JP4375636B2 - FM receiver - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an FM receiver in which an automatic tuning function of a broadcasting station is improved.
[0002]
[Prior art]
In general, in-vehicle radio receivers are provided with a station detection function called auto search. In this auto search, the reception frequency from each broadcasting station is checked in order, and when a frequency exceeding a certain electric field strength is detected, the search operation is stopped and the broadcasting station is received.
[0003]
FIG. 6 is a block diagram showing an example of an FM receiver having such a station detection function. In FIG. 6, 1 is an antenna, 2 is an FM front end, and 3 is an IF (intermediate frequency) amplifier circuit. An FM detection circuit 4 and an FM detection output terminal 5 are sequentially connected to the first output terminal of the IF amplifier circuit 3. On the other hand, an AM detection circuit 6 for obtaining a received electric field level signal (generally also called an S meter signal) is connected to the second output terminal of the IF amplifier circuit 3. An output terminal 7 of the AM detection circuit 6 is connected to an input terminal of a detection circuit 9 for a station detection signal (search operation stop signal) SD. A reference voltage Vs supply unit 8 is connected to an input terminal of the station detection signal detection circuit 9.
[0004]
In the conventional FM receiver having such a configuration, when the antenna 1 receives a radio wave from a specific broadcasting station during a search operation, the FM front end 2 and the IF amplifier circuit 3 process the radio wave. A signal is obtained, and this IF signal is detected by the AM detection circuit 6 to obtain a received electric field level signal. This received electric field level signal is compared with the reference voltage Vs in the detection circuit 9, and when the received electric field level signal is larger than the reference voltage Vs, the station detection signal SD is output from the output terminal 10 of the detection circuit 9, and the search operation is performed. Stop and continue receiving the station. On the other hand, when the received electric field level signal is smaller than the reference voltage Vs, the station detection signal SD is not output from the detection circuit 9, so the search operation is continued, and reception for this broadcast station is stopped, The presence / absence of the station detection signal SD is newly detected for the broadcasting station of the next frequency.
[0005]
[Problems to be solved by the invention]
By the way, in the conventional FM receiver, when the reception bandwidth is large, since the IF signal level does not vary due to modulation, the stop sensitivity does not vary. However, for the purpose of improving adjacent interference characteristics, a narrow band IF filter may be provided in front of the IF amplifier circuit to limit the reception bandwidth to a narrow band. In this case, since the high frequency component of the demodulated output is limited as compared with the case where a wideband IF filter is provided, the output of the received electric field level signal drops when the modulation degree is large, and accordingly, the station detection signal SD is output. It will not be done. When the stop time of the station detection signal SD exceeds a predetermined value, there is a problem that the search operation for the next broadcasting station is started.
[0006]
This point will be specifically described with reference to FIG. FIG. 7A shows the frequency shift of the FM modulated signal. The dotted line in FIG. 7B shows the case where the reception frequency band is narrow, and the solid line shows the case where the band is wide. When the frequency shift of the FM modulation signal shown in FIG. 7A increases, when the band of FIG. 7B is narrow, the band is outside the filter band, and the output from the IF amplifier circuit 3 decreases. Then, the level of the received electric field level signal obtained by AM detection of the output of the IF amplifier circuit 3 varies with the reference voltage Vs as a boundary as shown by the solid line in FIG. As a result, the station detection signal SD obtained by comparing the received electric field level signal with the reference voltage Vs is output as indicated by a thick solid line in FIG. As described above, in the prior art, although the reception signal level is sufficiently high, the station detection signal SD is not output because the reception frequency band is narrow, and the next search operation is started.
[0007]
In order to improve this, when the reception frequency band is narrow, the output signal from the AM detection circuit 6 is smoothed by a capacitor as shown by the one-dot chain line in FIG. Proposals have been made so that even when the high frequency component is cut, a decrease in the output level of that portion does not directly lead to a decrease in the output of the received electric field level signal. When the received electric field level signal is smoothed in this way, as shown in FIG. 7D, the stop time of the station detection signal SD is shortened, so that the search operation starts even if the level of the station detection signal SD decreases. Without receiving the broadcast station. However, when the capacitor is used in this way, the received electric field level signal has a delayed waveform as compared with the output signal from the AM detector circuit 6, so that the output timing of the station detection signal SD is delayed, and the response of the automatic channel selection operation. Sexuality gets worse.
