JP4161150B2 - FM receiver - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an FM receiver.
[0002]
[Prior art]
As an FM receiver, there is a receiver that reduces the influence of adjacent interference by automatically narrowing the passband width of an intermediate frequency filter when there is adjacent interference.
[0003]
[Problems to be solved by the invention]
However, the problem described below occurs only by detecting the adjacent interference wave and changing the passband width of the intermediate frequency filter.
[0004]
That is, in FM broadcasting, digital data such as character information is frequency-multiplexed and transmitted to an original audio broadcasting program. This FM character multiplex broadcasting employs a system called the DARC system in Japan, and adopts a DRIS system that is a modification of the DARC system in the United States and Europe.
[0005]
And in these systems, the standard for multiplexing digital data such as character information is
Subcarrier frequency: 76 kHz
Modulation method: LMSK
It is said that. The LMSK signal is frequency-multiplexed with the stereo composite signal of the original audio broadcast program, and the frequency-multiplexed signal is transmitted.
[0006]
In Europe, multiplex broadcasting of data called the RDS system has been put into practical use. Digital data transmitted by this RDS method is data relating to broadcasting stations, programs, etc., and the multiplexing standard is
Subcarrier frequency: 57 kHz
Modulation method: Balanced modulation. The balanced modulation signal is frequency-multiplexed with the stereo composite signal of the original audio broadcast program, and the frequency-multiplexed signal is transmitted.
[0007]
Therefore, when obtaining character information by FM character multiplex broadcasting, or when using data in RDS FM broadcasting, the passband width of the intermediate frequency filter is required to be wide corresponding to the LMSK signal and the balanced modulation signal. The
[0008]
However, when multipath occurs in the received wave, the adjacent interfering wave detection circuit malfunctions due to the multipath component, and as a result, the passband width of the intermediate frequency filter becomes narrow. Then, when receiving a text multiplex broadcast or an RDS broadcast, the level of the LMSK signal or the balanced modulation signal decreases, and the text multiplex broadcast or the RDS broadcast cannot be received normally.
[0009]
The present invention is intended to solve such problems.
[0010]
[Means for Solving the Problems]
In this invention,
A switch circuit for switching the bandwidth of the intermediate frequency signal between a wide band and a narrow band;
First and second bandpass filters to which an FM demodulated output or a detection output of the intermediate frequency signal is supplied;
First and second level detection circuits for detecting the levels of the output signals of the first and second bandpass filters, respectively;
A control signal forming circuit for forming a control signal for the switch circuit from a detection output of the first level detection circuit,
The first bandpass filter is a passband that allows high frequency components due to adjacent interference and multipath to pass through,
It said second band-pass filter is a pass band for passing the low frequency components of the high frequency components and low frequency components by the multi-path,
The control signal forming circuit is
When the detection output of the first level detection circuit is equal to or higher than a predetermined threshold level , the switching circuit switches the bandwidth of the intermediate frequency signal from the wide band to the narrow band, and
The FM receiver is configured such that the threshold level increases as the level of the detection output of the second level detection circuit increases .
Therefore, when there is multipath, the detection level of the adjacent interfering wave is changed.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an embodiment in which the present invention is applied to an FM receiver capable of receiving FM character multiplex broadcasting.
[0012]
That is, the FM broadcast wave signal is received by the antenna 11, and this FM broadcast wave signal is supplied to the front end circuit 12. The front end circuit 12 is configured in a superheterodyne system, and converts an FM broadcast wave signal having a target frequency into an intermediate frequency signal S12 (intermediate frequency f12 is 10.7 MHz, for example) and outputs the signal.
[0013]
The intermediate frequency signal S12 is supplied to the intermediate frequency filter 13 to remove unnecessary signal components and then supplied to the intermediate frequency filter 14. In this case, the intermediate frequency filters 13 and 14 are constituted by, for example, ceramic filters. Further, the pass band of the intermediate frequency filter 13 is set to a wide band such as f12 ± 90 kHz that can reliably obtain the LMSK signal of the text multiplex broadcasting. Furthermore, the pass band of the intermediate frequency filter 14 is set to a narrow band, for example, f12 ± 40 kHz, which can reduce the influence of adjacent interference and multipath when obtaining an audio output.
