GB2347805A - Electronic filter - Google Patents

Abstract

A band stop filter is disclosed having an input (20) connected to an output (22) via a signal line incorporating at least one lumped element delay circuit (24, 26) for producing a 90 degree phase shift in the signal. Ceramic resonators (28, 30, 32) are coupled to the signal line on either side of the or each delay circuit and are led to ground. The ceramic resonators serve as quarter wave lines. The filter serves to attenuate frequencies in a selected frequency band.

Description

DESCRIPTION ELECTRONIC FILTER The present invention is concerned with electronic filters and more specifically with band-stop filters.

The invention has applications in connection with radio receivers and this field will thus be considered first of all. The problem to which the present inventor first addressed himself is that transmissions to telephone pagers in the United Kingdom are at frequencies which can produce interference in the wave bands used to carry voice communications, including the bands used by emergency services. It can consequently prove necessary, for the sake of good reception in these bands, to filter out of the received signal a certain narrow range of frequencies at which the interference occurs.

Known circuits for this purpose comprise a band-stop filter interposed between the aerial and the radio receiver itself, the filter serving to pass the majority of frequency components of the received signal but to divert to ground those components in a range selected to correspond to that of the interference The filters conventionally used in such circuits have typically been based on electromagnetically resonant cavities whose dimensions are determined by the wavelengths to be filtered out of the signal. A typical example has a cavity defined by copper walls and an internal dimension of approximately 38cm. The resulting bulky unit proves difficult to accommodate eg. in a vehicle carrying the radio receiver. It is also subject to changes of ~t dimension, and hence of filtered frequency, with temperature. In practice it is found that the temperature fluctuations to which a vehicle mounted filter of this type is subject can impair its function.

A known type of band-stop filter is illustrated in Fig. 1. Filters of the type in question for removal of microwave frequency signal components can be realised in several known ways, eg. using microstrip or stripline construction, since the wavelength is short enough for devices realised in this way to be physically small. To a line 2 leading from the filter input 4 to its output 6 are connected at intervals a along the line a set of stub lines 8 of length b, each stub line being led to ground. The lengths a and b are chosen with respect to the frequency band to be filtered; specifically, they both correspond to one quarter of the wavelength in question.

The effect of such a filter is to remove/attenuate only components in a narrow band of frequencies, hence it is referred to as a"narrow band band-stop", or colloquially a "notch", filter.

At microwave frequencies the necessary quarter wavelength lines can be acceptably compact. However to adapt the filter to operate at lower frequencies and so longer wavelengths, particularly in the radio frequency band, requires much longer lines which cannot be accommodated in a conveniently sized unit.

Notch filters operable in the lower radio frequency band have previously been realised using technologies such as lumped element, crystal, helical resonator or cavity construction and have had restricted performance. They variously exhibit excessive loss (due to low quality factor, Q), low power handling capacity, poor temperature stability, large physical size and complex construction.

An object of the present invention is to provide an improved band-stop filter. It is desired that this filter should be capable of blocking selected frequencies in the radio frequency band.

An additional or alternative object of the present invention is to provide a band-stop filter which is an improvement over the known filters referred to above with respect to temperature stability and/or compactness.

In accordance with a first aspect of the present invention, there is a band-stop filter comprising an input, an output, a signal line for carrying a signal from the input to the output, a delay circuit connected in the signal line for producing a 90 degree phase shift in the signal and two ceramic resonators respectively coupled to the signal line on either side of the delay circuit, both ceramic resonators being led to ground and being adapted to serve as quarter wave lines, whereby the filter serves to attenuate components of the signal in a selected frequency band.

As compared with other ways to form the necessary quarter wave line, the ceramic resonator is advantageous in terms of dimensions and temperature stability. Use of a delay circuit in place of the quarter wave lines a of the above described prior art circuit allows for an acceptably compact construction even where the filter is to operate in the RF range.

The delay circuit is preferably a lumped element circuit.

To provide the required filter characteristic, a plurality of delay circuits may be connected in the signal line, both sides of each delay circuit being coupled to respective ceramic resonators each of which is led to ground.

In accordance with a second aspect of the present invention, there is a band stop filter comprising an input, an output, a signal line for carrying a signal from the input to the output and a ceramic resonator coupled between the signal line and ground, the ceramic resonator being adapted to serve as a quarter wave line at a selected frequency in the RF (radio frequency) range and so to attenuate a selected frequency band around the selected frequency.

Preferably, the selected frequency is in the range 10 to 1000 MHz. Still more preferably, the selected frequency is in the range 100 to 250 MHz. The latter range is of particular relevance in preventing interference between mobile radios and paging services, but it must be understood that the filter according to the present invention has applications in other fields.

In a particularly preferred embodiment a delay means is connected in the signal line and is such as to produce a 90 degree signal phase shift. The delay means is preferably a lumped element circuit. Preferably respective ceramic resonators are coupled to the signal line on both sides of the delay means, both resonators being led to ground.

To provide the required filter characteristic, a plurality of delay means may be connected in the signal line, both sides of each delay means being coupled to respective ceramic resonators and each ceramic resonator being led to ground.

