JPS6133723Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a sensitivity switching circuit for a receiver that can switch both AM and FM sensitivities with one switch.

Generally, this type of receiver is composed of an antenna 1, an FM front end 2, an IF filter 3, an IF amplifier 4, and an AM tuner 5, as shown in FIG. The conventional sensitivity switching circuit is constructed by providing a switch _S1 in series between the antenna 1 and the FM front end 2, and connecting a resistor R in parallel between the contacts of this switch _S1 . Furthermore, L is
It is an inductor. The input voltage of the antenna 1 is supplied to the FM front end 2 or the AM tuner 5 through a resistor R or directly through a switch _S1 . If the switch S ₁ is closed, the influence of the resistor R does not appear on the circuit, so that all the antenna input voltage is supplied to the FM front end 2 or the AM tuner 5. Also, when switch _S1 is open, resistor R is inserted in series in the path between antenna 1 and FM front end 2 or AM tuner 5.
The antenna input voltage is attenuated depending on the ratio with the input impedance of the AM tuner 5 before being input. Therefore, when switch _S1 is opened, the reception sensitivity is reduced.

Equivalent circuits for the above case are shown in FIGS. 2 and 3. FIG. 2 is for FM reception. Note that the inductor L in the circuit is AM
and FM interference. Here, the symbols in FIGS. 2 and 3 indicate the following, respectively.

1'...Antenna equivalent circuit 1''...AM antenna circuit Rg; Antenna impedance (Figure 2) e _F ; Antenna input voltage (Figure 2) Ri; Front-end input impedance (Figure 2) e _A ; Antenna input voltage ( (Figure 3) c _A1 , c _A2 ; Antenna capacitance (Figure 3) L _A ; AM antenna coil (Figure 3) c _A3 ; Antenna tuning capacitor (Figure 3) Here, in the case of FM in Figure 2, , the amount of attenuation can be varied without any problem by changing the value of the resistor R. However, in the case of AM shown in Fig. 3, although the amount of attenuation can be adjusted, the resistance R in this circuit
This lowers the Q of the resonant circuit, which adversely affects the performance of the receiver.

Further, FIG. 4 shows a simplified version of the one shown in FIG. In figure a, c _A is the parallel capacitance of the antenna capacitances c _A1 and c _A2 in Fig. 3, and in figure b, the parallel capacitance c _A and resistance R in figure a are converted into parallel equivalents, and c and R are respectively ’ is indicated. In addition, quantitatively, C=c _A
{1/1+(ωc _A R) ² }, R'=R{1+1/(ω
c _A R ) ² }, ω=2 π. Now, the resonance Q of this equivalent circuit is
Since it is represented by ωcR', the smaller the resistance R', that is, the larger the value of the resistance R, the lower the Q of the circuit and the worse the performance as a receiver. Therefore, performance such as image interference ratio and spurious interference ratio will deteriorate. Note that in FIG. 4 and FIG. 6, which will be described later, the inductor L is omitted because its inductor value is sufficiently smaller than that of the inductor _LA .

An object of the present invention is to provide a switching circuit that can attenuate the sensitivity without lowering the Q of the AM resonant circuit.

In other words, the sensitivity switching circuit of the receiver of the present invention is as follows.
As shown in Figure 5, there is a switch on the input side of the FM front end 2 and AM tuner 5.
Connect movable contact c of S ₂ , connect one contact a to antenna 1, connect capacitance c ₁ between contact a and movable contact c, and connect capacitance c between contact b and earth E. ₂ are connected in series. That is, this switch _S2 switches between the series capacitor _c1 and the parallel capacitor _c2 . The values of the capacitance c ₁ and the capacitance c ₂ are set to such values that the combined capacitance viewed from the AM tuner 5 side is always constant.

To give an example here, in a car radio, the antenna capacity, that is, the capacity c _A in Figure 7,
₁ and the capacitance c _A2 is 80 [pF], so c ₂ =80-c ₁ ×80/c ₁ +80 [pF].

First, for FM, as shown in Figure 6,
The input signal to the FM front end 2 is divided into an impedance ratio of capacitance c ₁ and capacitance c ₂ , so the capacitance
By setting the values of c ₁ and c ₂ to appropriate values, the desired degree of attenuation can be obtained.

Next, for AM, as shown in FIG. 7, an appropriate degree of attenuation can be obtained by capacitance c ₁ and capacitance c ₂ . Also, for the Q of the resonant circuit, the 8th
As shown in the figure, no unnecessary resistance component is generated in the equivalent circuit, and therefore the Q of the resonant circuit does not drop at all. In addition, c' in Figure 8 is c _A・c ₁
/c _A +c ₁ +c ₂ +c represents _A3 .

As described above, according to the present invention, it is possible to change the sensitivity for both AM and FM bands with a single switch, and the Q of the resonant circuit does not decrease.
Therefore, this switching circuit is ideal for applications such as car stereos.

[Brief explanation of drawings]

Figure 1 is a block diagram of the sensitivity switching circuit of a conventional receiver, Figure 2 is the equivalent circuit of the conventional receiver when receiving FM, Figure 3 is the equivalent circuit of the conventional receiver when receiving AM, and Figure 4 is the equivalent circuit of the conventional receiver when receiving AM. Fig. 5 is a block diagram of the sensitivity switching circuit of the receiver of the present invention, Fig. 6 is the equivalent circuit for FM reception of the present invention, and Fig. 7 is the equivalent circuit of the present invention. FIG. 8 is an equivalent circuit diagram that further simplifies the equivalent circuit of FIG. 7. 1...Antenna, 2...FM front end,
3...IF filter, 4...IF amplifier, 5...AM
Tuner, S ₂ ... Changeover switch, c ₁ , c ₂ ...
capacity.

Claims (1)

[Scope of utility model registration request] In a receiver capable of receiving AM and FM broadcasts using a single antenna, an input signal from the antenna is input to an FM front end or an AM tuner, and a common input side of the FM front end and AM tuner is used. , an antenna or a ground, a capacitor is connected in parallel to the antenna side path of the switch, and a capacitor is connected in series to the ground path. Sensitivity switching circuit.