JPS62159927A - Input circuit for diversity receiver - Google Patents

Abstract

PURPOSE:To decrease the effect on an AM tuner when an antenna input circuit of an FM diversity receiver is used in common to the AM tuner by inserting a series resonance circuit of FM band between an antenna input terminal branched to the AM tuner and a corresponding diode switch. CONSTITUTION:In order to decrease the loss of an antenna switching circuit 2, coupling capacitors (C1, C2) with a comparatively large capacitance are used to couple the antenna and the switching circuit. In case of an AM/FM double band receiver, the coupling capacitors (C1, C2) and bypass capacitors (C3, C4) are loads to the AM tuner input to decrease the sensitivity. A coil L4 is con nected to the antenna input side of the capacitor C1 to constitute a series reso nance circuit (L4, C1). The resonance point of the resonance circuit is set to a midpoint 83MHz in the FM band of 76-90MHz. In this case, since the capaci tor C1 is selected to be a capacitance giving less effect onto the AM band by selecting the value L4, the reduction of the input signal voltage to the AM tuner is prevented and the pass loss at the FM band is reduced.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to an antenna input circuit of a diversity receiver, and in particular, the influence on the AM tuner side when an antenna is shared between an AM tuner and an FM tuner is reduced by using a series resonant circuit. The aim is to reduce this by using

[Conventional technology]

An in-vehicle FM diversity receiver is configured as shown in FIG. 5, for example, and has multiple (two in this example) antennas IA.
, IB are selected by the antenna switching circuit 2. The selected high frequency signal is then converted into an intermediate frequency (IF) signal by the front end 3. 4 is an IF amplification and demodulation section, 5 is a stereo fj[ section, 6 is a power amplifier, and 7 is a speaker. The switching trigger detection circuit 8 is I
A switching signal is given to the switching control circuit 9 using the mute signal of the F stage 4 and the change in the pilot signal when multipath occurs. The switching control circuit 9 switches the signal path of the antenna switching circuit 2 by inverting the outputs (control signals) a and b according to the state of the switching signal.

10 is an AM tuner, and in this example, in order to share the antenna input with this AM tuner, A and M band signals are guided through a branch coil L of several μH which has a high impedance in the FM band.

FIG. 6 is a detailed diagram of a conventional antenna switching circuit 2, in which antenna power A and B are switched by voltages at control input terminals a and b.

L3 is the coil described above, and Ll and L2 are choke coils (both several μH) for preventing input signal leakage, and have high impedance in the FM band. C3,
Cs is a bypass capacitor, C1 and C2 are input coupling capacitors, and C5 is an output coupling capacitor. Diodes DI and D2 are for switching, and H2 of inputs a and b
Depending on the combination of L, one is turned on and the other is turned off.

[Problem that the invention seeks to solve]

FIG. 7 is an equivalent circuit in the AM band of the switching circuit 2 including the antenna seen from the AM tuner input. An equivalent circuit of an antenna (ANT) used in a vehicle-mounted AM receiver is expressed in a form in which the signal source voltage Vg is divided by the antenna equivalent capacitance and the cable capacitance. Therefore, if the stray capacitance up to the input circuit is large, the S/N will decrease. In other words, coil 1 in Figure 6
~L3 is several μH, so L3. Ll has low impedance in the AM band, and the switching diode D1 is
The equivalent circuit in the case of FF is represented by C1 and 03 in series,
This is added in parallel to 65 pF of the antenna equivalent circuit. Of these, the coupling capacitor CI uses several hundred pF to reduce loss in the FM band, and the bypass capacitor C3 uses several thousand pF, so the value of the combined capacitance of C1 and 03 is It becomes several hundred pF, and the input signal voltage decreases.

The present invention attempts to improve this point.

[Means for solving problems]

The present invention inserts an FM band series resonant circuit between the antenna input end branching to the AM tuner and the corresponding diode switch in the input circuit of a diversity receiver in which one of a plurality of antenna inputs is selected by a diode switch. It is characterized by the fact that

[Effect]

In a diversity receiver that uses switching diodes to switch antennas, there is some loss in the antenna switching circuit. Generally, in order to reduce this loss, a relatively large capacitance coupling capacitor (CI, C2) is used to couple the antenna and the switching circuit. However, if it is an FM-only device, there is no particular problem, but in the case of an AM/FM multi-band receiver, the coupling capacitors (CI, C2) and bypass capacitors (C3, C4) become a load on the AM tuner input, and the sensitivity decrease.

