# US 5,703,545 A · High frequency filter circuit

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- Number: US 5,703,545 A
- Document ID: US5703545A
- Status: Granted
- Kind: A
- Application number: 08/613,423
- Filed: 1996-03-11
- Granted: 1997-12-30
- Earliest claimed: 1995-03-17
- Assignees: Deutsche Thomson-Brandt GmbH
- Inventors: Gerald Dullberg; Herbert Peusens; Veit Armbruster
- Web page: https://patentgrep.com/patent/US5703545A

## CPC

- H03H 7/0123: {comprising distributed impedance elements together with lumped impedance elements}
- H03H: IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS (waveguides, resonators, lines or other devices of the waveguide type H01P)

## Abstract

A filter circuit having an amplifier stage, an upstream filter and a downstream bandpass filter. The filter includes, inter alia, a rejection circuit tuned to an intermediate frequency and a series circuit, tuned to an image frequency and formed by a capacitor and a .lambda./4 coaxial stub. The inductance formed by the coaxial stub is shared by the series circuit and the rejection circuit.

## Claims

### Claim 1

Filter circuit comprising:
an amplifier stage,
a bandpass filter coupled downstream of the amplifier stage, and
a filter coupled upstream of the amplifier stage and has at least one trap comprising a rejection circuit tuned to an intermediate frequency, and a series-tuned circuit tuned to an image frequency,
the rejection circuit and the series-tuned circuit have a common inductance.

### Claim 2 (depends on 1)

Filter circuit according to claim 1 wherein one of the rejection circuit and the series-tuned circuit is formed by a .lambda./4 coaxial stub.

### Claim 3 (depends on 2)

Filter circuit according to claim 2 wherein the .lambda./4 coaxial stub is connected in parallel with an input of the amplifier stage and is short-circuited at an end which is at reference potential, and .lambda. is the wavelength corresponding to the intermediate frequency.

### Claim 4 (depends on 2)

Filter circuit according to claim 2 wherein the series-tuned circuit is formed by a series circuit comprising a capacitor and the .lambda./4 coaxial stub.

### Claim 5 (depends on 4)

Filter circuit according to claim 4 wherein the capacitor is arranged between the input of the amplifier stage and a live end of the .lambda./4 coaxial stub.

### Claim 6 (depends on 2)

Filter circuit according to claim 2 wherein the .lambda./4 coaxial stub has a dielectric composed of ceramic material.

### Claim 7 (depends on 6)

Filter circuit according to claim 6 wherein a relative dielectric constant .epsilon.r of the ceramic material is approximately 88.

### Claim 8 (depends on 2)

Filter circuit according to claim 2 wherein a characteristic impedance Zo of the .lambda./4 coaxial stub is approximately 3.5 ohms.

### Claim 9 (depends on 1)

Filter circuit according to claim 1 wherein the resonant frequencies of the rejection circuit and of the series-tuned circuit are in the range from approximately 1 to 2 GHz, the resonant frequency of the series-tuned circuit being approximately 78 MHz below that of the rejection circuit.

### Claim 10 (depends on 1)

Filter circuit according to claim 1 wherein the input and an output impedances of the amplifier stage are approximately 50 ohms.

### Claim 11 (depends on 6)

Filter circuit according to claim 6 wherein the bandpass filter is a two-circuit ceramic filter tuned to a resonant frequency of the rejection circuit at a mid-frequency.

### Claim 12 (depends on 1)

Filter circuit according to claim 1 wherein the upstream filter comprises a plurality of cascaded traps.

### Claim 13 (depends on 12)

Filter circuit according to claim 12 wherein traps are coupled by means of passive coupling elements.

### Claim 14 (depends on 13)

Filter circuit according to claim 13 wherein the passive coupling elements are T attenuators and/or .pi. attenuators.

### Claim 15 (depends on 13)

Filter circuit according to claim 13 wherein the passive coupling elements have a same input impedance and output impedance as the amplifier stage.

### Claim 16 (depends on 13)

Filter circuit according to claim 13 wherein the passive coupling elements each have an attenuation of 3 db.