Dual bandwidth PLL for channel selection in satellite links

Abstract: A digital satellite broadcasting receiver includes: a channel selecting circuit having a channel selecting PLL; a carrier recovery circuit receiving an output from the channel selecting circuit and having a carrier recovery PLL; a switching circuit for switching a loop bandwidth of the channel selecting PLL; and a control circuit for controlling the switching circuit such that the loop bandwidth of the channel selecting PLL circuit is made narrower than in a normal receiving state and than at a channel switching, at the time of centering. Instead of channel selecting PLL, the loop bandwidth of the carrier recovery PLL may be made wider temporarily. The method of centering in the receiver includes the following steps. Namely, the step (#8) of narrowing the loop bandwidth of the channel selecting PLL; the step (#10, #15) of changing a synchronizing frequency of the channel selecting PLL so as to minimize the amount of carrier offset obtained from the demodulated signal; and the step (#18) of widening the loop bandwidth of the channel selecting PLL.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a digital satellite broadcasting receiver for receiving, demodulating and processing broadcasting signals from a satellite, for example, to output data. More specifically, the present invention relates to a digital satellite broadcasting receiver not susceptible to noise and almost free from the possibility of losing carrier lock in a PLL circuit for carrier recovery at the time of centering. Description of the Background Art In a digital satellite broadcasting receiver, a channel of the satellite broadcasting signal is selected by a channel selecting circuit, the signal is converted to an IF (Intermediate Frequency) signal and subjected to IQ demodulation to be an analog IQ signal of pseudo baseband. The analog IQ signal is further converted to a digital signal and demodulated at a QPSK (Quadrature Phase Shift Keying) demodulating section, subjected to a prescribed signal processing, and output as data. The QPSK demodulating section includes a carrier recovery section, which carrier recovery section detects any frequency error and an optimal phase. The channel selecting circuit has a PLL (Phase Locked Loop) and when a frequency divider in the PLL is provided with data for frequency division from a control circuit, the channel selecting circuit can select channels and change frequency stepwise. The QPSK demodulating section provides demodulated data. However, even at this time point, there still remains carrier offset. Therefore, centering of the satellite broadcasting receiver is necessary to minimize the offset. Conventionally, centering is performed in the following manner. Referring to Fig. 8, a carrier offset signal is read from the QPSK demodulating section (S5). Frequency of the PLL of the channel selecting circuit is changed one step by one step in a direction of reducing the amount of carrier offset (S10), and when it is detected that the amount of carrier offset attains to be smaller than the step width of the channel selecting PLL (S15), centering operation is completed (S20). This is because further correction is not possible when the amount of carrier offset is smaller than the step width of the channel selecting PLL. Such a conventional technique has quick response and is strong against shock noise when the loop bandwidth of the channel selecting PLL is wide. However, when the channel selecting frequency of the channel selecting PLL is shifted one step by one step at the time of centering, the carrier recovery PLL of the QPSK demodulating section in the succeeding stage cannot follow the channel selecting PLL, so that carrier lock is lost. Conversely, when the loop bandwidth of the carrier recovery PLL of the succeeding stage is made wider, there is less possibility that the carrier recovery PLL fails to follow the channel selecting PLL. However, it is more likely that characteristics against signal noise are deteriorated in normal reception, resulting in inferior bit error rate. EP 0782270 (having a priority date of 28 th December 1995 - ie. prior art under Article 54(3) EPC) discloses a tuning system for a digital satellite receiver with fine tuning provisions. EP 0782271 (having a priority date of 28 th December 1995 - ie. prior art under Article 54(3) EPC) discloses an arrangement for selectively controlling the response time of a type II PLL, especially one which includes a phase detector and an amplifier of a feedback type of integrator with an IC. WO 96/39745 discloses a tuner for digital satellite receiver comprising a single conversion stage which produces an IF signal at a low enough frequency to permit a SAW filter to be used to perform symbol shaping as well as the normal IF filtering function. US 5272452 discloses a PLL frequency synthesizer with variable bandwidth loop filter. SUMMARY OF THE INVENTION It is desirable to provide a digital satellite broadcasting receiver capable of preventing