JP2010213080A - Fm receiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent a band limit malfunction at a high degree of modulation while maintaining immunity to interference. <P>SOLUTION: This FM receiver is provided with: a second mixer 14 for converting an FM broadcasting signal into an intermediate frequency signal; an IF bandpath filter 15 connected to the latter part of the second mixer 14; an FM detector circuit 16 for detecting the intermediate frequency signal which has passed the bandpath filter; S meter circuits 22 and 23 for detecting a signal level of the FM broadcasting signal; and a control circuit 25 for controlling a passing band based on AC components superposed on an output signal of the S meter circuit 23 when the signal level of the FM broadcasting signal is small. The FM receiver is also provided with a load switching circuit 31 for changing a level value of the AC components to a direction in which the passing band of the IF bandpath filter 15 does not switch to a narrow band under a situation where disturbance waves do not exist, in response to a combination of the degree of modulation and the presence or absence of the disturbance waves. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an FM receiver that receives a frequency-modulated signal.

  Since the FM signal changes the frequency of the carrier wave based on an audio signal or the like, it requires a wider frequency band than the AM signal. For this reason, when the FM receiving apparatus receives a target FM signal, there is a possibility that the adjacent signal transmitted near the frequency (hereinafter referred to as “adjacent interference”) is received. Therefore, the adjacent interference is reduced by switching the bandpass filter for extracting the target FM signal to a narrow band.

  FIG. 4 is an overall configuration diagram of a conventional FM receiver. The FM receiver shown in FIG. 1 converts the FM signal into an IF frequency (for example, 10.7 MHz) by the first mixer 11 and inputs it to the second mixer 14 via the IF amplifier 12 and the IF filter 13. . After the IF frequency is converted to a lower IF frequency (for example, 450 kHz) in the second mixer 14, the frequency is selected by the band-variable IF bandpass filter 15 and then detected by the FM detection circuit 16. The signal demodulated by the detection circuit 16 is amplified by the mute circuit 17 and output as an audio signal. The band-variable IF bandpass filter 15 is switched between a narrow band and a wide band by a band control system circuit 20 described later. The AFC circuit 18 is composed of a circuit for detecting a variation in the local oscillation frequency and a circuit for controlling the oscillation frequency. The AFC circuit 18 detects and integrates the IF signal to obtain a DC voltage (AFC voltage) corresponding to the deviation from the IF. Thus, the correct IF is corrected by controlling the local oscillation frequency in the direction of correcting the AFC voltage.

  The FM receiver described above includes a band control system circuit 20 that switches the limited band of the IF bandpass filter 15 between a wide band and a narrow band for the purpose of improving the exclusion capability when adjacent interference occurs. In the band control system circuit 20, the path where the switching circuit 21 performs band control according to the electric field strength is switched to the path A or the path B. A voltage linearly proportional to the electric field strength generated by the S meter circuit (DC) 22 connected to the subsequent stage of the IF filter 13 is output to the switching circuit 21. The switching circuit 21 selects the path A if it is less than the medium electric field, and selects the path B if it is greater than or equal to the medium electric field.

  Here, in the path A, the AC component superimposed on the output signal of the S meter circuit (AC) 23 is extracted by the high pass filter 24 and output to the switching circuit 21. When the path A is selected because the electric field is less than the medium electric field, the AC component is supplied to the control circuit 25 via the switching circuit 21. The control circuit 25 controls the limited band of the IF bandpass filter 15 between a wide band and a narrow band according to the frequency of the AC component. In the path B, the noise amount of the modulation component output from the mute circuit 17 is extracted by the low-pass filter 26 and output to the switching circuit 21. When the path B is selected for the medium electric field or more, a noise component is given to the control circuit 25 via the switching circuit 21. The control circuit 25 determines that the noise component is adjacent interference, and controls the limited band of the IF bandpass filter 15 to a narrow band.

  As described above, in the conventional FM receiver, if the electric field strength is equal to or higher than the medium electric field, it is determined that adjacent interference occurs, and the adjacent band is removed by controlling the limited band of the IF bandpass filter 15 to a narrow band. It was.

JP 2008-199268 A

  However, in the case of the conventional FM receiver, since the noise component shows the same tendency when the modulation degree is high and when adjacent interference occurs, the limited band is switched to a narrow band even though adjacent interference does not occur. There is a problem that distortion increases.

