AUTOMATIC FREQUENCY SWEEPING METHOD FOR CONTROLLING A FREQUENCY SYNTHESIZING TYPE AUTOTUNER

Abstract: An automatic frequency sweeping method for controlling a frequency synthesizing type autotuners for broadcasting receivers comprises a first intermediate frequency counting method in which the intermediate frequency is counted for a relatively short first time and the presence or absence of the received signal is judged in a first range having a relatively wide intermediate frequency width and a second intermediate frequency counting method in which the intermediate frequency is counted for a second time longer than the first time and the presence or absence of the received signal is judged in a second range having an intermediate frequency width narrower than that of the first range. Thus, a high speed frequency sweeping is ordinarily executed by using the first intermediate frequency counting method, and when the presence of the received signal is judged in the first intermediate frequency counting method, a more precise judgment is made in accordance with the second intermediate frequency counting method.

Description: Background of the Invention Field of the invention The present invention relates to an automatic frequency sweeping method for controlling a frequency synthesizing type autotuners for broadcasting receivers, and more specifically to an intermediate frequency counting method for stopping the automatic frequency sweeping in the above mentioned method. Description of related art In general, broadcasting receivers incorporating therein frequency synthesizing type autotuners are so adapted to able to detect whether or not a broadcasting signal exists by counting an intermediate frequency. For example, in so-called FM band, assuming that a local oscillator is a lower side oscillator of the phase locked loop (PLL) type and a broadcasting signal exists at a frequency of 80MHz, if the lower side local oscillator generates an signal of 69.3 MHz, an intermediate frequency of 10.7MHz is obtained as a well known manner, and a result, it is possible to receive the broadcasting frequency signal of 80 MHz. At this time, if the oscillation frequency is deviated with 100 KHz, the intermediate frequency is also deviated with 100KHz from 10.7MHz. Therefore, taking into consideration the degree of modulation and noises of the broadcasting frequency, it is allowed to judge that a broadcasting frequency signal exists if the intermediate frequency is in the range of 10.7MHz±25KHz. Thus, in broadcasting receivers adapted to execute the automatic frequency sweeping in response to a key input, the automatic frequency sweeping operation is performed as follows. When the key input is given, the oscillation frequency is shifted by a frequency corresponding to one channel space by action of the PLL and associated controller. Then, after the PLL is locked, presence of the intermediate frequency is discriminated. If no broadcasting signal is detected, the oscillation frequency is shifted by the channel space again and presence of the intermediate frequency is discriminated. This operation is repeated, and when the broadcasting signal is judged to exist, the automatic frequency sweeping is stopped. For example, in the FM band of Japan, frequencies are allocated to FM broadcasting stations with frequency intervals of 100 KHz, and in the FM band of U. S. A., frequencies are allocated to FM broadcasting stations with frequency intervals of 200 KHz. Further, in Europe, frequencies are, as a general rule, allocated to FM broadcasting stations with frequency intervals of 100 KHz. Therefore, if a receiving frequency is changed (incremented or decremented) in accordance with the automatic frequency sweeping, the frequency of the local oscillator has to be changed with only an inter-station frequency (one channel space), and then, after the PLL is locked, a gate is opened for only 4 milliseconds so that the intermediate frequency signal is supplied to a so-called intermediate frequency counter. Thus, the intermediate frequency counter counts the intermediate frequency signal for a period of 4 milliseconds. Here, assuming that when the intermediate frequency is in the range of 10.7MHz±25KHz as mentioned above, it is judged that the broadcasting signal is present, the when the value of intermediate frequency counter is in the range of 42,800±100, the automatic frequency sweeping is stopped. If the value of the intermediate frequency counter is not in the above mentioned range, the frequency of the local oscillator is changed with 100KHz again, and the same operation is repeated. If the range for judging the presence of the broadcasting frequency is changed, it is possible to perform the automatic frequency sweeping in the AM band in the same manner. An automatic tuning system as mentioned above and which forms the basis of the preamble of claim 1 is disclosed in EP-A-0182 049. However, in fact, if the presence of the broadcasting signal is judged on the basis of only one counting