[0014]
In this way, a wide band intermediate frequency signal S13 is extracted from the intermediate frequency filter 13, and a narrow band intermediate frequency signal S14 is extracted from the intermediate frequency filter 14. These intermediate frequency signals S13 and S14 are supplied to one contact W and the other contact N of the switch circuit 15.
[0015]
Then, one of the intermediate frequency signals S13 and S14 is extracted from the switch circuit 15 as will be described later, and the extracted intermediate frequency signal is supplied to the FM demodulation circuit 17 through the limiter amplifier 16. Thus, the FM demodulating circuit 17 extracts the multiplexed signal of the stereo composite signal and the LMSK signal as the demodulated output signal S17 when receiving FM character multiplex broadcasting, and when not receiving FM character multiplex broadcasting, A stereo composite signal is extracted.
[0016]
Then, the demodulated signal S17 of the demodulating circuit 17 is supplied to the low-pass filter 18 to extract the stereo composite signal, and this stereo composite signal is supplied to the stereo demodulating circuit 19 to demodulate the left and right channel audio signals L and R. These signals L and R are supplied to the speakers 22L and 22R through the amplifiers 21L and 21R.
[0017]
At the time of reception of FM character multiplex broadcasting, the demodulated signal S17 is supplied to the bandpass filter 31 to extract the LMSK signal, and this LMSK signal is supplied to the display processing circuit 32 of the character multiplex broadcasting for decoding and error correction. Is used for display.
[0018]
By the way, when there is adjacent interference, the noise component (adjacent interference wave component) generated in the demodulated signal S17 due to the adjacent interference is distributed in a relatively high frequency band. When there are multipaths, noise components (multipath components) generated in the demodulated signal S17 due to the multipaths are distributed in a relatively low frequency band and a high frequency band.
[0019]
Therefore, the demodulated signal S17 is supplied to the bandpass filter 41. The band pass filter 41 has a pass band of, for example, 50 kHz to 200 kHz, and extracts a noise component due to adjacent interference and a noise component in a relatively high frequency band due to multipath. The output signal S41 of the band-pass filter 41 is supplied to the detection circuit 42 to obtain a DC voltage V42 having a level corresponding to the level of the signal S41, and this voltage V42 is supplied to the control signal forming circuit 43.
[0020]
In the forming circuit 43, a signal S43 that is "1" is formed when the voltage V42 is equal to or higher than the threshold level VTH, and this signal S43 is supplied to the switch circuit 15 as its control signal. When “=“ 0 ”, it is connected to the contact W, and when S43 =“ 1 ”, it is connected to the contact N.
[0021]
Further, the demodulated signal S17 of the demodulating circuit 17 is supplied to the band pass filter 44. The bandpass filter 44 has a passband of, for example, 10 kHz to 30 kHz, and extracts a relatively low frequency component due to multipath. Then, the output signal S44 of the band pass filter 44 is supplied to the detection circuit 45 to be a DC voltage V45 having a level corresponding to the level of the signal S44, and this voltage V45 is supplied to the control signal forming circuit 43.
[0022]
In the forming circuit 43, the magnitude of the threshold level VTH is controlled by the voltage V45. When the voltage V45 is larger than a predetermined value, the threshold level VTH is correspondingly increased.
[0023]
According to such a configuration, the output signal S41 of the bandpass filter 41 includes a noise component due to adjacent interference and a high-frequency noise component due to multipath. Therefore, the voltage V42 indicates the magnitude of the sum of adjacent interference and multipath. Further, the output signal S44 of the bandpass filter 44 includes a low-frequency noise component due to multipath. Therefore, the voltage V45 indicates the size of the multipath.
[0024]
When the multipath is sufficiently small and the adjacent interference is small, V42 <VTH, so S43 = "0" and the switch circuit 15 is connected to the contact W.
