JPH0818468A - Noise suppression circuit for fm receiver - Google Patents

Abstract

PURPOSE:To provide a noise suppression circuit for suppressing noise when radio waves are weak and a lot of the noise is present, making the current consumption of a low frequency amplification part small, reducing the degradation and damage of a speaker or an earphone and prohibiting influence on an auditory nerve in an FM receiver. CONSTITUTION:An output voltage is fixed in an amplitude limiter 2 and intermediate frequency signals (a) are frequency-detected in a frequency detector 3 and outputted as demodulation signals (c). Thereafter, they are passed through a filter and outputted to a preamplifier 5. Also, the intermediate frequency signals (a) are rectified and smoothed in an S meter voltage output circuit 8, outputted as an S meter voltage (b), inputted to the negative input terminal of an OP amplifier 9 and amplified. In the meantime, the preamplifier 5 is constituted so as to control an amplification factor by a voltage and to make the amplification factor smaller as the S meter voltage (b) gets smaller than a set value (as the noise is increased). In this case, the voltage of the positive input terminal of the OP amplifier 9 is variable, an offset voltage is adjusted by it and the previous set value is increased/decreased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit for suppressing noise output when an FM receiver receives a weak electric wave.

[0002]

2. Description of the Related Art FIG. 4 is a block diagram of an example of a conventional FM (frequency modulation) wave receiver. In the FM receiver, only the frequency component needs to be detected.
In the section 11, the gain is made sufficiently large to be amplified, and the amplitude limiter 1
After the amplitude is kept constant at 2 to cut the AM component and the like including pulse noise, the FM detector 13 demodulates. Therefore, if the demodulated signal e has a strength equal to or higher than a certain electric field strength of the radio wave input to the antenna, it is less likely to be affected by pulse noise, fading, etc. To be

However, since the output level of the demodulated signal e is constant irrespective of the received electric field strength, when the target radio wave is weak (the electric field strength is small), the amount of demodulated sound is small, so that the sound is not output. Large noise appears. In order to eliminate this annoying noise, an FM receiver for wireless communication is often provided with a mechanism called squelch, and an FM broadcast receiver is often provided with a mechanism called muting. For example, in the FM receiver shown in FIG. 4, only the noise component of the detected demodulated signal e is amplified and rectified by the squelch amplifier 14, and when the noise component is large, the squelch switch 16 is controlled to control the preamplifier 15. A squelch that turns off the output and turns off the output of the main amplifier 17 is provided.

As described above, the noise level or the signal strength is measured, and when the noise level is high or when a radio wave having a predetermined electric field strength level or less is input, the low frequency output (hereinafter, AF output) is completely cut. Therefore, weak radio waves that are hard to hear due to a lot of noise cannot be received.

In order to make the squelch operation of the FM receiver of FIG. 4 easier to understand, an explanation will be given using the graph of FIG. 5 showing an example of the relationship between the received electric field strength to the FM receiver and the noise level of the AF output. . First, as shown in FIG. 5, the AF (signal + noise) level from the speaker of the FM receiver is constant regardless of the received electric field strength. Further, at a received electric field strength of 10 dBμ or more, noise is suppressed to a constant level, and the S / N ratio at this time is usually 30 dB or more. However, the received electric field strength is 10 dB
As the value decreases from around μ, noise begins to increase, and at the end, it becomes just noise and cannot be heard.

Therefore, the squelch level of the squelch amplifier 14 for turning off the squelch switch 16 is about -10 dBμ in terms of received electric field strength, and about 15 dB from the noise level when the received electric field strength is sufficiently large in terms of S / N ratio. It has been set to a bad point. When the squelch mechanism operates at this point, the received electric field strength is −10 as shown in FIG.
The AF output from the speaker becomes 0 below dBμ. However, many FM receivers generate annoying noises such as “chuck” and “bock” at the moment this squelch operates.

In a receiver for FM broadcasting, the S / N ratio is the most important and it is not necessary to forcibly listen to weak radio waves with a poor S / N ratio. Was enough.