A specific embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram of a notch filter of known type; Fig. 2 is a circuit diagram of a filter circuit embodying the invention; Fig. 3 is a copy of a Vector Network Analyser plot showing the attenuation produced by the Fig. 2 circuit over a broad range of frequencies centred on the operative frequency of the filter; Fig. 4 is a similar plot covering a narrower range of frequencies centred on the operative frequency; and Figs. 5a and 5b are respectively side and end views of a ceramic resonator used in the Fig. 1 circuit.

The circuit illustrated in Fig. 2 comprises an input 20 connected to an output 22 via a pair of lumped element delay circuits illustrated in respective boxes 24,26 and connected to each other in series. Items 28,30 and 32 are quarter wave ceramic resonators, the first 28 being connected from a point between the input 20 and the first delay circuit 24, the second 30 being connected from a point between the two delay circuits to ground and the third 32 being connected from a point between the second delay circuit and the output to ground.

The notch frequency of the circuit (i. e. the frequency at the centre of the attenuated range of frequencies) is largely predetermined by the component values selected, particularly those of the ceramic resonators. A prototype circuit constructed by the inventor has a notch frequency of 153. 225 MHz.

The delay circuits 24,26 both serve to produce a 90 degree phase shift between their respective input sides 32 and output sides 34. Various circuits capable of achieving the necessary phase change will be known to the person skilled in the art, but the delay circuits used in the present exemplary embodiment comprise, connected between their input and output sides 32,34, a parallel combination of : i) an inductor 36; ii) a capacitor 38; and iii) a series connected pair of capacitors 40 between which is connected one side of an inductor 42, the other side of this inductor being led to ground.

Connections of the ceramic resonators 28,30,32 to the delay circuits are in each case via a respective variable trimmer capacitor 44, the signal being thereby loosely coupled to the resonators with low return loss and loading. The loading of the resonators by the trimmer capacitors can be used to provide fine adjustrnent of the resonant frequency of the resonators and hence of the frequency range attenuated by the filter.

The ceramic resonators themselves, one of which is illustrated in Fig. 5, are of conventional general type although in creating the prototype it was necessary for the inventor to have resonators tailor made with appropriate frequency response. The illustrated resonator comprises a substantially cuboidal block of ceramic material 50 which is silver plated on all of its outer faces except for one end face 52. A through going bore 54 leads from the end face 52 to the remote end face 56, and the input connected to the resonator is provided by a wire (not shown) conductively secured to the inner face of the bore. The other connection is to the outer plating; in the prototype apparatus the resonator simply rests on the device's metallic casing and so forrns the necessary connection to ground. In production devices this connection may be soldered.

The ceramic materials used in ceramic resonators typically have a relative permittivity in the range 10-100. At the resonant frequency a standing em wave can be supported in their interior.

In the present circuit, the ceramic resonators serve as quarter wave stubs at the notch frequency. That is, each resonator is effectively one quarter of a wavelength long at this frequency.

The circuit can be housed in a metal casing provided with conventional screened connectors for input and output. The prototype device has the three resonators laid side by side in the casing, taking up the bulk of its volume, but is acceptably compact-roughly 15 x 10 x 4cm.

The filter characteristic of the prototype circuit is illustrated in Figs. 3 and 4 by the curves labelled 60, the x axis being the input frequency and the y axis being a logarithmic (decibel) scale representing transmitted signal amplitude. The other curve in both figures represents the Voltage Standing Wave Ratio. The prototype achieves attenuation (at the notch frequency) in excess of 40 decibels and a very narrow passband. The components used, and hence the filter characteristic, are highly temperature stable. 25 watt signals can be handled by the prototype, which is Tx and Rx path compliant.

Claims (10)

1. A band-stop filter comprising an input, an output, a signal line for carrying a signal from the input to the output, a delay circuit connected in the signal line for producing a 90 degree phase shift in the signal and two ceramic resonators respectively coupled to the signal line on either side of the delay circuit, both ceramic resonators being led to ground and being adapted to serve as quarter wave lines, whereby the filter serves to attenuate components of the signal in a selected frequency band.

2. A band stop filter as claimed in claim 1 wherein the delay circuit is a lumped element circuit.

3. A band stop filter as claimed in claim 1 or claim 2 comprising a plurality of delay circuits connected in the signal line, both sides of the delay circuits being connected to respective ceramic resonators each of which is led to ground.

4. A band stop filter as claimed in any preceding claim adapted to filter selected frequencies in the radio frequency band.

5. A band stop filter as claimed in any of claims 1 to 3 adapted to filter selected frequencies in the range 10 to 1000 Mhz.

6. A band stop filter as claimed in any of claims 1 to 3 adapted to filter selected frequencies in the range 100 to 250 Mhz.

7. A band stop filter as claimed in any preceding claim wherein a trimmer capacitor is connected in series with at least one of the ceramic resonators.

8. A band stop filter as claimed in any preceding claim wherein the ceramic resonator has a relative permittivity in the range 10-100.

9. A band stop filter as claimed in any preceding claim having three ceramic resonators.

10. A band stop filter substantially as herein described with reference to and as illustrated in accompanying Figures 2 and 5.