JP2002176329A - High pass filter - Google Patents

Abstract

[PROBLEMS] To realize a high-pass filter having a matching circuit function with respect to a frequency for blocking passage with a small number of circuit elements. SOLUTION: Three capacitors C1 and C are provided in a signal transmission path.
2 and C3 are connected in series, and a capacitor C
A resistor R1 was connected in parallel with a capacitor C1 on the input terminal side of the high-pass filter circuit, which was grounded by coils L1 and L2 in which the capacitors C4 and C5 were connected in series.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-pass filter, and more particularly, to a high-pass filter having a matching function with respect to a pass rejection frequency.

[0002]

2. Description of the Related Art A BS signal (11.
7 to 12 GHz) is right-handed polarization, and similarly, right-handed polarization (12.2 to 12.
75 GHz), and it can be mixed with CS right-handed polarization and transmitted to the receiving end with a single cable. In this case, for example, the CS signal is usually down-converted into an IF signal of 1572 to 2072 MHz and transmitted, and the BS signal is normally 1035 to 1035 MHz.
The signal is down-converted to an IF signal of 1335 MHz and transmitted. For example, using a switching circuit 5 as shown in FIG.
The selected CS signal and BS signal are transmitted to the receiving end.

A switching circuit 5 shown in FIG.
First input terminal 6 for inputting a signal, CS right-handed polarization and BS
It has a second input terminal 7 and an output terminal 8 for inputting a signal IF signal. The high frequency switch 9 switches the left and right rotations of the CS signal and outputs the same, and the BS signal is always output. . The high-frequency switch 9 is connected to the a-side when receiving a CS left-handed polarization signal from the first input terminal 6, and outputs the signal via the high-pass filter 11.
To receive the CS right-handed polarization signal, the signal is switched to the b side to output the signal through the high-pass filter 11 while blocking the passage of the BS signal. Then, the BS signal having a lower frequency than the CS signal is output through a low-pass filter while blocking the passage of the CS signal.

[0004]

However, in the case of the switching circuit 5, when receiving a CS right-handed polarization signal, a high-pass filter 11 that allows only the CS signal to pass therethrough, for example, as shown in FIG. If formed by a filter circuit, the impedance becomes high in the IF band of the BS signal, impedance matching cannot be performed, and total reflection occurs. For example, as shown in the frequency characteristics of the pass loss and the return loss in FIG. , The return loss becomes large, which is an obstacle when receiving a BS signal.

For this reason, a matching circuit corresponding to the BS signal frequency is provided at a stage preceding the high-pass filter circuit so that total reflection does not occur. A high-pass filter including this matching circuit is formed, for example, as shown in FIG. 6, where 11a is a high-pass filter circuit, 11b is a matching circuit, and the matching circuit 11b is a resistor R10, a capacitor C17, and a coil L
It was formed using three elements of thirteen. By providing such a circuit on the input side of the filter circuit 11a, the passing loss and the return loss of the high-pass filter 11 have been improved. As described above, the high-pass filter 11 provided in the switching circuit 5 requires the matching circuit 11b in addition to the high-pass filter circuit 11a, so that the installation space must be secured on the substrate, and the number of elements increases, so that the cost is increased. Had become high.

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to realize a high-pass filter having a matching circuit function with respect to a frequency for blocking passage with a small number of circuit elements.

[0007]

In order to solve the above-mentioned problems, the invention according to claim 1 comprises a plurality of capacitors connected in series to a signal transmission line, and at least a coil provided between the plurality of capacitors and a ground. In the high-pass filter, a resistor is connected in parallel to one of the plurality of capacitors on the signal input side. The invention according to claim 2 is characterized in that the value of the resistor is set to 0.5 to 1.7 times the characteristic impedance of the signal transmission line.

[0008]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a circuit diagram showing an example of a high-pass filter according to the present invention, in which three capacitors C1 to C3 are provided in series on a signal transmission line,
It has a coil L1 in which a capacitor C4 is connected in series between the input side capacitors C1 and C2 and the ground, and has a coil L2 in which a capacitor C5 is connected in series between the output side capacitors C3 and C2 and the ground. And the signal input terminal 1
A resistor R1 is provided in parallel with the capacitor C1 on the side. 2 is a signal output terminal.