[0008]

However, in the FM receiver for wireless communication, the squelch is open when the propagation state of the radio wave is bad or the radio wave is weak because it is too far from the communicating party. Until the squelch works, you have to hear the target signal from among the ferocious noises. Such noise may cause hearing loss such as tinnitus or deafness to a person who constantly receives the noise. Also, the squelch itself, which eliminates noise, produces noise each time it is operated, which is not desirable for the auditory nerve. These sounds are heard when listening with earphones like an outside police officer.
There was a problem that it had a greater adverse effect on the auditory nerve.

The present invention has been proposed in view of the above problems, and in an FM receiver, even when an input radio wave is weak and there is a lot of noise, the low frequency amplification unit is operated until the squelch operates. An object of the present invention is to provide a noise suppression circuit that consumes less current, has less deterioration or damage to speakers or earphones, and does not adversely affect the auditory nerve.

[0010]

In a noise suppression circuit for an FM receiver according to a first aspect of the present invention, in order to solve the above problems, an electric field strength detecting means for outputting a control signal according to the electric field strength of a received radio wave. And a low frequency amplifier that reduces the output according to the control signal.

Further, in the noise suppression circuit of the FM receiver according to claim 2, a high frequency amplifier for amplifying the received radio wave and outputting a high frequency signal, and an S for rectifying the high frequency signal and outputting an S meter voltage. A meter voltage output circuit,
An amplitude limiter that limits the amplitude of the high-frequency signal and outputs a fixed output voltage, a frequency detector that frequency-detects the output of the amplitude limiter and outputs a demodulation signal, and a low-frequency amplifier that amplifies the demodulation signal. In a noise suppression circuit of an FM receiver including a frequency amplifier, control signal output means for amplifying the S meter voltage and outputting a control signal, and input, output or amplification of the low frequency amplifier according to the control signal. The output control means for controlling the rate to reduce the output is provided. The control signal may be analog data or digital data.

[0012]

In the noise suppression circuit of the FM receiver according to the first aspect, the electric field strength detecting means first outputs the control signal according to the electric field strength of the received radio wave. Since the output of the low frequency amplifier is reduced according to this control signal, the output of the low frequency amplifier is reduced when the electric field strength of the received radio wave is low.

In the noise suppression circuit of the FM receiver according to the second aspect, the received radio wave is first amplified by the high frequency amplifier and is output as a high frequency signal to the S meter voltage output circuit and the amplitude limiter. The amplitude limiter limits the amplitude of the input high frequency signal to make the output voltage constant, and then outputs the high frequency signal to the frequency detector. After that, it is detected by a frequency detector and amplified by a low frequency amplifier. On the other hand, the S output from the S meter voltage output circuit
The meter voltage is amplified by the control signal output means and output as a control signal. The output control means is controlled according to the control signal to control at least one of the input, output and amplification factor of the low frequency amplifier, and the output of the low frequency amplifier is reduced. Since the output of the low frequency amplifier is reduced by this output control means, the output of the low frequency amplifier is reduced in proportion to the electric field strength of the received radio wave.

[0014]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a block diagram of an embodiment of a noise suppression circuit of an FM receiver according to the present invention, FIG. 2 is a graph showing the relationship between electric field strength and noise level using the circuit of the embodiment of FIG. 1, and FIG. It is an input / output characteristic view of an amplifier.

In FIG. 1, reference numeral 1 denotes an RF.I which amplifies a received radio wave, frequency-converts it, and outputs it as an intermediate frequency signal a.
F part, 2 is an amplitude limiter for limiting the amplitude of the intermediate frequency signal a to keep the output voltage constant, 3 is a frequency detector for detecting the output of the amplitude limiter 2 and outputting it as a demodulated signal c, 4
Is a squelch amplifier which detects and amplifies only noise included in the demodulated signal c, rectifies and outputs the rectified signal. Reference numeral 5 is a preamplifier that amplifies the demodulated signal c and has an amplification factor (hereinafter, gain) of 20 dB. The gain is controlled by a voltage from an OP amplifier 9 described later. 6 is a squelch amplifier 4
Preamplifier 5 when the noise detected in
The squelch switch 7 for cutting the output signal of is a main amplifier connected to the squelch switch 6 for amplifying the output signal and driving the speaker. The gain of the preamplifier 5 may be kept constant, an electronic volume or an attenuator may be provided in front of or behind the preamplifier 5, and the input voltage or the output voltage of the preamplifier 5 may be controlled by voltage control.

