GB2164804A - Stripline filters for transmission systems - Google Patents

Abstract

A filter suitable for a demodulator circuit in a receiver of a fibre optic broadband transmission system, such as a cable TV system, comprises a double side printed circuit board, one side providing several stripline resonators (5, 6, 7, 8 and 9), adjacent resonators having capacitive and inductive coupling, the reverse side of the printed circuit board (10) forming an earth plane, each resonator being folded, preferably twice, to provide at least two or preferably three limbs which are parallel and rectilinear (5a,5b,5c), to provide a cheap, reproduceable filter for use in the frequency range 100MHz to 1GHz. A particular form of channel selection circuit incorporating the filter is also described (Figs. 5 and 6). <IMAGE>

Description

SPECIFICATION Filters for transmission systems This invention relates to filters for information transmission systems and circuits incorporating such filters.

An electrical band pass filter is used to select a signal or signals of known frequency from signals of other frequencies. The most effective filters require many sections to achieve minimal disturbance of the pass band as well as high selectivity. Such a filter is normally built from a network of inductors and capacitors several of which are adjusted in production to achieve the optimum response.

Such filters are relatively expensive to manufacture in view of the adjustment required and thus do not provide the "reproduceability" necessary for economical production in large numbers.

Surface acoustic wave devices are also used as filters and although these provide very good "reproduceability", they are nevertheless expensive to produce and have relatively high loss, of the order of 25Db.

Another type of filter can be made of resonant transmission lines. This is common at frequencies above 1GHz and they are used in microwave systems in strip-line form. A book by A.J. Zverev entitled "Handbook of Filter Synthesis" shows such a filter in Fig. 1.4.

A broadband cable TV system requires primary link equipment incorporating a transmit terminal and a receive terminal, the receive terminal being located at the hub or distribution point. In an optical system there would be required at the receiver an optical receiver and FM demodulators for separating the various channels transmitted along the fibre optical primary link. Frequencies typically adopted lie in the range of 300 to 400MHz and it is particularly although not exclusively for this application that the filter requirement has arisen.

It is an object of the present invention to satisfy this requirement and provide a filter at low cost with high reproduceability and minimal loss.

According to the present invention there is provided a bandpass filter comprising a plurality of strip-line resonators formed on the surface of a substrate, the opposite surface of the substrate having metal cladding to form an earth potential conductor, each resonator being folded at least once.

Preferably the fold in the resonator produces at least two limbs formed as parallel rectilinear tracks. In one particular embodiment the resonator has three limbs formed as parallel rectilinear tracks and constituting a "flattened" spiral.

Such a filter can very conveniently be formed by etching a double sided printed circuit board, leaving one side clad in metal and the other side etched to form the individual resonators. Earth connections to the appropriate points on the resonators can be formed by a plated through hole to the earth plane on the reverse side.

According to another aspect of the present invention there is provided a channel selection circuit comprising a printed circuit board carrying components providing means for demodulating a frequency modulated carrier signal, and a filter for selecting the demodulated signal, said filter comprising a printed circuit having a ground plane on one side and a plurality of printed resonators on the opposite side, each resonator being folded at least once and having an earth connection to the ground plane on the opposite side of the board, said printed circuit filter being mounted on the printed circuit board parallel thereto and providing a filter centre frequency in the range 100MHz to 1GHz.

In order to improve conductivity and increase the Q of the filter, the resonators are overplated by the same process as for the plated through holes.

Such a filter is particularly suitable for use in a channel selection circuit in a broadband FM primary link operating in the 100MHz to 1GHz frequency range.

In order that the invention can be clearly understood reference will now be made to the accompanying drawings which: Fig. 1 shows a three-section microwave filter implemented according to the prior art; Fig. 2 shows a two-section filter with folded elements, according to an embodiment of the present invention; Fig. 3 shows a two-section filter with folded elements according to another embodiment of the present invention; Fig. 4 shows a five-section filter according to yet another embodiment of the present invention; Fig. 4A is an equivalent circuit diagram of the filter of Fig. 4; Fig. 5 shows in cross section and somewhat schematically a physical construction for a channel demodulator circuit incorporating a filter according to an embodiment of the present invention, and Fig. 6 shows the block circuit diagram of the demodulator circuit of Fig. 5.

Referring to Fig. 1 there is shown a threesection microwave filter implemented according to the prior art, as described in the Handbook by A.J. Zverev referred to earlier. Each resonator is one quarter wave long. The three resonate elements are coupled by being in close proximity so that the electrical and magnetic fields of adjacent resonators provide the desired coupling. It will be noted that opposite ends of each alternate resonator are grounded. This is necessary to ensure that electrical and magnetic coupling do not act in opposite senses. The current is shown by ar rows to emphasise this aspect.

Referring to Fig. 2 there is shown a twosection filter with folded elements, according to a first embodiment of the present invention. Referring to Fig. 2 the filter has two resonators 1 and 2 formed by etching a printed circuit board which is double sided and the reverse side of which (not shown) represents the ground plane or earth. Resonator 1 is folded to produce two limbs 1a and ib which are rectilinear and parallel. Similarly the second resonator 2 is folded to produce two limbs 2b, 2a which are rectilinear and parallel.

