Abstract:
In a receiver supporting multiple data rates, a sweep control range is calculated on the basis of the data rate of the carrier of the channel to be selected and sweep control is executed on the oscillated frequency by VCO  8  within the sweeping frequency range so that synchronization locking to the carrier is attained and thereby an erroneous channel selection is prevented.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a receiver for receiving a digital modulated signal such as a QPSK signal. 
     2. Description of the Related Art 
     In digital broadcast making use of a communications satellite, video and audio are broadcasted by having the carrier subjected to digital modulation. In the communications satellite (CS) audio broadcast, digital modulation having a data rate of 24.576 Mbps is used. In video-audio broadcast practiced in Japan, digital modulation having a data rate of 42.192 Mbps is used. 
     The radio wave from the satellite is a frequency band of 11.2 GHz. The radio wave received by an antenna is converted by a converter into a first intermediate frequency of a frequency band of 1-2 GHz. Conventionally, a dielectric resonator is used as the local oscillator in the converter. Since the dielectric resonator lacks in stability, the frequency given by the local oscillator sometimes exhibits a drift of around ±2 MHz. Therefore, to tune to the first intermediate frequency signal from the antenna, the voltage-controlled oscillator (VCO) control loop requires a synchronization locking control means for a frequency range of at least exceeding ±2 MHz or, preferably, about ±5 MHz. 
     A conventional art example for solving such a problem is disclosed in Japanese Laid-open Patent No. S 62-136152. 
     FIG. 5 is a block diagram showing a conventional receiver for a QPSK modulated signal. 
     The conventional art example will be described with reference to FIG.  5 . 
     Mixer  102  mixes a signal of a frequency band of 11.2 GHz received by antenna  101  with an output signal of local oscillator  103  to output a first intermediate frequency signal of a frequency band of 1 GHz. Multiplier  102  and local oscillator  103  constitute a converter attached to the antenna. 
     Mixer  104  mixes the first intermediate frequency signal with an output signal of local oscillator  105  for channel selection and thereby outputs a second intermediate frequency signal of a frequency band of 400 MHz. 
     The second intermediate frequency signal limited for bandwidth by bandpass filter (BPF)  106  is supplied to Quardrature Phase Shift Keying (QPSK) demodulator  200 . 
     QPSK demodulator  200  is made up of detectors  201  and  202 , phase shifter  203 , waveform shapers  204  and  205 , multipliers  206  and  207 , subtractor  208 , adder  209 , low-pass filter (LPF)  210  and voltage-controlled oscillator (VCO)  211 . 
     Phase shifter  203  shifts 90 degrees the phase of the output signal of VCO  211 . 
     Adder  209  adds the output signal of low-frequency oscillator  110 , passed through switch  111 , to the output signal of subtractor  208 . 
     Switch  111  is closed when demodulator  200  is in its asynchronous state, while it is opened when demodulator  200  is in its synchronous state. 
     When demodulator  200  is brought into its synchronous state, a detection signal is delivered from synchronous pattern detector (SPD)  109  in digital signal processor (DSP)  108 . Switch  111  is controlled by this detection signal. 
     The capture range of QPSK demodulator  200  is around ±500 kHz, while the lock range thereof is ±several MHz. 
     When local oscillator  103  is drifting several MHz, QPSK demodulator  200  sometimes becomes unable to perform synchronization locking. In such case, switch  111  is closed and a low-frequency signal is added to a phase error signal, so that VCO  211  is swept and, thereby, synchronization locking control is performed. 
     This makes use of the characteristic of the VCO control loop that it has a wide lock range while it has a narrow capture range, and, hence, the sweeping expands the apparent capture range of the loop. By means of such a configuration, even when the carrier frequency is off-centered, it can be captured and demodulated. 
     Recently, as applications making use of communications satellites, uses of broadcast communications, such as inter-company communications, have become increased. In such communications applications, not so large an amount of transmitted data is required as in ordinary video broadcasting. Therefore, the data rate is 8 Mbps, 4 Mbps, or so, which is considerably lower than the data rate around 40 Mbps in ordinary broadcasting. 
     Since the carrier bandwidth is in direct proportion to the data rate, the space between adjacent carriers can be set narrower according as the data rate becomes lower. Hence, efficiency in utilization of the frequency can be improved. 
     When the data rate is 8 Mbps, the distance between channels in the carrier disposition is about 5.4 MHz, while it is about 2.7 MHz when the data rate is 4 Mbps. 
     If synchronization locking control over a frequency range of ±5 MHz, in accordance with the data rate for conventional broadcasting, is applied to a carrier at a low data rate of 8 Mbps or 4 Mbps, there arises a problem that a wrong channel comes to be selected. 
     SUMMARY OF THE INVENTION 
     The present invention was made to solve the above mentioned problem encountered in a receiver arranged to support multiple data rates. In order to solve the problem, the receiver of the present invention comprises: 
     tuning means for converting a desired modulated carrier wave selected from a first intermediate frequency signal received by an antenna into a second intermediate frequency signal; 
     a bandpass filter for limiting the second intermediate frequency signal for bandwidth; 