If this coupling capacitor is made smaller, the decrease in sensitivity of the AM tuner is alleviated, but the passage loss on the FM tuner side increases and the sensitivity of FM decreases. Therefore, in the present invention, an LC series resonant circuit is inserted in front of the switching diode of the antenna switching circuit, taking advantage of the fact that the impedance becomes low near the resonance point of series resonance. The value of the capacitor in this circuit should be set so that it has little influence on the AM band. When configuring the diversity switching circuit as an adapter outside the receiver, use a similar series resonant circuit on the output side.

〔Example〕

FIG. 1 is a circuit diagram showing an embodiment of the present invention, which differs from FIG. 6 in that a coil L4 is connected to the antenna input side of a capacitor C1 to form a series resonant circuit La, CI. The resonance point of this resonant circuit is set at 83 MHz at the center of the 76 MHz to 90 MHz FM band. In this case L4
By selecting the value of , CI can be set to a capacity that has little influence on AMi, so it is possible to prevent a drop in the input signal voltage to the AM chainer and to reduce the passing loss in the FM band.

FIG. 2 is an equivalent circuit in the FM band of FIG. 1.

rd represents the loss resistance of La and CI and the series resistance of the switching diode D1, and can be ignored if it is sufficiently low compared to the output resistance Rg of the signal source and the input resistance RL of the front end. In that case, the output voltage Eo at point 0 is expressed by the following equation.

If the resonance frequency of La and C'+ is expressed as C0, the above equation becomes as follows. The transmission ratio in this case is generally , and at the resonant frequency ω0. Changes in the transmission ratio at frequencies other than the resonant frequency are expressed by the following equation, and it can be seen that if the resonant frequency is selected around the FM broadcast pantono, there will be no significant level drop even at band edges.

FIG. 3 is a circuit diagram showing a second embodiment of the present invention. This example is an example of application to a method (adapter type) in which the diversity antenna switching circuit is provided outside the receiver, with an LC series resonant circuit L! on the output side as well as on the input side. 1 and C5 were inserted. This is to prevent the resistor R and capacitor C3 or C4 of the switching circuit from becoming a load on the AM tuner when the AM and 4'FM common outputs are connected to the receiver. Le is a coil for AM branching.

FIG. 4 is a circuit diagram of a third embodiment of the present invention, in which parallel tuning circuits CVI and L are provided before switching diodes DI and D2.
7 and Cv2. This is the one in which Le is connected. This tuning circuit is of a type in which the capacitance of the variable capacitance diode Cv1°Cv2 is varied by a tuning voltage.

The series resonant circuit according to the present invention is La and C+ on the input side.

〔Effect of the invention〕

As described above, according to the present invention, the antenna input circuit of the FM diversity receiver is connected to the AM tuner (LW, MW).
, SW band), the influence on the AM tuner side can be reduced.

[Brief explanation of drawings]

Figure 1 is a circuit diagram of the first embodiment of the present invention, and Figure 2 is its F.
3 and 4 are circuit diagrams showing second and third embodiments of the present invention, FIG. 5 is a block diagram of an FM diversity receiver with an AM tuner, and FIG. 6 is an equivalent circuit diagram in the M band. 7 is a block diagram of a conventional FM diversity antenna switching circuit, and FIG. 7 is an equivalent circuit diagram of the same in the AM band. In the figure, LA and IB are antennas, 2 is an antenna switching circuit,
3 is an FM front end, Dl and D2 are switching diodes, La, C+ and L5°C5 are series resonant circuits, and 10 is an AM tuner. Applicant: Fujitsu Ten Ltd. Representative Patent Attorney Minoru Aoyagi Control input cutout for all January 1st actual operation circuit diagram Figure 1 % 1 FM equivalent circuit diagram 81 Figure 2

Claims (1)

[Claims] In the input circuit of a diversity receiver in which one of multiple antenna inputs is selected by a diode switch, an FM band series resonant circuit is inserted between the antenna input end branching to the AM tuner and the corresponding diode switch. Input circuit of the diversity receiver featuring features.