[0025]
Therefore, in this case, since the broadband intermediate frequency signal S13 is selected by the switch circuit 15 and supplied to the FM demodulation circuit 17, an LMSK signal by FM character multiplex broadcasting can be obtained normally, and as a result, The character by FM character multiplex broadcasting can be displayed normally.
[0026]
The multipath is sufficiently small, but the voltage V45 increases as the adjacent disturbance increases. When V45 ≧ VTH, S43 = "1" and the switch circuit 15 is connected to the contact N. Accordingly, in this case, since the narrow band intermediate frequency signal S14 is selected by the switch circuit 15 and supplied to the FM demodulation circuit 17, character display by FM character multiplex broadcasting is not possible, but the FM broadcast is appropriately listened to. be able to.
[0027]
On the other hand, although the adjacent interference is small, when the multipath becomes large, the voltage V42 increases, but the voltage V45 also increases, and the threshold level VTH increases corresponding to this voltage V45. Therefore, since the relationship of V42 <VTH is maintained, S43 = "0" and the switch circuit 15 is connected to the contact W.
[0028]
Therefore, also in this case, the broadband intermediate frequency signal S13 is selected by the switch circuit 15 and supplied to the FM demodulation circuit 17. Since the character information in FM character multiplex broadcasting is transmitted by digital data, the character information can be obtained normally even if there is some multipath, and as a result, the characters by FM character multiplex broadcasting are displayed normally. can do.
[0029]
Thus, according to the above-described FM receiver, the detection level of the adjacent interfering wave is changed when multipath occurs, so that the intermediate frequency band is not narrowed by multipath. Therefore, FM character multiplex broadcasting can be normally received even under multipath conditions.
[0030]
In the above description, the FM character multiplex broadcast is received. However, when the RDS FM broadcast is received, the balanced modulation signal may be extracted by the band-pass filter 31 and processed.
[0031]
Further, if an intermediate frequency signal is extracted from a part of the limiter amplifier 16 and detected, a signal including an adjacent interference wave and multipath can be obtained. Therefore, this signal is supplied to the bandpass filters 41 and 44 and the same. Processing can also be performed.
[0032]
【The invention's effect】
According to the present invention, when receiving FM character multiplex broadcasting, RDS FM broadcasting, or the like, the intermediate frequency band is not narrowed by multipath.
[Brief description of the drawings]
FIG. 1 is a system diagram showing an embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 ... Front end circuit, 13 and 14 ... Intermediate frequency filter, 15 ... Switch circuit, 16 ... Limiter amplifier, 17 ... FM demodulation circuit, 18 ... Low pass filter, 19 ... Stereo demodulation circuit, 21L and 21R ... Amplifier, 22L and 22R ... Speaker, 31 ... Band pass filter, 32 ... Display processing circuit, 41 ... Band pass filter, 42 ... Level detection circuit, 43 ... Control signal forming circuit, 44 ... Band pass filter, 45 ... Level detection circuit

Claims (2)

A switch circuit for switching the bandwidth of the intermediate frequency signal between a wide band and a narrow band;
First and second bandpass filters to which an FM demodulated output or a detection output of the intermediate frequency signal is supplied;
First and second level detection circuits for detecting the levels of the output signals of the first and second bandpass filters, respectively;
A control signal forming circuit for forming a control signal for the switch circuit from a detection output of the first level detection circuit,
The first bandpass filter is a passband that allows high frequency components due to adjacent interference and multipath to pass through,
It said second band-pass filter is a pass band for passing the low frequency components of the high frequency components and low frequency components by the multi-path,
The control signal forming circuit is
When the detection output of the first level detection circuit is equal to or higher than a predetermined threshold level , the switching circuit switches the bandwidth of the intermediate frequency signal from the wide band to the narrow band, and
An FM receiver in which the threshold level increases as the detection output level of the second level detection circuit increases . The FM receiver according to claim 1,
Pass band of the first band-pass filter is 50 k Hz ~ 200 k Hz,
FM receiver as is described above the passband of the second bandpass filter 10 k Hz ~ 30 k Hz.

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