     orthogonal detecting means supplied with an output signal of the bandpass filter for delivering an I baseband signal and a Q baseband signal orthogonal with each other; 
     a voltage-controlled oscillator for feeding a signal for detection to the orthogonal detecting means; 
     AD converters for converting the I and Q baseband signals into digital signals; 
     digital demodulator means supplied with the I and Q baseband signals digitized by the AD converters for delivering a frequency error signal, a synchronism detection signal, and a demodulated signal; 
     sweeping control means supplied with the frequency error signal and sweeping data for delivering a control voltage of the voltage-controlled oscillator; and 
     a microcomputer supplied with a channel select command including carrier wave information and data rate information of the first intermediate frequency, the frequency error signal, and the synchronism detection signal, in which the microcomputer delivers a tuning frequency setting value to the tuning means, and delivers the sweeping data to the sweeping control means. 
     By virtue of the structure as described above, the sweeping frequency range of a desired carrier is calculated on the basis of the data rate of the carrier of the selected channel and sweep control is executed within the thus calculated sweeping frequency range so that synchronization locking to the carrier is attained and, thereby, an erroneous channel selection can be prevented. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a configuration of a receiver as an exemplary embodiment of the present invention. 
     FIG. 2 is a block diagram showing a configuration of a tuner means in the receiver. 
     FIG. 3 is a characteristic diagram of a VCO in the receiver. 
     FIG. 4 is a characteristic diagram of a VCO in the receiver. 
     FIG. 5 is a block diagram showing a configuration of a conventional receiver. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An exemplary embodiment of the present invention will be described with reference to the accompanying drawings. 
     FIG. 1 is a block diagram showing a receiver according to the embodiment of the invention. 
     With reference to FIG. 1, antenna portion  1  converts a received radio wave into a first intermediate frequency signal (of a frequency band of 1 GHz-2 GHz ) and delivers the signal. 
     As tuner  3 , that of a PLL synthesizer type as shown in FIG. 2 is generally used. Tuner  3 , as shown in FIG. 2 is made up of mixer  22 , local oscillator  23  formed of a VCO, variable divider (programmable divider)  24 , phase comparator  25 , reference oscillator  26 , and loop filter  27 . The first intermediate frequency signal is supplied to terminal  31 , while a tuning frequency setting value from a microcomputer is supplied to terminal  33 . The frequency dividing value of variable divider  24  is established in accordance with the tuning frequency setting value. As a result, a desired carrier frequency is selected and the selected second intermediate frequency signal of 479.5 MHz is delivered to terminal  32 . 
     The second intermediate frequency signal is limited for bandwidth in bandpass filter (BPF)  4  and supplied to orthogonal detector  5 . 
     Orthogonal detector  5  uses the output signal of VCO  8  to convert the second intermediate frequency signal into an I baseband signal (hereinafter called “I signal”) and a Q baseband signal (hereinafter called “Q signal”) which are 90° out of phase with each other. The I signal and Q signal are converted by ADC  61  and  62 , respectively, into digital signals and the same are supplied to digital demodulator  7 . Digital demodulator  7  is generally formed of an LSI. 
     Digital demodulator  7  demodulates the received signals and delivers the demodulated signal. Demodulator  7  also delivers a synchronism detection signal, which is at a high level when the intermediate frequency signal and the output signal of VCO  8  are in phase and at a low level when they are out of phase, and a frequency error signal as the control voltage for VCO  8 . 
     The frequency error signal is passed through selector circuit  10  constituting sweeping controller and, then, converted by DAC  11  into an analog signal for controlling the oscillation frequency of VCO  8 . 
     Channel selection information, which includes carrier frequency information and data rate information of a selected channel, is supplied to microcomputer  12  through terminal  13 . Microcomputer  12  performs the following functions: 
     (1) to set the value of tuning frequency in tuner  3 ; 
     (2) to set the value of data rate of the selected channel in demodulator  7 ; and 
     (3) to set initial values in synchronization locking section  9  constituting sweeping controller. 
     Microcomputer  12  identifies that the synchronism detection signal is at a high level and confirms that channel selection has been completed normally. 
     Even after the completion of channel selection, microcomputer  12  continues read of the frequency error information delivered from modulator  7 . In this state, if a new channel select command is received, microcomputer  12  establishes the current frequency error signal in the synchronization locking section  9 . Then, in accordance with the new channel select command, it sets up the tuning frequency in tuner  3  and sets up the data rate in demodulator  7 . 
     When the synchronism detection signal is at a low level in the state where the A/D converted I signal and Q signal are fed into demodulator  7 , microcomputer  12  performs the following operations (1)-(4) in order that the intermediate frequency signal and the output signal of VCO  8  synchronize with each other: 
     (1) Calculation of sweep-controlling frequency range. 
     The sweep-controlling frequency range is set to be the carrier disposition interval. The carrier disposition interval is assumed to be equal to the carrier bandwidth. Here, the carrier bandwidth is obtained from Equation 1, 
     
