Automatic frequency control circuit

BACKGROUND OF THE INVENTION

1. Field of the Invention

0001The present invention relates to an automatic frequency control (AFC) circuit for use in a receiver of radio communication or satellite communication equipment. More particularly, the present invention is concerned with an AFC circuit providing frequency stability sufficient for use in receiver in a single channel per carrier (SCPC) system.

2. Description of the Related Art

0002In a SCPC system, since a bandwidth occupied by a single channel is very narrow (several dozen kHz) compared with a carrier frequency (more than several hundred MHz), and tends to be narrower every year to raise the utilization factor of frequencies, a high level of stability of a received frequency is strongly recommended to ensure stable communication.

0003However, the stability of a frequency of a signal received from a satellite is limited to a certain level because of unstable influences from a local oscillator mounted on the satellite, the Doppler effect, and the instability of a high-frequency circuit of a receiver.

0004Therefore, an AFC system is employed to ensure stable communication. In the AFC system, a pilot signal not including modulation components is transmitted in a specific frequency position within the used band, and the frequency of a local oscillator of the receiver is automatically controlled so that the frequency of a received pilot signal is converted into a precise frequency in a frequency converter.

0005The local oscillator consists of a voltage controlled oscillator (VCO). A received radio-frequency signal or an intermediate frequency signal is converted into an intermediate frequency signal or a lower intermediate frequency signal by being mixed with an output of the VCO in the frequency converter. The pilot signal is extracted from an output of the frequency converter through a band pass filter having a narrow passband and is compared with a reference frequency output from a reference oscillator in a phase comparator. An output of the phase comparator is applied through a loop filter to the VCO as a control signal, to thereby control the frequency and the phase of the extracted pilot signal so as to be equal to those of the reference signal. Since frequencies of the other received signals within that band fluctuate simultaneously with the received pilot signal, the fluctuations of the other received signals are compensated for in converted signals.

0006Usually, the pilot signal is transmitted only from a master station or a reference station supervising a whole communication network. Therefore, the pilot signal is not obtained when a transmission of the pilot signal is terminated for some reason or other, when the receiver is used in a system where the pilot signal is not transmitted, or when a loop test is carried out within a part of the transmission paths.

0007Conventionally, in order to maintain receiver functions even when the pilot signal is not obtained, a switch is provided between the VCO and the loop filter for selecting the output of the loop filter or a fixed bias voltage. If the pilot signal is obtained, the switch is set so as to select the output of the loop filter, and if the pilot signal is not obtained, the switch is set so as to select the fixed bias voltage.

0008In the above construction, since frequency stability of outputs of the frequency converter is determined by the stability of the VCO while the pilot signal is not obtained, a high level of stability is required in the VCO. However, since the stability of the VCO is determined by an oscillation frequency, a variable range, and the quality factor (Q) of a resonator, etc., it is very difficult to obtain a VCO having a high level of stability as well as an ultra high operating frequency (more than several hundred MHz) and a variable range necessary for searching over the assigned frequency band to catch the pilot signal. For example, frequency stability of 10^-6 is realized by using a crystal oscillator as a reference oscillator when the pilot signal is obtained, whereas the same frequency stability cannot be realized in practice when the pilot signal is not obtained, because frequency stability of the VCO is generally about 10^-4. In this case, if the output frequency of the VCO is 1,120 MHz to convert 1,190 MHz into 70 MHz, the output frequency of the VCO fluctuates over a range of 112 kHz (1,120 MHz 10^-4). Therefore, if the bandwidth of a channel is 45 kHz, a designated channel cannot be received because the width of the fluctuation is larger than the bandwidth.

0009Meanwhile, the band pass filter for extracting the pilot signal from the output of the frequency converter has a narrow passband such that the width of a 3dB passband is, for example, 30 kHz in a 80 MHz band, and a crystal filter is often used as the band pass filter. Although a single frequency is usually used as the frequency of the pilot signal, a spare frequency is reserved in another frequency in some systems, and it is previously announced that the spare frequency will be used for the pilot signal when the pilot signal in the current frequency is terminated for some reason or other. In that system, if the frequency of the pilot signal is switched from current to spare, a central frequency of the band pass filter and the frequency of the reference oscillator must be changed in accordance with the changed pilot frequency.

0010The change may be possible if a plurality of the band pass filters and the reference oscillators having individual frequencies are provided within the receiver and each one of those is selected according to the pilot frequency. However, this solution is not economical because the band pass filters having a narrow passband, such as the crystal filters, are often expensive and the reference oscillator is also expensive.

