JPH0420001A - Low pass filter circuit - Google Patents

Abstract

PURPOSE:To interrupt undesired radio waves whose frequency is not only 2nd and 3rd harmonic frequency but also 4th harmonic frequency or over by providing a capacitor with a smaller capacitance than that of plural capacitors in parallel with the plural capacitors so as to form dual pi type filters interrupting higher frequency wave than the frequency wave to be interrupted. CONSTITUTION:A 1st pi type filter circuit 6 to interrupt 2nd-3rd harmonic waves is formed by connecting three capacitors 3-5 whose capacitance is respectively 5-8pF in parallel between a strip line 1 and ground. Moreover, a 2nd pi type filter circuit 9 to interrupt 4th harmonic waves or over is formed by connecting a capacitor 7 whose capacitance is 0.5-1pF in parallel with a capacitor 4 and connecting a capacitor 8 whose capacitance is 0.5-1pF in parallel between the capacitors 3, 4. Thus, not only 2nd-3rd harmonic waves of a pass band but also 4th harmonic waves or over are sufficiently interrupted, and since only capacitors of a small capacitance are added, the low pass filter is inexpensively realized.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a low-pass filter circuit used in an 800M) Iz band transmitter and transceiver.

"Prior Art" This type of conventional low-pass filter circuit is constructed by connecting three chip capacitors (3), (4), and (5) to a 50Ω strip line (1), as shown in Figure 5. The equivalent circuit is shown in FIG.

[Problem to be solved by the invention] The conventional circuit has the characteristics shown by the dotted line (B) in Figures 3 and 4, and is designed to block the 2nd to 3rd harmonics of the transmission output carrier wave in the 800M band. was. That is, in the dotted line characteristic CB) in FIGS. 3 and 4, 850M)h (
7) It is designed to block the second harmonic (point B2) and third harmonic (point B3) from the passband (point B1).

However, in recent years, as the frequency of high-frequency transformers has been extended to higher frequencies, and measuring instruments have become capable of measuring high frequencies, it is now necessary to block unnecessary radio waves not only at the 2nd to 3rd harmonics, but also at the 4th harmonic and above. It has become impossible to do so. However,
In the characteristic line (B) of FIG. 4, it was not possible to sufficiently block the fourth harmonic wave as in the case of CB4 points).

An object of the present invention is to obtain a filter circuit that can sufficiently block the fourth harmonic or higher by simply adding a simple element to a conventional circuit.

[Means for Solving the Problems] The present invention provides a π-type low-pass filter circuit in which a plurality of capacitors are connected in parallel to each other in a strip line to cut off frequencies several times higher than the transmission output carrier wave. A capacitor with a smaller capacity than these capacitors is added in parallel with the capacitor to form a double π type in order to cut off frequencies even higher than the frequency to be cut off.

1 action" The first π-type filter, which is made of a capacitor with a large capacity among the capacitors connected in parallel, blocks the 2nd to 3rd harmonics of the pass band, and the second filter, which consists of a capacitor with a small capacitance, blocks the 4th harmonic and above. Block it with a π-type filter.

"Example" Hereinafter, one embodiment of Ippon Shinmei will be described based on the drawings.

In FIG. 1, (1) is a strip line. Assuming that the passband of the transmission output carrier wave is 800M) lz band,
This stripline (1) is made to 50Ω.

Between this strip line (1) and ground is 5 to 8 PF.
The three capacitors (3), (4), and (5) are connected in parallel with each other to form a first π-type filter circuit (6) for blocking second to third harmonic waves. Additionally, a 0.5-19F capacitor (7) is connected in parallel with the capacitor (4), and a 0.5-1ρF capacitor (8) is inserted in parallel between the capacitors (3) and (4). Second π-type filter circuit (9) for blocking the fourth harmonic or higher
is formed.

Here, up to the 400MHz band, the strip line (1) has a resistance of 50Ω, but in the 800MHz band it is slightly below 50Ω. Therefore, the strip line (1) can be considered as the L component, and therefore the circuit of FIG.
This is equivalent to the circuit shown in the figure.

In this circuit, the first π-type filter circuit (6) blocks the 2nd to 3rd harmonics of the transmitted output carrier wave, as shown at points A2 and A3 of the solid line characteristics (A) in Figures 3 and 4. . Further, in the second π-type filter circuit (9), even the '44th harmonic is sufficiently blocked, as shown at point A4 of the solid line characteristic (A) in FIG.

"Effects of the Invention" Since the present invention is configured as described above, 2 to 3 of one passband are
Not only harmonics but also quadruplets and higher harmonics are sufficiently blocked. Moreover, since only a small capacitance capacitor is added, it can be provided at low cost.

[Brief explanation of the drawing]

FIG. 1 is an electric circuit diagram showing an embodiment of the low-pass filter circuit according to the present invention, FIG. 2 is an equivalent circuit diagram of the same, FIGS. 3 and 4 are filter characteristic diagrams, and FIG. 5 is a conventional low-pass filter. The electric circuit diagram of the circuit, FIG. 6 is an equivalent circuit diagram of the same. (1)...Strip line, (3) (4) (5
) (7) (8)... Capacitor, (6)... First π-type filter, (9)... Second π-type filter. Applicant: Fujitsu General Ltd. Figure p → GHz

Claims (1)

[Claims] (1) In a π-type low-pass filter circuit in which a plurality of capacitors are connected in parallel to each other on a strip line to cut off frequencies several times higher than the transmission output carrier wave, 1. A low-pass filter circuit characterized in that a small-capacity capacitor is added and a double π type is used to cut off frequencies higher than the frequency to be cut off.