JPH0744471B2 - Miting circuit - Google Patents

Description

TECHNICAL FIELD The present invention relates to a muting circuit used in an FM receiver.

2. Description of the Related Art Generally, FM broadcast channels are provided in steps of 50 KHz (or 100 KHz), but in some areas, they may be provided in steps other than 50 KHz, such as 86 KHz steps. In receivers that receive FM broadcasts in 50KHz steps, the IF (intermediate frequency) is set to 10.7MHz, and 86KHz
In the receiver that receives the step FM broadcast, IF 1
It is set to 0.72MHz.

Problems to be Solved by the Invention In an FM receiver provided with a changeover switch for selectively receiving 50KHz step broadcasting and 86KHz step broadcasting,
When a muting circuit for muting inter-station noise is provided, the following problems occur due to the difference in IF.

As shown in Fig. 4A, the muting is performed in the ± 35MHz muting width around 10.7MHz by performing the null adjustment for the step of 50KHz. 86KH
When switching to the z step, the center shifts to 10.72 KHz within the same muting width as shown in Figure B, so the + side muting width becomes narrower. In this case, if there are variations in parts, temperature changes, etc., there may be a break in the bass.

Further, as shown in FIG. 3A, by performing null adjustment in 86 KHz steps, it is possible to perform muting with a muting width of ± 50 KHz around 10.72 MHz. When this is switched to the 50KHz step, the center shifts to 10.7MHz within the same muting width, as shown in Figure B,
The muting width on the + side becomes wider. As a result, a part of the stations adjacent by one step is received.

When the number of steps is increased, the muting width is also increased.

Means for Solving the Problems In the embodiment, a shunt circuit is provided by the resistors R ₂ and R ₁ for shunting the current i _B from the current i ₇ flowing through the resistor R ₀ , and this shunt circuit is step-switched by the switch 30. It is designed to be operated selectively according to.

The center of the muting width can be shifted by the amount of change in IF caused by the action step switching, thereby eliminating the deviation of the center of muting and changing the muting width.

Embodiment 1 In FIG. 1, reference numeral 1 is a muting switching circuit for changing the muting width and the center of muting added according to the present invention in accordance with the switching of steps. The switching circuit 1 includes a switch 30 that can select a voltage V _A or a ground voltage, a FET 31 and a resistor R ₁ that are connected in parallel with a resistor R ₀ and can set a muting width in two steps, and a switch 30. And a resistor R ₂ connected to the common terminal. This common terminal is also connected to the gate of the FET 31 via the resistor R ₃ . Therefore, the FET 31 is turned on or off depending on the state of the switch 30 and has a predetermined on-resistance value when turned on, but is included in the resistor R ₁ for simplification.
Known circuits are used as the IC circuit 2 and other circuits. In addition, in the IC circuit 2, the terminal pins indicated by and are using the pin numbers of the actual IC circuit, but will be referred to as terminals 7 to 12 and 14 hereinafter.

In FIG. 1, the IF signal output from the IF amplifier 3 is
It is added to the quadrature detection circuit 4 and also has a terminal 8
The quadrature input signal is phase-shifted by about 90 ° by L ₁ and the phase shifter 5 connected to. This input signal is the terminal 9
Is input to the detection circuit 4. The detection output of the detection circuit 4 is applied to the MPX circuit 6 to demodulate the R and L signals. Along with this, DC outputs (S curve outputs) of opposite phases obtained from the detection circuit 4 are supplied to the AFC amplifier 18 composed of the current mirror circuits 15, 16 and 17. At the output terminal 7 of the AFC amplifier 18, for example, an S-curve output voltage v ₇ that decreases when the frequency of the IF signal increases becomes available. This voltage
v ₇ is added to the AFC tuning meter drive circuit (not shown).

Based on the power supply voltage V _cc which is supplied to the terminal 11, from the power supply circuit provided separately not shown, a DC reference voltage V ₁₀ of the terminal 10, for example 5.6V being applied. This voltage V ₁₀ is applied to the phase shifter 5 and the detection circuit 4, and is also applied to the zero volt switch 20 constituting the muting circuit 19. A resistor R ₀ is connected between the terminals 10 and 7. The resistance value of the resistor R ₀ determines the muting width of the muting circuit 19 as described later.

Next, the operation of the muting circuit 19 when the muting switching circuit 1 is not taken into consideration will be described.