0011Alternatively, the change may also be possible by exchanging the band pass filters and the reference oscillators according to the pilot frequency. However, the exchange of the filters and oscillators requires the shutdown of the system and requires excessive labor for the shutdown and exchange.

0012The problem of changing the pilot frequency also arises when the pilot frequency is unknown at the stage of shipping because the pilot frequency is not unique but is different if the satellite used or manner of usage threrof is different.

0013Furthermore, if the pilot frequency is not a typical value, an oscillator specially made to order must be used as the reference oscillator. The specially made oscillator is more expensive than the typical oscillators.

0014EP-A-0 199 887 relates to a FM radio receiver for weak signals which includes two phase-locked loops for reducing noise level. The receiver has a mixer and an oscillator circuit supplying an intemediate frequency signal to an intermediate frequency amplifier via a filter circuit. A phase-locked loop and a narrow bandpass filter are coupled to the junction between the intermediate frequency filter and the intermediate frequency amplifier, supplying a signal to a phase detector of a asecond phase-locked loop. Furthermore, a frequency divider is provided for selectively dividing the frequency of the input signal supplied thereto and for outputting a signal to the phase comparator.

SUMMARY OF THE INVENTION

0015It is an object of the present invention to provide an AFC circuit that can deal with a discrepancy or a change of a pilot frequency and oscillators or the provision of various filters and oscillators.

0016It is another object of the present invention to provide an AFC circuit, wherein a typical low cost oscillator can be used even when the pilot frequency is not of typical value.

0017In accordance with claim 1 of the present invention there is provided an automatic frequency control circuit supplied with input signals including information carrying signals and a pilot signal not carrying information, comprising: <ul id="ul0001" list-style="none" compact="compact"><li>frequency conversion means for converting the input signals into converted signals having frequencies different than the frequencies of the input signals by mixing a local oscillation signal with the input signals;</li><li>a first frequency divider for selectively dividing a frequency of the pilot signal by a first division number to thereby output a frequency-divided pilot signal;</li><li>oscillation means for outputting a reference signal having a reference frequency;</li><li>a first phase comparator for comparing a frequency and a phase of the reference signal output from the oscillation means with those of the frequency-divided pilot signal, to thereby output a comparison result;</li><li>a first variable oscillator for outputting the local oscillation signal having a frequency varying in accordance with the comparison result output from the first phase comparator to the frequency conversion means; and</li><li>means for setting the first division number at the first variable frequency divider.</li></ul>

0018The automatic frequency control circuit according to the present invention is further developed as described in the subclaims 2 to 5.

BRIEF DESCRIPTION OF THE DRAWINGS

0019<ul id="ul0002" list-style="none" compact="compact"><li>Figure 1 is a diagram showing an example of disposition of signals in a SCPC system;</li><li>Figure 2 is a block diagram showing a construction of an AFC circuit according to an embodiment of the present invention;</li><li>Figure 3 is a block diagram showing a construction of an AFC circuit as a modification of the AFC circuit of Fig. 2;</li><li>Figure 4 is a block diagram showing a construction of a unit 120 as a modification of the unit 120 of Fig. 3;</li><li>Figure 5 is a block diagram showing a construction of a unit 120 as another modification of the unit 120 of Fig. 3;</li><li>Figure 6 is a diagram for explaining an operation of the AFC circuit of Fig. 5;</li><li>Figure 7 is a block diagram showing a construction of an AFC circuit as another modification of the AFC circuit of Fig. 3; and</li><li>Figure 8 is a diagram for explaining an operation of the AFC circuit of Fig. 7.</li></ul>

DESCRIPTION OF THE PREFERRED EMBODIMENTS

0020Figure 1 shows an example of disposition of signals in a received signal of a SCPC system. As shown in Fig. 1, channel slots CH1 to Chn for channel 1 to channel n are assigned at constant intervals and a pilot signal from a reference station is transmitted in a frequency position within the band. In an AFC circuit, a frequency of a local oscillator is controlled so that a fequency of the received pilot signal is converted into a frequency equal to a fequency of a reference oscillator. Since frequencies of received signals of the channel 1 to the channel n fluctuate simultaneously with that of the received pilot signal, the fluctuations of the fequencies of the received signals are compensated for in the converted signals.

0021Figure 2 is a block diagram showing a construction of an AFC circuit according to an embodiment of the present invention, which deals with a discrepancy or change of the pilot frequency without exchange of the narrow band filters and the reference oscillators or provision of various filters and oscillators, and permits the use of a typical low cost oscillator as the reference oscillator.

