The present invention relates to a microwave switched multiplexer. More particularly, but not exclusively, the present invention relates to a microwave switched muitiplexer comprising n (n>1) transmission channels coupled at each end to input and output microwave resonant waveguides, each transmission channel comprising input output and center resonators, the admittances of the input microwave resonant waveguide, input resonator; center resonator and output resonator being in the ratios as claimed. The present invention also relates to a mobile telecommunications device including such a microwave switched multiplexer.

Switched multiplexers are constructed from the parallel connection of H channel filters each of which can be switched on or off at the same time thus prOducing a frequency dependent device which can have 2 ^N different states. Such devices have been available for over 30 years but suffer from the fact that the frequency selective filters had increasing group delay and loss at the cross over point between chahneis and hence if two adjacent channels were switched on the increased loss and group delay occurred at the center of the combined channels.

One method used to overcome this problem is to power divid between alternate channels and recombine at the output. However, this technique increases the overall loss by 6dB. Also the rapid change in group delay at the cross over frequency makes phase tracking of devices (a necessity in interferometer systems) extremely difficult, Typically 8 or more channels are required

Microwave switched multiplexers having a plurality of transmission channels each comprising resonators coupled together in cascade are also known. Such multiplexers however suffer from high loss, in particular effectively impedance matching to the multiplexer across the passband of the multiplexer has to date proved impossibie, resulting in igh loss.

The present invention seeks to overcome the problems of the prior art.

Accordingly, the present invention provides a microwave switched multi exer having a bandpass Af between frequencies f1 and 12, M~ ft-f2, the multiplexer comprising an input microwave resonant waveguide; ah output microwave resonant waveguide; and, n transmission channels where n^1, each transmission channel coupled to the input microwave resonant waveguide and the output microwave resonant waveguide, each transmission channel having a transmission bandpass at a center frequenc within M. the center frequencies of the transmission channels being equally spaced a part; ir 1 reque n cy by Mf each transmission channel comprising

(a) an input resonator coupled to the input microwave resonant waveguide;

(b) an output resonator coupled to the output microwave resonant waveguide;

(c) a center resonator Coupled to the input and output resonators, the three resonators being coupled together in cascade;

(d) a tuning mechanism connected to the center resonator and adapted to be switched between oh and off states, in the on state the resonant frequency of the center resonator being the same as that of the input and output resonators and; in the off state the resonant frequenc of the center resonator being outside the bandpass Δί; the ! admittances of the input microwave resonant waveguide, input resonator, center resonator and output resonator being in the ratio where x Is in the range 0,9 to 1 ,1 and where y is in the range 0.5 to 1.5 the admittances of the input, center and output resonators having absolute vaiues such that for every transmission channel when the tuning mechanis of the transmission channel is in the on state and the tunirig mechanisms of all other transmission channels are in the off state the width of the transmission bandpass of the transmission channel is substantiall Δί/η

The microwave switched multipiexer according to the invention has no additional loss at the cross Oyer frequencies from one transmission channel to the next Further, the group delay remains fiat (linear phase) throug the cross over when adjacent channels are switched on. Further, one can impedance match to th microwave switched multipiexer across the passband with almost no additional loss.

Preferably y is in the range 0.8 to 12, more preferabl 0.9 to 1.1. more preferably 0.95 to 1.05.

Preferabl x is in the rang 0.95 to 1.05, more preferably 0.97 to 1.03, more preferabl 0.99 to 1.01 ,

Preferably n is odd. Alternatively n is eve ;

Preferably the admittance of the output microwave resonant waveguide is equal to the admittance of the input microwave resonant waveguide.

Preferably each resonato comprises an Ϊ electricall conductin resonant cavity comprising first and second end faces and a side wall extending therebetween, the resonator further comprising a dielectric mernber arranged within the cavity and spaced apart from the end faces; the distance between the first and second end faces defining the length of the cavity, the width of the cavity in a plane normal to the length being at least four times the length.

Preferably the dielectric member of the center resonator comprises an aperture extending therethrough, the tuning mechanism comprising a rod which can be inserted or removed from th aperture to switch the center resonator between on and off states.

Alternatively each resonator comprises an electrically conducin cavity comprising first and second end faces and a side wall extending therebetween, the resonator further Gomprising a resonator post extending from an end face part way to the opposite end face.

Preferably the tuning mechanism comprises an electrical switch extending between the resonator post of the center resonator to the spaced apart end face.

Alternativel each resonator is an FBAR or S W resonator.

