Television receiver for terrestrial broadcasting

Abstract: In a television receiver in which image frequency signals of desired signals are removed when television broadcasting signals are frequency-converted through a local oscillator and mixers. The local oscillator generates quadrature signals that are frequency-converted by respective mixers, an output signal of one mixer is phase-shifted and added to an output signal of the other mixer in order to offset only the image frequency signals. The local oscillator and a phase shifter operate so as to follow the frequency change of the intermediate frequency signal in order to stabilize the suppression of the image frequency signal.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a television receiver for terrestrial broadcasting effectively used for multichannel television receivers. 2. Description of the Prior Art Television broadcastings use the terrestrial or CATV system. With the recent trend that the broadcasting channel has been increasingly multiplied, the CATV system broadcasting as multiplied as 100 channels have been announced. As a result, television receivers are required to be more improved in immunity than would be obtained in the prior art. In such receiver, particularly, a tuner performs the frequency conversion of an input signal by a local oscillation signal. That is, the heterodyne system has been adopted which selects a desired signal as an intermediate frequency signal. Accordingly, an image frequency signal can be generated. In case that the intermediate frequency is largely apart in value from the input frequency, the image frequency signal is also existed in the close vicinity of the input signal, so that it is difficult to be removed by a simple selecting filter. Under such a circumstance, considerations are made recently on the fact that the upper and lower frequency signals of the local oscillation signal are changed in phase when frequency-converted into the intermediate frequency signal and as a result, such a method that the image frequency signals after frequency conversion are offset has been investigated. One example is disclosed in the Japanese Laid-Open Patent Application No. 2-113608, which comprises two mixers, a biaxial orthogonal local oscillation signals generator and a phase shifter for phase-shifting one of the output signals of the mixers. With such structure, however, the phase shifter has a frequency limitation on the range of 90-degree phase shift, becoming impossible to follow the frequency change of a converter and resulting in the phase-shift amount of not being 90 degrees. Thus, there arises a problem that the image rejection performance is rapidly degraded. In addition, another example is disclosed in the Japanese Laid-Open Patent Application No. 3-60206, in which two mixers and a biaxial orthogonal local oscillation signals generator are provided and the phases of the output signals of the two mixers are compared to feedback the result to one of phase shifters thereby to offset the image frequency signals, and further in which the amplitudes of the output signals of the two mixers are compared to obtain the phase difference and the amplification factors of variable gain amplifiers which are disposed so as to receive the outputs of the mixers are changed in response to the phase difference thereby to offset the image frequency signals. With this structure, hovewer, it is very difficult to change closely the amplitudes of image frequency signals from the output signals of the mixers and as a result, it is required that there is no conversion gain difference between the two mixers in advance. In addition, it is required to equip with two phase shifters or two variable gain amplifiers, resulting in the redundant circuit arrangement. GB-A-2244611 discloses a synchronous television receiver which includes an I signal block and a Q signal block which together with a base band 90 degrees phase shifter. obtained in phase video signals from the two signal blocks. An adder is provided for adding the in phase signals, and a phase equaliser corrects the phase distortion of the adder output. The present invention provides a television receiver comprising: channel selecting means for generating a channel selection signal for selecting a desired signal of high frequency signals input from an antenna; frequency converting means for frequency-converting the desired signal of high frequency signals into an intermediate frequency signal in response to the channel selection signal: local oscillation signal generating means for generating biaxial orthogonal local oscillation signals which are phase-shifted from each other by 90 degrees; first mixing means and second mixing means for mixing the intermediate frequency signal with the biaxial orthogonal local oscillation signals. respectively; phase shifting means for shifting a phase of an output signal of the first mixing means by 90 degrees; adding means for adding an output signal of the phase shifting means and an output signal of the second mixing means; and detection means for detecting an output signal of the adding means to obtain a television signal, characterized by further comprising: frequency variation control means for feeding a variation of the intermediate frequency outputted from the detecting means back to the local oscillation signal generating means to control the frequency of the biaxial orthogonal local oscillation signals; and phase-shift amount setting means for setting a phase-shift amount of at least one of the local oscillation signal generating means and the phase shifting means so as to maintain the phase difference of 90 degrees between the biaxial orthogonal local oscillation signals and the phase difference of 90 degrees between input and output signals of the phase shifting means. In one embodiment the phase shift amount setting means comprises a phase difference detector for detecting a phase difference between the biaxial orthogonal local oscillation signals; and a phase-shift amount setter for setting the phase-shift amount of the local oscillation signal generating means depending on the detected phase difference. In another embodiment the phase shift amount setting means comprises a phase difference detector for detecting a phase difference between the output signals of the first and second mixing means; and a phase-shift amount setter for setting the phase-shift amount of the local oscillation signal generating means depending on the detected phase difference. This invention thus provides a television receiver which has means of controlling a phase shifter so as to have a phase-shift of 90 degrees and means of following the input frequency change to enable variation in the local oscillation frequency and the phase shifter thereby to suppress the image frequency signal so that the image rejection performance can be extremely improved over would be obtained previously. With the structures as above, the image frequency signal can be suppressed by following event to the frequency change of the intermediate frequency signal. Also, the image frequency signal can be rejected without using the phase shifter on the mixer side. