JPS6327479Y2 - - Google Patents

Description

[Detailed Description of the Invention] Field of the Invention The present invention relates to a noise reduction circuit for a radio receiver, particularly an AM radio receiver.

Prior Art The AGC circuit of a conventional AM radio receiver uses a circuit as shown in Fig. 1.

In Figure 1, 1 is an intermediate frequency amplification (IF) circuit, 2 is a detection circuit, Q ₁ is a detection transistor,
3 is a smoothing circuit, 4 is an AGC (automatic gain control) circuit,
5 is AGC voltage ripple filter circuit, R ₁ , C ₁
are resistors and capacitors that constitute the AGC voltage ripple filter circuit, _Q2 is a grounded emitter transistor, 10 is a DC amplifier for AGC voltage amplification, and the output terminal 6 of the DC amplifier is an intermediate frequency amplification (IF) circuit 1. is connected to the AGC voltage input terminal 7.

Next, the operation of the circuit shown in FIG. 1 will be explained.

When a normal signal as shown in Fig. 2 A appears as an intermediate frequency (IF) signal at the output terminal (point a) of the IF circuit 1, the signals appear at points b, C, and d in Fig. 1, respectively. Voltages as shown in Figure 2 B, C, and D appear. The voltage at point c controls transistor Q ₂ , drives DC amplifier 10 , and controls the gain of IF circuit 1 .

That is, when the received input signal is large, the gain of IF circuit 1 is decreased, and when the received input signal is small, the gain of IF circuit 1 is increased.
The AGC circuit 4 operates so that the DC level of the voltage at point b is always constant.

By the way, noise may be superimposed on the IF signal appearing at point a, as shown in Figure 3A, due to ignition noise or discharge noise of a car, or noise generated when switching circuits such as when the power is turned on and off or when switching bands. At this time, the voltages at points b, c, and d in Fig. 1 become as shown in Fig. 3 B, C, and D, respectively, and even though noise is generated, point c in Fig. 1 is noisy. The voltage waveform is the same as when it is not occurring.
This is because the time constant of the AGC voltage ripple filter circuit 5 is considerably larger than the time constant of the smoothing circuit 3 of the detection circuit 2. As a result, noise appeared at point d, making it unpleasant to the ears.

For this reason, radios have traditionally been equipped with a muting circuit that works with a changeover switch, or have an ignition noise removal circuit, in order to eliminate the noise that occurs when switching circuits such as when turning the power on and off or switching bands. receiver is known.

Problems with the prior art Noise can be removed by providing a muting circuit linked to the changeover switch or an ignition noise removal circuit, but on the other hand, the circuit configuration becomes complicated and the cost increases. It may not be possible to use it in low-cost radio receivers.

Purpose of the invention In view of the above, the object of the present invention is to obtain a noise reduction circuit for a radio receiver that reduces noise with a simple circuit.

Embodiment of the invention FIG. 4 is a circuit diagram showing an embodiment of the invention used in an AM radio receiver.

In FIG. 4, the same parts as in FIG. 1 are designated by the same numbers as in FIG. 1.

The circuit shown in FIG. 4 is characterized in that a series circuit 8 consisting of a diode D ₁ and a resistor R ₂ is connected in parallel to the AGC voltage ripple filter circuit 5. The other configurations are the same as in FIG. 1.

Next, the operation of the circuit shown in FIG. 4 will be explained.

Pulse noise has now entered the received input signal, and a
If the voltage waveform at point B becomes as shown in FIG. 5A, the voltage waveform at point b after detection includes noise as shown in FIG. 5B. Next, considering the voltage waveform at point c, when there is no noise, point c has a voltage of E ₁ [V] as shown in FIG. 2C. Here, the conduction voltage of diode _D1 is E _D1
[V], when a voltage exceeding (E ₁ + E _D1 ) [V] appears at point b, the excess voltage is applied to E ₁ [V] at point c through diode D ₁ and resistor R ₂ . They overlap, resulting in a voltage waveform as shown in FIG. 5C. As a result, the AGC circuit 4 operates, and the gain of the IF circuit 1 decreases by the time the noise is generated, and the output terminal (point a) of the IF circuit 1 has a voltage waveform as shown in Figure 5 D. appear. Therefore, point b and point c are
The voltage waveforms are as shown in Figure 5 E and F, respectively.
The audio signal that appears at point d has a distorted waveform in the noisy part, as shown in Figure 5 (g).
Harsh noise has been reduced.