An S-meter voltage output circuit 8 rectifies and smoothes the intermediate frequency signal a and outputs it as an S-meter voltage b.
Reference numeral 8a is an S-meter that operates by the S-meter voltage b, which represents the strength of the received signal, and 9 is an OP amplifier that amplifies the S-meter voltage b and outputs it as a control voltage d. Also,
A constant voltage is applied across the variable resistor R,
The offset voltage applied to the positive input terminal of the amplifier 9 can be adjusted. Here, the intermediate frequency signal a is a high frequency signal, the control voltage d is a control signal, the RF / IF unit 1, the S meter voltage output circuit 8 and the OP amplifier 9 are electric field intensity detecting means, and OP
The amplifier 9 corresponds to the control signal output means, and the preamplifier 5 corresponds to the low frequency amplifier and the output control means.

Next, the operation of the noise suppression circuit of the FM receiver of FIG. 1 will be described with reference to the graph of FIG. Figure 2
In the graph of, the squelch circuit including the squelch amplifier 4 and the squelch switch 6 is not operated, and the output of the preamplifier 5 and the input of the main amplifier 7 are connected to each other. The AF output in FIG. 2 is the output from the speaker.

First, the received electric field strength reaching the antenna of the RF / IF unit 1 in FIG. 1 is, for example, 30 dB in FIG.
When μ is sufficiently large, the noise level is sufficiently smaller than the AF level as in the conventional FM receiver. next,
When the received electric field strength becomes 10 dBμ or less, in the case of the conventional example, the noise level gradually rises as shown by the dashed line, and finally the noise level reaches the AF level. Normally, the squelch switch 6 cuts the demodulated signal c by this time. However, according to the noise suppression circuit of FIG. 1, the S meter voltage b output from the S meter voltage output circuit 8 is inverted and amplified by the OP amplifier 9 and added to the preamplifier 5 as the control voltage d. The gain can be kept small. Therefore, the overall AF level decreases as indicated by the solid line as the received electric field strength decreases. Naturally, the noise level included therein also decreases according to the AF level, and as a result, the noise level does not increase.

The circuit operating point A for starting the operation of this circuit is set by changing the offset voltage of the OP amplifier 9 by the semi-fixed resistor R. This will be described with reference to the input / output characteristic diagram of FIG. As shown in the figure, as the S meter voltage b decreases (noise increases), the control voltage d increases, the gain of the preamplifier 5 decreases (attenuation increases), and the output voltage decreases. Therefore, the AF output from the speaker becomes small. Here, the relationship between the control voltage d and the gain of the preamplifier 5 is that the gain is 20 dB (attenuation 0 dB) at 0 volt, and 1
It is assumed that the gain is 10 dB (attenuation 10 dB) at volt, and the gain is 0 dB (attenuation 20 dB) at 2 volts or more.

The above example will be specifically described with reference to FIG. In the case of the solid line input / output characteristic graph, the control voltage d is 0 V when the S meter voltage b is 2.0 V or higher, and the gain of the preamplifier 5 is 20 dB, which is the maximum. However, the S meter voltage b
The gain of the preamplifier 5 becomes smaller and the output voltage thereof becomes smaller as the voltage becomes smaller than 2.0 V (noise becomes large). Therefore, the AF output of the main amplifier 7 also becomes smaller. By the way, when the S meter voltage b is 1.5 volts, the control voltage d is 1 volt and the gain is 10 dB. When the S meter voltage b is 1.0 volt or less, the control voltage d is 2 volts and the gain is 0 dB.

When the variable resistor R of FIG. 1 is moved, the offset voltage of the OP amplifier 9 changes, so that the input / output characteristic graph of FIG. 3 moves up or down. Now, according to the input / output characteristic graph of the upper one-dot chain line moved in this way, S
The gain starts to drop from the meter voltage b of 2.25 V, 1.5
At the time of the volt, the control voltage d is also 1.5 volt and the gain is 5 dB.
Becomes That is, by adjusting the variable resistance R, A
The F output is attenuated 5 dB earlier than the solid line input / output characteristics. In FIG. 2, it means that the circuit operating point A has moved to the side (right) having a larger electric field strength. If the variable resistor R is moved in the opposite direction to the previous case, according to the input / output characteristic graph of the lower dashed line in FIG.
However, the gain starts to drop from 1.75 V, and at 1.5 V, the control voltage d also becomes 0.5 V and the gain is 15 dB. That is, by adjusting the variable resistance R, the AF output is attenuated by 5 dB later than the input / output characteristics indicated by the solid line. In FIG. 2, it means that the circuit operating point A has moved to the smaller electric field strength (left). In other words, the state where the noise is large becomes longer. In this way, the starting point of operation of the circuit is adjusted.