[0008]
The present invention has been proposed in order to solve the above-described problems of the prior art, and its purpose is that the stop sensitivity does not vary greatly even when the reception frequency band is narrow, and a response delay occurs. An object of the present invention is to provide an FM receiver having an automatic channel selection function.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, the invention of claim 1 is an FM receiver that outputs a station detection signal and stops a channel selection operation based on a comparison between an electric field level of a received signal and a predetermined reference value. , An FM detection circuit using the reception signal as an input signal, and a circuit that outputs the modulation signal as a modulation degree detection signal by full-wave rectifying the output signal from the FM detection circuit , A modulation degree detection unit that detects the output voltage by using the output signal , a reference voltage supply unit, a resistor connected to the reference voltage supply unit, and the modulation degree detection unit connected to the resistor as input from, and a transistor in which the modulation degree detecting unit is applied changes voltage in accordance with the modulation index changes continuously detected, as an input of the transistor, and applying the varying voltage, the reference value The continuous A reference voltage control unit for changing, characterized in that it comprises a.
[0010]
According to the invention of claim 1 having such a configuration, even if the received signal has a high degree of modulation and its high frequency component deviates from the received frequency band and the received electric field level decreases, the received signal By reducing the reference voltage in accordance with the degree of modulation, the station detection signal is continuously output. As a result, the channel selection operation can be performed stably regardless of the modulation degree of the received signal.
[0012]
The received signal is detected by the FM detection circuit, and the reference voltage is controlled by the modulation degree detection signal obtained by full-wave rectification of the detection signal. In this reference voltage control by the modulation degree detection signal, when the modulation degree is high and the received electric field level signal falls, the reference voltage is pulled down by the resistor R and the transistor T using the modulation degree detection signal, and this is detected by the station detection signal. Give to the circuit.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of an FM receiver of the present invention will be specifically described with reference to the drawings.
[0014]
(1) Configuration of Embodiment FIG. 1 is a block circuit diagram showing an embodiment of the present invention. An IF amplifier circuit 3 and an FM detector circuit 4 are provided on the output side of the FM front end 2, and the IF amplifier circuit. 3 is connected to an AM detection circuit 6 for detecting a received electric field level signal, which is the same configuration as the prior art shown in FIG. In the present embodiment, a modulation degree detection circuit 20 that detects the modulation degree of the detection output output from the FM detection circuit 4 is provided on the output side of the FM detection circuit 4. In the present embodiment, a reference voltage control circuit 30 for changing the reference voltage Vs is provided. The output of the modulation degree detection circuit 20 is input to the reference voltage control circuit 30.
[0015]
As shown in FIG. 2, the modulation degree detection circuit 20 includes a phase inversion buffer 21 and a rectifying diode 22 provided in parallel, and the input FM detection signal is modulated only with a positive component. The full-wave rectification is performed to the detection signal. That is, the modulation degree detection in the modulation degree detection circuit 20 is based on the use of the FM detection output signal as shown in FIG. The FM detection is performed using an FV converter (frequency-DC voltage conversion), and the DC voltage changes to positive when the deviation of the FM reception signal becomes positive, for example. Therefore, the modulation degree of the FM reception signal can be determined by the detection output voltage. The reference voltage may be varied by the detection output voltage when the bandwidth of the intermediate frequency is overloaded with a plus and minus over with the modulation degree. At this time, since there is a plus side and a minus side over the band, the FM detection signal is full-wave rectified by the modulation detection circuit of FIG. 2 to control the reference voltage.
[0016]
On the other hand, the reference voltage control circuit 30 includes a resistor R connected to the reference voltage supply unit 8 and a transistor T connected to the resistor R as shown in FIG. The output terminal of the detection circuit 20 is connected. The transistor T and the resistor R are connected to the input terminal of the detection circuit 9 for the local detection signal.