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a television receiver according to a first embodiment of this invention; Fig. 2 is a block diagram of a television receiver accoridng to a second embodiment of this invention; Fig. 3 is a block diagram of a television receiver accoridng to a third embodiment of this invention; Fig. 4 is a block diagram of a television receiver according to a fourth embodiment of this invention; Fig. 5 is a block diagram of a biaxial orthogonal phase difference varibale local oscillator of this invention; Fig. 6 is a block diagram of a biaxial orthogonal local oscillator of this invention; Fig. 7 is a block diagram of a biaxial orthogonal phase difference variable local oscillator of this invention; Fig. 8 is a circuit diagram of a voltage variable oscillator of this invention; Fig. 9 is a diagram showing signal phases of a biaxial orthogonal local oscillator; Fig. 10 is a block diagram of an equalizer consisting of a digital circuit; Fig. 11 is a circuit diagram of a phase difference detector and a phase-shift amount setter; Fig. 12 is a block diagram of a television receiver according to a fifth embodiment of this invention; Fig. 13 is a block diagram of a television receiver according to a sixth embodiment of this invention; Fig. 14 is a input signal vector of two mixers; Fig. 15 is a input signal vector diagram of an adder; Fig. 16 is a diagram showing the detection axes and signal vectors in the fifth and sixth embodiments; Fig. 17 is a block diagram of a television receiver according to a seventh embodiment of this invention; Fig. 18 is a block diagram of phase synchronizer for synchronizing with an output signal of a mixer and a frequency divider; Fig. 19 is a diagram showing a magnitude characteristic of an intermediate frequency band; and Fig. 20 is a diagram showing frequency spectra of an intermediate frequency signal, a local oscillation signal, an image frequency signal and a desired frequency signal. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 is a block diagram of a television receiver according to a first embodiment of this invention, in which high frequency signals sent from an antenna 1 are subjected to frequency conversion through a converter 2. In this case, a desired signal of the high frequency signals is converted into a first intermediate frequency signal in response to a channel selection control signal supplied from a channel selecting circuit 3. The desired signal converted into the first intermdiate frequency signal is removed its unnecessary signals therefrom through a band-pass filter 4 and delivered to mixers 5 and 6. A local oscillator 7 generates biaxial orthogonal local oscillation signals to be delivered respectively to the mixers 5 and 6 as well. As a result, the mixers 5 and 6 generate second intermediate frequency signals. However, the local oscillation signals are orthogonal in phase to each other, so that the second intermediate frequency signals of the mixers 5 and 6 are orthogonal in phase to each other. The frequency relationship of the firts and second intermediate frequency signals is shown in Fig. 20. In which, the second intermdiate frequency siganl 82 is generated by mixing the desired signal 80 as the first intermediate frequency signal and the local oscillation signal 79. In this case, an image frequency signal 81 having an image frequency of the second intermediate frequency signal which is small in frequency by the second intermediate frequency is also mixed from the local oscillation signal 79 to form the second intermediate frequency signal. The mixers 5 and 6 generate the desired signals and image frequency signals in their output signals as shown above. Fig. 14 is a vector diagram of the desired signals and the image frequency signals, in which the desired signals are indicated by the solid line vectors 63 and 65, and the image frequency signals are indicated by the broken line vectors 64 and 66. In Fig. 1, the mixer 5 is supplied with a signal ΦI from the local oscillator 7 to generate the desired siganl vector 63 and the image frequency signal vector 64. also, the mixer 6 is supplied with a signal ΦQ from the local oscillator 7 to generate the desired signal vector and the image frequency signal vector 66. Subsequently, the signal of the vectors 65 and 66 are sent to the phase shifter 15 to be subjected to the phase shift of 90 degrees and as a result, the vectors 65, 64 and 63 are equal in phase and the vectors 64 and 66 are opposite in phase as shown in Fig. 15. These signals are supplied to an adder 9 to perform that the signals opposite in phase are offset. Namely, only the image frequency signal is eliminated. The remained desired signals are sent through a band-pass filter 10 to a detector 11 to be detected whereby the television signal is taken out and sent through a signal processing circuit 12 to a display circuit 13 to be displayed. On the other hand, the detector 11 follows the input signal frequency when to be synchronized with a video carrier for detection, so that the input frequency variation component signal can be detected by a loop signal of its phase synchronizer or the signal that is frequency-detected. The signal thus detected is sent to an AFC driver 14 to be converted into a frequency signal and supplied to the local oscillator 7. Thus, the frequency change due to the converter 2 is suppressed. Furthermore, in order that the phase difference between the local oscillation signals ΦI and ΦQ is maintained at 90 degrees, the phase difference therebetween is detected through a phase difference detector 16 and supplied to a phase-shift amount setter 17 for controlling the phase shift of the signal ΦQ of the local oscillator 7. Also, a necessary phase-shift amount set in advance when the