In the above explanation, we have explained the case where pulse noise occurs, but it is not pulse noise, but rather large noise such as that of a power switch.
Even if noise occurs due to large DC fluctuations during ON/OFF or band switching, the AGC circuit operates in the same way and can reduce the noise.

In this way, by connecting a series circuit consisting of a diode and a resistor in parallel to the AGC voltage ripple filter circuit, noise components can be reduced.

The circuit in Figure 4 is an example of a circuit where the DC levels at points b and c do not differ much. However, if the DC level at point b is higher than the DC level at point c, the circuit shown in Figure 4 This circuit is not convenient. In other words, if there is a difference greater than the conduction voltage of diode _D1 between points b and c, diode _D1 will turn on even if there is no noise.
, and no longer exhibits normal AGC characteristics. Further, even if there is no difference between point b and point c that is greater than the conduction voltage of diode _D1 , normal AGC characteristics will not be exhibited if diode _D1 becomes conductive at the peak of the modulated signal.

Therefore, in this case, as shown in Figure 6,
In parallel with the AGC voltage ripple filter circuit 5,
A series circuit 8 consisting of a capacitor C ₂ , a diode D ₁ and a resistor R ₂ is connected.

Note that the resistor R ₂ is for controlling the noise reduction effect, and the larger the value of the resistor R ₂ , the smaller the noise reduction effect, and the smaller the value of the resistor R ₂ , the larger the noise reduction effect. Therefore, this is limited to cases where the DC levels at point b and point c are the same or higher at point c, but as the simplest circuit example, as shown in Figure 7, a circuit connected in parallel to the AGC voltage ripple filter circuit 5 is , a noise reduction effect can be obtained even if only the diode _D1 is connected. However, in this case, the noise reduction effect may be too effective and the audio output may feel interrupted, so be careful.

As another example in which the DC level at point b is higher than the DC level at point c, as shown in _FIG . The voltage at point c may be increased. That is, a reverse bias voltage may be applied to the diode _D1 . However, in this case, the AGC characteristics change slightly depending on the resistance _R3 , so care must be taken.

In each of the above embodiments, the gain of the IF circuit 1 is lowered when the DC level at point b or c rises, and when the DC level at point b or c falls.
Although the case of an AGC circuit with a characteristic that increases the gain of the IF circuit 1 has been described, the present invention is not limited to such an embodiment, but also applies to an AGC circuit with characteristics opposite to those described above.
It can also be applied to circuits. In this case, the direction of diode _D1 is shown in Fig. 4, Fig. 6,
The connection may be made in the opposite direction to that shown in FIGS. 7 and 8.

Effects of the invention As described above, the invention has a simple circuit configuration in which a circuit for generating an AGC voltage according to noise when noise occurs is connected in parallel to the AGC voltage ripple filter circuit. can reduce noise.

Therefore, the present invention is particularly useful for low-cost radio receivers.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing a conventional radio receiver, Figure 2 A, B, C, D and Figure 3 A, B, C, D are voltage waveform diagrams at each point in Figure 1. Figure 4 is a circuit diagram showing an embodiment of the noise reduction circuit for a radio receiver according to the present invention, and Figure 5 shows a, b, c, d, ho,
F, G are voltage waveform diagrams at each point in Figure 4,
7 and 8 are principal circuit diagrams showing other embodiments of the present invention, respectively. 4...AGC circuit, 5...AGC voltage ripple filter circuit, _D1 ...diode, _R2 ...resistance,
8...Series circuit.

Claims (1)

[Scope of utility model registration claims] (1) AGC circuit connected to the detection circuit
The AGC voltage ripple filter circuit is configured with a resistor and a capacitor, and in order to generate an AGC voltage according to the noise when noise occurs, the above-mentioned
A noise reduction circuit for a radio receiver, characterized in that a circuit including at least a diode is connected in parallel to a resistor constituting an AGC voltage ripple filter circuit. (2) The noise reduction circuit for a radio receiver according to claim 1, wherein the circuit for generating an AGC voltage according to noise is a series circuit consisting of a diode and a resistor. (3) Noise reduction in a radio receiver according to claim 1, wherein the circuit for generating an AGC voltage according to noise is a series circuit consisting of a capacitor, a diode, and a resistor. circuit. (4) The noise reduction circuit for a radio receiver according to claim 1, wherein a reverse bias voltage is applied to the diode.