JP2919511B2 - Multipath interference removal circuit - Google Patents

Description

The present invention relates to a multipath interference elimination circuit that prevents noise from being generated from a receiver when multipath interference is received.

(B) Conventional technology In an on-vehicle radio receiver such as a car radio, the electric field strength of a received signal changes according to the traveling of the automobile. In particular, when traveling in a valley of a building or the like, there is a problem that the electric field strength becomes extremely weak and the noise level rises sharply. In order to prevent this, the receiver is generally provided with a soft muting (hereinafter referred to as mute) circuit, and as shown in FIG. Let me
We are trying to reduce noise. In FIG. 2, a solid line (a) indicates a signal level when soft mute is not applied, a dashed line (b) indicates a signal level when soft mute is applied, and a solid line (c) indicates soft mute. The dashed line (d) indicates the noise level when soft mute is applied. Thus, if the soft mute is applied in this manner, it is possible to prevent the noise level from sharply increasing at the time of a weak electric field, and to improve the audibility.

(C) Problems to be Solved by the Invention However, since the soft mute circuit has an original property of gradually muting, it requires a capacitor to cope with a sudden change in electric field strength such as at the time of multipath interference. I couldn't do that, and some countermeasures were waiting.

(D) Means for Solving the Problems The present invention has been made in view of the above points, and has a detecting means for detecting the electric field strength of a received signal, and a mute drive signal corresponding to an output signal of the detecting means. A driving stage to generate, a mute signal generating means for generating a soft mute signal according to the mute drive signal, a mute means to mute a signal path according to the soft mute signal, and an output signal of the detection means. AC signal detection means for detecting an AC signal having a predetermined level or higher and controlling the gain of the IF amplification stage.

(E) Operation According to the present invention, the occurrence of multipath interference can be detected by detecting an AC signal of a predetermined level or more included in the detecting means for detecting the electric field intensity. Since the gain of the IF amplifier circuit is controlled by the output signal of the AC signal detection means, the IF signal is output when multipath interference occurs.
By reducing the gain of the amplifier circuit, noise can be reduced.

(F) Embodiment FIG. 1 is a circuit diagram showing one embodiment of the present invention.
Is the antenna, (2) is the RF received by the antenna (1)
An RF amplifier circuit for amplifying the signal; (3) a mixing circuit for mixing the amplified RF signal with a local oscillation signal obtained from a local oscillation circuit (4) to generate an IF signal; (6) is a detection circuit that performs FM detection of the IF signal, (7) is a mute circuit that mutes the output signal of the detection circuit (6), and (8) is a level detection of the IF signal. A detection circuit for detecting the electric field strength of the received signal by
(9) is a drive circuit for generating a mute drive signal according to the output signal of the detection circuit (8), and (10) is a capacitor (1).
(1) a mute signal generating circuit for generating a soft mute signal in response to an output signal of the driving circuit (9);
2) an AC signal detection circuit for detecting an AC signal of a predetermined level or more contained in the output signal of the detection circuit (8), and (13) determining that the electric field strength of the received signal has become a predetermined level or less. And a determination circuit for stopping the operation of the AC signal detection circuit (12).

Next, the operation will be described. When the electric field strength of the received signal received by the antenna (1) is large, the output level of the detection circuit (8) for detecting the electric field strength of the received signal becomes large, and the mute drive signal is output from the drive circuit (9). No signal is generated, and the mute signal generation circuit (10) and the mute circuit (7) do not operate. Therefore, the output signal of the detection circuit (6) passes through the mute circuit (7) and is led out to the output terminal (14).

When the electric field strength of the received signal decreases, the detection circuit (8) detects the decrease, and a mute drive signal is generated from the drive circuit (9) accordingly, and charging of the capacitor (11) is started. As the capacitor (11) is charged,
The soft mute signal generated from the mute signal generation circuit (10) increases, and accordingly, the conduction of the mute circuit (7) is reduced. As a result, as shown by the dashed lines (b) and (d) in FIG. 2, the signal and the noise level are reduced according to the electric field strength, and the noise can be reduced by the soft mute.