of the intermediate frequency for 4 milliseconds, possibility of mis-judgment cannot be denied. Therefore, the counting operation of 4 milliseconds is repeated three times and if at least two of three counting operations are in the range mentioned above, it is then judged that the broadcasting signal is present. As seen from the above, the conventional automatic frequency sweeping has required at least 12 milliseconds for counting the intermediate frequency. Therefore, the sweeping from a minimum frequency channel to a maximum frequency channel will take a very long time, particularly in a band such as FM band including a number of channels. Summary of the Invention Accordingly, it is an object of the present invention to provide an automatic frequency sweeping method which has overcome the above mentioned defect of the conventional one. Another object of the present invention is to provide an automatic frequency sweeping method for controlling a frequency synthesizing type autotuner for broadcasting receivers, which is capable of certainly detecting a broadcasting frequency with a very short time and stopping the automatic frequency sweeping. The above and other objects of the present invention are achieved in accordance with the present invention by an automatic frequency sweeping method for controlling a frequency synthesizing type autotuner for broadcasting receivers, which tuner includes a phase locked loop for controllably generating a local oscillation signal, a mixer receiving a received RF signal and the local oscillation signal for generating an intermediate frequency signal, an intermediate frequency counter receiving the intermediate frequency signal through a gate, and a controller coupled to the phase locked loop, the intermediate frequency counter and the gate being also associated with a key input for instructing the automatic frequency sweeping, the controller operating to open the gate so as to allow the intermediate frequency counter to count the intermediate frequency and to stop the automatic frequency sweeping on the basis of presence or absence of the received signal judged with reference to the count value of the intermediate frequency counter, the method comprising a first step of intermediate frequency counting in which the intermediate frequency is counted during opening the gate for a relatively short first time period and the presence or absence of the received signal is judged in a first range having a relatively wide intermediate frequency width and a second step of intermediate frequency counting in which the intermediate frequency is counted during opening the gate for a second time period longer than the first time period and the presence or absence of the received signal is judged in a second range having a intermediate frequency width narrower than that of the first range, wherein a high speed frequency sweeping is ordinarily executed by using the first step of intermediate frequency counting, and when the presence of the received signal is judged in the first step of intermediate frequency counting, a more precise judgment is made in accordance with the second step of intermediate frequency counting. The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings. Brief Description of the Drawings Figure 1 a block diagram of a broadcasting receiver including a frequency synthesizing type autotuners for broadcasting receivers, capable of embodying an automatic frequency sweeping method in accordance with the present invention; Figure 2 a flow chart of illustrating one embodiment of the automatic frequency sweeping method in accordance with the present invention, which can be realized by the controller shown in the frequency synthesizing type autotuners shown in Figure 1; and Figure 3 a flow chart of illustrating another embodiment of the automatic frequency sweeping method in accordance with the present invention, which can be realized by the controller shown in the frequency synthesizing type autotuners shown in Figure 1. Description of the Preferred embodiments Referring to Figure 1, there is shown a block diagram of a broadcasting receiver including a frequency synthesizing type autotuner for broadcasting receivers, capable of embodying an automatic frequency sweeping method in accordance with the present invention. The shown receiver comprises an antenna 1 and a high frequency stage 2 having an input connected to the antenna 1 and adapted to execute a tuning and high frequency amplification. An output of the high frequency stage 2 is connected to a mixer 3, which is also connected to receive an output of a local oscillator 10, so that the mixer 3 generates an intermediate frequency signal representative of a difference between the frequency of the received signal from the high frequency stage 2 and the frequency of the oscillation signal of the local oscillator 10. The intermediate frequency signal is applied to an intermediate frequency amplifier 4 so that it is amplified by the intermediate frequency amplifier 4 