JPS60236523A - Muting circuit of fm radio receiver - Google Patents

Abstract

PURPOSE:To ensure the satisfactory muting operations in a channel selection mode, a detuning mode, etc. despite the presence of a broadcast station having an extremely large input field intensity without using a special noise detecting circuit and with a simple and inexpensive constitution, by providing the frequency characteristics to detect also the high band noise component when the output signal of an FM wave detecting circuit is detected. CONSTITUTION:A broadcast station has an extremely large input field intensity, and the output signal of an intermediate frequency amplifier circuit 5 is increased up to the bottom part level of the frequency characteristics of an intermediate frequency signal owing to the blocking characteristics of a front end, the characteristics of an intermediate frequency filter, the earth state, etc. Then a noise signal of a high level is obtained at a frequency band outside an S curve frequency range at the output side of an FM wave detecting circuit 6 even in case the level of said bottom part exceeds a prescribed level Vs. The high band noise component of said noise signal is supplied to a comparator 11d via a buffer amplifier circuit 11a. Thus a muting circuit 8 has a muting ON state.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a muting circuit for an FM radio receiver.

Background technology and its problems The first muting circuit for conventional FM radio receivers
Something like the one shown in the figure has been proposed. That is, in FIG. 1, (1) shows an antenna for receiving FM radio broadcasting,
The FM radio broadcast signal transmitted by this antenna (1) is supplied to the mixing circuit (3) via the high frequency amplifier circuit (2), and the local oscillation signal of the local oscillation circuit (4) is supplied to this mixing circuit (3). The intermediate frequency signal obtained at the output side of this mixing circuit (3) is passed through an intermediate frequency amplifier circuit (5) to F
The FM demodulated signal is supplied to the M detection circuit (6), and the FM demodulated signal obtained at the output side of this FM detection circuit (6) is sent to the multiplexer (7).
The left signal R and the right signal R obtained at the output side of the multiplexer (7) are respectively led out to the left signal output terminal (8L) and the right signal output terminal (8R) via the mute circuit (8). Do as you like. Also, this intermediate frequency amplification circuit (5)
The output signal of this signal level detection circuit (9) is supplied to a signal level detection circuit (9) which obtains a high level signal "'1" on its output side when the output signal is higher than a predetermined level Vs, and the output signal of this signal level detection circuit (9) is ANDed. The output signal of the FM detection circuit (6) is supplied to one input terminal of the circuit a, and the output signal of the FM detection circuit (6) is supplied to the tuning state detection circuit. This tuning state detection circuit αυ utilizes the S curve of the detection output signal of this FM detection circuit (6), and when the output signal is below a predetermined absolute level IVD1 of this S curve characteristic, the output signal becomes a high level signal "1". For example, the output signal of the FM detection circuit (6) is supplied to the input side of the buffer amplifier circuit (lla), and the output signal of the FM detection circuit (6) is supplied to the input side of the inverter circuit (llb). ) and the inverter circuit (llb), output signals having a phase difference of 180 degrees from each other are obtained, and the output sides of the buffer amplifier circuit (lla) and the inverter circuit (llb) are connected to the DC blocking capacitor (
A comparator circuit (lid) which is connected to each other via a . so that it is supplied to The output signal of this comparison circuit (lid), that is, the output signal of the tuning state detection circuit αυ, is supplied to the other input terminal of the AND circuit (II), and the simulator circuit (8) is controlled by the output signal of this AND circuit. That is, when the output signal of the AND circuit aO is a high level signal ``1'', the mute circuit (8) is muted off and the multiplexer (
The output signal of 7) is output from the left and right signal output terminals (8L) and (
8R) respectively, and a single octopus AND circuit (when the output signal of 1G is a low level signal "0", a mute circuit (
8) is muted on so that the output signal of the multiplexer (7) is not supplied to the left and right signal output terminals (8L) and (8R).