  As shown in FIGS. 5 (a) and 5 (b), when there is no adjacent disturbance, the signal (AC component) has a threshold of the electric field strength until the modulation degree is medium (75.0k) in the region below the intermediate electric field. The value is not exceeded. On the other hand, as shown in FIG. 5 (d), when interference occurs, the AC component exceeds the sensitivity threshold value of the electric field strength, so that the band is switched to a narrow band. However, as shown in FIG. 5C, even when there is no adjacent interference, if the degree of modulation increases (130.0 k), the AC component exceeds the sensitivity threshold of the electric field strength. As is apparent from a comparison between FIG. 5C and FIG. 5D, the signal component when the modulation degree is high and the noise component when the adjacent disturbance occurs show the same tendency and are difficult to discriminate. As a countermeasure, it is conceivable to lower the sensitivity threshold value. However, this causes a problem that the sensitivity threshold value is low even when adjacent interference occurs, so that the narrow band is not switched.

  FIG. 6 shows distortion characteristics in a narrow band and a wide band during high modulation (130 k dev). It can be seen that the distortion characteristics are extremely deteriorated when the electric field strength is reduced in a narrow band. Therefore, in the region below the medium electric field, distortion is increased if the band is controlled to a narrow band despite no adjacent interference.

  The present invention has been made in view of this point, and an object of the present invention is to provide an FM receiver that can prevent a band limiting malfunction at a high modulation degree while maintaining the interference elimination capability.

  An FM receiver of the present invention includes a frequency conversion circuit that converts a received FM broadcast signal into an intermediate frequency signal, and a bandpass filter that is connected to a subsequent stage of the frequency conversion circuit and that allows the passband to be changed and allows the intermediate frequency signal to pass. A detection circuit for detecting the intermediate frequency signal that has passed through the bandpass filter, a level detection circuit for detecting the signal level of the FM broadcast signal, and when the signal level of the FM broadcast signal is smaller than a predetermined value, A control circuit for controlling a pass band of the band-pass filter based on an AC component superimposed on an output signal of the level detection circuit; a modulation degree detection circuit for detecting a modulation degree of the FM broadcast signal from the detection signal; An interference wave detection circuit for detecting presence / absence of an interference wave with respect to the FM broadcast signal from the detection signal, a detected modulation degree, and presence / absence of the interference wave Depending on the combination, in a situation where there is no interference wave in a direction not switch the passband of the bandpass filter to a narrow band, characterized by comprising a change circuit for shifting the level value of the AC component.

  According to this configuration, in accordance with the combination of the detected modulation degree and the presence / absence of the jamming wave, in a situation where no jamming wave exists, the passband of the bandpass filter is not switched to a narrow band. Since the level value is shifted, it is possible to prevent a band limiting malfunction in which the pass band of the band pass filter is switched to a narrow band with a high modulation degree, and in the situation where an interference wave exists, it is also possible to return the AC component level value. It is also possible to maintain the interference elimination capability that eliminates the interference wave.

  According to the present invention, in the FM receiver, the change circuit has a high level in which the output signal of the modulation degree detection circuit indicates “high modulation degree”, and the output signal of the interference wave detection circuit has “no interference wave”. The level value of the AC component is changed to a side where a wide band is set in the band pass filter.

  With this configuration, if the combination of “high modulation degree” and “no interference wave” is used, the level value of the AC component is shifted to the side where the wide band is set in the bandpass filter. Therefore, it is possible to apply a wide band limitation and suppress the occurrence of distortion.

  According to the present invention, in the FM receiver, the change circuit has a high level in which the output signal of the modulation degree detection circuit indicates “high modulation degree”, and the output signal of the interference wave detection circuit indicates “there is an interference wave. The level value of the AC component is changed to a side where a narrow band is set in the band pass filter.

  With this configuration, the level value of the AC component is changed to the side where the narrow band is set in the band-pass filter by looking at the combination of “high modulation degree” and “with interference wave”, so that it is difficult to discriminate with the conventional method. Also in the case, the interference wave can be appropriately eliminated without taking a countermeasure for lowering the sensitivity threshold fixedly for preventing the band limitation malfunction for the “high modulation degree”.