0022The AFC circuit of Fig. 2 comprises a unit 120 for extracting the pilot signal and for converting a frequency of the extracted pilot signal. The unit 120 consists of a variable frequency divider 122, a read only memory 124 and a manual switch 126.

0023Generally, the level of the pilot signals is higher than that of modulated signals. Therefore, the variable frequency divider 122 selectively divides the frequency of the pilot signal by a division number N₁ by having a threshold level lower than the level of the pilot signal and higher than the level of the modulated signals. The division number N₁ of the variable frequency divider 122 is supplied from the read only memory 124 and the value N₁ read out from the read only memory 124 is controlled by the manual switch 126. After synchronism in the AGC loop is established, the following equation is realized<maths id="math0001" num="(3)"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">f</mtext></mrow><mrow><mtext>r</mtext></mrow></msub><mtext> = </mtext><msub><mrow><mtext mathvariant="italic">f</mtext></mrow><mrow><mtext>P</mtext></mrow></msub><msub><mrow><mtext>/N</mtext></mrow><mrow><mtext>1</mtext></mrow></msub></mrow></math><img file="EP0779710A2_D0001.tif" /></maths> i.e.,<maths id="math0002" num="(4)"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">f</mtext></mrow><mrow><mtext>P</mtext></mrow></msub><msub><mrow><mtext> = N</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><mtext>·</mtext><msub><mrow><mtext mathvariant="italic">f</mtext></mrow><mrow><mtext>r</mtext></mrow></msub></mrow></math><img file="EP0779710A2_D0002.tif" /></maths> Therefore, if the value of <i>f</i>_r is selected from among common divisors of various <i>f</i>_P's, the change of the pilot frequency <i>f</i>_P is realized only by changing the division number N₁ by switching the manual switch 126, and a typical value can be used as the <i>f</i>_r.

0024Figure 3 is a block diagram showing a construction of an AFC circuit as a modification of the AFC circuit of Fig. 2.

0025In the AFC circuit of Fig. 3, the unit 120 further comprises a PLL consisting of a phase comparator 128, a loop filter 130, a voltage controlled crystal oscillator 132, and a 1/N₂ frequency divider 134. The voltage controlled crystal oscillator has high frequency stability and a narrow variable range.

0026The output signal of the variable frequency divider 122 is fed to one input terminal of the phase comparator 128. An output signal of the phase comparator 128 is input through the loop filter 130 to the voltage controlled crystal oscillator 132 as a control signal therefor. An output signal of the voltage controlled crystal oscillator 132 is fed to one input of the phase comparator 18 as an output of the PLL. The output signal of the voltage controlled crystal oscillator 132 is also fed to the 1/N₂ frequency divider 134, whose output signal is fed to the other input terminal of the phase comparator 128.

0027After synchronism in the PLL is established, a frequency of the output signal of the voltage controlled crystal oscillator 132 becomes<maths id="math0003" num=""><math display="block"><mrow><msub><mrow><mtext>N</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msub><mrow><mtext>/N</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><mtext>·</mtext><msub><mrow><mtext mathvariant="italic">f</mtext></mrow><mrow><mtext>P</mtext></mrow></msub></mrow></math><img file="EP0779710A2_D0003.tif" /></maths> where N₂ is constant and N₁ is supplied from the read only memory 124 according to a setting in the switch 126.

0028After synchronism in the AGC loop is established, the following equation is realized<maths id="math0004" num="(5)"><math display="block"><mrow><msub><mrow><mtext mathvariant="italic">f</mtext></mrow><mrow><mtext>r</mtext></mrow></msub><msub><mrow><mtext> = N</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msub><mrow><mtext>/N</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><mtext>·</mtext><msub><mrow><mtext mathvariant="italic">f</mtext></mrow><mrow><mtext>P</mtext></mrow></msub></mrow></math><img file="EP0779710A2_D0004.tif" /></maths>

0029For example, suppose that the frequency and bandwidth of an input signal of a demodulator (the output signal of the frequency converter 10) are 140 MHz and 36 MHz, respectively, and that the frequency of the pilot signal is 155,025 MHz in the 140 MHz band in a normal state (in the case of "A" of Fig. 14). If the division number N₂ in the frequency divider 134 is 200, and a typical oscillator of 5 MHz (<i>f</i>_r = 5 MHz) is used as the reference oscillator 20, then the pilot signal can be caught by setting the division number N₁ to 6201, according to the equation (5). In this situation, frequencies of input signals of the phase comparator 128 are 25 kHz (= 155,025 MHz/6201).

