JPS5951791B2 - Antenna switching circuit for car radios, etc. - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an antenna switching circuit for a car radio or the like.

Conventionally, some car radios and the like equipped with AM-FM bands have an antenna that can be used for both bands.

In this type of antenna, as shown in FIG. 1, in order to share the antenna AT, the antenna circuit includes a branching circuit consisting of a coil L and a converter tower TC, and resistors R, , R.

The attenuator has an attenuator, and a far/near switch that turns the attenuator on and off. I However, in this case, far/near switching is appropriate for FM radio waves but inappropriate for AM radio waves, and therefore, on the AM side, the R of the AM tuner is
It was necessary to change the gain at the same time in the F amplifier.

In addition, in a varactor tuning type tuner, the antenna input terminal is set to AM high impedance and FM low impedance, and the far/near switch S is set to exclude the attenuator, that is, set to the far side Dx (field strength of incoming radio waves is low). However, due to the influence of the FM input terminal, there was some loss in AM, which caused problems with high-sensitivity tuners.

The present invention seeks to eliminate such conventional drawbacks.

The embodiment shown in FIG. 2 will be described below.

The circuit shown in FIG. 2 shows the main part of the circuit of a car radio that is equipped with AM-FM bands and uses a car antenna AT in common for both bands. In the figure, 1 is an antenna switching circuit, 2 is an AM-FM tuner, and 3 is an antenna switching relay drive circuit.

The antenna circuit 1 sends AM radio waves among the incoming radio waves caught by the car antenna AT to the AM input circuit of the AM-FM tuner 2 through a coil L capable of blocking FM radio waves.
The M radio wave is connected to a capacitor C with a sufficiently small reactance for the FM band (for example, 220
0pF) or a capacitor C with a sufficiently large reactance for the AM band.

AM-
It is designed to be sent to an appropriate location in the coil of the FM input circuit of the FM tuner 2. The antenna switching relay drive circuit 3 is
The far/near switch S and the AM-FM switch SO constitute an AND circuit, and the far/near switch S is set to the near side LO (field strength of incoming radio waves is high), and the AM/FM switch SO is connected to the FM side, and conversely, when they are connected to the far side Dx and AM, the switching transistor Tr is turned on, and accordingly the switching relay coil RC is energized and activated. It's getting old. The switching relay coil RC operates the switching relay contact RS of the antenna circuit 1, and when the switching relay contact RS is not energized, the switching relay contact RS is set to the a side, that is, the F from the car antenna AT.
M radio waves pass through capacitor C1 to AM-FM tuner 2
This is the side that sends the signal to the FM input circuit of the
ie, to the side where it is sent through a series circuit of capacitor C2 and resistor R.

As shown in Fig. 2, when the AM-FM changeover switch SO is set to the FM side and the far/near changeover switch S is set to the far side Dx,
The switching relay coil RC is not energized, the switching relay contact RS is on the a side, and the FM radio waves from the car antenna AT are sent to the FM input circuit of the AM-FM tuner 2 through the capacitor C1.

At this time, the FM radio wave causes almost no loss because the capacitor C1 has a sufficiently small reactance with respect to the FM band. Furthermore, in this situation, when the far/near switch S is switched to the near side LO, the switching relay coil RC is energized and the switching relay contact RS is switched to the b side. As a result, the FM radio waves from the car antenna AT are transferred to the capacitor C2.
The signal is sent to the FM input circuit through the resistor R and the capacitor C2, whose value is appropriately selected, and the resistor R provides appropriate attenuation. A numerical example of this is as follows.

The impedance of the power antenna AT is FM band (76~
90M field, center 83M pressure), low impedance of about 75Ω, AM band (525-1610K field, center 1000K)
The impedance of the FM input circuit is low impedance of approximately 75 Ω, and the impedance of the AM input circuit is 10 Ω.
It has a high impedance of KΩ or more. Now, for example, C
1=2200PF. Reactance XC in the FM band (83M pressure) at C1 and C2 when C2=1PF
l, XC2 are, and the series impedance Z1 of C2 and R is given by.

Therefore, for the far side Dx in FM, the reactance of C1 is
l=0.87Ω, which is small, and FM input causes almost no loss.

Also, in the case of the near side LO, the rearatance of C2 is XC2
= 1.92KΩ, and the series impedance Z1 between C2 and R is a little larger than this, and as a result, the FM input is appropriately attenuated. Since the FM radio waves are blocked by the coil L, they are not sent to the AM input circuit.

Next, when the AM-FM changeover switch SO is switched to the AM side, the energization relationship to the changeover relay coil RC by the operation of the far/near changeover switch S, and therefore the connection relationship of the changeover relay contact RS, changes to the above-mentioned FM reception case. If the far/near changeover switch S is connected to the near side LO, this will connect the switching relay contact RS to the a side, but in the case of this connection to the a side, there will be a The AM radio waves are moderately attenuated by a grounding path consisting of a capacitor C and a coil I of the FM input circuit, and the rest are brought to the AM input circuit via a coil L.

Furthermore, when the far/near changeover switch S is connected to the far side Dx, the changeover relay contact RS (7) is connected to the b side. For AM radio waves from antenna AT, capacitor C2 is connected to A.
Since the reactance is sufficiently large for the M band, no loss occurs. If this is illustrated numerically following the above, it will be as follows.

The reactances XCl and XC2 in the AM band (1000K pressure) for C1 (2200pF) and C2 (1pF) are:
, and the series impedance Z2 of C2 and R is given by.

Therefore, for the near side LO in AM, a portion of the AM radio wave has a relatively low reactance of C1, XCl = 72.3
Ω, and a low impedance FM input circuit to ground, and the AM input is appropriately attenuated.

In addition, in the case of the far side Dx, the reactance of C2 is XC2
= 159KΩ, and the series impedance Z2 between C2 and R becomes even higher.As a result, the AM input hardly flows to q and R, and according to the present invention, which does not cause loss, both FM reception and AM reception are good. Furthermore, there is no need to control the gain of the RF amplifier as in the conventional method during AM reception, which simplifies the circuit and is advantageous in terms of cost.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of a conventional example, and FIG. 2 is a circuit diagram of an embodiment of the present invention. 1. Antenna switching circuit, 2. AM-FM tuner,
3...Antenna switching relay drive circuit, RC...Switching relay coil, RS...Switching relay contact, AT...Power-antenna, SO...AM-FM switching switch, S...Far/near switching switch.

Claims (1)

[Claims] 1. In an antenna switching circuit for a car radio or the like that is equipped with AM/FM bands and has an antenna common to both bands, one capacitor C_1 with a relatively high capacitance such as 2200 pF or more is connected between the antenna AT and the FM input circuit, 1p
Switching relay contact R
Antenna A
A coil L capable of blocking FM radio waves is interposed between T and the AM input circuit, and an AND circuit capable of mutually switching is configured with an AM/FM changeover switch So and a far/near changeover switch S. The above switching relay contact RS via the circuit
An antenna switching circuit for a car radio or the like, characterized in that the switching relay coil RC for driving is configured to be energized, and the switching relay contact RS is switched by mutual operation of both switching switches So and S.