  In the FM receiver, the change circuit is configured to be capable of switching a load resistor that shifts the level value of the AC component to a higher side with respect to an output stage of the level detection circuit, between a connected state and a disconnected state, When the output signal of the modulation degree detection circuit is at a high level and the output signal of the interference wave detection circuit is at a low level, the load resistor is connected, and the output signal of the modulation degree detection circuit is at a high level, In addition, when the output signal of the jamming wave detection circuit is at a high level, the load resistor can be disconnected.

  In the FM receiver, when the signal level of the FM broadcast signal is higher than a predetermined value, a control signal based on the output signal of the detection circuit and the output signal of the jamming wave detection circuit is input to the control circuit. It is good also as composition to do.

  ADVANTAGE OF THE INVENTION According to this invention, the FM receiver which can prevent the band-limiting malfunction by a high modulation degree can be provided, maintaining interference exclusion capability.

The block diagram of the FM receiver which concerns on one embodiment of this invention Configuration diagram of load switching circuit in the above embodiment The figure which shows the matrix for the switching determination in the load switching circuit in the said embodiment Configuration diagram of a conventional FM receiver The figure which shows the relationship between modulation intensity and electric field strength at the time of disturbance occurrence and noise comparison Diagram showing distortion characteristics in wide and narrow bands

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a configuration diagram of an FM receiver according to an embodiment of the present invention. The same parts as those of the FM receiver shown in FIG. In the present embodiment, in the path A, the load that shifts the output signal superimposed with the AC component to the high level side as a whole with respect to the output stage of the high-pass filter 24 is configured to be freely loaded or removed. That is, a switching circuit 31 that connects or disconnects a load resistor to the output stage of the high-pass filter 24 and a determination unit 32 that instructs the switching circuit 31 to connect / disconnect the load resistor are provided.

FIG. 2 shows a conceptual diagram of the switching circuit 31 and the determination unit 32.
The switching circuit 31 includes a load resistor 31a having one end connected to the transmission line L of the path A, and a switch 31b provided between the other end of the load resistor 31a and the ground. By connecting the load resistor 31a to the output stage of the high-pass filter 24, it is possible to shift the wideband output signal including the AC component of the high-pass filter 24 to the high level side. That is, when the switch 31b is turned on, the output signal including the AC component of the high pass filter 24 is shifted to the high level side, and when the switch 31b is turned off, the output signal of the high pass filter 24 is shifted to the low level side.

  The determination unit 32 includes an adjacent disturbance detection unit 32a that detects the presence or absence of adjacent interference based on the AFC voltage output from the AFC circuit 18, and a modulation degree detection unit 32b that detects the modulation degree from the audio signal output from the mute circuit 17. And a determination logic unit 32c that determines load switching from the adjacent interference detection result and the modulation degree detection result. The determination logic unit 32c determines the connection / disconnection of the load corresponding to the combination of the adjacent interference detection path status (Low / High) and the modulation degree detection path status (Low / High) shown in FIG. ing. The switching circuit 31 and the determination logic unit 32c constitute a change circuit that shifts the level value of the AC component.

  As shown in FIG. 3, in the situation where there is no adjacent interference, the AFC circuit 18 has the AFC voltage within a predetermined range on the plus side or minus side near 0 at the tuning point (low level range). On the other hand, in the situation where the adjacent disturbance occurs, the AFC voltage becomes a high level range where the AFC voltage is outside the predetermined range on the plus side or the minus side at the tuning point.

  The adjacent disturbance detection unit 32a determines that the adjacent disturbance detection path status is “None” if the AFC voltage is in a low level range near 0 where the AFC voltage is within a predetermined range on the plus side or the minus side at the tuning point ( Low determination). Further, if the AFC voltage is in a high level range that is outside the predetermined range on the plus side or the minus side at the tuning point, the adjacent disturbance detection path status is determined to be “present” (high determination).

  Further, as shown in FIG. 3, the peak value of the sine wave audio signal output from the mute circuit 17 falls within a predetermined range (low level range) up to a certain degree of modulation. On the other hand, if the modulation degree exceeds a certain numerical value and becomes overmodulated, the peak value does not fall within a predetermined range (high level range).

  If the modulation degree detected from the peak value of the audio signal is a low level range within a predetermined range, the modulation degree detection unit 32b determines that the modulation degree detection path situation is a low modulation degree (Low determination). Further, if the modulation degree is in a high level range that is outside the predetermined range, the modulation degree detection path status is determined to be a high modulation degree (High determination).