increase in noise in normal receiving state while maintaining carrier lock of the carrier recovery PLL at the time of centering, and to provide the method thereof. It is also desirable to provide a digital satellite bmadcasting receiver capable of preventing increase in noise in normal receiving state while allowing a carrier recovery PLL, to well follow an operation of a channel selecting PLL at the time of centering, and to provide the method thereof. It is desirable to provide a digital satellite broadcasting receiver in which response of a channel selecting PLL is made slow enough to allow a carrier recovery PLL to follow at the time of centering and the response for channel selection is made quick in normal receiving state, and to provide the method thereof. According to the present invention there is provided a digital satellite broadcasting receiver as claimed in claim 1. In the digital satellite broadcasting receiver structured as described above, the bandwidth of the first PLL circuit for channel selection is wide at the time of channel selection and in the normal receiving state, so that response for channel selection is quick and therefore the receiver is less susceptible to shock noise. At the time of centering, the bandwidth of the first PLL circuit for channel selection is made narrower, so that the second PLL circuit of the carrier recovery circuit easily follows the operation of the PLL circuit, and therefore it is less likely that the carrier lock is lost. Preferably, the channel selecting circuit has a first function for tuning, based on an applied control signal, with a channel selecting frequency designated by the control signal, and a second function of changing one step by one step an oscillation frequency of the first PLL circuit. The channel selecting circuit further controls the switching circuit when it performs the second function such that the loop bandwidth of the first PLL circuit is made narrower than when it performs the first function. According to another aspect of the present invention, there is provided a digital satellite broadcasting receiver as claimed in claim 7. The method of centering in a digital satellite broadcasting receiver in accordance with a still further aspect of the present invention includes the steps of: narrowing loop bandwidth of a PLL circuit for channel selection included in a channel selecting circuit; in the PLL circuit of which loop bandwidth is made narrower, changing a synchronizing frequency of the PLL circuit so that an amount of carrier offset obtained from a demodulated signal is minimized; and after the amount of a carrier offset obtained from the demodulated signal is minimized, widening the loop bandwidth of the PLL circuit. According to a further aspect there is provided a method of centering in a digital satellite broadcasting receiver as claimed in claim 9. According to a further aspect there is provided a method of centering a digital satellite broadcasting receiver as claimed in claim 11. In order that the present invention be more readily understood, specific embodiments thereof will now be described with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a digital satellite broadcasting receiver in accordance with an embodiment of the present invention. Fig. 2 is a block diagram showing a detailed structure of a channel selecting PLL section in the digital satellite broadcasting receiver shown in Fig. 1. Fig. 3 is a circuit diagram showing a detailed structure of a low pass filter in the channel selecting PLL section shown in Fig. 2. Fig. 4 is a detailed block diagram of a QPSK demodulating section of the digital satellite broadcasting receiver shown in Fig. 1. Fig. 5 is a flow chart showing an operation of a control circuit at the time of centering in the digital satellite broadcasting receiver in accordance with a first embodiment. Fig. 6 is a flow chart showing an operation of the channel selecting PLL in the digital satellite broadcasting receiver in accordance with the first embodiment. Fig. 7 is a flow chart showing an operation of the control circuit at the time of centering in the digital satellite broadcasting receiver in accordance with a second embodiment. Fig. 8 is a flow chart of the control circuit at the time of centering in a conventional digital satellite broadcasting receiver. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1, the digital satellite broadcasting receiver includes a channel selecting section 11 receiving an RF (Radio Frequency) signal, an SAW (Surface Acoustic Wave) filter 12 acting on an IF signal 2 provided as an output from channel selecting section 11, an IQ demodulating section 14 receiving an output from SAW filter 12 and a local signal 13, an AD (Analog-to-Digital) converting section 15 for converting analog I and Q signals 3 output from IQ demodulating section 14 to digital signals, a QPSK demodulating section 16 receiving I and Q signals 4 output from AD converting section 15 and providing output data 5, and a control section 17 for overall control of the digital satellite broadcasting receiver. Control section 17 includes a microcomputer and a program executed by the microcomputer, for example. Referring to Fig. 2, channel selecting section 11 includes a mixer 20 for converting the received RF signal to the IF signal, and a PLL section 27 for applying a local oscillation signal to mixer 20. PLL section 27 in channel selecting section 11 will be hereinafter referred to as channel selecting PLL. Channel selecting PLL 27 includes a voltage controlled oscillator (VCO) 21 which is under PLL control and providing an output to mixer 20, and a programmable frequency divider 22 for dividing an output frequency of oscillator 21 by a factor N. The dividing factor N of frequency divider 22 is applied from control section 17 through a terminal 25 shown in Fig. 1. Channel selecting PLL 27 further includes a phase comparator 23 receiving a reference frequency f REF and an output from frequency divider 22, and a low pass filter 24 receiving an output from phase comparator 23. An output from low pass filter 24 is applied to oscillator 21 and oscillation frequency of oscillator 21 is controlled thereby. Referring to Fig. 3, low pass filter 24 has a control terminal 36 and two terminals 31 and 32 receiving an output from phase comparator 23. A signal "1" or "0" is applied from control section 17 shown in Fig. 1 to control terminal 36. Low pass filter 24 includes an inverter 33, P channel MOS transistors 34 and 39, N channel MOS transistors 35 and 40, resistors R1 and R2, a buffer 41, an NAND gate 37, an AND gate 38 and a capacitor C. Inverter 33 has an input connected to terminal 31. Inverter 33 has an output connected to the gate of transistor 34. Transistor 34 has its source connected to a power supply voltage V DD . Transistor 34 has its drain connected to the drain of transistor 35 and further to one end of resistor R1. Transistor 35 has its gate connected to terminal 32 and its drain grounded. NAND gate 37 has one input connected to terminal 31 and the other input connected to control terminal 36, respectively. NAND gate 37 has an output connected to the gate of transistor 39. AND gate 38 has one input connected to terminal 32 and the other input connected to control terminal 36, respectively. AND gate 38 has an output connected to the gate of transistor 40. Transistor 39 has its source connected to the power supply potential V DD . Transistor 39 has its drain connected to the drain of transistor 40 and further to one end of resistor R2. Transistor 40 has its source grounded. The other terminals of resistors R1 and R2 are connected to each other, to an input of buffer 41 and to one end of capacitor C. The other end of capacitor C is connected to an output of buffer 41. Referring to Fig. 4, QPSK demodulating section 16 shown in Fig. 1 has input terminals 41 and 42 to which digital I and Q signals are applied, respectively, a terminal 48 for data DA and a terminal 47 for a clock CLK, and includes a Nyquist filter 43, a derotator 44, an automatic gain control (AGC) circuit 45, a carrier offset evaluater 46, an I 2 C bus interface 49, a register 50, a carrier phase tracking loop 51, a DCO 52 and a processing circuit 53. Nyquist filter 43 has two inputs connected to terminals 41 and 42, respectively. Nyquist filter 43 has its output connected to derotator 44 and carrier offset evaluater 46. Derotator 44 is for substantially demodulating I and Q signals, and its output is connected to AGC circuit 45, processing circuit 53 and carrier phase tracking loop 51. Processing circuit 53 is for performing various processing of the demodulated I and Q signals, and provides an output to output terminal 54. Carrier phase tracking loop 51 has its output connected to an input of DCO 52. DCO 52 has its output connected to derotator 44. Carrier offset evaluater 46 has its output connected to I 2 C bus interface 49. I 2 C bus interface 49 has two inputs connected to clock terminal 47 and data terminal 48, and an output connected to register 50. Register 50 has its output connected to carrier phase tracking loop 51. By applying an output from carrier phase tracking loop 51 to DCO 52, carrier recovery is performed, and a recovered carrier signal is supplied to derotator 44. Carrier phase tracking loop 51 also has a PLL circuit (hereinafter referred to as "carrier recovery PLL"), of which bandwidth is set by data stored in register 50. An amount of carrier offset output from carrier offset evaluater 46 is transmitted through I 2 C bus interface 49 from data terminal 48 to control section 17 of Fig. 1. Control section 17 passes set value data to register 50 through terminal 48. The set value data is stored in register 50 through I 2 C bus interface 49. The set value data stored in register 50 determines the bandwidth of the carrier recovery PLL. Referring to Fig. 3, in operation, low pass filter 24 of channel selecting PLL 27 performs the following function. At the time of normal operation and channel switching, "0" is applied to control terminal 36. As a result, a