There is electro-magnetic and capacitive cou pling between the resonators. Both elements are grounded at the same end such as to maintain the same sense of magnetic coupling as shown in Fig. 1. Input IN and output OUT connections are made on the limbs la and 2a, respectively, at a predetermined distance from the connection of those limbs to the ground plane.

Referring now to Fig. 3 another embodiment of the invention is described in which the resonators of Fig. 2 are further folded to produce a lower frequency of operation. These resonators can be considered to be based upon spirals, elongated to increase the region of coupling between each other. It is also analogous to a helical resonator filter, but implemented in a single plane with spiral instead of helical resonator configuration. Referring to Fig. 3 the first resonator 3 has been folded to produce three limbs 3a, 3b, and 3c all rectilinear and parallel to each other and connected in series. Similarly the resonator 4 has three limbs produced by folding, 4a, 4b, and 4c, connected in series. The sense of the magnetic coupling is indicated by the arrows, as previously.Input IN and output OUT are derived at limbs 3a and 4a respectively at a predetermined distance from the connections to the ground plane. Once again this would be implemented in double sided printed circuit board, etched on one side to provide the resonators and the cladding on the opposite side (not shown) providing the ground plane.

In both the embodiments of Fig. 2 and Fig. 3, plated through holes are provided to make connection between the ground plane and the ends of the resonator limbs 1a . 2a, 3a, 4a.

A discussion of helical resonator configurations mentioned above can be found in chapter 9 of the above-mentioned book by A.J.

Zverev.

Referring now to Fig. 4 there is shown a five-element filter formed according to a further embodiment of the present invention. It is constructed of double side printed circuit board material. One side (the side visible) carries the pattern shown and the other carries the earth plane. The resonators 5, 6, 7, 8 and 9 are each folded to provide three limbs such as 5a, 5b, Sc which as before are rectilinear and parallel. Each resonator has a further folded section providing limbs 5d and 5e which are grounded by a plated through hole 5f to the ground plane cladding on the reverse side of the printed circuit board. The outline of the printed circuit board is indicated by the dashed line 10, not shown in the previous embodiments in Figs. 2 and 3.

The length of the further sections 5d and 5e relative to the main spirals controls the characteristic impedance for the input and output connections. These short sectons have been folded because this simplifies the experimental optimisation. Instead small spirals could also have been used. The relative length of the main spiral of a resonator such as 5 to the associated further folded section also modifies their mutual coupling which permits fine adjustment of bandwidth. Preferably the metallic cladding of the printed filter is increased in thickness by additive plating of copper or silver process to increase the Q-factor of the resonators. This can be the same process as for the plated through holes.

The substrate material chosen has a dielectric constant which is stable over the operational temperature range and in this embodiment is made of polyester glass.

The overall dimensions of the embodiment of Fig. 4 is about two inches square with a substrate thickness of one sixteenth of an inch. The track thickness and track separation is also approximately one sixteenth of an inch.

Fig. 4A is an equiavlent circuit of the filter of Fig. 4 and the five sections 5, 6, 7, 8 and 9 are shown. Each section comprises a parallel connection of inductor L and capacitor C4 and coupling capacitance between adjacent sections is represented by series capacitance C3. The input and output terminals IN and OUT are shown as coming from a tap on the inductor of section 5 and section 9, respectively. The sections 5d and 5e shown in Fig.

4 form the part of L between the tap and grounded end of resonator 5.

The coupling capacitance C3 between adjacent resonators is controlled by the distance between the resonators. There is also a degree of magnetic coupling between the adjacent resonators and the polarity is arranged so that the magnetic and capacitive coupling reinforce.

The frequency of resonance depends on the total length of each resonator-the longer the length the lower the resonant frequency.

The thickness of the printed circuit board of the filter affects the Q and affects the ratio of L to C4. But has little if any effect on the resonant frequency.

We have that the self capacitance C4 increases with the width of the track and also increases as the thickness of the board decreases. The coupling capacitance C3 between adjacent sections increases if the resonators are closer together and the magnetic coupling between the inductive portions L also in creases as the resonators are made closer together. The input and output impedance is controlled by the position of the tap on inductor L of resonators 5 and 9.

The filter described in Figs. 4 and 4A has a resonant frequency of 343MHz and a bandwidth of about 10% i.e. approximately 30MHz. It can select a broadband frequency modulated channel carrying for example, television, multichannel voice or data by radio, microwave, co-axial cable, satellite or optical fibre. The relatively high percentage bandwidth i.e. around 10%, allows very cheap construction of common printed circuit board material, particularly suitable for frequencies in the range 100MHz to 1GHz.

A particular application of the filter of Fig. 4 is shown is Figs. 5 and 6. Referring firstly to Fig. 6 there is shown a demodulator circuit for channel selection in a receive terminal in an optical fibre wideband primary link equipment.