channel selection is made, is sent from the channel selecting circuit 3 to the phase-shift amount setter 17 as a channel selection signal. According to this embodiment, as explained above, such a frequency control that feeds the variation of the input signal frequency to the mixers back to the local oscillator can be executed and furthermore, the phase difference of the biaxial orthogonal signals of the local oscillator is detected thereby to make variable the phase-shift amount of the orthogonal signals of the local oscillator. Consequently, the image rejection performance of a wide frequency bandwidth capable of following even to the frequency change of the input signal can be obtained. Fig. 2 is ablock diagram of a television receiver according to a second embodiment of this invention, in which high frequency signals from an antenna 1 are subjected to frequency conversion through a converter 2. In this case, a desired signal of the high frequency signals is converted into a first intermediate frequency signal in response to a channel selection control signal supplied from a channel selecting circuit 3. The desired signal thus converted into the first intermediate frequency signal is removed its unnecessary signals therefrom through a band-pass filter 4 and delivered to mixers 5 and 6. A local oscillator 7 generated biaxial orthogonal local oscillation signals to be delivered respectively to the mixers 5 and 6 as well. As a result, the mixers 5 and 6 generate second intermediate frequency signals. However, the local oscillation signals are orthogonal in phase to each other, so that the second intermediate frequency signals by the mixers 5 and 6 are orthogonal in phase to each other. The frequency relationship of the first and second intermediate frequency signals is shown in Fig. 20, in which the second intermediate frequency signal 82 is generated by mixing the desired signal 80 as the first intermediate frequency signal and the local oscillation signal 79. In this case, an image frequency signal having an image frequency of the second intermediate frequency signal which is small in frequency by the second intermediate frequency is also mixed from the local oscillation signal 79 to form the second intermediate frequency signal. The mixers 5 and 6 generate the desired signals and image frequency signals in their output signals as shown above. Fig. 14 is a vector diagram of the desired signals and image frequency signals, in which the desired signals are indicated by the solid line vectors 63 and 65, and the image frequency signals are indicated by the broken line vectors 64 and 66. In Fig. 2, the mixer 5 is supplied with a signal ΦI from the local oscillator 7 to generate the desired signal vector 63 and the image frequency signal vector 64. Also, the mixer 6 is supplied with a signal ΦQ from the local oscillator 7 to generate the desired signal vector 65 and the image frequency signal vector 66. Subsequently, the signals of the vectors 65 and 66 are sent to the phase shifter 15 to be subjected to the phase shift of 90 degrees and as a result, the vectors 65, 64 and 63 are equal in phase and the vectors are opposite in phase as shown in Fig. 15. These signals are supplied to an adder 9 to perform that the signals opposite in phase are offset. Namely, only the image frequency signal is eliminated. The remained desired signals are sent to an A/D converter 18 to be converted into digital signals, then, supplied through a digital detector 19 to an equalizer 20 to be waveform-shaped and sent to a detector 21 for performing the signal processing of the television signal and supplied to a display device 13 to be displayed. On the other hand, the equalizer 20 shown in Fig. 10 waveform-shape the input signal string through a complex multiplier 57 and a digital filter 54 and outputs. In this case, if the input signal of the digital signal detector 19 contains the frequency variation component, the orthogonally axial phase difference can be detected by a phase error detector 55, and the oscillation frequency of a numerical-controlled oscillator 56 is changed in response to the signal thus detected so as to follow the input frequency of the digital signal detector 19. This signal is supplied to the complex multiplier 57 to perform the detection synchronized with the input frequency. The signal thus obtained of the phase error detector 55 is sent to an AFC driver 14 to be converted into a frequency control signal and supplied to the local oscillator 7. Therefore, the frequency change due to the converter 2 is suppressed. In addition, in order that the phase difference between the local oscillation signals ΦI and ΦQ is maintained at 90 degrees, the phase difference therebetween is detected through the phase difference detector 16 and supplied to a phase-shift amount setter 17 for controlling the phase shift of the signal ΦQ of the local oscillator 7. Also, a necessary phase-shift amount set in advance when the channel selection is made is supplied from the channel selecting circuit 3 to the phase-shift amount setter 17 as a channel selection signal. According to this embodiment, as explained above, such a frequency control that feeds the variation of the input frequencies to the mixers back to the local oscillator can be executed even in digital broadcasting receiving and furthermore, the phase difference of the biaxial orthogonal signals is detected thereby to make variable the phase-shift amount of orthogonal signals of the local oscillator. Consequently, the image rejection performance of wide frequency band capable of following even to the frequency change of the input signal can be obtained. Fig. 3 is a block diagram of a television receiver according to a third embodiment of this invention, in which high frequency signals sent from an antenna 1 are subjected to frequency conversion through a converter 2. In this case, a desired signal of the high frequency signals is converted into a first intermediate frequency signal in response to a channel selection control signal supplied from a channel selecting circuit 3. The desired signal converted into the first intermediate frequency signal is removed its unnecessary signals therefrom through a band-pass filter 4 and delivered to mixers 5 and 6. A local oscillator 24 generates biaxial orthogonal local oscillation signals to be delivered respectively to the mixers 5 and 6 as well. The mixers 5 and 6 generate second intermediate frequency signals. However, the