[0017]
(2) Operation of the Embodiment In the FM receiver of the present embodiment having such a configuration, the FM signal output from the IF amplifier circuit 3 of FIG. 1 is detected by the FM detection circuit 4, and FIG. The detection signal shown in FIG. This detection signal is full-wave rectified by the modulation degree detection circuit 20 shown in FIG. 2 to become a modulation degree detection signal shown in FIG. Control of the reference voltage Vs by the modulation degree detection signal is performed by applying a change voltage corresponding to the level of the modulation degree detection signal to the input of the transistor T in the reference voltage control circuit 30 as shown in FIG. The DC voltage of the reference voltage Vs given to the circuit 9 is divided and controlled by the resistor R and the transistor T. That is, as shown in FIG. 5C, when the modulation degree is high and the received electric field level signal falls, the reference voltage is pulled down by the resistor R and the transistor T by the output signal from the modulation degree detection circuit 20, The station detection signal SD is supplied to the detection circuit 9.
[0018]
On the other hand, the IF amplification circuit 3 outputs a received electric field level signal corresponding to the electric field strength, and this is applied to the detection circuit 9 for the station detection signal. The other input of the detection circuit 9 is supplied with the reference voltage controlled by the modulation degree detection circuit 20, and the voltage is compared with the voltage of the received electric field level signal, as shown in the straight line portion of FIG. Thus, when the voltage of the received electric field level signal is higher than the reference voltage, the detection circuit 9 outputs the station detection signal SD for stopping the seek as shown in FIG. Further, the received electric field level signal decreases when the modulation degree is high and the frequency shift exceeds the IF bandwidth. In this embodiment, the modulation degree detection circuit 20 and the reference voltage control circuit 30 allow the received electric field level signal to be reduced. As shown in the curved portion of the valley, the reference voltage Vs input to the detection circuit 9 is pulled down, so that the received electric field level signal exceeds the reference voltage Vs, and the influence of the change in the modulation degree is eliminated. The detection signal SD can be determined.
[0019]
【The invention's effect】
According to the FM receiver of the present invention having the above-described configuration, it is possible to stably receive radio waves of a selected broadcast station even when the reception band is narrow, and at the time of channel selection operation. It becomes possible to provide an FM receiver in which no response delay occurs.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a first embodiment of an FM receiver according to the present invention.
FIG. 2 is a circuit diagram showing a modulation degree detection circuit portion in the embodiment of FIG. 1;
FIG. 3 is a waveform diagram for explaining the operation of the modulation degree detection circuit of FIG. 2;
4 is a circuit diagram showing an example of a reference voltage control unit in the embodiment of FIG.
FIG. 5 is a waveform diagram for explaining the operation of each unit in the embodiment of FIG. 1;
FIG. 6 is a block diagram showing a configuration of a conventional FM receiver.
FIG. 7 is a waveform diagram for explaining a defect of a conventional FM receiver.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Antenna 2 ... FM front end 3 ... Amplifier circuit 4 ... Detection circuit 5 ... Detection output terminal 6 ... Detection circuit 7 ... Output terminal 8 ... Reference voltage supply part 9 ... Detection circuit 10 ... Output terminal 20 ... Modulation degree detection circuit 21 ... Phase inversion buffer 22 ... Diode 30 ... Reference voltage control circuit Vs ... Reference voltage SD ... Station detection signal R ... Resistance T ... Transistor

Claims (1)

In the FM receiver that outputs the station detection signal and stops the channel selection operation based on the comparison between the electric field level of the received signal and a predetermined reference value,
And FM detection circuit for a received signal and the input signal, and a circuit for outputting an output signal from the FM detection circuit as a full-wave rectification to the modulation degree detecting signal, the modulation of the received signal which changes from moment to moment, the A degree-of-modulation detector that detects the output voltage by determining the output voltage ;
A reference voltage supply unit, a resistor connected to the reference voltage supply unit, and an input from the modulation degree detection unit connected to the resistor, to the continuously changing modulation degree detected by the modulation degree detection unit A transistor to which a change voltage corresponding to the transistor is applied,
As the input of said transistor, and applying the varying voltage, a reference voltage control section for continuously changing the reference value,
An FM receiver comprising:

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