JPS5831777B2 - antenna input circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an antenna input circuit for a vehicle-mounted electronically tuned AM receiver.

Automotive antennas are generally capacitive, and as shown in FIG. 1, they are represented by an antenna electromotive force EA, an antenna capacitance Ca in series with it, and a cable capacitance cb in parallel with these.

For whip antenna, Ca = 15pF, Cb =
It is about 65 pF.

When electronically tuning the receiver with a capacitive antenna,
Tuning variable capacitors using voltage variable capacitance diodes (varactor diodes) have problems such as insufficient capacitance variation range and low S/N ratio.

Therefore, in the antenna input circuit of a conventional electronic tuning tuner, (a) the antenna output is first received by a field effect transistor and then passed through the tuning circuit, using an untuned antenna input type, or (b) two varactor diodes (one is Countermeasures have been taken such as using a tuned input circuit (one connected in series and the other connected in parallel).

However, in case a), it is not possible to directly amplify the radio waves of each broadcasting station input to the antenna, select them using a tuning circuit, and amplify only the desired radio waves, so there is a drawback that cross-modulation is likely to occur. In case b), cross-modulation does not occur because the tuning circuit selects and amplifies the signal, but it has the drawbacks of increasing cost and making tracking adjustment difficult because two diodes are used.

Taking these points into consideration, the present invention has developed a varactor diode 1.
The aim is to construct a tunable antenna input circuit using the two antennas.

The circuits shown in FIGS. 2 and 3 are conceivable as the tunable antenna input circuit using one diode.

In FIG. 2, a transformer TR is used, and its primary side with a small number of turns is connected to the cable termination P1 of the antenna circuit, and its secondary side is connected to a varactor diode C.

This reduces the capacitance seen from the secondary side, and by appropriately selecting the turns ratio, the capacitance change ratio of diode C (maximum capacitance Cmax/minimum capacitance (Cm1n)) is approximately 15, which is the frequency band of AM broadcasting in Japan. 5 30 ) G (z to 16
This means that it can cover 0.00 KHz.

However, this method has a problem in that the S/N ratio is significantly lowered in the lower frequency portion, and hence in the portion where the capacitance of the diode C is large, as shown by the curve C1 in FIG.

This is generated by dividing the antenna electromotive voltage EA by the capacitances Ca, Cb, and C.

In Figure 3, a varactor diode C is connected in series and resonates with a tuning inductance L. However, if this is done as it is, the minimum capacitance Cm1n required for the diode C to cover the above broadcasting frequency band will be extremely small, and it will not actually exist. Since this is difficult to achieve with a varactor diode, a capacitance Cg is added to make Cm1n a dog.

However, in this case, as shown by curve C2 in Figure 9, S
/N decreases significantly.

The present invention aims to bring out the good characteristics of both of these circuits, that is, to make the characteristics shown in Figure 3 appear in the low frequency part and the characteristics shown in Figure 2 in the high frequency part. The circuit of an electronically tuned AM receiver connected to a capacitive antenna is represented by an antenna electromotive force, an antenna capacitance in series with the electromotive force, a cable capacitance in parallel with these, etc. The antenna input circuit includes a circuit connected to the antenna and having a winding of a transformer, a voltage variable capacitance diode in series with the winding, and an additional capacitor, so that the continuous current flows through a part of the winding of the transformer. and a circuit connected to the transformer so as to pass through the transformer.

The basic circuit of the present invention is shown in FIG. In Figure 4, TR is the second
As shown in the figure, the transformer has a primary winding with fewer turns than the secondary winding, and the primary winding is connected in series with the capacitor Cg, and the secondary winding is connected between the terminal P2 and the ground.

This circuit is intended to effectively utilize the current of the additional capacitor Cg that flows in vain in FIG.

The number of 1.2 turns of the transformer TR is T2. T1, turns ratio T2
/T1 is m, and the inductance on the secondary winding side is R.

Then, the resonant frequency f of this circuit.

is expressed by the following equation (1), and its tuning capacitance co is (2)
It is expressed by the formula.

When considering the S/N ratio of this circuit, if the noise source is the loss of the resonant circuit, that is, the thermal noise of the resistor, then the resistance r is from the Q of the resonant circuit.

Here ω. is the resonance angular frequency, and co is the tuning capacitance expressed by equation (2).

