. An AFT output is obtained based on a result of this output. As a result, it is possible to obtain the AFT output that is dependent on only the predetermined reference signal.
Further, the phase-locked loop circuit has a frequency divider circuit for setting a frequency of the second output signal to a sum of the video intermediate frequency and the frequency of the reference signal. Since the phase-locked loop circuit is provided with a frequency divider circuit, it is possible to obtain a signal having a frequency that is a sum of the predetermined video intermediate frequency and the frequency of the reference signal as the second signal that is output from the phase-locked loop circuit according to the setting of a frequency dividing ratio of this frequency divider circuit.
Further, the phase-locked loop circuit has a frequency divider circuit for setting a frequency of the second output signal to a difference between the video intermediate frequency and the frequency of the reference signal. Since the phase-locked loop circuit is provided with a frequency divider circuit, it is possible to obtain a signal having a frequency that is a difference between the predetermined video intermediate frequency and the frequency of the reference signal as the second signal that is output from the phase-locked loop circuit according to the setting of a frequency dividing ratio of this frequency divider circuit.
Further, the reference signal is generated based on a sub-carrier frequency of a video color signal. Since a sub-carrier frequency usually generated in a television receiver is used as the reference signal, it is not necessary to prepare a special circuit for generating the reference signal.
According to the AFT circuit of another aspect of the present invention, a first output signal generated by a conventional automatic phase-controlled loop circuit and a reference signal generated based on a signal of a video intermediate frequency are multiplied together. A phase difference detector circuit and an edge detector circuit input a result of this multiplication, and output a result of an edge detection such as a signal of a pulse density corresponding to a frequency difference between the video intermediate frequency and the first output signal. An AFT output is obtained based on a result of this output. As a result, it is possible to obtain the AFT output that is dependent on only the predetermined reference signal. At the same time, it is possible to exclude a phase-locked loop circuit that generates a stable signal for carrying out a multiplication with the first output signal.
Further, the reference signal is a signal having a frequency that is one half of the video intermediate frequency. Therefore, it is possible to select one half of a video intermediate frequency as the frequency of the reference signal.
Further, the edge detector circuit carries out an edge detection by using a trigger clock as the reference signal. Since the edge detector circuit uses a trigger clock as the reference signal, it is possible to depend on the stability of the reference signal for carrying out the edge detection.
Further, the AFT control voltage generator circuit generates an AFT control voltage that resultantly changes linearly in proportion to a difference between the frequency of the video intermediate signal and the video intermediate frequency. Therefore, in a similar manner to that of the analog AFT circuit, it is possible to obtain an AFT output that changes linearly according to a frequency difference between the frequency of the video intermediate signal and the video intermediate frequency.
Further, the AFT control voltage generator circuit generates the AFT control voltage by integrating a signal obtained based on a result of an edge detection of the edge detector circuit. Therefore, it is possible to structure the AFT control voltage generator circuit by a low-pass filter and a charge pump circuit.