local oscillation signals are orthogonal in phase to each other, so that the second intermediate frequency signals of the mixers 5 and 6 are orthogonal in phase to each other. Fig. 20 shows the relationship of the frequencies of the firts and second intermdiate frequency signals. In which, the second intermdiate frequency signal 82 is generated by mixing the desired signal as the first intermediate frequency siganl 80 and the local oscillation signal 79. In this case, an image frequency signal having an image frequency of the second intermediate frequency signal which is small in frequency by the second intermediate frequency is also mixed form the local oscillation signal 79 to form the second intermediate frequency signal. The mixers 5 and 6 generated the desired signals and image frequency signals in their output signals as shown above. Fig. 14 is a vector diagram of the desired signals and image frequency signals, in which the desired signals are indicated by the solid line vectors 63 and 65, and the image frequency signals are indicated by the broken line vectors 64 and 66. In Fig. 3, the mixer 5 is supplied with a signal ΦI from the local oscillator 24 to generate the desired signal vector 63 and the image frequency signals 64. Also, the mixer 6 is supplied with a signal ΦQ from the local oscillator 24 to generate the desired siganl vector 65 and the image frequency signal vector 66. Subsequently, the signals of the vectors 65 and 66 are sent to a variable phase shifter 22 to be subjected the phase shift of 90 degrees and as a result, the vectors 65, 64 and 63 are equal in phase and the vectors 64 and 66 are opposite in phase as shown in Fig. 15. These signals are supplied to an adder 9 to perform that the signals opposite in phase are offest. Namely, only the image frequency signal is eliminated. The remained desired signals are sent through a band-pass filter 10 to a detector 11 to detect the television signal. The television signal thus detected is sent to a signal processing circuit 12 for performing signal processing and supplied to a display device 13 to be displayed. The detector 11 phase-synchronizes with the input signal to detect. In addition, a frequency detector detects an input frequency variation component. The frequency variation component thus detected is sent to an AFC driver 14 to be converted into a frequency control signal and supplied to a local oscillator 24. Therefore, the frequency change due to the converter 2 is suppressed. Also, in order that the phase difference between an input signal and output signal of the variable phase shifter 22 is maintained at 90 degrees, the phase difference therebetween is detected by a phase difference detector 23 and supplied to a phase-shift amount setter 17 for controlling the phase shift of the variable phase shifter 22. Also, a necessary phase-shift amount set in advance when the channel selection is made, is supplied from the channel selecting sircuit 3 to the phase-shift amount setter 17 as a channel selection signal. According to this embodiment, as explained above, such a frequency control that feeds the variation of the input signal frequencies to the mixers back to the local oscillator can be executed and furthermore, the phase-shift amount of the variable phase shifter for performing the phase shift of the second intermediate frequency signal is detected thereby to make varibale the phase-shift amount so as to be followed to the change of the input frequency. Consequently, the image rejection performance of a wide frequency bandwidth capable of following even to the frequency change of the input signal can be obtained. Fig. 4 is a block diagram of a television receiver according to a fourth embodiment of this invention, in which high frequency signals sent from an antenna 1 are subjected to frequency conversion through a converter 2. In this case, a desired signal of the high frequency signals is converted into a first intermediate frequency signal in response to a channel selection control signal supplied from a channel selecting circuit 3. The desired signal converted into the first intermediate frequency signal is removed its unnecessary signal therefrom through a band-pass filter 4 and delivered to mixers 5 and 6. A local oscillator 7 generates biaxial orthogonal local oscillation signals to be delivered respectively to the mixers 5 and 6 as well. As a result, the mixers 5 and 6 generate second intermediate frequency signals. However, the local oscillation signals are orthogonal in phase to each other, so that the mixers 5 and 6 are orthogonal in phase to each other. The frequency relationship of the first and second intermediate frequency signals is shown in Fig. 20, the explanations on which are omitted in this embodiment because of being the same as in the third embodiement. In addition, the explanations on Fig. 14 showing a vector diagram of the desired signal and image frequency signal are omitted here for the same reason as above. The signals of the vector 65 and 66 are sent to a variable phase shifter 22 to be subjected to the phase shift of 90 degrees and as a result, the vectors 65, 64 and 63 are equal in phase and the vectors 64 and 66 are opposite in phase as shown in Fig. 15. These signals are supplied to an adder 9 to perform that the signals opposite in phase are offset. Namely, only image frequency signal is eliminated. The remained desired signals are sent thorugh a band-pass filter 10 to a detector 11 to detect the television signal. The signal thus desired is sent to a signal processing circuit 12 to be signal-processed and supplied to a display 13 to be displayed. The detector 11 phase-synchronizes with the input signal to detect. In addition, a frequency detector detects an input frequency variation component. The frequency variation component thus detected is sent to an AFC driver 14 to be converted into a frequency control signal and supplied to the local oscillator 7. Therefore, the frequency change due to the converter 2 is suppressed. Also, in order that the phase difference between an input signal an output signal of the variable phase shifter 22 is maintained at 90 degrees, the phase difference therebetween is detected by a phase difference detector 23 and supplied to a phase shift amount setter 27 for controlling the phase shift of the variable phase shifter 22. Also, a necessary phase-shift amount set in advance when the channel selection is made is supplied to the phase-shift amount setter 