Therefore, the generated noise※※VN is becomes.

Here, k is Boltzmann's constant, T is temperature, and B is band.

Therefore, the S/N ratio at the time of resonance at the terminal P2 is as shown in equation (5).

As is clear from Equation (5), the capacitance of the varactor diode C (also denoted by the same symbol C) is included in the denominator and numerator, and even if C is changed for channel selection, the S/N will not change. Mother and child tend to cancel each other out and become constant.

In fact, substitute each value for Ca, Cb... and choose Cg and m appropriately (for example, Cg = 1500 p
F, m=0.15), and the variation range of C is 530 to 16, which is a value of about 15 that can be achieved with commercially available varactor diodes.
Can cover the broadcasting frequency band of 00KHz and has a S/N ratio of
is as shown by curve C3 in FIG. 9, and there is no significant drop at any frequency, and a good S/N characteristic can be obtained.

Since a transistor amplifier circuit is placed next to the antenna input circuit, the output impedance is required to be low.

Therefore, the output impedance at terminal P2 in FIG. 4 becomes considerably high, so as shown in FIG.
It is preferable to provide a tap in the next winding and use this as the output terminal P3.

The circuit shown in FIG. 4 can also be modified as shown in FIG.

As shown in Fig. 7a, the transformer in Fig. 4 is replaced with an autotransformer TRa, and then the ground point is set to point P1 as shown in Fig. 7a, and a large capacitance capacitor Cd not related to resonance If the circuit is added and the circuit is rearranged upside down on the circuit diagram, it will look like C in the same diagram, which is obvious from the fact that this is nothing but the circuit in FIG.

If the output of this circuit is also taken out from the connection point P4 between capacitors C and Cd, a low output impedance can be obtained due to the additional capacitor Cd.

FIG. 8 shows a specific circuit in which a DC bias circuit is added to the varactor diode C shown in FIG.

vB is the diode bias voltage for tuning, which is applied to diode C through resistors R1, R2 and the winding of transformer TR.

Ce is a capacitor for high frequency bypass.

As explained above, according to the present invention, it is possible to configure a tunable antenna input circuit that can cover the AM broadcasting frequency range with only one voltage variable capacitance diode, and the S/N characteristics are excellent from the low frequency region to the high frequency region. g Can be flattened.

[Brief explanation of drawings]

Figure 1 is an equivalent circuit diagram of a capacitive antenna, Figures 2 and 3
The figure is a circuit diagram showing the conventional flow, Figure 4 is a circuit diagram showing the basic form of the present invention, Figure 5 is a circuit diagram explaining how to take out the output of the circuit in Figure 4, and Figure 6 is a modification of the present invention. Schematic diagram showing an example,
FIG. 7 abc is a circuit diagram explaining the modification procedure, FIG. 8 is a specific circuit diagram of FIG. 6, and FIG. 9 is an S/N characteristic diagram. In the drawing, EA is the antenna electromotive force, Ca is the antenna capacity, and C
b is a cable capacitance, TRTRa is a transformer, C is a voltage variable capacitance diode, and Cg is an additional capacitor.

Claims (1)

[Claims] 1. An antenna electromotive force, an antenna capacitance in series with the electromotive force,
In-vehicle electronically tuned AM connected to a capacitive antenna represented by an equivalent circuit consisting of cable capacitance in parallel with these
An antenna input circuit of the receiver includes a circuit connected to the antenna and having a winding of a transformer and a voltage variable capacitance diode in series with the winding, and an additional capacitor so that the current flowing through the capacitor is connected to the winding of the transformer. and a circuit connected to the transformer so as to pass through a part of the antenna input circuit. 2. The transformer has a primary winding with a small number of turns and a secondary winding with a large number of turns, and the series circuit between the additional capacitor and the primary winding and the series circuit between the voltage variable capacitance diode and the secondary winding are connected to each other. The antenna input circuit according to claim 1, wherein the antenna input circuit is connected to a capacitive antenna in parallel. 3. The transformer consists of a single-turn transformer with a tap, and of both ends of the winding of the transformer, the end with fewer turns between it and the tap is connected to the capacitive antenna, and the other end has a variable voltage. An antenna input circuit according to claim 1, characterized in that a capacitive diode is also connected to the tap with an additional capacitor.