Referring to Fig. 6 a carrier signal in the range 60MHz to 180MHz carrying a TV transmission is amplified in an RF amplifier 20 and then mixed in a half-ring balanced mixer 21 with a local oscillator signal 22. The local oscillator is remotely tunable. A buffer amplifier 23 feeds the signal at 100ohms impedance to the input IN of the printed filter 10 as shown in Fig. 4 and Fig. 4A, and tuned to 343MHz. The output OUT is fed to another buffer amplifier 24 having an input impedance of 500hms, and overal between buffer amplifier 23 and 24, in terms of voltage gain there is a reduced loss i.e. an impedance transformation, giving a voltage gain of about 3dB.

The loss of the filter is otherwise approximately 9dB.

The signal is then limited in limiter 25 and proceeds to a frequency discriminator 26 which converts frequency to voltage. This circuit uses a phase locked loop which provides further selectivity over and above that provided by the filter 10.

Finally the signal is amplified in amplifier 27.

The physical implemention of the circuit in somewhat schematic form, is shown in Fig. 5.

A printed circuit board 11 carries surfacemounted components such as 12, 13 and 14 necessary to provide the amplifiers, mixer, local oscillator, limiter and frequency discriminator of the circuit of Fig. 6. On the opposite side of the printed circuit board 11 is mounted the printed circuit filter 10 with its ground plane cladding adjacent the underside of the board 11 and the etched resonators facing away from the board 11. The input and output connections IN and OUT extend through the printed circuit filter 10 and through the printed circuit board 11 to make contact with the appropriate printed circuit track (not shown) on the surface of board 11.

The aperture in the substrate of filter 10 is clear of the cladding on the reverse side. Each connnection plated through holes such as 5f make earth connections between the ends of the resonators and the cladding on the reverse side and these are also connected to the appropriate earth connection tracks on the demodulator printed circuit board 11. Input and output connection terminals for the demodulator are indicated by reference numerals 15 and 16 respectively.

In a particular embodiment there are four channel demodulators such as shown in Figs.

5 and 6, each operating at a different frequency (60MHz, 100MHz, 140MHz and 180MHz) the frequency being determined by the local oscillator. In this embodiment the filters 10 of all four channels are identical and have the same resonant frequency.

The filter described is particularly suitable for the application described in Figs. 5 and 6, and is extremely cheap to produce. The art work to produce the printed circuit resonators defines all the parameters of the filter and is thus very easily and accurately reproduceable.

There is no need for separate electrical components.

Claims (13)

1. A bandpass filter comprising a plurality of strip-line resonators formed on the surface of a substrate, the opposite surface of the substrate having metal cladding to form an earth potential conductor, each resonator being folded at least once.

2. A filter as claimed in claim 1, wherein a resonator comprises at least two limbs formed as parallel rectilinear tracks electrically connected together at one common end and one of the limbs being earthed.

3. A filter as claimed in claim 2, wherein a resonator has three limbs formed as parallel rectilinear tracks.

4. A filter as claimed in any preceding claim having a passband in the range 100MHz to 1GHz.

5. A filter as claimed in any preceding claim, wherein the resonators are etched from one side of a double side printed circuit board.

6. A filter as claimed in claim 5, wherein the resonators are over plated to reduce resistance and increase 0.

7. A filter as claimed in any preceding claim, wherein a resonator is connected to ground by a plated through hole connected to the metal on the reverse side of the substrate.

8. A filter comprising a plurality of strip-line resonators formed side by side on the surface of a substrate with metal on the reverse side to form an earth potential conductor, each resonator having a first elongate limb, a second elongate limb extending generally parallel to the first limb, the limbs being connected together at one end, the first limb being grounded at its second end by connection through the substrate to the earth potential conductor, adjacent resonators being in close proximity to provide inductive and capacitive coupling between them, at least one resonator having an input connection on its first limb spaced a predetermined distance from the earth connection, and at least one other resonator having an output connection spaced a predetermined distance from the earth connection on the first limb, the resonators having been etched from a double sided printed circuit board.

9. A filter substantially as hereinbefore described with reference to and as illustrated in Fig. 2, Fig. 3, Fig. 4 and Fig. 4A.

10. A channel selection circuit comprising a printed circuit board carrying components providing means for demodulating a frequency modulated carrier signal, and a filter for selecting the demodulated signal, said filter comprising a printed circuit having a ground plane on one side and a plurality of printed resonators on the opposite side each resonator being folded at least once and having an earth connection to the ground plane on the opposite side of the board, said printed circuit filter being mounted on the printed circuit board parallel thereto and providing a filter centre frequency in the range 100MHz to 1GHz.

1 1. A channel selection circuit as claimed in claim 10, wherein the positions of input and output terminals on the filter in relation to earth connections, are so arranged as to provide an impedance transformation.

12. A circuit as claimed in claim 11, wherein the impedance transformation in the direction of filter input filter output, is 2:1.

13. A channel selection circuit substantially as hereinbefore described with reference to Figs. 5 and 6 of the accompanying drawings.