whose output is connected to a detector 5. An output of the detector 5 is connected to a stereo demodulator 6 where an audio signal is reproduced. The audio signal is amplified by an low frequency amplifier 7 and then inputted to a loudspeaker 8. Further, the intermediate frequency signal is supplied through a gate 11 to an intermediate frequency counter 13 which can be formed of for example a shift register. A count output of the intermediate frequency counter 13 is coupled to a controller 16. On the other hand, the above mentioned local oscillator 10 is composed of for example a voltage controlled oscillator (VCO) and an output of the VCO 10 is connected not only to the mixer 3 but also to a programmable frequency divider 15. This frequency divider 15 has a frequency division ratio which is programmed or set by the controller 16. An output of the frequency divider 15 is one input of a phase comparator 12 whose other input is connected to receive an output of a reference frequency generator 14. In addition, an output of the phase comparator 12 is supplied through a low pass filter 9 to a VCO which constitutes the local oscillator 10. Thus, a phase locked loop is formed by the programmable frequency divider 15, the phase comparator 12, low pass filter 9 and the VCO 10. The controller 16 is also coupled to an indicator 17 and key input 18. In addition, if the controller 16 is composed of a single chip microcomputer, it can be understood to persons skilled in the art that the controller can control the associated elements, such as key input 18, programmable frequency divider 15, the gate 11, the intermediate frequency counter 13 and the indicator 17. Referring to Figure 2, there is shown a flow chart of illustrating the automatic frequency sweeping operation in accordance with the present invention, which can be realized by the controller 16 in response to a depression of a button in the key input 18 for an automatic frequency sweeping operation in an up direction. This embodiment is made to comply with the FM band of Japan and U. S. A. However, the FM band of Japan is different from FM band of U. S. A. in the channel spacing, as mentioned above. Therefore, operation will be first described on the receiving of the FM band of Japan. In Japan, the local oscillator 10 is of a lower oscillation for superheterodyne detection, and the intermediate frequency is 10.7MHz. In addition, the reference frequency generator 14 is set to generate a reference signal of 100 KHz. Under these condition, if one wishes to receive a FM broadcasting of 80MHz, the controller 16 operates to set the frequency division ratio of 693 to the programmable frequency divider 15, so that phase locked loop (PLL) composed of the programmable frequency divider 15, the phase comparator 12, the low pass filter 9 and the local oscillator 10 will generates the local oscillation signal of 69.3 MHz. Thus, the broadcasting signal of 80 MHz received by the antenna 1 and outputted from the high frequency stage 2 is converted by the mixer 3 to the intermediate frequency of 10.7MHz. As a result, the broadcasting signal of 80MHz can be listened. In this condition, if the button in the key input 18 for an automatic frequency sweeping operation in an up direction is depressed, the controller 16 operates as illustrated in the flow chart of Figure 2. In Step A, the programmable divider 15 is set to the frequency division ratio "694" since the channel space is 100 KHz in Japan. As a result, the oscillation frequency of the local oscillator 10 is incremented to 69.4MHz. The setting of the frequency division ratio needs about 10 milliseconds. In Step B, the receiver is put in a wait condition until the PLL is brought into a stabilized condition and therefore the stabilized intermediate frequency is outputted from the intermediate frequency amplifier 4. This wait time is about 10 milliseconds. In Step C, the gate 11 is opened for only 1 millisecond, so that the intermediate frequency of the intermediate frequency amplifier 4 is counted by the intermediate frequency counter 13 for only 1 millisecond, and the count value of the counter is read out and written into the controller 16. This operation is repeated three times. In Step D, the controller 16 decides whether or not at least two of the three intermediate frequency count values written into the controller 16 in the process of Step C are in the range of 10,700±45. If two of the three are in this range, it goes to Step E. If not so, Steps A, B, C and D are repeated again. The operation of the above mentioned Steps B and C is called a "first intermediate frequency counting method". The intermediate frequency count value of 10,700±45 means that the intermediate frequency is in the range of 10.7MHz±45KHz. Since the gate 11 is opened for only a very short time such as 1 milliseconds, a detection is made in a wide range such as 10.7MHz±45KHz in order to prevent a misjudgment. Here, assuming that a broadcasting frequency upwardly adjacent to the broadcasting frequency