When the muting circuit of an FM radio receiver as shown in Fig. 1 receives a broadcasting station whose input electric field strength is less than a medium electric field, the output signal of the intermediate frequency amplifier circuit (5) is as shown in Fig. 2 A4C. In this case, as shown in FIG.
'' is obtained, and the S-curve characteristic of the detection output signal of the FM detection circuit (6) is as shown in Fig. 2B.
The frequency range of the high level signal ``1'' of the output signal of the tuning state detection circuit ttn, in which a signal below the predetermined absolute level I Vol of this S curve characteristic becomes the no-repel signal ``1'', is shown in Figure 2. As shown, there is a predetermined range a near the center frequency fO and the entire frequency range before and after the S-curve range.

Therefore, in this case, a signal with a signal level as shown in FIG. 2C is supplied to one input terminal of the AND circuit, and a tuning state signal as shown in FIG. 2 is supplied to the other input terminal of the AND circuit. Since the mute control signal obtained at the output side of the circuit (101) is a high level signal "1" only in a predetermined frequency range a as shown in FIG. 2E, this mutes the mute circuit (8) off. It is possible to perform good muting when selecting a station or when out of tune.

However, when there is a broadcasting station where the input electric field strength is very large (for example, the input signal level is 100 dBμ or more), the output of the intermediate frequency amplifier circuit (5) may be affected by front-end blocking, the characteristics of the FI intermediate frequency filter, the grounding condition, etc. As shown in FIG. 3A, the level of the tail of the frequency characteristic of this intermediate frequency signal also increases, causing a phenomenon in which the level of the tail reaches a predetermined level (1)8 or higher. Therefore, the frequency range of the high level signal "'1" obtained at the output side of the Hiro signal level detection circuit (9) at this time is as shown in Fig. 3C.
If the frequency range of the S-curve characteristic of the detection output signal of the detection circuit (6) is wider than that of the S-curve characteristic (see Figure 3B), then the output signal of the tuning state detection circuit fiυ is as shown in Figure 3 in the same manner as described above. Therefore, the output signal of AND circuit a is as shown in Fig. 3E.
As shown in the figure, the center frequency f is a high level signal in a certain range of frequencies 2 and 3 above and below the frequency range a and outside the S curve frequency range.
1", this frequency range S and C will mute off. In this case, when tuning and detuning in this frequency range S and C, noise will be generated from the speakers etc., and this AND circuit aO For example, when the output signal of is used as an auto-stop signal for a synthesizer receiver, In particular, there were inconveniences such as being extremely uncomfortable.

In order to eliminate this inconvenience, a system as shown in FIGS. 4 and 5 has been proposed in which the frequency ranges A and C are detected and the mute circuit (8) is turned on by the detection signal. ing.

In the example of FIG. 4, the output signal of the FM detection circuit (6) in the example of FIG. The noise detection circuit (121) outputs a low level signal "0" when it detects noise in the frequency ranges C and C, and outputs a high level signal "1" at other times. For example, as shown in Fig. 5, the input terminal (12a) to which the output signal of the FM detection circuit (6) is supplied is connected to the input side of a high-pass filter (12b) that passes a frequency signal of 100 KHz or more. , the noise signal obtained at the output side of this high-pass filter (12b) is supplied to the amplitude detection circuit (12d) via the amplifier circuit (12c), and the output signal of this amplitude detection circuit (12d) is at a predetermined level or higher. When the amplifier circuit (12e) output terminal (1
A low level signal "0" is obtained at the output terminal (12f), and when this output signal is below a predetermined level, a high level signal "1" is obtained at the output terminal (12f) by the amplifier circuit (12e). . The output signal of this noise detection circuit +121 is supplied to a third input terminal of an AND circuit. The other parts of FIG. 4 are constructed in the same manner as in FIG. 1.

