JPS6079830A - Fm receiver - Google Patents

Abstract

PURPOSE:To eliminate noise at strong electric field and weak electric field without giving a large change to a low frequency signal by changing the noise eliminating period in response to an electric field strength of a received radio wave. CONSTITUTION:A signal IF amplified by an intermediate frequency amplifier 4a is inputted to a detector 5 and also a signal Vc proportional to the electric field strength of an input signal is detected by a level detection circuit 4b. The signal subjected to detection 5 and low frequency amplification 6, 7 is fed to an LPF8 and an HPF9. A high level pulse noise N1 through the HPF and an internal noise N2 of each circuit are inputted to a noise detection circuit 22 through a buffer 21. The signal Vc and an output signal VO of a meter circuit 23 go to high level and its inverting signal VD' goes to low level at a strong electric field, a transistor (TR) Q1 is turned off, the pulse width of a monostable multivibrator 26 operated by an N1 noise output of the circuit 22 is widened and a gate circuit 11 is closed. The circuit 11 cuts off a voice signal via the LPF8 when the noise N1 is generated, its voltage is held (27) and fed to an amplifier 26. The Q1 is turned on at a weak electric field and the circuit 11 is interrupted with a pulse having a narrower width to that of the noise N2.

Description

[Detailed description of the invention] 〔Technical field〕 The present invention relates to an FM receiver having a noise removal function.

[Background technology]

Regarding FM receivers for automobiles, it is known that ignition noise and wiper noise generated when the automobile is running appears in the audio signal. Ignition noise and wiper noise are so-called pulse noises that have a pulse-like waveform, so they are very difficult to hear even when receiving FM broadcasts.

On the other hand, when the received radio wave has a weak field strength, the AGC circuit may increase the gain of the high frequency amplification circuit or the intermediate frequency amplification circuit. In this case, in addition to the pulse noise, internal noise generated from a high frequency amplification circuit or the like is also amplified. Therefore, when the electric field strength is low, the noise mixed into the audio signal becomes even greater, making it even more difficult to hear.

For this reason, FM receivers are provided with various noise removal circuits. The noise removal circuit detects the noise and performs a noise removal operation when noise is mixed into the received radio waves.

However, prior to inventing the present invention, the present inventors investigated and found that the following defects were present.

That is, the noise removal circuit performs noise detection at a predetermined time after noise detection, regardless of the field strength of the received radio waves. Therefore, studies by the present inventors have revealed that when noise removal is performed, the missing portion of the audio signal becomes large when the S/C ratio deteriorates significantly in a state of weak electric field strength.

[Purpose of the invention]

An object of the present invention is to provide an FM receiver that can remove fibers such as pulse noise in accordance with the electric field strength of received radio waves.

The above and other objects and novel features of the present invention include:
It will become clear from the description of this specification and the accompanying drawings.

[Summary of the invention]

A brief summary of the invention disclosed in this application is as follows.

That is, the transistor Q is driven into an off state or a conductive state by a signal whose level changes in proportion to the electric field strength, for example, a signal indicating a tuned state, and the time constant that determines the operating time of the monostable multivibrator 26 is set by the transistor Q1. The monostable multivibrator 26 is controlled by
The purpose of the present invention is to vary the opening time of the gate circuit 11 in synchronization with the generation of noise by the pulse width of the pulse signal obtained from the pulse width, and to remove noise in proportion to the electric field strength of the received radio wave. It is.

〔Example〕

An embodiment of an FM receiver to which the present invention is applied will be described below with reference to FIGS. 1 and 3. Incidentally, FIG. 1 shows the circuit configuration of the main part of an FM receiver made into a semiconductor integrated circuit (hereinafter referred to as IC), and the circles surrounding the numbers indicate external connection terminals of the IC.

1 is the receiving antenna, 2 is the front f)", lt.

This is a ceramic filter for intermediate frequencies. The intermediate frequency signal IF obtained from the ceramic filter 3 is supplied to the intermediate frequency amplification circuit 4 via the No. 1 terminal. Normally, that is, when automatic gain control AGC is not applied, the voltage level of this intermediate frequency signal air changes in accordance with the electric field strength of the radio waves received by the antenna 1. Note that the intermediate frequency amplification circuit 4 actually has three stages (
In particular, the amplifier circuit 4a is composed of amplifying circuits 4a of not necessarily three stages but four or five stages (generally n stages), and level detection circuits 4b provided correspondingly.

However, for convenience of illustration, FIG. 1 shows a one-stage amplifier circuit 4a and a corresponding level detection circuit 4b.

The output signal 2° of the amplifier circuit 4a is supplied to the detection circuit 5. Note that although the detection circuit 5 is constituted by a quadrature chair detection circuit, illustration and explanation of the phase shift circuit will be omitted. The detected output VD is then amplified by the low frequency amplifier circuit 6 and supplied to the buffer amplifier 7. The output signal ■□ of the buffer amplifier 7 is supplied via the second terminal to a low-pass filter 8 and a bypass filter 9, which are composed of a capacitor and a resistor.