of 80MHz is 82.5 MHz, the controller 16 repeats the operation of the Steps A, B, C and D 25 times. In the 25th operation, the local oscillator 10 generates the oscillation signal of 71.8MHz and therefore the broadcasting signal of 82.5MHz is converted in the intermediate frequency signal of 10.7MHz. As a result, the controller 16 will decide in the Step D that at least two of the three intermediate frequency count values are in the range of 10,700±45, and therefore, it goes to Step E. In the Step E, similarly to the Step C, the gate 11 is opened and the intermediate frequency signal is counted by the intermediate frequency counter 13 and then result of the counting is read out and written into the controller 16. This operation is repeated three times. But, the the open time of the gate 11 in the step E is 4 milliseconds. In the next Step F, the controller 16 decides whether or not at least two of the three intermediate frequency count values written in the process of Step C are in the range of 42,800±100. If two of the three are in this range, it is judged that the broadcasting frequency is present and therefore the automatic frequency sweeping operation is stopped. If not so, the detection of the broadcasting signal in accordance with the first intermediate frequency counting method composed of the steps C and D has been in error and the operation is returned to the Step A, again. The intermediate frequency count value of 42,800±100 means that the intermediate frequency is in the range of 10.7MHz±25KHz. The above step has an open time (4 milliseconds) of the gate 11 greater than the first intermediate frequency counting method and detects the presence and absence of the broadcasting in a range narrower than that of the first intermediate frequency counting method. Namely, a more precise intermediate frequency counting is executed. The operation of the above mentioned Steps E and F is called a "second intermediate frequency counting method". As mentioned above, the first and second intermediate frequency counting methods will permit that in the case of absence of a broadcasting signal, the time required for incrementing or decrementing the frequency of a received signal by one channel space is shortened by about 10 milliseconds as compared with the conventional method. In the above mentioned example, about 250 milliseconds are reduced in total. Next, the operation for receiving the FM band of U. S. A. will be described. As mentioned above, the channel space of U. S. A. practice, the channel space is 200 KHz, and the local oscillator 10 is of an upper oscillation. Further, the intermediate frequency is 10.7MHz. In addition, the reference frequency generator 14 is set to generate a reference signal of 100 KHz. Under these condition, now assume that a FM broadcasting of 88.9MHz is received and therefore the frequency division ratio of 693 is set to the programmable frequency divider 15, so that the local oscillator 10 will generates the local oscillation signal of 99.6 MHz. In this condition, if the button in the key input 18 for an automatic frequency sweeping operation in an up direction is depressed, the controller 16 operates in the Step A to increment the frequency division ratio "694" set in the programmable divider 15 by 2, namely to "998". As a result, the oscillation frequency of the local oscillator 10 is incremented by 200KHz. Thereafter, the first intermediate frequency counting method composed of the Steps B, C and D is executed for detection of the broadcasting signal. Thus, the operation of the Steps A, B, C and D is repeated 12 times while incrementing the local oscillation frequency by 200KHz. In the twelfth operation, if it is judged that at least two of the three intermediate frequency count values are in the range of 10,700±45, it goes to Step E. At this time, the programmable frequency divider 15 is set to "1020" (namely, the oscillation frequency of the local oscillator 10 has adjusted to 102.0MHz). Therefore, it is judged that a broadcasting signal is present in the channel of 91.3MHz, and accordingly the automatic frequency sweeping operation is stopped. But, if at least two of the three intermediate frequency count values are out of the range of 10,700±45, the operation is returned to the Step A. The operation the Steps E and F is the same as that mentioned above in connection with the Japanese FM band receiving operation. Turning to Figure 3, there is shown a flow chart of illustrating the automatic frequency sweeping operation in accordance with the present invention, which can be realized by the controller 16 in response to a depression of a button in the key input 18 for an automatic frequency sweeping operation in an up direction, but which is made to comply with the FM band of Europe. As mentioned above, a high speed and precise automatic frequency sweeping can be executed by using the first and second intermediate frequency counting methods. As a general rule, the channel space of the European FM band is 100KHz, but locally 