JPH0370319A - Fm multipath noise reduction circuit - Google Patents

Abstract

PURPOSE:To surely reduce multipath noise by providing a control circuit comprising a control resistor, a control capacitor and a control transistor(TR) between an S-meter output terminal of an FM intermediate frequency circuit and an electrolytic capacitor circuit. CONSTITUTION:A control transistor(TR) is provided between a variable resistor VR and ground, a control resistor R4 is provide between th emitter and the base of the TR and other control resistor R5 is provided between the base and ground. Moreover, a control capacitor C3 is provided between a connecting point of the resistors R4, R5 and one terminal of a variable resistor VR. The control circuit is constituted of various control means. Thus, multipath noise is surely reduced from a weak to a strong electric field.

Description

[Detailed Description of the Invention] [Industrial Field of Application] This invention relates to FM multipath noise reduction circuits, and in particular, it can be used to reduce electric fields from weak to strong electric fields without significantly increasing the number of components. This invention relates to an FM multipath noise reduction circuit that can reliably reduce multipath noise throughout the range.

[Prior Art] -a. FM broadcast waves are easily affected by the influence of topography and the like, and the electric field strength changes significantly. Therefore, especially in vehicle-mounted FM receivers, skip noise may occur or be affected by multipath noise, resulting in a significant reduction in S/N. Various efforts have been made to overcome these shortcomings.

FIG. 2 is a block diagram showing a conventional FM multipath noise reduction circuit of this type. In this Figure 2, (
1) is an FM intermediate frequency circuit (FM-IF) that amplifies an intermediate frequency (IF) signal corresponding to a received signal from a receiving antenna (not shown). (2) is a noise canceller circuit. (NK), and (3)
is a stereo demodulation circuit (MPX) from which left and right channel signals as stereo signals can be obtained. A parallel circuit consisting of a capacitor (C1) and a resistor (R1) is inserted between the output terminal (S) of the FM intermediate frequency circuit (1) and the ground. A variable resistor (VR) is provided between the output terminal (S) of 1> and the input terminal (A) which functions as a high-cut control section of the stereo demodulation circuit (3). 6. A capacitor (C2) is provided between the input terminal (A) of this stereo demodulation circuit (3) and the ground.
In addition, a resistor (R3) is provided between the input terminal (B) functioning as the stereo control section of the stereo demodulation circuit (3) and the ground, and the input terminal (B) of the stereo demodulation circuit (3) Between A) and input terminal (B),
A separate resistor (R2) is inserted.

Next, its operation will be explained. FM intermediate frequency circuit (
The DC voltage level of the S meter output from the output terminal (S) of 1) is monitored, and this DC voltage level normally saturates at about 5 V when the input is 50 to 60 dBμ or more. Moreover, when this is 50 to 60 dBμ or less, the voltage gradually decreases. As this voltage decreases, the high-cut control voltage at the input terminal (A) and the stereo control voltage at the input terminal (B) of the stereo demodulation circuit (3) are respectively decreased. This increases the high-cut portion of the output stereo signal and reduces the separation, thereby reducing multipath noise. By the way, in the case of this conventional example, since it depends only on DC voltage, the input is 50 to 6
When it is 0 dBμ or more, the DC voltage is saturated, so the multipath noise reduction effect described above cannot be expected.

[Problems to be Solved by the Invention] Since the conventional device has the configuration and operates as described above, it is not possible to reduce multipath noise when a certain electric field exceeds a certain level. There was a problem with it disappearing.

This invention was made to solve the above problems, and it is possible to reliably reduce multipath noise from weak electric fields to strong electric fields without requiring a significant increase in the number of parts. An object of the present invention is to obtain an FM multipath noise reduction circuit that can perform the following.

[Means for Solving the Problems] The FM multipath noise reduction circuit according to the present invention has F
Based on the control voltage from the charging/discharging electrolytic capacitor circuit provided between the S meter signal output terminal of the M intermediate frequency circuit and the high-cut control signal input terminal and stereo control signal input terminal of the stereo demodulation circuit, DC compatible with S meter signal! C voltage level and AC! C voltage level and adjusts a stereo control voltage and a high-cut control voltage according to the output voltage, the FM multipath noise reduction circuit having an S meter output terminal of the FM intermediate frequency circuit and the S meter output terminal of the FM intermediate frequency circuit; A control circuit including a control resistor, a control capacitor, and a control transistor is provided between the electrolytic capacitor circuit and the electrolytic capacitor circuit.

[Function] In this invention, the operation of a control circuit including a control resistor, a control capacitor, and a control transistor provided between the S meter output terminal of the FM intermediate frequency circuit and the electrolytic capacitor circuit, Depending on the DC voltage level and AC voltage level of the S-meter signal output terminal, the voltages at the stereo control terminal and high-cut control terminal of the stereo demodulation circuit are reduced to generate multipath signals.
Reduce noise.

