GB2336037A

GB2336037A - Radio frequency filtering

Info

Publication number: GB2336037A
Application number: GB9807058A
Authority: GB
Inventors: Andrew David Palfreyman; Richard John Hellicar
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 1998-04-01
Filing date: 1998-04-01
Publication date: 1999-10-06
Anticipated expiration: 2018-04-01
Also published as: GB9807058D0; GB2336037B

Abstract

A radio frequency filter circuit comprises first and second terminals (A, C), a ground connection (32), a transmission line (26, 27, 28) connected between the first and second terminals and forming a spiral inductor (28), and a capacitive device (30) connected between the second terminal and the ground connection. The spiral inductor (28) is formed such as to provide a quarter wavelength transmission line filter having a notch frequency at a first predefined frequency, and such that the inductance of the spiral inductor and capacitance of the capacitive device provide a further notch frequency at a second predefined frequency.

Description

2336037 RADIO FREQUENCY FILTERING

The present invention relates to radio frequency filtering. DESCRIPTION OF THE RELATED ART

When processing radio frequency (RP) signals, it is often necessary to provide an amplifier for amplification of the signals. Such amplifiers commonly require the use of a DC bias and supply voltages (VDC), but such DC voltage sources can provide leakage paths along which RP signals can be lost. Accordingly, it is necessary to isolate the DC voltage source from the RP signal path.

Figure 1 of the accompanying drawings shows a first previously considered circuit for achieving such isolation. An amplifier 5 receives a RP input 1 and provides a RP output 3. A RP filter 7 is connected between the RP filter output 3 and an applied DC voltage MC). The RP filter 7 comprises a length L of printed transmission line 9, e.g. microstrip which is connected between the RP output 3 and the applied DC voltage VDC, and a capacitor 11 which is connected between the applied DC voltage VDC and ground 13. The line 9 provides a short length of transmission line having well known characteristics.

The transmission line will act as a filter at certain frequencies defined by the length of the line. Accordingly, if the main frequency of the RP signal being amplif ied by amplif ier 5 is F,, the f ilter 7 is designed to present an effective open circuit impedance at the f requency PO, in order to isolate the RP f rom VDC. The filter characteristics of the line 9 are achieved by choosing its length such that:

is = 4L where X. = wavelength of the fundamental frequency, F..

(1) Accordingly, length L should be equal to a quarter wavelength of the fundamental frequency of operation of the amplifier 5.

In addition, the line 9 also provides a short circuit at a frequency Fsc where:

XX = 2L (2) Thus, the circuit provides an open circuit at F. and a short circuit at 2FO. The capacitor 11 is chosen so as to operate as a short circuit at radio frequency so that any RF signal transmitted through the microstrip 9 is conducted to earth, whereas the DC voltage signal VDC is isolated from earth. The circuit can therefore isolate F. from VDC and also reduce any second harmonic (2F.) components of the RF signal.

Figure 2 shows a second previously-considered arrangement for isolating a DC voltage source VDC from RF in an amplifier 16. The amplifier 16 receives a radio frequency input 15 and provides a radio frequency output 17, as before. A coiled inductor 19 is provided between the RF output 17 and the DC input VDC.

Such an inductor is designed to provide a high impedance RF choke, at the fundamental RF frequency FO amplified by the amplifier circuit and, of course, provides increasing impedance with increasing frequency.

However, the circuits of Figures 1 and 2 are of limited use, since they can only provide effective or optimum filtering at a very restricted choice of frequencies.

It is therefore desirable to provide a radio frequency filter circuit which can provide multiple isolation or notch frequency characteristics, as well as being simple and economical to produce on a printed circuit board.

SUMMARY OF THE PRESENT INVENTION

According to a first aspect of the present invention, there is provided a radio frequency filter circuit comprising:

first and second terminals; a ground connection; a transmission line connected between the first and second terminals and forming a spiral inductor; and a capacitive device connected between the second terminal and the ground connection, the spiral inductor being formed such as to provide a quarter wavelength transmission line filter having a notch frequency at a first predefined frequency, and such that the inductance of the spiral inductor and capacitance of the capacitive device provide a further notch frequency at a second predefined frequency.

Such a circuit can be used for multi-notch filtration, for example for isolation of an RF line from a DC source.