25KHz or 50KHz. Therefore, it is desirable to able to receive a broadcasting signal shifted by only 25KHz or 50KHz. Accordingly, if the frequency changing (incrementing/decrementing) step width of the local oscillator 10 is set to 25KHz, the automatic frequency sweeping can be executed in the flow chart shown in Figure 2 (where the frequency changing amount is changed from 100KHz or 200KHz to 25KHz). However, the automatic frequency sweeping of this method will require a period of time four times in the area where the channel space is 100KHz. On the contrary, for example, if the frequency changing step width of the local oscillator 10 is set to 100KHz, when the automatic frequency sweeping is started from 90.125MHz, the received frequency is changed in the order of 90.225MHz, 90.325MHz, ..... . As a result, it is not possible to receive or precisely tune a broadcasting signal in an area in which the channel space is 100KHz. Incidentally, in the European FM band, the local oscillator is of the upper oscillation and the intermediate frequency is 10.7MHz. In addition, the reference frequency generator 14 is set to generate the reference frequency of 25KHz in the following embodiment. Now, assume that the broadcasting signal of 99.0MHz is received, the programmable frequency divider 15 is set to the frequency division ratio of "4388". In this condition, if a button in the key input 18 for an automatic frequency sweeping operation in an up direction is depressed, the controller 11 operates as follows in accordance with the flow chart of Figure 3. In Step G, the programmable divider 15 is incremented by "1" and therefore set to the frequency division ratio "4389". As a result, the oscillation frequency of the local oscillator 10 is incremented by 25KHz. In Step H, the receiver is put in a wait condition until the PLL is brought into a stabilized condition and therefore the stabilized intermediate frequency is outputted from the intermediate frequency amplifier 4. In the succeeding Step I, the gate 11 is opened for 4 milliseconds, so that the intermediate frequency of the intermediate frequency amplifier 4 is counted by the intermediate frequency counter 13, and the count value of the counter is read out and written into the controller 16. This operation is repeated three times. In Step J, if at least two of the three intermediate frequency count values written into the controller 16 in the process of Step J are in the range of 42,800±50, it is judged that the broadcasting signal is present and therefore the automatic frequency sweeping is stopped. If not so, it goes to Step K. In Step K, there is judged whether or not the oscillation frequency of the local oscillator 10 is "XX.X75MHz". If so, it goes to the step L. But, if not so, it returns to the Step G again. This detection of "XX.X75MHz"can be easily executed by checking the frequency division ratio in the programmable frequency divider 15. Now, assuming that the frequency of a broadcasting signal upwardly adjacent to the broadcasting signal of 99.000MHz is 101.125MHz, the controller 16 repeats the operation of the Steps G, H, I, J and K three times and goes to Step L. In the course of the three operations of the Steps G, H, I, J and K, the oscillation frequency of the local LSC 10 is sequentially incremented from 109.700MHz in the order of 109.725MHz, 109.750MHz and 109.775MHz. In addition, whether or not the intermediate frequency is in the range of 10.7MHz±12.5KHz is determined. Furthermore, the open time of the gate 11 is set to a relatively long time such as 4 milliseconds and the presence of the intermediate frequency is judged in a narrower range. Accordingly, this intermediate frequency counting method can be called the "second intermediate frequency counting method". In Step L, the programmable divider 15 is further incremented by "1" and therefore the oscillation frequency of the local oscillator 10 is incremented from 109.775MHz to 109.800MHz. In the next Step M, the operation is waited, similarly to the Step H. In Step N, the gate 11 is opened for 1 millisecond, so that the intermediate frequency of the intermediate frequency amplifier 4 is counted by the intermediate frequency counter 13, and the count value of the counter is read out and written into the controller 16. This operation is repeated three times. In Step O, there is judged whether or not at least two of the three intermediate frequency count values written into the controller 16 in the process of Step O are in a predetermined range. In this embodiment, the range in which the presence or absence of the intermediate frequency is detected is from 10,687 to 10,788, namely 10,700 ± 13 / 88. The intermediate frequency count value in the range of 10,700± 13 / 88 means that the intermediate frequency is in the range of 10.612MHz to 10.713MHz. With this feature, if the oscillation frequency of the local oscillator 10 is at 109.800MHz and if a broadcasting signal exists at the frequency of 99.175MHz, the intermediate frequency becomes 