17 as a channel selection signal. The phase difference of the biaxial orthogonal signals ΦI and ΦQ of the local oscillator 7 is detected by phase-detecting the second intermdiate frequency signals sent from the mixers 5 and 6 by a phase difference detector 26 and the result thus obtained is supplied to a phase-shift amount setter 17 for controlling the phase shift of the signal ΦQ of the local oscillator 7. Also, a necessary phase-shift amount set in advance when the channel selection is made is supplied to the phase-shift amount setter 17 as a channel selection signal. According to this embodiment, as explained above, such a frequency control that feeds the variation of the input signal frequencies to the mixers back to the local oscillator can be executed and furthermore, the phase-shift amount of the varibale phase shifter for performing the phase shift of the second intermediate frequency signal is detected thereby to make variable the phase-shift amount so as to be followed to the change of the input frequency. Consequently, the image rejection performance of a wide frequency band capable of following even to the frequency change of the input signal can be obatined. Fig. 5 is a block diagram of the local oscillator 7, in which a voltage-controlled oscillator (VCO) 28 changes the oscillation frequency in response to a frequency control voltage, and the oscillation signal thereof is supplied directly as the in-phase signal ΦI. On the other hand, the oscillation signal from the VCO 28 is sent to a variable phase shifter 29 to change the phase-shift amount in response to an external control signal. The output signal of the variable phase shift 29 is the orthogonal signal ΦQ. Fig. 6 is a block diagram of the local oscillator 24, in which a voltage-controlled oscillator (VCO) 28 changes the oscillations frequency in response to a frequency control voltage, and the oscillation signal thereof is sent to a flip-flop 30 and the output signal thereof is supplied as the in-phase signal ΦI. On the other hand, the oscillation signal from the VCO 28 is snet through an inverter 32 to a flip-flop 31 and the output signal thereof is the orthogonal signal ΦQ. The waveforms of these signals are shown in Fig. 9. In which, the oscillation waveform of the VCO 28 is indicated at waveform 51. The in-phase signal ΦI becomes as shown at waveform 52 by effecting the action, of flip-flop 30 at a rise b which is synchronized with a rise a of the waveform 51, and the orthogonal signal ΦQ becomes as shown at 53 by effecting the action of the flip-flop 31 at a rise d synchronized with a rise c of the waveform 51. Fig. 7 is a block diagram of the local oscillator 7 of this embodiement, in which an address generator 25 generates an address signal X of a memory in response to the frequency control voltage to deliver thorugh a fixed delay circuit 33 and variable delay circuit 34 to a SIN(X) table 35 and COS(X) table 36. The variable delay circuit 34 delays a read address signal X in response to an external control signal to change the phase shift amount. An output signal of the SIN(X) table 35 is supplied as the in-phase signal ΦI. On the other hand, an output signal of the COS(X) table 36 is supplied as the orthogonal signal ΦQ. Fig. 8 is a circuit diagram of the VCO circuit 28 of this embodiment. The frequency control signal is sent via a resistor 37 to a resonance circuit consisting of variable capacity elements 38 and 39, a resonator 41 and an oscillator 40 to change its resonant frequency. Thus, the oscillation frequency can be changed. Oscillation is made thorugh a transistor 46 and capacitors 42, 49 and 48 to generate a specified oscillation frequency determined by the resonance circuit. The oscillation signal is taken out from the emitter terminal of the transistor 46. A DC power source 44 and resistors 43, 50 and 47 supply a bias voltage of the transistor 46, and a capacitor 45 is used for grounding. Fig. 11 is a circuit diagram showing concrete examples of the phase difference detector 16 and phase-shift amount setter 17. The phase difference detector 16 consists of a double-balance detector 58 for detecting a phase difference between two signals to generate a detection output signal generate a detection output signals so that if the phase difference between the two signals is 90 degrees, it outputs zero (0), if it is small than 90 degrees, it outputs a negative voltage, and if it is large than 90 degrees, it outputs a positive voltage. Also, a phase-shift amount setter 17 consists of an operational amplifier 59 for mixing a control voltage from the channel selecting circuit 3 and a detection voltage from the phase difference detector 16. Fig. 12 is a block diagram of a television receiver according to a fifth embodiment of this invention, in which high frequency signals form an antenna 1 are subjected to frequency conversion through a converter 2. In this case, a desired signals of the high frequency signals is converted into a first intermediate frequency signal in response to a channel selection control signal from a channel selecting circuit 3. The desired signal thus converted into the first intermediate frequency signal is removed its unnecessary signals therefrom through a band-pass filter 4 and delivered to mixers 5 and 6. A local oscillator 24 generates biaxial orthogonal local oscillation signals to be delivered respectively to the mixers 5 and 6. Thus, the mixers 5 and 6 generate second intermdiate frequency signal. However, the local oscillation signals are orthogonal in phase to each other, so that the second intermediate frequency signals of the mixers 5 and 6 are orthogonal in phase to each other. The frequency relationship of the first and second intermediate frequency signals is shown in Fig. 20. In which, the second intermediate frequency signal 82 is generated by mixing the desired signal 80 as the first intermediate frequency signal and the local oscillation signal 79. In this case, an image frequency signal 81 having an image frequency of the second intermediate frequency which is small in frequency by the second intermediate frequency is also mixed from the local oscillation signal 79 to form the second intermediate frequency signal. The mixers 5 and 6 generate the desired signals and the image frequency signals. Fig. 14 is a vector diagram of the desired signals and image frequency signals, in which the desired are indicated by the solid line vectors 63 and 65, and the image frequency signals