In any aspect of the invention, the transmission line can, for example, be a microstrip line, a coplanar line, or strip-line.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 and 2 show respective circuit diagrams of previously-considered radio frequency filters; and Figure 3 shows a circuit diagram of a radio frequency filter circuit embodying the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Figure 3 shows a radio frequency filter circuit embodying the present invention, which is connected to the output of an amplifier circuit. The filter provides an RF filter function for the signal output is giving from the amplifier circuit. The filter can be used to provide isolation from a DC source.

A radio frequency (RF) input 21 is connected to an amplifier 24 which produces a RF output 22. A microstrip spiral inductor 28 and microstrip lines 26 and 27 and a capacitor 30 connect the RF output 22 the DC voltage VDC to ground or earth 32. A DC voltage can be connected to the microstrip line 27 and capacitor 30 to provide DC biasing for the amplifier.

The microstrip spiral inductor 28 combined with the capacitor 30 provides a filter arrangement for the circuit. The inductor described with reference to Figure 3 is a microstrip inductor, but could be provided by any transmission line element, such as strip-line, coplanar, etc.. The use of the term "spiral,, simply indicates that the inductor has a number of turns, and does not limit the shape to be, for example, of the circular screw-thread type.

The microstrip spiral inductor 28 provides a printed transmission line filter in combination with the microstrip lines 26 and 27. The notch frequency of the microstrip filter is determined by the length of microstrip between points A and C in Figure 3. The length of this microstrip is measured as the length of strip 26, plus the length of strip 27, plus the length of strip needed to provide the spiral inductor 28. The notch frequency (Ff) of the microstrip filter is given by:

A. =2A C f F -- - L f SC (3) (4) and so the microstrip filter provides an effective short circuit impedance at Ff, and an open circuit is impedance at Ff/2.

The microstrip filter notch frequency can be chosen as desired. For example, the microstrip filter can provide a notch filter centred on a harmonic of the fundamental frequency or at a particularly noisy frequency, in order to filter out that component from the RF path.

In addition, the circuit of Figure 3 provides a lumped element L-C filter circuit. The L and C values are chosen to provide a further notch frequency for the circuit.

If the circuit is to be used for isolation, the inductance of the spiral inductor 28 provides a RF choke at the fundamental frequency (F.) of the amplifier circuit. Such an arrangement provides increasing impedance with increasing frequency. Thus, at the fundamental frequency of the amplifier circuit 24, RF signals can be isolated from the DC voltage VDC and from ground.

The inductance of the spiral inductor 28 is governed by the number of spiral turns, and their spacing from one another. The inductance characteristics can therefore provide desired choke characteristics at the fundamental frequency used in the RF circuit.

The notch frequencies provided by the transmission line filter and the lumped L-C circuit can then be chosen according to the specific use of the circuit. The transmission line filter can be adjusted by adjusting the length of the link between the amplifier 24 and the inductor 28. Ideally, the inductor to amplifier distance AB would be chosen so as to avoid coupling between the inductor and the RF output 22.

Claims

CLAIMS is 1. A radio frequency filter circuit comprising:

first and second terminals; a ground connection; a transmission line connected between the first and second terminals and forming a spiral inductor; and a capacitive device connected between the second terminal and the ground connection, the spiral inductor being formed such as to provide a quarter wavelength transmission line filter having a notch frequency at a first predefined frequency, and such that the inductance of the spiral inductor and capacitance of the capacitive device provide a further notch frequency at a second predefined frequency.
2. A filter as claimed in claim 1, wherein the first terminal is for connection to an RF line, and the second terminal is for connection to a DC voltage source.
3. A filter as claimed in claim 1, wherein the first and second terminals provide respective RF input and output ports of the filter.
4. A f ilter as claimed in claim 1, 2 or 3, wherein the spiral inductor provides a radio frequency choke at a first predefined frequency.
5. A filter as claimed in any one of the preceding claims, wherein the transmission line is of microstrip.
6. A filter as claimed in any one of claims 1 to 5, wherein the transmission line is of strip-line.
7. A printed circuit board carrying a radio frequency filter circuit as claimed in any one of the preceding claims, wherein the spiral inductor is formed within the plane of the circuit board.
8. A radio frequency filter circuit substantially as hereinbefore described with reference to, and as shown in, Figure 3 of the accompanying drawings.