The low-pass filter 8 has a detection output ■ shown in Fig. 2 (6).
9 to obtain a low frequency signal S having a predetermined frequency or less. Moreover, the bypass filter 8 has a detection output ■.

This is to obtain noise components N, . . . Nt below a predetermined frequency.

The low frequency signal S obtained from the low pass filter 8 is supplied to the gate circuit 11 via the third terminal and the low pass filter amplifier 10.

On the other hand, from the bypass filter 9, noise components N1.
Nt is obtained. The noise component N is the ignition noise of the automobile or the pulse noise generated by the drive of the wiper or the like. Further, the noise component N is internal noise generated from each circuit below the front end 2. The noise component N+ -Nt is filtered through the bypass filter 9.
The signal is supplied to the buffer amplifier 21 via the No. 4 terminal.

A noise component tooth is provided after the buffer amplifier 21.

N! A noise detection circuit 22 is provided for detecting the level of . The noise detection circuit 22 detects the threshold 4 hold voltage Vth and the noise components N1 and N shown in FIG.
, make a comparison with . And threshold 4 hold voltage V
A noise component Nt having a higher level than th is detected. In addition, in the case of a strong electric field, the internal noise N is not detected due to the circuit operation described later, and the noise detection circuit 2 becomes zero.
2, a pulse-like detection signal 2 is obtained. The noise N is then supplied as a trigger signal to the monostable multivibrator 26 almost in synchronization with the time point t when the noise N is generated.

Incidentally, the level detection circuit 4b provided in the intermediate frequency amplification circuit 4 outputs an output voltage V whose level changes in accordance with the electric field strength of the received radio wave. , vc' are obtained. Output voltage V
C is supplied to the meter circuit 23. Further, one output voltage Vc' is supplied to the front end 2 via the No. 5 terminal as a control voltage for performing AGC.

In the case of a strong electric field as described above, it is usual to apply AGCl, for example, RFAGC, in the front end 2 to hold the electric field strength at a constant value.

Therefore, the noise component N! generated from the front end 2!
Even if it is amplified in each subsequent circuit, the AGC
The 8/N ratio of the output signal {circle around (2)} improves by the amount that is applied and the limiter applied by I F Arnp 4a.

Therefore, even if the internal noise Nt is obtained from the bypass filter 9, its voltage level is lower than the threshold voltage Vth.
If the level is lower than that, no detection output will be obtained due to the internal noise N.

The output signal ■o of the meter circuit 23 is supplied to the meter 24 via the No. 5 terminal as a signal indicating the electric field strength. The meter 24 has an indicated value that is proportional to the electric field strength, and the indicated value becomes large when the electric field is strong.

Note that the output signal Vo is also supplied to the inverter 25. When the electric field is strong, the output signal ■. is at a high level,
Output signal of inverter 25■. ′ becomes a low level. Then, the transistor Qt is turned off in a strong electric field.

A capacitor C and l resistors R, , R, are connected to the monostable multivibrator 26 via a No. 7 terminal. These constitute a time constant circuit of the monostable multivibrator 26. When transistor Q1 is off, resistor R7 can be ignored. Therefore, the monostable multivibrator 26 operates between T=C and .R1. From the monostable multi-by-break 26, a pulse signal P having a time width corresponding to the above-mentioned T is output. is obtained and supplied to the gate circuit 11.

The gate circuit 11 is in an open state for a time corresponding to the above-mentioned T. Therefore, the low frequency signal S is not transmitted for one hour. However, a hold circuit 27 is provided downstream of the gate circuit 11 via the No. 7 terminal. The hold circuit 27 is a time constant circuit composed of a capacitor, a resistor, and the like.

When the gate circuit 11 operates in the open state as described above,
The voltage level at the output end of the gate circuit 11 is held by the hold circuit 27 at the voltage level immediately before it becomes open. Then, after one hour has passed, the gate circuit 11
The voltage level at the output end of the signal S changes again in accordance with the voltage level of the low frequency signal S.

As a result, the buffer amplifier 28 is supplied with a low frequency signal 8o from which the noise components N1 and Nt have been removed from the detection output VD shown in FIG. Signal S.

is supplied to a demodulation circuit (not shown) via terminal No. 8.

Next, the circuit operation when the received radio wave is a weak electric field will be described.

In this case, the output voltage of the level detection circuit 4b is ■. .

■, ′ will be at a low level. Generally, RFAGC is not performed in the front end 2 when the electric field is weak. Regarding the detection output VD, since the electric field strength is weak, the voltage level of the low frequency signal S decreases.

However, since RFAGC and the like are not performed, the voltage level of the internal noise N generated in the front end 2 and the like is amplified by the amount that AGC and the like are not performed. Furthermore, if AGC is applied to control the gain of the amplifier circuit during a weak electric field, internal noise will be further amplified.