are indicated by the broken line vectors 64 and 66. In Fig. 12, the mixer 5 is supplied with a signal ΦI from the local oscillation 24 to generate the desired signal vector 63 and the image frequency signal vector 64. Also, the mixer 6 is supplied with a signal ΦQ from the local oscillation signal 24 to generate the desired signal vector 65 and the image frequency signal vector 66. By mixing these signals by an adder 9, a vector diagram as shown in Fig. 16 can be obtained. This diagram is similar to that shown in Fig. 14 excepting that the vectors have their beginning points coupled with each other in the case of Fig. 16. These mixed desired signals 63, 65 and image frequency signals 64, 66 are sent through a band-pass filter 10 to a detector 11. On the other hand, the output signal of the mixer 6 is supplied to a phase synchronizer 60 to generate a signal synchronized with the second intermediate frequency. The signal thus obtained is supplied through a 45-degree phase shifter 61 to the detector 11 as a reference carrier. The signal axis I shown in Fig. 16 is the reference carrier axis as a detection axis. Analyzing through the axial system thus phase-shifted, the image frequency signal 64 becomes the vector Oe and the image frequency signal 66 becomes the vector Oh, which are reverse voltage signals. Also, the desired signal 63 becomes the vector Of and the desired signal becomes the vector Og, which are in-phase signals. As a result, the phase of the detection axis I is changed so that the respective voltages Oe and Oh of the image frequency signals have values whose absolute values are equal, making possible to eliminate the image frequency signal. The phase-shift amount detected when channel-setting is supplied in advance from the channel selecting sircuit 3 to the phase synchronizer 60. The detector 11 is formed of a phase synchronizer for phase-synchronizing with the input frequency or a frequency detector thereby to detect the change of the input frequency. The output signal of the detector 11 is delivered to an AFC driver 14 to generate a control signal for suppressing the frequency variation and sent to the local oscillator 24. In addition, the detector 11 detects the television signal to send out to a signal processing circuit 12 for signal-processing and then, sent to a display circuit 13 to be displayed. According to this embodiment, as explained above, the phase shifter of the second intermediate frequency band is not needed, and the image frequency signal is eliminated by making variable the detection axis of the detector while following the second intermediate frequency signal. In addition, such a frequency control that feeds the variation of the input frequencies to the mixers back to the local oscillator can be provided. Consequently, the image rejection performance of a wide frequency band capable of following even to he frequency change of the input signal can be obtained. Fig. 13 is ablock diagram of a television receiver according to a sixth embodiement of this invention, in which high frequency signals from an antenna 1 are subjected to frequency conversion through a converter 2. In this case, a desired signal of the high frequency signal is converted into a first intermediate frequency signal in response to a channel selection control signal supplied from a channel selecting circuit 3. The desired signal thus converted into the first frequency signal is removed its unnecessary signals through a band-pass filter 4 and delivered to mixers 5 and 6. A local oscillator 24 generates biaxial orthogonal local oscillation signals to be delivered respectively to the mixers 5 and 6. Thus, the mixers 5 and 6 generates second intermediate frequency signals. However, the local oscillation signals are orthogonal in phase to each other, so that the second intermediate frequency signals of the mixers 5 and 6 are orthogonal in phase to each other. The frequency relationship of the first and second intermediate frequency signals is shown in Fig. 20. In which, the second intermdiate frequency signal 82 is generated by mixing the desired signal 80 as the first intermediate frequency signal and the local oscillation signal 79. In this case, as image frequency signal having an image frequency of the second intermediate frequency signal which is small in frequency by the second intermediate frequency is also mixed from the local oscillation signal 79 to form the second intermediate frequency signal. The mixers 5 and 6 generate the desired signals and the image frequency signals. Fig. 14 is a vector diagram of the desired signals and image frequency signals, in which the desired signals are indicated by the solid line vectors 63 and 65, and the image frequency are indicated by the blocken line vectors 64 and 66. In Fig. 13, the mixer 5 is suppplied with a signal ΦI from the local oscillator 21 to generate the desired signal vector 63 and the image frequency signal vector 64. Also, the mixer 6 is supplied with a signal ΦQ from the local oscillator 24 to generate the desired signal vector 65 and the image frequency signal vector 66. These signal are sent to an adder 9, whose vector diagram is shown in Fig. 16. This diagram is similar to that shown in Fig. 14 excepting that the vectors have their beginning points coupled with each other in the case of Fig. 16. These mixed desired signals 63, 65 and image frequency signals 64, 66 are sent though a band-pass filter 10 to a detector 11. On the other hand, the output signal of the mixer 6 is supplied to a phase synchronizer 60 to generate a signal synchronized with the second intermediate frequency. The signal thus obtained is supplied through a ±45-degree phase shifter 62 to the detector 11 as a reference carrier. The signal axis I shown in Fig. 16 is the reference signal axis as a detection axis. Analyzing the axial system thus phase-shifted, the image frequency signal by becomes the vector Oe and the image frequency signal 66 becomes the vector Oh, which are reverse voltage signals. Also, the desired signal 63 becomes the vector Of and the desired signal 65 becomes the vector Og, which are in-phase signals. As a result, the phase of the detection axis I is changed so that the respective voltages Oe and Oh of the image frequency signals have values whose absolute values are equal, making possible to eliminate the image frequency signal. In the other hand, the detection using the axis Q reveals that the image frequency signals are added, thus the desired