       
         carrier bandwidth=(data rate)×(1+roll-off rate)×½  1 
       
     
     where it is set such that roll-off rate=0.2. 
     In executing the sweep control, sweeping is carried out over the above-obtained frequency range on both sides of the intermediate frequency. 
     (2) Calculation of VCO-sweeping voltage range based on the sweeping frequency range. 
     Relationship between VCO-oscillated frequency (Fvco (MHz)) and VCO controlling voltage (Va (v)) is expressed as equation 2. 
     
       
           Fvco=k Va+C   2 
       
     
     where k is the conversion sensitivity between input voltage to VCO and generated frequency therefrom, and C is the VCO-oscillated frequency at the time when the VCO controlling voltage is at its minimum (Va_min). (Refer to FIG. 3.) 
     When the sweeping frequency range is expressed as F=Fvco 1 −Fvco 2  and the VCO sweeping voltage range is denoted by Va, then, Va is expressed as equation 3. 
     
       
           Va=F/k.   3 
       
     
     (3) Calculation of sweep-control-value range from the sweeping voltage range. 
     As shown in FIG. 4, there is one-to-one correspondence between VCO sweeping voltage (Va) and the sweep-control-value (Ds). Here, the VCO sweep-control-value is given by two&#39;s-complement numbers, so that 00h corresponds to the center value of VCO controlling voltage (Va_center), 7Fh to the maximum value (Va_max), and 80h to the minimum value (Va_min). 
     Microcomputer has the correspondences between them recorded for example in a ROM table, and based on the correspondences, obtains the sweep-control-value range corresponding to the sweeping voltage range with the center value of the sweep-control-value range (corresponding to (Va_center)) in the center. 
     (4) Sweeping of VCO-oscillated frequency. 
     Microcomputer, in order to bring about a synchronized state by effecting synchronization locking to the carrier, sweeps VCO  8  through selector  10  and DAC  11  over the sweep-control-value range obtained in (3) above to attain synchronization locking to the carrier. 
     Hence, microcomputer splits up the sweep-control-value range obtained in (3) above into sections each thereof having the width of the sweep-control-value corresponding to the lock range and, thereupon, applies the sweep-control-value of each section to DAC  11  for allowing the locking to the carrier to be made. 
     In the case of QPSK modulated wave, for example, the lock range (MHz) to the carrier is equivalent to ⅛ of the data rate (Mbps). 
     By making sweep control as described in (1)-(4) above, synchronization locking to the carrier is achieved. 
     Although the above described embodiment has been constructed by a combination of a demodulating LSI and a VCO circuit, it is also possible to realize the VCO in a digital manner within the demodulating LSI so that the demodulating LSI contains all of the structure of the VCO control loop. 
     According to the present invention as described above, synchronization locking can be attained by calculating the sweeping frequency range for a searched carrier based on the data rate of the carrier of the selected channel and performing sweep control over the sweeping frequency range. Therefore, such an effect can be obtained that erroneous selection of other channels can be prevented.