10.625KHz which can be detected in the above mentioned range. Namely, the intermediate frequency detection can be executed until a frequency higher than the center frequency of the intermediate frequency by 75KHz or 3 channel spaces. In the Step O, if no broadcasting signal is detected, it goes to Step P where the frequency division ratio set in the programmable frequency divider 15 is incremented by only "4" so that the oscillation frequency of the local oscillator 10 is incremented by 100KHz. Thus, the automatic frequency sweeping composed of the steps P, M, N and O is repeatedly executed. But, the range for judging the presence or absence of the broadcasting signal is set as mentioned above, a broadcasting signal which is allocated with the frequency space of 25KHz or 50KHz can be detected. As mentioned hereinbefore, if the next broadcasting signal is of 101.125MHz, the automatic frequency sweeping operation composed of the steps P, M, N and O is repeated 21 times and as a result, the oscillation frequency of the local oscillator 10 is brought into 111.8MHz and the intermediate frequency is put to 10.675MHz. Accordingly, the presence of the broadcasting signal is detected in the step O. The above mentioned intermediate frequency counting method in the step O is such that the open time of the gate is relatively very short (1 millisecond) and the range for judging the presence or absence of the broadcasting signal is relatively wide. Therefore, this method is called the "first intermediate frequency counting method". Even if the presence of the broadcasting signal is detected in the step O, it is not actually sure where the broadcasting signal is in the range of 101.100MHz to 101.175MHz. Therefore, the operation is returned to the steps I, J, K, G and H so that the presence or absence of the broadcasting signal is executed for every 25KHz. Thus, the broadcasting signal of 111.125MHz can be received. As mentioned above, when no broadcasting signal is present, the detection for a broadcasting signal is executed in accordance with the first intermediate frequency counting method composed of the steps M, N, O and P, while changing (incrementing or decrementing) 100 KHz which corresponds to four channel spaces in an area having the channel space of 25KHz. Therefore, the automatic frequency sweeping can be carried out at a speed of four times. As a result, when it is judged in the first intermediate frequency counting method that the broadcasting signal is present, the second intermediate frequency counting method composed of the steps I, J, K, G and H is executed so that a more precise tuning can be achieved and also it is possible to receive a broadcasting signal shifted by 25KHz unit or 50 KHz unit. In addition, even if the received frequency has been for example 90.125MHz when the automatic frequency sweeping starts, the received frequency is shifted to 90.200MHz by advancing to the step L, and thereafter, the automatic frequency sweeping can been executed by a 100KHz step. Therefore, a broadcasting signal can be received in an area having the channel space of 100KHz. As mentioned above, in a frequency area in which when no broadcasting signal is present, a high speed frequency sweeping is executed in accordance with the first intermediate frequency counting method having a short frequency counting time and a wide intermediate frequency count range for detecting the presence of the broadcasting signal, and thereafter, when it is judged in the first intermediate frequency counting method that the broadcasting signal is present, a more precise sweeping is executed in accordance with the second intermediate frequency counting method having a long frequency counting time and a narrow intermediate frequency count range for detecting the presence of the broadcasting signal. In addition, the frequency counting time and the intermediate frequency count range can be controlled by the controller and therefore, it is easy to execute the auto frequency sweeping with a frequency counting time and an intermediate frequency count range which are different from those mentioned above, respectively. Therefore, it is possible to perform the automatic frequency sweeping not only in the FM band but also in the AM band. Furthermore, when the automatic frequency sweeping is made in a frequency down direction, it is possible to execute the sweeping by decrementing 100KHz (or 200KHz), 25KHz and 100KHz in the steps A of Figure 2 and in the steps G and P of Figure 3, respectively. Thus, the broadcasting signal selection operation can be hastened, and therefore, a desired broadcasting station can be searched in a short time. Particularly, when an automobile is on the run, receivable broadcasting signals are often replaced. In such case, the high speed frequency sweeping is very convenient not only since it is possible to quickly find out a desired broadcasting signal, but also it is possible to reduce a driver's load in the case that the selection operation is repeated many times.