signal being suppressed. Therefore, the use of this axis for the detection signal of the image frequency signal makes possible that the image frequency signal is more accurately suppressed than would be made in the fifth embodiment. Fig. 18 is a block diagram of the phase synchronizer 60 and the ±45-degree phase shifter 62 shown in Fig. 13. The phase synchronizer 60 consists of an offset voltage generator 83 for changing the phase angle when synchronizing and a synchronizer 73 for synchronizing with the input signal. The signal thus synchronized is sent to a quarter frequency divider 74 forming the ±45-degree phase shifter 62 to generate the orthogonal signals I and Q. The detector 11 is formed of a phase synchronizer for phase-synchronizing with the input frequency or a frequency detector to detect the change of the input frequency. The output signal of the detector 11 is sent to an AFC driver 14 to generate a control signal or suppressing the frequency change and sent to the local oscillator 24. In addition, the detector 11 detects the television signal to send out to a signal processing circuit 12 for signal-processing and then, sent to a display circuit 13 to be displayed. According to this embodiment, as explained above, the phase shifter of the second intermediate frequency band is not needed, and the image frequency signal is eliminated by making variable the detection axis of the detector while following the second intermediate frequency signal. In addition, such a frequency control that feeds the variation of the input frequencies to the mixers back to the local oscillator can be provided. Consequently, the image removal performance of a wide frequency band capable of following even to the frequency change of the input signal can be obtained. Fig. 17 is a block diagram of a television receiver according to a seventh embodiment of this invention, in which high frequency signals from an antenna 1 are subjected to frequency conversion by a converter 2. In this case, a desired signal of the high frequency signals in converted into a first intermediate frequency signal in response to a channel selection control signal supplied from a channel selecting circuit 3. The desired signal thus converted into the first intermediate frequency signal is removed its unnecessary signals through a band-pass filter 4 and delivered to mixers 5 and 6. A local oscillator 24 generate biaxial orthogonal local oscillation signals to be delivered respectively to the mixers 5 and 6. Thus, the mixer 5 and 6 generate second intermediate frequency signals. However, the local oscillation signals are orthogonal in phase to each other, so that the second intermediate frequency signals of the mixers 5 and 6 are orthogonal in phase to each other. The frequency relationship of the first and second intermediate signals is shown in Fig. 20. In which, the second intermediate frequency signal 82 is generated by mixing the desired signal 80 as the first intermediate signal and the local oscillation signal 79. In this case, an image frequency signal 81 having an image frequency of the second intermediate frequency signal which is small in frequency by the second frequency signal is also mixed from the local oscillation signal 79 to form the second intermediate frequency signal. The mixers 5 and 6 generate the desired signals and the image frequency signals. Fig. 14 is a vector diagram of the desired signals and image frequency signals, in which the desired signals are indicated by the solid line vectors 63 and 65, and the image frequency signals are indicated by the broken line vectors 64 and 66. In Fig. 17, the mixer 5 is supplied with a signal ΦI from the local oscillator 24 to generate the desired signal vector 63 and the image frequency signal vector 64. Also, the mixer 6 is supplied with a signal ΦQ from the local oscillator 24 to generate the desired signal vector 65 and the image frequency signal vector 66. Then, the output signal of the mixer 6 is sent through a phase shifter 15 to an adder 9 and the output signal of the mixer 5 is also sent to the adder 9, the vector diagram of which is shown in Fig. 15. As clear from Fig. 15, the image frequency signals 64 and 66 are offset. The remained desired signals of the adder 9 are sent through a band-pass filter 10 to a NTSC detector 67. In the NTSC detector 67, the detection is made analogically to generate the television signal, then, signal-processed through a signal processing circuit 12 and sent to a display circuit 13 to be displayed. On the other hand, the output signal of the band-pass filter 10 is supplied to a digital detector 69 to perform the digital signal detection, then, expanded through an expander 68 and displayed by the display circuit 13. The NTSC detector 67 is formed of a phase synchronizer for phase-synchronizing with the input frequency or a frequency detector thereby to detect the change of the input frequency. The output signal of the NTSC detector 67 is supplied through a switch circuit 71 to the local oscillator 24 as a control signal for suppressing the frequency change. Also, in the NTSC broadcasting case, a trapping circuit 72 is operated by a discriminator 70 for discriminating a modulated signal from the output signals of the NTSC detector 67 and digital signal detector 69, and the switch circuit 71 is closed to feed the variation of the input frequency back to the local oscillator 24. Also, in the digital broadcasting case, the trapping circuit 72 is not operated and the switch circuit 71 is being remained open. The first intermediate frequency band characteristic in this case is shown in Fig. 19, in which the solid line curve 75 shows the characteristic of the band-pass filter 4 and the broken line curve 76 shows the characteristic thereof when the trapping circuit 72 is being operated. The NTSC signal is closer in detection limit on the image frequency signal than the digital signal and an the other hand, the digital signal is closer in the amplitude characteristic in the frequency band than the NTSC signal. According of this embodiment, as explained above, the television receiver of this invention can be used in common for the NTSC and digital broadcastings. Furthermore, such a frequency control that feeds the variation of the input frequency to the mixers back to the local oscillator can be provided. Consequently, the image rejection performance of a wide frequency band capable of following even to the input frequency change can be obtained.