  The determination logic unit 32c forms a determination logic for load switching by combining the detection result of the adjacent disturbance detection path situation and the detection result of the modulation degree detection path situation. That is, as shown in FIG. 3, when the adjacent disturbance detection path situation is Low determination and the modulation degree detection path situation is Low determination, and the adjacent disturbance detection path situation is Low determination, and the modulation degree detection path situation is High. In the case of determination, it is determined as “load connection”. In particular, even if the modulation degree detection path state is High determination (high modulation degree), if the adjacent disturbance detection path state is Low determination, the load resistor 31a is inserted between the output terminal of the high pass filter 24 and the ground. Thus, the wideband output signal including the AC component is shifted to the high level side. In addition, when the adjacent disturbance detection path status is High determination and the modulation degree detection path status is Low determination, and when the adjacent disturbance detection path condition is High determination and the modulation degree detection path status is High determination, It is determined as “load connection”. In particular, even when the modulation degree detection path status is Low determination, when the adjacent interference detection path status is High determination and adjacent interference has occurred, the ground side of the load resistor 31a is opened to include the AC component. The wideband output signal is shifted to the low level side.

  Next, the band switching operation of the IF bandpass filter 15 in the FM receiver configured as described above will be described. A voltage that is linearly proportional to the input electric field strength is input to the switching circuit 21 from the S meter circuit (DC) 22. The switching circuit 21 determines whether or not the current electric field strength is at the middle electric field strength level based on the output voltage of the S meter circuit (DC) 22, and selects the path A if the electric field strength is weaker than the middle electric field strength. If it is strong, route B is selected. When the switching circuit 21 selects the path A, the control circuit 25 compares the AC component superimposed on the output signal output from the high-pass filter 24 of the path A with the sensitivity threshold value to limit the IF bandpass filter 15. Switch the band. Specifically, if the AC component does not exceed the electric field strength sensitivity threshold, a wide band is set, and if the AC component exceeds the electric field strength sensitivity threshold, the band is switched to the narrow band.

  At this time, the output signal of the path A input to the control circuit 25 is corrected by the load switching circuit 31 to shift to the strong electric field strength side or the low electric field strength side according to the determination logic shown in FIG. In the situation where the adjacent interference detection path status is Low determination and the adjacent interference is not generated, the determination logic unit 32c displays the load input ON determination result when the modulation degree detection path status is Low determination and the modulation degree is small. The load resistor 31a is inserted between the output terminal of the high-pass filter 24 and the ground, and the wideband output signal including the AC component is shifted to the high level side.

  In the present embodiment, even when it is detected that the modulation degree detection path situation is a high modulation degree by High judgment, the adjacent disturbance detection path situation is Low judgment and no adjacent disturbance has occurred. If there is, the determination result of turning on the load is output to the switch 31b. As a result, the load resistance 31a is inserted between the output terminal of the high-pass filter 24 and the ground even in a high modulation degree state, and the wideband output signal including the AC component is shifted to the high level side. In the control circuit 25 to which the signal of the path A is input, the output signal shifts to a high level side, so that the electric field strength sensitivity threshold is lowered, and the high modulation degree signal has the sensitivity threshold. It is possible to prevent switching to a narrow band beyond this.

  Thus, even in the case shown in FIGS. 5C and 5D, which is difficult to discriminate with the conventional method, the sensitivity threshold value is lowered only in the case shown in FIG. Can be prevented from switching to a narrow band.

  In addition, the determination logic unit 32c determines that the load is turned off if the adjacent disturbance detection path status is High and the adjacent interference is generated, even if the modulation degree detection path status is Low and the modulation degree is low. The result is output to the switch 31b, the load resistor 31a connected to the output terminal of the high pass filter 24 is opened, and the wideband output signal including the AC component is shifted to the low level side. As a result, the sensitivity threshold in the control circuit 25 increases, so that the limited band of the IF bandpass filter 15 can be switched to a narrow band without waiting for the switching circuit 21 to switch to the path B.