The noises N+ and Nt are subtracted from the output signal □ of the buffer amplifier 7 by a noise pass filter 9.

The noise NI-Nt is supplied to the noise detection circuit 22 through the same circuit operation as described above.

The voltage levels of the noises Nt and Nt are higher than the threshold voltage Vth, as shown in FIG. 3(G). Therefore, the monostable multivibrator 26 has a detection signal ■ synchronized with the pulse noise NI and the internal noise N.
2 will be supplied.

By the way, in the case of a weak electric field, the output voltage of the level detection circuit 4b is ■. will also be at a low level. And meter circuit 2
The output signal Vo of No. 3 also becomes a low level, and the meter 24 takes an indicated value proportional to the weak electric field strength.

Output signal V. is phase inverted by the inverter 25,
Output signal VQ' becomes high level. Therefore, transistor Q operates in the on state and resistor R.

One end of the terminal is grounded via the No. 9 terminal and the transistor Q1.

At this time, the resistance values seen from terminal 7 are resistors R1, IR,
becomes the combined resistance. The resistance value of the combined resistance R8 is the resistance R,
becomes smaller than the resistance value of Therefore, when the time constant with the capacitor CI is included, T=C, .Ro. T=C+・
No is also smaller than the time constant T in the case of a strong electric field. As a result,
From the monostable multipipe lake 26, a pulse signal P with a narrow time width as shown in FIG. will be obtained.

Since the gate circuit 11 operates in an open state during the rise time of the pulse signal PC, its time width becomes small. Hold circuit 27 operates in the same manner as described above. Therefore, the low frequency signal So obtained from the gate circuit 11 is
As shown in the figure (g), from the time when the noises Nt and Nt are generated, the waveform has a level held by the hold circuit 27 for a time period corresponding to T=C,·RO.

However, in the case of a weak electric field, the time width during which the high level is held by the hold circuit 27 is small, so the low frequency signal S0 is
The noises N, , N, are removed, and a waveform with less distortion corresponding to the level change of the signal S supplied to the gate circuit 11 is obtained.

Note that although the above-described embodiment deals with circuit operations at the bottom of a strong electric field and at a weak electric field, the same circuit operation as described above is also performed at an intermediate point between the two.

That is, the output signal VO of the meter circuit 23 changes in an analog manner in response to the electric field strength. Therefore, the transistor Q does not simply turn on or off, but its collector current IC (not shown) is controlled in inverse proportion to the electric field strength. As a result, the combined resistance Ro is also inversely proportional to the electric field strength, and the time constant determined by T=self·Ro is also the same. Therefore, the open time of the gate circuit 11 is maximum υ when the electric field strength is strong, and is gradually changed to a minute time width as the electric field strength becomes weaker.

〔effect〕

By varying the noise removal period in accordance with the electric field strength of the received radio waves, (1) the effect of being able to remove pulse noise that is particularly harsh at a strong electric field strength with a good 8/N ratio; is obtained.

(2) At the time of weak electric field strength where the S/N ratio deteriorates, the noise removal period can be varied to a minute time, so it is possible to remove not only the pulse noise but also the internal noise that causes the 87N ratio to deteriorate. , the effect that a large waveform change does not appear in the low frequency signal and good signal reception can be achieved is obtained.

Although the invention made by the present inventor has been specifically explained based on the embodiments above, the present invention is not limited to the above embodiments, and can be modified in various ways without departing from the gist thereof. Needless to say.

For example, transistor Q8 can be configured as a PNP transistor.

[Application field]

In the above description, the invention made by the present inventor is mainly applied to a PM receiver, which is the background field of application, but the invention is not limited thereto.

For example, the FM receiver may be for automobile or home use. Further, the present invention can also be applied to communication devices that use FM waves.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an embodiment of an FM receiver to which the present invention is applied; Fig. 2 (A) @ (Qo is a circuit diagram showing circuit operation in a strong electric field; Fig. 3 (A) is a circuit diagram showing circuit operation in a strong electric field; Circuit diagram showing circuit operation in a weak electric field, 2...Front end, 4...Intermediate frequency circuit, 5.
...Detection circuit, 8..Low pass filter, 9.. Bypass filter, 11.. Gate circuit, 22.. Noise detection circuit, 23.. Meter circuit, 26.. Monostable multivibrator, Q+-) transistor % R1,
, R1,...Resistor, C4...Capacitor, ■.・・・
・Detection output, S...Low frequency signal, N,...Pulse noise, N,...Internal noise, Vo...Output signal, P
o...Pulse signal, T...Time constant, ■th...Threshold voltage, IC...Cow conductor integrated circuit. Figure 2 t. Figure 3

Claims (1)

[Claims] 1. PM reception characterized by obtaining a control signal whose level changes in proportion to the electric field strength of received radio waves, and configured to vary the noise removal period of the audio signal obtained from the received radio waves according to this control signal. Machine.