second charge pump section constituted by transistors 39 and 40 does not operate. Only a first charge pump section constituted by transistors 34 and 35 is made operable. Rate of charging/discharging capacitor C is defined by a time constant CR1 of capacitor C and resistor R1. Conversely, at the time of centering, "1" is applied to control terminal 36, as will be described in the following. At this time, not only the first charge pump section but also the second charge pump section is made operable. The rate of charging/discharging capacitor C is defined by a time constant C•R1•R2/(R1+R2). This value is smaller than the time constant CR1 when the first charge pump section only operates. Accordingly, when the first and second charge pumps both operate, bandwidth per one step is smaller than when only the first charge pump section operates. More specifically, by changing charge pump current, which is the base current of the transistor constituting LPF 24, which is an active filter of channel selecting PLL 27, time constant of the time constant circuit including R1, R2 and C is changed, so that the speed of response and loop bandwidth of the channel selecting PLL are changed. Control section 17 operates in the following manner when centering of the satellite broadcasting receiver is performed. Referring to Fig. 5, an amount of carrier offset from QPSK demodulating section 16 is received (#5), and the loop bandwidth of channel selecting PLL 27 is made narrower than in normal receiving state or than at the time of channel selection (#8). The loop bandwidth can be made narrower by applying "1" to control terminal 36 shown in Fig. 3, as described above. Thereafter, a channel selecting frequency of channel selecting PLL 27 is shifted by one step in a direction of reducing carrier offset (#10). It is determined whether the amount of carrier offset is smaller than the step width of channel selecting PLL 27 (#15). When the amount of carrier offset is smaller than the step width, control proceeds to #18, and the loop bandwidth of channel selecting PLL 27 is returned to the original width by applying "0" to control terminal 36 of Fig. 3 (#18). By the above described process performed by control section 17, centering of the satellite broadcasting receiver is completed (#20). When it is determined in step #15 that the amount of carrier offset is not smaller than the step width of the channel selecting PLL, control returns to #10, and the channel selecting frequency of the channel selecting PLL is further shifted by one step. Channel selecting section 11 operates in the following manner. Referring to Fig. 6, first, channel selecting section 11 receives a control signal (#100). Channel selecting section 11 determines whether the control signal designates change of the channel selecting frequency by one step, or designates a channel selecting frequency (#110). When it is determined that the control signal designates change of the channel selecting frequency by one step, then channel selecting section 11 narrows the loop bandwidth of channel selecting PLL 27 (#120). Thereafter, channel selecting section 11 selects a channel by changing the channel selecting frequency step by step (#130). After completion of such channel selecting operation, channel selecting section 11 sets the loop bandwidth of channel selecting PLL 27 to the original value (#140). When it is determined in step #110 that the control signal designates a channel selecting frequency, then channel selecting section 11 selects a channel designated by frequency division data corresponding to the designated frequency, applied to terminal 25 of Fig. 2 (#150). In this manner, in the present embodiment, response of channel selecting PLL is made slow by narrowing loop width of channel selecting PLL 27 at the time of centering of the satellite broadcasting receiver. Therefore, carrier recovery PLL of QPSK demodulating section 16 in the succeeding stage well follows the operation of channel selecting PLL 27, whereby possibility of losing carrier lock is reduced. In normal receiving state, the channel selecting PLL has wide bandwidth. Therefore, the PLL has quick response and is strong against oscillation and noise. The problem of deteriorated bit error rate even in the normal reception experienced when the loop width of the carrier recovery PLL is made wider, can be avoided. Further, possibility of losing carrier lock of the carrier recovery PLL is smaller at the time of centering as well as at the time of channel selection. Fig. 7 shows a flow chart of operation at the time of centering, of control section 17 in the digital satellite broadcasting receiver in accordance with a second embodiment of the present invention. The digital satellite broadcasting receiver in accordance with the second embodiment is the same as the digital satellite broadcasting receiver in accordance with the first embodiment except the operation of control section 17 at the time of centering. Therefore, detailed description of the hardware and so on is not repeated. Referring