In the example in FIG. 4, a noise detection circuit (
121 is provided, and the mute circuit (8) is also controlled by the output signal of this noise detection circuit (121), so even if there is a broadcasting station with an extremely high input electric field strength, the noise detection circuit (121) can detect a predetermined frequency near the center frequency fo. By turning off the mute circuit (8) in the range a, good muting such as special detuning can be performed when selecting a station.

However, in the example shown in FIG. 4, the tuning state detection circuit (
It is not necessary to provide a noise detection circuit a2 in addition to In, and this noise detection circuit (121 is relatively complicated as shown in FIG. 5, for example), which makes the circuit configuration complicated and relatively expensive. was there.

OBJECTS OF THE INVENTION In view of the above, an object of the present invention is to perform good muting with a relatively simple configuration.

Summary of the Invention The present invention provides a muting circuit for an FM radio receiver in which a muting circuit is turned on and off in relation to the output signal level of an intermediate frequency amplification circuit and the detected output signal of an FM detection circuit. The present invention provides a frequency characteristic for detecting the output signal of the circuit so that high-frequency noise components can also be detected. Muting can be performed.

Embodiment An embodiment of the muting circuit of the FM radio receiver of the present invention will be described below with reference to FIG. In FIG. 6, parts corresponding to those in FIG. 1 are designated by the same reference numerals, and detailed explanation thereof will be omitted.

In this example of FIG. 6, the tuning state detection circuit α of the example of FIG.
A capacitor yt (tt
e) and a parallel circuit of a capacitor f (llf) and a coil (l1g), and connect the connection point between the output terminal of this buffer amplifier circuit (lla) and the capacitor (lie) so that the input signal is at a predetermined absolute level. Connected to the input terminal of a comparison circuit (lid) that detects whether the level is below IVD1. In this case, the capacitor (lie) and the capacitor (l
Frequency characteristics of a series circuit with a parallel circuit of lf) and a coil (l1g) As shown in Figure 7, the impedance is extremely low in the audio frequency band, and the signals in this band cancel each other out, and the output terminal of the buffer amplifier circuit (lla) and The connection point of the capacitor (lie), that is, the input terminal of the comparator circuit (lid), should not be
) and the tank circuit of the coil (l1g) are set to the frequency of high-frequency noise components, for example, 100 KHz, and this high-frequency noise component signal of about 100 KHz is supplied to the input terminal of this comparison circuit (lid). Do as you wish.

The rest of FIG. 6 is constructed similarly to FIG. 1.

In the example shown in FIG. 6, when receiving a broadcasting station whose input electric field strength is less than a medium electric field, the mute control signal obtained at the output side of the AND circuit (II) operates in the same manner as explained in FIG. As shown in Fig. 2E, only the predetermined frequency range a becomes a high level signal "1", which allows the mute circuit (8) to be muted off. Good muting can be performed in such situations.

In addition, due to the presence of a broadcasting station with a very large input electric field strength, blocking of the front end, characteristics of the intermediate frequency filter, grounding conditions, etc., the output signal of the intermediate frequency amplifier circuit (5) may be lower than the intermediate frequency as shown in Figure 3A. The level of the tail of the frequency characteristic of the signal also increases, and the level of this tail reaches the predetermined level v
Even if it becomes more than B, in this case, a high level noise signal is obtained at the output side of the FM detection circuit (6) in a frequency band other than the S carer frequency range, and this high level noise signal is A high-frequency noise component is supplied to a comparison circuit (lid) via a buffer amplifier circuit (lla), and when there is a high-frequency noise component of this high level, that is, a predetermined level VD or higher, the output signal of this comparison circuit (lid) is is low level signal 1
At this time, the output signal of the AND circuit a1 also becomes a low level signal 10'', and the simulator circuit (8) turns on the mute signal. Therefore, in the example shown in Fig. 6, even if there is a broadcasting station with a very large input electric field strength, the mute circuit (8) can be controlled well, and the mute circuit (8) can be controlled well when selecting a station, detuning, etc. Muting can be performed. Also, in Fig. 6, compared to Fig. 1, only a capacitor (llf) and a coil (l1g) are provided, so compared to the examples in Figs. 4 and 5, the configuration is extremely simple and can be obtained at low cost. can.

