JPS6334358Y2 - - Google Patents

Description

[Detailed description of the invention] This invention relates to a noise suppression device for removing multipath noise in an FM receiver.
An IF amplifier is connected, this attenuator is driven by the level detection output to apply AGC, and by setting a large time constant of the AGC loop, a momentary drop in the antenna input level is detected, and the electronic volume circuit and high-cut control By controlling the circuit, separation control circuit, etc., the noise suppression device can be operated reliably only when multipath noise occurs, whether in strong electric field areas or medium/weak electric field areas. The point is that it can be done.

Generally, when multipath noise occurs, the antenna input level decreases, so conventionally, a multipath noise suppression device has been operated directly using a signal meter voltage representing the antenna input level.

Figure 1 shows a block diagram of such a conventional device, and when the signal meter voltage taken from the middle of the IF circuit drops below a predetermined value, the electronic volume circuit, high-cut control circuit, and separation control circuit are activated. It is now possible to control multipath noise to suppress multipath noise.

However, in such a device, when multipath noise occurs in a strong electric field area, the signal meter voltage may not fall to a predetermined value, so the noise suppression device may not operate.
If the operating point is set high to prevent this, there is an inconvenience that the noise suppression device will operate even when no multipath noise is occurring in areas with medium or weak electric fields. .

The purpose of this invention is to improve the shortcomings of the conventional devices mentioned above, and to create a noise suppression device that operates reliably only when multipath noise occurs, whether in strong electric field areas or medium/weak electric field areas. It is to obtain.

An embodiment of this invention will be described with reference to FIG.

The front end output is led to a control signal IF amplifier via an attenuator, and its output signal is input to a level detection circuit.

The output signal of the level detection circuit is input to an electronic volume circuit, a high cut control circuit, and a separation control circuit, respectively.
Control its behavior.

In addition, the output signal of the level detection circuit is connected to an attenuator to prevent the input level of the control signal IF amplifier from exceeding a predetermined value.
AGC is applied.

The time constant of the AGC loop is set sufficiently large,
The attenuation amount of the attenuator does not follow sudden changes in the antenna input level.

The reason for installing the IF amplifier for the control signal is that AGC must be applied from a place where the antenna input level is relatively low, so the tuner can obtain a sufficient S/N to obtain the signal meter voltage (control signal voltage). This is because it is necessary to amplify the level.

Next, its operation will be explained with reference to FIG.

Now, as the antenna input level gradually increases, the attenuator operates at point A, and the signal meter voltage (control signal voltage), which is the output signal of the level detection circuit, does not increase any further, as shown by the solid line. (If the attenuator does not operate, it will rise as shown by the dotted line).

When multipath noise occurs at point B, the antenna input level drops by about 20 dB. However, since the time constant of the AGC is set sufficiently large, the attenuator's attenuation does not follow sudden changes in the antenna input level and maintains the (large) attenuation just before multipath noise occurs. Therefore, when the antenna input level decreases due to the occurrence of multipath noise, the signal meter voltage is sufficiently attenuated and falls below the control start voltage C.
As a result, the electronic volume circuit, high cut control circuit, and separation control circuit are controlled and multipath noise is suppressed.

Fig. 4 shows another embodiment, which differs from the previous embodiment in that instead of obtaining the AGC signal for driving the attenuator from the level detection circuit of the control signal IF amplifier, it is used inside the IF circuit. The operation is the same as that of the previous embodiment.

As explained above, the noise suppression device for an FM receiver of this invention connects an IF amplifier for control signals to the front end via an attenuator, drives this attenuator with level detection output to apply AGC, and loops the AGC loop. By setting the time constant of The noise suppression device can be reliably operated only when multipath noise occurs in both electric field areas and medium/weak electric field areas.

In the above embodiment, the electronic volume circuit, high-cut control circuit, and separation control circuit are all controlled, but it is not necessary to do so, and only two or one of them are controlled. Even if only one is controlled, a certain effect can be obtained.

[Brief explanation of the drawing]

Fig. 1...Block diagram of a conventionally known noise suppression device, Fig. 2...Block diagram of one embodiment of this invention, Fig. 3...Explanatory diagram of its operation, Fig. 4...Block diagram of another embodiment. .

Claims (1)

[Scope of utility model registration request] A control signal IF amplifier connected to the front end via an attenuator, and an AGC circuit that drives the attenuator with the output of a level detection circuit connected to the control signal IF amplifier or the output of a level detection circuit included in the IF circuit. By setting a large time constant of the AGC loop, it detects an instantaneous drop in the antenna input level, and uses the output of the level detection circuit connected to the control signal IF amplifier to detect at least the electronic volume circuit and the high A noise suppression device for an FM receiver, characterized in that it controls one of a cut control circuit and a separation control circuit to remove multipath noise.