to Fig. 7, at the time of centering, control section 17 of the digital satellite broadcasting receiver in accordance with the second embodiment receives an amount of carrier offset from QPSK demodulating section 16 (#5), and widens the loop bandwidth of the carrier recovery PLL of QPSK demodulating section 16 (#28). More specifically, different from the first embodiment in which the loop bandwidth of channel selecting PLL 27 is made narrower, the loop bandwidth of carrier recovery PLL is made wider. Thereafter, the channel selecting frequency of channel selecting PLL 27 is shifted by one step in a direction of reducing carrier offset (#10). Whether the amount of carrier offset is smaller than the step width of channel selecting PLL 27 is determined (#15). When the amount of carrier offset is smaller than the step width, control proceeds to #30, where the loop bandwidth of carrier recovery PLL is returned to the original width (#30). By the above described process performed by control section 17, centering of the satellite broadcasting receiver is completed (#20). When it is determined in step #15 that the amount of carrier offset is not smaller than the step width of channel selecting PLL 27, control returns to #10, and the PLL in the QPSK demodulating section is further shifted by one step. In the digital satellite broadcasting receiver in accordance with the second embodiment, response of the PLL in QPSK demodulating section is made faster at the time of centering. Therefore, the PLL in QPSK demodulating section can follow the operation of channel selecting PLL 27. In the normal state or at the time of channel selection, the bandwidth of the PLL in QPSK demodulating section is returned to the original value. Therefore, characteristics against signal noise are not deteriorated in normal reception, and tendency of inferior bit error rate can be avoided.

Claims: A digital satellite broadcasting receiver, comprising: a channel selecting circuit (11) for outputting a frequency of a selected channel, and having a first PLL circuit (27) which has a frequency that may be manipulated for channel selection; a carrier recovery circuit (16) for receiving an output from said channel selecting circuit (11) and having a second PLL circuit (51); switching means (24) for switching a loop bandwidth of said first PLL circuit (27); means for carrying out a centering operation, that is an operation to minimise the carrier offset of the carrier recovered by said carrier recovery circuit; and a control circuit (17) for controlling said switching means (24) such that the loop bandwidth of the first PLL circuit (27) at the time of the centering operation is made narrower than in a normal receiving state or than at a time of channel switching, wherein said means for carrying out a centering operation is adapted to cause the frequency of the first PLL circuit to be shifted in one-step increments in the direction of reducing carrier frequency offset, until it is determined that the carrier frequency offset is less than a step width of the first PLL circuit (27), whereafter the channel selecting circuit is adapted to return the loop bandwidth of the first PLL circuit to its original value. A digital satellite broadcasting receiver according to claim 1, wherein said channel selecting circuit (11) has a first function (150) for tuning, based on an applied control signal, with a channel selecting frequency designated by said control signal, and a second function (120-140) for changing stepwise an oscillation frequency of said first PLL circuit; and said channel selecting circuit (11) further controls said switching means (24) such that the loop bandwidth of said first PLL circuit (27) is made narrower when said second function is performed, than when said first function is performed. A digital satellite broadcasting receiver according to claim 2, wherein said switching means (24) includes time constant switching means (37,38) for switching time constant of a time constant element (R1, R2, C) of an active filter of a loop in said first PLL circuit (27). A digital satellite broadcasting receiver according to claim 3, wherein said time constant switching means (37,38) includes means (37,38) responsive to a control signal from said control circuit (17) for switching a charge pump current to said time constant element (R1, R2, C). A digital satellite broadcasting receiver according to claim 1, wherein said switching means (24) includes time constant switching means (37,38) for switching time constant of a time constant element (R1, R2, C) of an active filter of a loop of said first PLL circuit (27). A digital satellite broadcasting receiver according to claim 5, wherein said time constant switching means (37,38) includes means (37,38) responsive to a control signal from said control circuit (17) for switching charge pump current to said time constant element (R1, R2, C). A digital satellite broadcasting receiver, comprising: a channel selecting circuit (11) having a first PLL circuit (27); a carrier recovery