Claims: A television receiver comprising: channel selecting means (3) for generating a channel selection signal for selecting a desired signal of high frequency signals input from an antenna (1); frequency converting means (2) for frequency-converting the desired signal of high frequency signals into an intermediate frequency signal in response to the channel selection signal; local oscillation signal generating means (7, 24) for generating biaxial orthogonal local oscillation signals which are phase-shifted from each other by 90 degrees: first mixing means (6) and second mixing means (50) for mixing the intermediate frequency signal with the biaxial orthogonal local oscillation signals, respectively; phase shifting means (15, 22) for shifting a phase of an output signal of the first mixing means by 90 degrees: adding means (9) for adding an output signal of the phase shifting means and an output signal of the second mixing means; and detection means (11, 18-20) for detecting an output signal of the adding means to obtain a television signal. characterized by further comprising: frequency variation control means (14) for feeding a variation of the intermediate frequency outputted from the detecting means back to the local oscillation signal generating means (7, 24) to control the frequency of the biaxial orthogonal local oscillation signals; and phase-shift amount setting means (16, 17, 23, 26, 27) for setting a phase-shift amount of at least one of the local oscillation signal generating means (7) and the phase shifting means (22) so as to maintain the phase difference of 90 degrees between the biaxial orthogonal local oscillation signals and the phase difference of 90 degrees between input and output signals of the phase shifting means. A television receiver according to claim 1, wherein the phase-shift amount setting means (16, 17) comprises: a phase difference detector (16) for detecting a phase difference between the biaxial orthogonal local oscillation signals; and a phase-shift amount setter (17) for setting the phase-shift amount of the local oscillation signal generating means (7) depending on the detected phase difference. A television receiver according to claim 1, wherein the phase-shift amount setting means (16, 17, 23, 26, 27) comprises: a phase difference detector (26) for detecting a phase difference between the output signals of the first and second mixing means (6, 5); and a phase-shift amount setter (17) for setting the phase-shift amount of the local oscillation signal generating means (7) depending on the detected phase difference. A television receiver according to claim 3, wherein the phase-shift amount setting means (16, 17, 23, 26, 27) further comprises: another phase difference detector (23) for detecting a phase difference between input and output signals of the phase shifting means (22); and another phase-shift amount setter (27) for setting the phase-shift amount of the phase shifting means (22) depending on the phase difference detected by said another phase difference detector (23). A television receiver according to any of claims 2 to 4, wherein the channel selecting means (3) supplies a phase-shift amount preset for a selected channel to the phase-shift amount setter (17), and the phase-shift amount setter (17) sets the phase-shift amount of the local oscillation signal generating means (7) depending further on the phase-shift amount supplied from the channel selecting means. A television receiver according to any of claims 1 to 5, wherein the local oscillation signal generating means (7) comprises: a voltage-variable frequency control oscillator (28) which is responsive to an output signal of the frequency variation control means (14) for generating a first oscillation signal; and a variable phase shifter (29) which is responsive to an output signal of the phase-shift amount setter (17) for shifting a phase of the first oscillation to obtain a second oscillation signaL the first and second oscillation signals being output as the biaxial orthogonal local oscillation signals. A television receiver according to any of claims 1 to 5, wherein the local oscillation signal generating means (7) comprises: a numerical address generator (25) which is responsive to an output signal of the frequency variation control means (14) for generating an address signal; a fixed delay circuit (33) for delaying the address signal by a predetermined fixed delay amount; a sine function signal memory (35) for generating a sine function signal stored therein in response to an output of the fixed delay circuit; a variable delay circuit (34) which is responsive to an output signal of the phase-shift amount setter (17) for delaying the address signal; and a cosine function signal memory (36) for generating a cosme function signal stored therein in response to an output of the variable delay circuit, the sine and cosine function signals being outputted as the biaxial orthogonal local oscillation signals. A television receiver according to any of claims 1 to 7, wherein the detecting means (18-20) comprises: an A/D converter (18) for converting the output signal of the adding means (9) to a digital signal; a digital signal detector (19) for digitally detecting an output signal of the A/D converter; and a wave-form equalizer (20) for wave-form shaping an output signal of the digital signal detector, and wherein the frequency variation control means (14) converts a phase error signal of the wave-form equalizer into a frequency control signal of the local oscillation signal generating means (7, 24). A television receiver according to claim 1, wherein the phase-shift amount setting means (23, 17) comprises: a phase difference detector (23) for detecting a phase difference between input and output signals of the phase shifting means (22); and a phase-shift amount setter (17) for setting the phase-shift amount of the phase shifting means (22) depending on the detected phase difference. A television receiver according to claim 9, wherein channel selecting means (3) supplies a phase-shift amount preset for a selected channel to the phase-shift amount setter (17), and the phase-shift amount setter (17) sets the phase-shift amount of the phase shifting means (22) depending further on the phase-shift amount supplied from the channel selecting means. A television receiver according to any of claims 1, 9 and 10, wherein the local oscillation signal generating means (24) comprises: a voltage-variable frequency control oscillator (28) which is responsive to an output signal of the frequency variation control means (14) for generating an oscillation signal; a first frequency-divider (30) for frequency-dividing the oscillation signal of the voltage-variable frequency control oscillator by a predetermined dividing ratio to obtain a first frequency-divided oscillation signal: and a second frequency-divider (32, 31) for phase-inverting and frequency-dividing the oscillation signal of the voltage-variable frequency control oscillator by said predetermined dividing ratio to obtain a second frequency divided oscillation signal, the first and second frequency-divided oscillation signals being outputted as the biaxial orthogonal local oscillation signals.

Inventors: HAYASHI DAISUKE, KANNO IPPEI, OZEKI HIROAKI, SAKASHITA SEIJI, HAYASHI, DAISUKE, KANNO, IPPEI, OZEKI, HIROAKI, SAKASHITA, SEIJI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1993
Main classification: H03D7
IPC codes: H03D7, H03B1, H03D3, H04N5, H03B5