[Example] Hereinafter, an example of the present invention will be described with reference to the drawings. 1st
The figure is a block diagram illustrating an embodiment of the present invention. In this figure, (1) is an FM intermediate frequency circuit (F
8(2) is a noise canceller circuit (M-IF) which amplifies the intermediate frequency (IP) signal corresponding to the received signal from the receiving antenna (not shown).
NK), and (3) is a stereo demodulation circuit (MP
X) from which left and right channel signals as stereo signals can be obtained. And F.M.
A parallel circuit consisting of a capacitor (C1) and a resistor (R1) is inserted between the output terminal (S) of the intermediate frequency circuit (1) and the ground.
1) output terminal (S) and the input terminal (A) that is used as a high-cut control section of the stereo demodulation circuit (3).
) is connected to a variable resistor (
VR) is provided. A capacitor (C2) is provided between the input terminal (A) of this stereo demodulation circuit (3) and the ground, and an input terminal that functions as a stereo control section of the stereo demodulation circuit (3) A resistor (R3) is provided between the terminal (B) and the ground, and the input terminal (A) of the stereo demodulation circuit (3)
A separate resistor (R2) is inserted between the input terminal (B) and the input terminal (B).

The process up to this point is similar to the conventional example described above, but in the embodiment of the present invention, the variable resistor (VR) til
A control transistor (TR) is provided between I and the ground side, with an emitter connected to the former side and a collector connected to the latter side. A control resistor (R4) is provided between the emitter and base of the transistor (TR),
A separate control resistor (R5) is provided between the base and ground. Moreover, between the connection point of the resistors (R4) and (R5) and the connection point of the output terminal (S) of the FM intermediate frequency circuit (1) and the resistance (R1),
A control capacitor (C3) is provided.

A desired control circuit is constituted by these various control means.

Next, the operation of the above embodiment shown in FIG. 1 will be explained. S from the output terminal (S) of the FM intermediate frequency circuit (1)
The voltage level of the meter output is monitored. Now, suppose that the electric field is 50 to 60 dBμ or more and multipath noise is generated.At this time, although the DC voltage is in a saturated state, a pulsed AC voltage is also generated, and this pulsed The AC voltage is made to change more toward the lowest side than the DC saturation voltage, and this pulsed AC voltage causes the S meter output voltage to drop momentarily. At this time, the transistor (T
The base voltage of the transistor (TR) is pulled down via the capacitor (C3) so that the voltage at the base of the transistor (TR)
is turned on, and its emitter-collector resistance value decreases. Therefore, the charge in the charging/discharging capacitor (C2) is discharged as a collector current to the transistor (TR), and the potential of the capacitor (C2) decreases. Thus, the voltages for the high cut and stereo controls are reduced to provide multipath noise reduction. Note that when multipath noise does not exist, the transistor (TR) is reverse biased and in an off state, and the operation in this case is similar to that of the conventional example.

[Effects of the Invention] As explained above, the FM multipath noise reduction circuit according to the present invention has an S-meter signal output terminal of an FM intermediate frequency circuit, a high-cut control signal input terminal of a stereo demodulation circuit, and a stereo control Based on the control voltage from the charging/discharging electrolytic capacitor circuit provided between the signal input terminal and the output voltage, stereo control is performed depending on the DC voltage level and AC voltage level corresponding to the S meter signal. FM multipath noise reduction circuit adapted to adjust voltage and high-cut control voltage, the FM multipath noise reduction circuit comprising:
A control circuit consisting of a control resistor, a control capacitor, and a control transistor is provided between the S meter output terminal of the intermediate frequency circuit and the electrolytic capacitor circuit. Pulse ACffi pressure is also generated in the S meter output, and this pulse AC voltage changes significantly to the negative side, and this pulse AC voltage causes the S meter output voltage to drop momentarily. Utilizing this property, high-cut and high-cut output signals can be achieved.
Since it performs operations such as control and stereo separation control, it has the effect of reliably preventing the generation of multipath noise due to electric field fluctuations of the vehicle-mounted FM receiver. .

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention, and FIG.
The figure is a block diagram showing a conventional system. (1 is the FM intermediate frequency circuit (FM-IF), (2 is the noise canceller circuit (NK), (3 is the stereo demodulation circuit (MPX), (R4, (R5)
) is a control resistor, (C3 is a control capacitor, (TR is a control transistor.) In Figure 5, the same symbols indicate the same or equivalent parts.

Claims (1)

[Claims] (1) The control voltage from the charging/discharging electrolytic capacitor circuit provided between the S meter signal output terminal of the FM intermediate frequency circuit and the high-cut control signal input terminal and stereo control signal input terminal of the stereo demodulation circuit. Based on the output voltage, the output voltage controls the stereo control voltage and the high-cut voltage, depending on the DC and AC voltage levels corresponding to the S-meter signal.
An FM multipath noise reduction circuit configured to adjust a control voltage, the circuit including a control resistor, a control capacitor, and a control capacitor between the S meter output terminal of the FM intermediate frequency circuit and the electrolytic capacitor circuit. An FM multipath noise reduction circuit, characterized in that it is provided with a control circuit made up of transistors.