When multipath interference occurs, a large-level AC signal is generated in the output signal of the detection circuit (8) for detecting the electric field strength. The AC signal detection circuit (12) detects the large-level AC signal and determines that multipath interference has occurred. Said
The IF amplifier circuit (5) is constituted by, for example, a six-stage limiter amplifier, and the electric field strength of the received signal is controlled by the first-stage, third-stage, and fifth-stage limiter amplifiers of the six-stage limiter amplifier. Is detected by applying a signal obtained from the above to a detection circuit (8). The output signal of the AC signal detection circuit (12) acts on a sixth-stage limiter amplifier to control its gain. In FIG. 1, IF (1) in the IF amplifier circuit (5) indicates limiter amplifiers in the first to fifth stages, and IF (2) indicates a limiter amplifier in the sixth stage. However, the AC signal detection circuit (12) detects the AC signal caused by the multipath interference, and the IF amplification circuit (5)
A control signal is applied to the IF (2). Then, the IF
The gain of (2) decreases, and the overall gain of the IF amplifier circuit (5) decreases. The decrease in the gain of the IF amplifier circuit (5) is substantially the same as the state in which the mute is applied, and the received signal and the noise level are reduced. Therefore, this operation can reduce noise at the time of multipath interference.

The output signal of the AC signal detection circuit (12) is a limiter amplifier that does not hinder detection of the electric field strength, ie, IF
Since the voltage is applied only to (2), the gain reduction operation using the AC signal detection circuit (12) does not adversely affect the electric field strength detection operation.

By the way, when the electric field strength of the received signal is weak, the white noise level rises, and this white noise is mixed as an AC signal into the output signal of the detection circuit (8) for detecting the electric field strength. Then, there is a danger that the AC signal detection circuit (12) detects the AC signal caused by the white noise and performs the gain reduction operation of the IF amplification circuit (5). In the present invention, in order to prevent such a malfunction, the signal indicating the electric field strength of the received signal, that is, the level of the DC signal obtained from the drive circuit (9) is determined, and the AC signal detection circuit (1) is determined.
A determination circuit (14) for stopping the operation of 2) is provided.
By providing this discriminating circuit (14), it is possible to distinguish between a weak electric field (large AC signal and small DC signal) and a medium-strong electric field when multipath disturbance occurs (large AC signal and large DC signal).

(G) Effects of the Invention As described above, according to the present invention, it is possible to reduce noise when multipath interference occurs. Also, when the electric field strength of the received signal is weak, the operation of the AC signal detection circuit can be stopped, so that the soft mute circuit can be operated normally and the noise can be reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a characteristic diagram for explaining the present invention. (5) IF amplifier circuit, (7) Mute circuit,
(8) a detection circuit, (10) a mute signal generation circuit, (12) an AC signal detection circuit, (13) a discrimination circuit.

Claims (2)

(57) [Claims] 1. A detecting means for detecting an electric field strength of a received signal, a driving stage for generating a mute drive signal according to an output signal of the detection means, and a mute for generating a soft mute signal according to the mute drive signal Signal generating means; mute means for muting a signal path according to the soft mute signal; AC signal detecting means for detecting an AC signal having a predetermined level or more contained in an output signal of the detecting means; Determining that the intensity has fallen below a predetermined level, and stopping the operation of the AC signal detecting means based on the determination; controlling the gain of the IF amplification stage based on the output signal of the AC detecting means. A multipath interference elimination circuit characterized in that: 2. The IF amplification circuit includes a plurality of limiter amplifiers, the electric field strength of a received signal is detected from an arbitrary stage, and the AC signal detecting means detects the electric field strength of the received signal. 2. The multipath interference canceling circuit according to claim 1, wherein the gain of a limiter amplifying unit disposed downstream of the circuit is reduced.