  In the present embodiment, when the adjacent disturbance detection path status is High determination and the modulation degree detection path status is High determination, the load input OFF determination result is output to the switch 31b and is output to the output terminal of the high-pass filter 24. The connected load resistor 31a is opened, and the limited band of the IF bandpass filter 15 is switched to a narrow band.

  Thus, even in the case shown in FIGS. 5C and 5D, which is difficult to discriminate with the conventional method, the sensitivity threshold value is increased only in the case shown in FIG. Can be surely switched to a narrow band.

  When the electric field is greater than or equal to the medium electric field, the switching circuit 21 selects the path B, and the control circuit 25 switches the limited band of the IF bandpass filter 15 to a narrow band.

  As described above, according to the present embodiment, the switching circuit 31 that connects or disconnects the load resistor 31a to the output stage of the high-pass filter 24, and the switching circuit 31 is instructed to connect / disconnect the load resistor 31a. Since the determination unit 32 is provided and the load connection / disconnection is determined based on the combination of the adjacent disturbance detection path status (Low / High) and the modulation degree detection path status (Low / High), the interference rejection capability is improved. A band limiting malfunction at a high modulation degree can be prevented while maintaining it.

  In addition, the present invention is not limited to the above embodiment, and when the signal level of the FM broadcast signal is higher than a predetermined value, the output signal of the AFCF circuit 18 and the output signal of the adjacent disturbance detection unit 32a are used. A control signal based on the control circuit 25 may be input to the control circuit 25.

  The present invention can be applied to limit band switching control of an IF bandpass filter mounted on an FM receiver.

DESCRIPTION OF SYMBOLS 11 1st mixer 12 IF amplifier 13 IF filter 14 2nd mixer 15 IF band pass filter 16 FM detection circuit 17 Mute circuit 18 AFC circuit 20 Band control system circuit 21 Switching circuit 22 S meter circuit (DC)
23 S meter circuit (AC)
24 high-pass filter 25 control circuit 26 low-pass filter 31 switching circuit 31a load resistor 31b switch 32 determination unit 32a adjacent interference detection unit 32b modulation degree detection unit

Claims (5)

A frequency conversion circuit that converts the received FM broadcast signal into an intermediate frequency signal;
A band-pass filter connected to a subsequent stage of the frequency conversion circuit and capable of varying a pass band and passing the intermediate frequency signal;
A detection circuit for detecting an intermediate frequency signal that has passed through the bandpass filter;
A level detection circuit for detecting a signal level of the FM broadcast signal;
A control circuit for controlling a pass band of the band-pass filter based on an AC component superimposed on an output signal of the level detection circuit when the signal level of the FM broadcast signal is smaller than a predetermined value;
A modulation degree detection circuit for detecting a modulation degree of the FM broadcast signal from the detection signal;
An interference wave detection circuit for detecting presence / absence of an interference wave with respect to the FM broadcast signal from the detection signal;
A change that changes the level value of the AC component in a direction in which the passband of the bandpass filter is not switched to a narrow band in a situation where no interference wave is present, depending on the combination of the detected modulation degree and the presence or absence of the interference wave Circuit,
An FM receiver characterized by comprising:   When the output signal of the modulation degree detection circuit is at a high level indicating “high modulation degree” and the output signal of the interference wave detection circuit is at a low level indicating “no interference”, the change circuit is 2. The FM receiver according to claim 1, wherein a level value of the component is changed to a side where a wide band is set in the band pass filter.   When the output signal of the modulation degree detection circuit is at a high level indicating "high modulation degree" and the output signal of the interference wave detection circuit is at a high level indicating "interference", the change circuit 3. The FM receiver according to claim 1, wherein the level value of the component is changed to a side where a narrow band is set in the band pass filter. The change circuit is configured to be capable of switching a load resistor that shifts the level value of the AC component to a higher side with respect to the output stage of the level detection circuit between a connected state and a disconnected state,
When the output signal of the modulation degree detection circuit is at a high level and the output signal of the interference wave detection circuit is at a low level, the load resistance is not connected,
4. The load resistor is disconnected when the output signal of the modulation degree detection circuit is at a high level and the output signal of the interference wave detection circuit is at a high level. The FM receiver in any one. When the signal level of the FM broadcast signal is higher than a predetermined value, a control signal based on the output signal of the detection circuit and the output signal of the jamming wave detection circuit is input to the control circuit. The FM receiver in any one of Claims 1-4.

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