Claims: An automatic frequency sweeping method for controlling a frequency synthesizing type autotuner for broadcasting receivers, which tuner includes a phase locked loop for controllably generating a local oscillation signal, a mixer (3) receiving a RF signal and the local oscillation signal for generating an intermediate frequency signal (IF), an intermediate frequency counter (13) receiving the intermediate frequency signal through a gate (11) and a controller (16) coupled to the phase locked loop, the intermediate frequency counter (13) and the gate (11) being also associated with a key input (18) for instructing the automatic frequency sweeping, the controller (16) operating to open the gates (11) so as to allow the intermediate frequency counter (13) to count the intermediate frequency and to stop the automatic frequency sweeping on the basis of presence or absence of the received signal judged with reference to the count value of the intermediate frequency counter (13), said method being characterized by comprising a first step of intermediate frequency counting in which the intermediate frequency is counted during opening the gate for a relatively short first time period and the presence or absence of the received signal is judged in a first range having a relatively wide intermediate frequency width, and a second step of intermediate frequency counting in which the intermediate frequency is counted during opening the gate for a second time period longer than the first time period and the presence or absence of the received signal is judged in a second range having a intermediate frequency width narrower than that of the first range, wherein a high speed frequency sweeping is ordinarily executed by using the first step of intermediate frequency counting and when the presence of the received signal is judged in the first step of intermediate frequency counting, a more precise judgment is made in accordance with the second step of intermediate frequency counting. An automatic frequency sweeping method as claimed in claim 1, wherein the intermediate frequency is 10.7 MHz, the first time period is 1 ms and the first range is 10,700±45, so that when the count value of the intermediate frequency counter (13) is judged to be out of the first range in the first step of intermediate frequency counting, the local oscillation frequency is changed by 100 kHz or 200 kHz. An automatic frequency sweeping method as claimed in claim 2, wherein in the step of intermediate frequency counting the count value of the intermediate frequency counter relating to the first range is sampled three times, and it is then judged that the count value of the intermediate frequency counter is in the first range if at least two of the three samples indicate that the count value of the intermediate frequency counter was in the first range. An automatic frequency sweeping method as claimed in claim 3, wherein the second time period is 4 ms and the second range is 42,800±100 so that when the count value of the intermediate frequency counter is judged to be out of the second range in the second step of intermediate frequency counting, the operation is returned to the first step of intermediate frequency counting. An automatic frequency sweeping method as claimed in claim 4, wherein in the second step of intermediate frequency counting the count value of the intermediate frequency counter relating to the second range is sampled three times and it is then judged that the count value of the intermediate frequency counter is in the second range if at least two of the three samples indicate that the count value of the intermediate frequency counter was in the second range. An automatic frequency sweeping method as claimed in claim 1, wherein the second step of intermediate frequency counting is executed at the start of the automatic frequency sweeping until the intermediate frequency is changed to XX.X75MHz, and thereafter, the first step of intermediate frequency counting is executed, so that when the count value of the intermediate frequency counter is judged to be out of the first range, the local oscillation frequency is changed by 100 kHz. An automatic frequency sweeping method as claimed in claim 6, wherein the intermediate frequency is 10.7MHz, the first time period is 1 ms and the first range starts from 10,700+13 and terminates at 10,700-88. An automatic frequency sweeping method as claimed in claim 7, wherein in the first step of intermediate frequency counting the count value of the intermediate frequency counter relating to the first range is sampled three times, and it is then judged that the count value of the intermediate frequency counter is in the first range if at least two of the three samples indicate that the count value of the intermediate frequency counter was in the first range. An automatic frequency sweeping method as claimed in claim 8, wherein the second time period is 4 ms and the second range is 42,800±50, so that when the count value of the intermediate frequency counter is judged to be out of the second range in the second step of intermediate frequency counting, the operation is returned to the first step of intermediate frequency counting. An automatic frequency sweeping method as claimed in claim 9, wherein in the second step of intermediate frequency counting the count value of the intermediate frequency counter relating to the second range is sampled three times, and it is then judged that the count value of the intermediate frequency counter is in the second range if at least two of the three samples indicate that the count value of the intermediate frequency counter was in the second range. An automatic frequency sweeping method as claimed in claim 1, wherein the intermediate frequency is 10.7MHz and wherein the first and second time periods are 1 ms and 4 ms, respectively, and wherein the first range starts from 10,700+13 and terminates at 10,700-88 and the second range is 42,800±50, wherein the second step of intermediate frequency counting is executed at the start of the automatic frequency sweeping while changing the local oscillation frequency by 25KHz until the intermediate frequency is changed to XX.X75MHz, and thereafter the first step of intermediate frequency counting is executed while changing the local oscillation frequency by 100KHz until the count value of the intermediate frequency counter is in the first range, and then the second step of intermediate frequency counting is executed again while changing the local oscillation frequency by 25KH7 until the presence of the received signal is detected.

Inventors: ICHIKAWA, MASAOMI
Applicants: NEC CORPORATION
Application year: 1988
Main classification: H03J5
IPC codes: H03J7, H03J1