Specifically, the values of each element of the high-pass filter are as follows: capacitors C1 = C3 = 1.5 pF, C2 = 1
pF, C4 = 3 pF, C5 = C7 = 2 pF, coil L1 = L2 = L3 = 4.7 nH, and resistance R1 = 75Ω. The capacitance of the capacitor C6 provided at the signal output terminal 2 is 0.5 pF. The frequency characteristics of the excess loss and the return loss in this case are as shown in FIG.
The passing loss at 72 MHz hardly changes, but 1335 M
It can be seen that the return loss in Hz is improved by about 10 dB, but the BS signal is prevented from passing through the IF band.

As described above, merely providing the resistor R1 in parallel with the capacitor C1 constituting the filter provides the IF of the BS signal.
The reason why the reflection of the band can be eliminated will be described. First, in FIG. 1, the reactance X of the capacitor C1 at 1200 MHz which is almost the center frequency of the IF band of the BS signal.
c is because it is Xc = 1 / (2πfc) = 88.4Ω, the impedance Z of the circuit viewed from this from the input ^{side, Z = R1 × Xc 2 /} (R1 2 + Xc 2) -jR1 2 × Xc / ( R1 ² + Xc ² ) = 75 × 7815 / (75 ² +7815) −j75 ² × 88.4 / (75 ² +7815) = 43.6−j37.0 When the characteristic impedance is 50Ω and the reflection coefficient Re is obtained. Re = | (Z−50) / (Z + 50) | = | 0.076−j0.365 | = 0.373.

This is converted to a voltage standing wave ratio (VSWR) of VSWR = 2.2, which means that the reflection of the BS signal at 1200 MHz, which is almost the center frequency of the IF band of the BS signal, is slight, and the matching is difficult. It is possible to take. If the VSWR is 2.5 or less, there is no practical problem. When the value of the resistor R1 at the VSWR 2.5 is obtained, R1 is 22 Ω to 85 Ω. Also,
When normalized by the characteristic impedance Z ₀ , the resistance R 1 becomes 0.
5Z ₀ ~1.7Z ₀ the (Ω).

As described above, by simply adding one resistor R1 to the high-pass filter, it is possible to form a circuit that can be substituted for the high-pass filter provided with the matching circuit of FIG. There is no need to provide a matching circuit. Therefore, a circuit can be formed on the substrate in a small space,
Cost reduction can be achieved. In the above-described embodiment, the values of the respective elements are determined by setting the pass band of the high-pass filter to 1.572 MHz or more and the stop band to 1335 MHz or less. The value of each element may be changed in accordance with.

[0013]

As described in detail above, according to the present invention,
By adding only one circuit element to the high-pass filter, matching can be achieved with respect to the frequency at which passage is blocked, and there is no need to separately provide a matching circuit including a plurality of elements. Therefore, a circuit can be formed in a small space on the substrate, and the cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a high-pass filter showing an example of an embodiment of the present invention.

FIG. 2 is a diagram illustrating frequency characteristics of a pass loss and a return loss of the high-pass filter of FIG. 1;

FIG. 3 is a circuit block diagram of a switching circuit having a high-pass filter.

FIG. 4 is a circuit diagram of a conventional high-pass filter.

FIG. 5 is a diagram illustrating frequency characteristics of pass loss and return loss of the high-pass filter of FIG. 4;

FIG. 6 is a circuit diagram in which a matching circuit is provided in the high-pass filter of FIG.

[Explanation of symbols]

1 ··· Signal input terminal, 2 ··· Signal output terminal, C1, C
2, C3, C4, C5, C6, C7 .. capacitor, L
1, L2, L3 ... coil, R1 ... resistance.

Claims (2)

[Claims] 1. A high-pass filter having a plurality of capacitors connected in series to a signal transmission path and at least a coil between the plurality of capacitors and a ground, wherein one of the plurality of capacitors on a signal input side is provided. A high-pass filter characterized by connecting a resistor in parallel to a capacitor. 2. The high-pass filter according to claim 1, wherein the value of the resistance is 0.5 to 1.7 times the characteristic impedance of the signal transmission line.