0030When the pilot signal is changed to a spare signal having a frequency of 132.025 MHz (in the case of "B" of Fig. 14), the frequency of the signal supplied from the variable frequency divider 122 to the phase comparator 128 becomes 21.29 kHz (= 132,025 MHz/6201), and then the PLL and the AFC loop do not operate. In this case, by changing the division number N₁ from 6201 to 5281 for spare, the frequency of the output signal of the frequency divider 122 becomes 25 kHz (= 132,025 MHz/5281), and then the PLL and the AFC loop normally operates.

0031Figure 4 is a block diagram showing a construction of the unit 120 as a modification of the unit 120 of Fig. 3. The unit 120 of Fig. 4 comprises a controller 136. When the controller 136 detects a pull out signal output from the phase comparator 128, the controller 136 automatically changes the N₁ value from current to spare.

0032Figure 5 is a block diagram showing a construction of a unit 120 as another modification of the unit 120 of Fig. 3. The unit 120 of Fig. 16 further comprises a division number scanner 138 for scanning the division number N₁ of the variable frequency oscillator 122, and a scan controller 140 for controlling the division number scanner 138. When the scan controller 140 detects the pull out signal, the scan controller 140 commands the division number scanner 138 to scan the division number N₁ until the pull out signal vanishes. The division number scanner 138 scans the division number N₁ of the variable frequency divider 122 over a prescribed range at constant intervals in an increase or decrease direction.

0033Figure 6 is a diagram explaining an example of operations of the AFC circuit described with reference to Fig. 5. Suppose that a frequency and a bandwidth of an input signal of a demodulator are 140 MHz and 36 MHz, respectively, and that a frequency interval between channels is 50 kHz. If the pilot signal frequency is 132.025 MHz for a current and 155.025 MHz for a spare, the division number N₁ is set to 5281 in a normal state.

0034When the pilot signal frequency is changed to 155.025 MHz for some reason, the pull out signal is output from the phase comparator 128 because the AGC loop cannot follow up out of its variable range. The scan controller 140 detects the pull out signal and the division number N₁ is successively changed to 4963, 4965, 4967, ... by the division number scanner. The target frequency, which is capable of being caught by the AFC, is also changed to values such as 124.075, 124.125, 124.175, ..., according to the value of N₁. During this period, the phase comparator 128 continues outputting the pull out signal. When the division number N₁ becomes 6201, the target frequency becomes equal to the pilot signal frequency 155.025 MHz. Then the pilot signal is caught by the AFC, and after synchronism has been established, the pull out signal vanishes. As the pull out signal vanishes, the scan controller 140 commands the division number scanner 138 to stop scanning, and the AFC continues operating.

0035Figure 7 is a block diagram showing a construction of an AFC circuit as another modification of the AFC circuit of Fig. 3. The AFC circuit of Fig. 7 comprises a variable frequency divider 142 instead of the frequency divider 134 of Fig. 3. The read only memory 144 retains a division number N₂ for the variable frequency divider 142 as well as the division number N₁ for the variable frequency divider 122.

0036Suppose that the communication channels are disposed at channel intervals of 50 kHz and the pilot signal frequency is 155.025 MHz in a system 1, as shown in Fig. 6, and that the channel intervals are 45 kHz and the pilot signal frequency is 15.0845 MHz in a system 2, as shown in Figure 8. The frequency of the reference signal is 5 MHz.

0037In the system 1, the pilot signal having the frequency of 155.025 MHz can be caught by the AFC by setting as N₁ = 6201 and N₂ = 200 (the case of "A" in Fig. 18). Even when the frequency of the pilot signal is changed, the changed frequency can be dealt with by changing the target frequency by a unit of 50 kHz by changing the division number N₁. In the system 2, the pilot signal having the frequency of 150.845 MHz can be caught by the AFC by setting N₁ = 30169 and N₂ = 1000 (the case of "B" in Fig. 7). Even when the frequency of the pilot signal is changed, the changed frequency can be dealt with by changing the target frequency by a unit of 45 kHz by changing the division number N₁.

0038The replacement of the frequency divider 134 with the variable frequency divider 142 is also possible in the AFC circuits described with reference to Figs. 4 and 5. Furthermore, the change of the N₂ is possible in the manual and automatical process.