FIG. 8 shows a specific example of the present invention. Fig. 8 shows the application of the present invention to a semiconductor integrated circuit that has been sold and used previously.This semiconductor integrated circuit uses a quadrature detection circuit as an FM detection circuit (6). A phase adjustment circuit fi3 is provided on the output side to obtain an FM detection signal at one output terminal (6a) and an AFC signal whose phase is 180 degrees different from the FM detection signal for AFC at the other output terminal (6b). I was trying to get it. This example focuses on this point and connects the output terminals (6
A buffer amplifier circuit (lla) is installed between a) and (6b), respectively.
and (llb) to capacitor (l1g) and wry
A series circuit with a parallel circuit of the differential (llf) and a coil (l1g) is connected, and this buffer amplifier circuit (llji)
and an output terminal (lli) connected to the input terminal of the comparator circuit (lid) in FIG. 6 from the connection point of the capacitor (lie).
The rest of the structure is the same as that shown in FIG. 6. In this Figure 8, (5F) (5b) is an input terminal to which an output signal from the intermediate frequency amplifier circuit (5) is supplied, (6C) is a 90 degree phase shifter constituting a quadrature detection circuit, and (6d) ) is a comparison voltage input terminal, (6e) is a battery for comparison voltage, and 10B is a power supply terminal to which positive DC voltage is supplied.

It is easy to understand that when the example in FIG. 8 is applied to FIG. 6, the same effect as described in the explanation of FIG. 6 above can be obtained. A capacitor (lie) and a capacitor (llf) between the respective output terminals of the circuit (lla) and the inverter circuit (llb) or between the respective output terminals of the buffer amplifier circuits (lla) and (llb).
In place of this, a series resonant circuit consisting of a series circuit of a capacitor (11) and a coil (1"lk) is provided as shown in FIG. Of course, if this frequency characteristic is made to have a low impedance in the audio frequency band (1) and a high impedance at a high frequency of 100 KHz as shown in Fig. 10, the same effect as described above can be obtained. It goes without saying that the invention is not limited to the above-described embodiments, and that various other configurations can be made without departing from the gist of the invention.

Effects of the Invention According to the present invention, good muting can be achieved at the time of tuning, detuning, etc. even when there is a broadcasting station with a very large input electric field strength, with a simple and inexpensive configuration without providing a special noise detection circuit. There are profits that can be made.

[Brief explanation of drawings]

1 and 4 are block diagrams showing an example of a muting circuit of a conventional FM radio receiver; FIGS. 2, 3, and 7 are diagrams for explaining the present invention; and FIG. The figure is number 4
Fig. 6 is a block diagram showing an embodiment of the muting circuit of the FM radio receiver of the present invention; Fig. 8 is a connection diagram showing a specific example of the main parts of the present invention; Fig. 9 The figure is a connection diagram showing another example of the main part of the present invention, FIG. 10 is used for explanation of FIG. 9, a mute circuit (8), and (9) ti
t signal level detection circuit, the head is an AND circuit, (111 is a tuning state detection circuit, (lla) is a buffer amplifier circuit, (l
lb) is an inverter circuit, (lid) is a comparison circuit, (l
ie) and (1, 1f) are capacitors, (l1g)
is a coil. -L-□ri Figure 2 Figure 3

Claims (1)

[Claims] In the muting circuit of the FM radio receiver, the muting circuit is turned on and off in relation to the output signal level of the intermediate frequency amplification circuit and the detection output signal of the FM detection circuit, and the output signal of the FM detection circuit is detected. A muting circuit for an FM radio receiver is characterized in that it has a frequency characteristic to detect even high-frequency noise components.