If the center frequency of the IF signal is, for example, 10.7 MHz (in the case of 50 KHz steps), perform null adjustment using L _{2 of the} phase shifter 5,
Adjust so that v _{7 at} 10.7MHz is v ₇ = V ₁₀ i ₇ = i ₁₀ ). The value of the resistor R ₀ is set so that a muting width of ± 35 KHz can be obtained centered on 10.7 MHz. That is, a current i ₇ flowing through the resistor R ₀ flows in or out of the terminal 7 depending on the frequency, and an S-curve output voltage v ₇ varying around the reference voltage V ₁₀ is obtained. In the case of this embodiment, for example, IF
When becomes higher than 10.7MHz, i ₇ flows in from the V ₁₀ side and v ₇ drops.

This voltage v ₇ is applied to the bases of the transistors 21 and 22 that form the zero volt switch 20 so that both transistors 2
Compared to a reference voltage V ₁₀ applied to 1,22 emitters. Therefore, when v ₇ deviates from the center of the S curve due to detuning, the transistor 21 or 2 becomes conductive. For example, when v ₇ rises to a predetermined value, the transistor 21 becomes conductive and the transistors 23 and 24 become conductive. When v ₇ drops to a predetermined value, the transistor 22 becomes conductive, the transistor 25 becomes conductive, and the transistors 23 and 24 become conductive.

When the transistors 23 and 24 are turned on, their emitter outputs are taken out as muting signals through the resistors 26 and 27 and the diode 28.

This muting signal is output from terminal 12 and the MPX
Muting the input signal of the circuit 6.

Along with this, part of the output of the IF amplifier 3 is a noise detection circuit.
Added to 29. This noise detection circuit 29 outputs a muting signal when there is a lot of noise and outputs the MPX circuit 6
It is designed to be added to.

The muting width by the muting signal obtained from the muting circuit 19 is determined by the change of v ₇ ,
This v ₇ is determined by the value of R ₀ .

Next, the operation when the muting switching circuit 1 according to the present invention is added will be described.

First, when the changeover switch 30 is closed to the 86 KHz side, the voltage V _A
There applied to the gate of FE31 through the resistor R ₃ of current limiting, the FET31 is turned on. As a result, the resistor R ₁ is connected in parallel with the resistor R _0, and the muting width is first determined by the combined resistance value of these R ₁ and R ₀ , and the final muting is determined by the resistance value of the resistor R ₂ and the voltage V _A. The width is determined.
In this case, current i ₇ of the terminal 7 with flowing R _0, R _1, R ₂
The current i _B is shunted to. Next, null adjustment is performed by L ₂ of the phase shifter 5, and when IF is 10.72 MHz, v ₇ = V ₁₀ (i ₁₀ = 0, i ₇
= I _B ). As a result, as shown in FIG. 2A, a muting width equal to positive and negative with a center of 10.72 MHz is obtained.

Next, if switch 30 is switched to the 50 KHz side in this state, F
Since the gate of ET31 is grounded, this FET31 becomes non-conductive. Therefore, R ₁ is separated, and the muting width is determined by the combined resistance value of R ₀ and R ₂ which is grounded. In addition, a split flow i _B = v ₇ / R ₂ is generated.

This change in i _B is the IF10.7MHz at the 50KHz step and 86
By selecting the magnitude of R ₂ so that it becomes equal to the magnitude of the current corresponding to 20 KHz, which is the difference from IF 10.72 MHz at the KHz step, i ₁₀ = 0, i ₇ = i _B , and As shown in FIG. 2B, it is possible to obtain a muting width equal to positive and negative with a center at 10.7 MHz.

In the above operation, R ₁ is associated with changing the muting width, and R ₂ is associated with changing the muting width and shifting the center of muting.

EFFECTS OF THE INVENTION In accordance with the switching of the frequency step between stations, the center of the muting width can be shifted by the amount of the change in IF caused by the step switching to eliminate the deviation of the center of the muting and change the muting width. be able to.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a diagram for explaining the effect of muting obtained by the present invention, and FIGS. 3 and 4 are diagrams for explaining conventional muting. . In the reference numerals used in the drawings, 1 ... Muting switching circuit, 4 ... Quadrature detection circuit, 7, 10 ... Terminal, _R0 ... Resistor.

Claims (1)

[Claims] 1. A resistor is connected between a first terminal to which a reference voltage is applied and a second terminal to which an S-curve output voltage of an FM detector is applied, and the reference voltage and the S-curve are connected. In the muting circuit that compares the output voltage and obtains the muting signal by this comparison output, while providing the switching means of the frequency step between stations,
Providing a shunt circuit for shunting the current flowing through the resistor,
A muting circuit adapted to selectively operate the shunt circuit according to the step switching.