The final AF level output B in the state where there is no signal at all and only noise changes the amplification degree of the OP amplifier 9 (slope of the graph in FIG. 3) or controls the preamplifier 5. The input sensitivity of the voltage is changed or the main volume of the main amplifier 7 is changed to adjust the voltage. When the squelch circuit including the squelch amplifier 4 and the squelch switch 6 is operated, the output signal of the preamplifier 5 is cut at the squelch operating point C in FIG. 2, and the AF output becomes 0 as shown by the thick broken line. Here, the squelch operating point C is of course variable.

In the noise suppression circuit of the present invention, the preamplifier 5 may be directly controlled by the S meter voltage, in which case the circuit becomes simple. An A / D converter is used instead of the OP amplifier 9 to convert the S meter voltage into digital data, which is shifted to adjust the operation start point of the circuit, and then D / A converted to perform preamplification. A gain of 5 may be controlled. Further, the preamplifier 5 may be a digitally controlled amplifier to control the gain of the digital data as it is. According to the noise suppression circuit of the embodiment, OP
By changing the offset voltage applied to the positive input terminal of the amplifier 9, the operation starting point of this circuit can be changed.

[0024]

The noise suppression circuit of the FM receiver according to the present invention is configured as described above, and the volume is automatically reduced when the received radio wave is weak. And the deterioration or damage of the speaker or earphone due to excessive noise input is reduced. Also, due to the intense noise when the received radio wave is weak,
The acoustic nerve is less likely to be damaged. Moreover, since the existing S meter voltage is used, it can be configured with a few additional circuits. Furthermore, since the volume of the FM receiver decreases when the radio wave is weak, this has the effect that the distance between the own station and the partner station can be estimated like an amplitude-modulated (AM) wave.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of a noise suppression circuit of an FM receiver according to the present invention.

FIG. 2 is a graph showing the relationship between electric field strength and noise level in an example using the circuit of FIG.

FIG. 3 is a graph of input / output characteristics of an OP amplifier.

FIG. 4 is a block diagram of an example of a conventional FM receiver with squelch.

FIG. 5 is a graph showing an example of the relationship between electric field strength and noise level when the squelch is not operated in the conventional FM receiver.

FIG. 6 is a graph showing an example of the relationship between electric field strength and noise level when squelch operates in a conventional FM receiver.

[Explanation of symbols]

1 RF / IF unit (electric field strength detection means) (high frequency amplifier) 2 amplitude limiter 3 frequency detector 4 squelch amplifier 5 preamplifier (low frequency amplifier) (output control means) 6 squelch switch 7 main amplifier 8 S meter voltage output circuit (Field strength detecting means) 8a S meter 9 OP amplifier (field strength detecting means) (control signal output means) a intermediate frequency signal (high frequency signal) b S meter voltage c demodulation signal d control voltage (control signal)

Claims (2)

[Claims] 1. A field strength detection means for outputting a control signal according to the field strength of a received radio wave, and a low frequency amplifier for reducing the output according to the control signal.
A noise suppression circuit for an FM receiver, comprising: 2. A high frequency amplifier for amplifying a received radio wave to output a high frequency signal, an S meter voltage output circuit for rectifying the high frequency signal and outputting an S meter voltage, and limiting an amplitude of the high frequency signal. An FM receiver including an amplitude limiter that outputs a fixed output voltage, a frequency detector that frequency-detects the output of the amplitude limiter and outputs a demodulation signal, and a low-frequency amplifier that amplifies the demodulation signal. In the noise suppression circuit, the control signal output means for amplifying the S meter voltage and outputting a control signal, and controlling the input, output or amplification factor of the low frequency amplifier according to the control signal to reduce the output. A noise suppression circuit for an FM receiver, comprising: an output control means.