circuit (16) receiving an output from said channel selecting circuit (11) and having a second PLL circuit (51); switching means (50) for switching a loop bandwidth of said second PLL circuit (51); means for carrying out a centering operation, that is an operation to minimise the carrier offset of the carrier recovered by said carrier recovery circuit; and a control circuit (17) for controlling said switching means (50) such that the loop bandwidth of said second PLL circuit (51) at the time of said centering operation is made wider than in a normal receiving state or than at a time of channel switching, wherein said means for carrying out a centering operation is adapted to cause the frequency of the first PLL circuit to be shifted in one-step increments in the direction of reducing carrier frequency offset, until it is determined that the carrier frequency offset is less than a step width of the first PLL circuit (27), whereafter the channel selecting circuit is adapted to return the loop bandwidth of the first PLL circuit to its original value. A digital satellite broadcasting receiver according to claim 7, wherein said switching means (50) includes a register for storing and applying to said second PLL circuit (51) a set value for switching the bandwidth of said second PLL circuit (51) applied from said control circuit (17). A method for centering a digital satellite broadcasting receiver, wherein said receiver comprises: a channel selecting circuit (11) for outputting a frequency of a selected channel, and having a first PLL circuit (27) which has a frequency that may be manipulated for channel selection; a carrier recovery circuit (16) for receiving an output from said channel selecting circuit (11) and having a second PLL circuit (51); switching means (24) for switching a loop bandwidth of said first PLL circuit (27); and means for carrying out a centering operation, that is an operation to minimise the carrier offset of the carrier recovered by said carrier recovery circuit; wherein said method comprises the steps of: controlling said switching means (24) from a control circuit (17) such that the loop bandwidth of the first PLL circuit (27) at the time of the centering operation is made narrower than in a normal receiving state or than at a time of channel switching, and said means for carrying out a centering operation causing the frequency of the first PLL circuit to be shifted in one-step increments in the direction of reducing carrier frequency offset, until it is determined that the carrier frequency offset is less than a step width of the first PLL circuit (27), whereafter the channel selecting circuit returns the loop bandwidth of the first PLL circuit to its original value. A method according to claim 9, wherein said step of narrowing the loop bandwidth of said PLL circuit (27) includes the step of making smaller a time constant of a time constant element (R1, R2, C) constituting an active filter of said PLL circuit (27); and said step of widening the loop bandwidth of said PLL circuit (27) includes the step of making larger the time constant of said time constant element (R1, R2, C). A method for centering a digital satellite broadcasting receiver, wherein said receiver comprises: a channel selecting circuit (11) having a first PLL circuit (27); a carrier recovery circuit (16) receiving an output from said channel selecting circuit (11) and having a second PLL circuit (51); switching means (50) for switching a loop bandwidth of said second PLL circuit (51); and means for carrying out a centering operation, that is an operation to minimise the carrier offset of the carrier recovered by said carrier recovery circuit; wherein said method comprises the steps of: controlling said switching means (50) from a control circuit (17) such that the loop bandwidth of said second PLL circuit (51) at the time of said centering operation is made wider than in a normal receiving state or than at a time of channel switching, and said means for carrying out a centering operation causing the frequency of the first PLL circuit to be shifted in one-step increments in the direction of reducing carrier frequency offset, until it is determined that the carrier frequency offset is less than a step width of the first PLL circuit (27), whereafter the channel selecting circuit returns the loop bandwidth of the first PLL circuit to its original value. A method according to claim 11 wherein said step (#28) of widening the loop bandwidth of said carrier recovery PLL circuit (51) includes the step of storing a first set value for widening the bandwidth in a register (50) storing a value for setting the bandwidth of said carrier recovery PLL circuit (51); and said step (#30) of narrowing the loop bandwidth of said carrier recovery PLL circuit (51) includes the step of storing a second set value for narrowing the bandwidth in said register (50).

Inventors: IKEDA HITOSHI, IKEDA, HITOSHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1998
Main classification: H04L27
IPC codes: H03L7, H04L27