Preferably the microwave switched multiplexer further comprises a microwave source connected to the input microwave resonant waveguide, the microwave source being adapted to provide a microwave signal having a frequency between ft and - tn a further aspect of the invention there is provided a mobiie tefecomrnunications device comprising at least one microwave svvitched multiplexer as ..claimed its any one of claims 1 to 12,

The present invention will now be described by way of example only and not n any limitative sense with reference to the accompanying drawings in which

Figure 1 shows an equivalent circuit for Υ ₀ (ρ) of a transmissio line;

Figure 2 shows an equivalent circuit for Y _e (p) of a transmission line;

Figure 3 shows an equivalent circuit for a combinatio of Y¾(p) and Y _e (p) of a transmission line;

Figure 4a shows insertion loss and return loss for a 20 transmission channel multiplexer not according to the invention with all channels switched on;

Figure 4b shows the insertion loss and group delay for the multiplexer of figure 4a;

Figure 5a shows the insertion loss and return loss for the multiplex©^

one channel switched off;

Figure 5b shows the grou delay for the multiplexer of figure 4a with one channel switched off; Figure 6a shows the insertion loss an return joss for the multiplexer of figure 4a with one channel switched on;

Figure 6b shows the group delay for the multiplexer of figure 4a with one channel switched on;

Figure 7a shows the insertion loss and return loss for th© multiplexer of figure 4a with th e channels switched off; of figure 4a with three channels switched off;

Figure 8a shows the insertion loss and return loss for the multiplexer of figure 4a with three channels switched on;

Figure 8b shows the group delay for the muitiptexer of figure 4a with three channels switched off;

Figure 9a shows the insertion loss and return loss for the multiplexer of figure 4a with two channels switched off;

^' Figure. 9b shows the grou delay for the multiplexer of figure 4a with two channels switched off;

Figur 10a shows the insertion loss and return loss for the multiplexer of figure 4a with two channels switched on; Figure 10b shows the group delay for the multiplexer of figure 4a with two channels switched on;

Figure 11a shows the insertion !oss and return loss for the multiplexer of figure 4a with four channels switched off;

Figure 11b shows the group delay for the multiplexer of figure 4a with four channels switched off;

Figure 12a shows the insertion loss and return loss for the multiplexer of figure 4a with four channels switched On;

Figure 12b shows the group delay for the multiplexer of figure 4a with four channels switched on;

Figure 13a shows the insertion loss and return loss for the multiplexer of figure 4a ith all channels switched on and the resonators having finite (¾

Figure 13b shows the group delay for the multiplexer of figure 4a with all channels switched on an d the resonators having finite; Q .

Figure 14(a) shows insertion loss and return loss fo a 20 channel microwave switched multiplexer according to the invention with all channels switched on; Figure 14(b shows the group delay for the multiplexer of figure 14(a);

Figures 15(a) and 15(b) are equivalent plots to figures 14(a) and 14(b) with one channel turned off;

Figures 16(a) and 16(b) are equivalent plots to figures 14(a) and 14(b) with three channels turned on;

Figures 17(a) and 17(b) are equivalent plots to figures 14(a) and 4(b) with all channels ^" on but with the resonators having finite G;

Figure 18 shows an embodiment of a multiplexer according to the invention in cross section;

Figure 19 shows a transmission channel of a further embodiment of a multiplexer according to the invention; and,

Figure 20 shows the bandpass of a single transmission channel within the bandpass and close to the edges.

The key aspect of the design of the microwave switched multiplexer according to the invention is that it is designed as a parallel connection of resonant elements suc thai n th 'all on' state the device represents a known circuit with no loss (in the lossless case) and constant group delay (linear phase) at all frequencies. Such a circuit is equivalent to a length of t ansmission line matched to the load and source resistance (normalised to 1 Ohm) and defined by a transfer matrix - m - C 3

Where c - cosh (π and s—sinh (πρ), where p is the normalised complex frequency variable.

The refiectien coefficient Snip) and the transmission coefficient Sia( ) are defined in terms of even Y _e (p) and odd Y ₀ {p) admittances of the network as

W ere from the above one has

Y _e t ) tanh(— )

And hence Sn(p) ~ 0 and S1¾p)

Hence, the circuit is matched at all frequencies With a linear phase of πω (p=jw)..

To form tri equivalent parallel eonnection of circuit elements a partial fraction expansion of Y _e (p) and Yofp) is performed ~ Hence,

Thus, the equivalent circuit for Y¾(p) is as shown in figure 1

For Υ _ε {ρ),

This may be decomposed into the sum of two infinite series as

Thus, the equivalent circuit for Y _s (p) is as shown in figure 2

Combining the even and odd mode networks gives the circuit as shown in figure 3. The circuit of figure 3 can be Viewed as a multiplexer comprising an infinite number of transmission channels equally spaced apart: i frequency. Each transmission channel comprises three resonators connected together in cascade, an input resonator coupled to an input waveguide for receiving the microwav signal, an output resonator coupled to an output waveguide and a center resonator con necied between the two. The resonators are connected in cascade: Jn eac transmission channel the admittances of the resonators are; in the ratio ΐτίΖ:4Ιπ:πί2 (and in fact have these absolute values in this normalised model). The admittances of equivalent resonators in each channel are the same (te a l input resonators have the same admittance, ail cente resonators have the same admittance and all output resonators have the same admittance). The transmission channels are spaced equally apart in frequency. Whilst the transmission channels are shown physically spaced apart the input resonators are each connected to a common signal Input point. Similarly the output resonators are connected to a common signal output point.

This idealised multiplexer with an infinite number of transmission channels ha the property that it is matched at all frequencies and has a constant delay at all frequencies. To convert this multiplexer into a switched multiplexer a tuning mechanism is connected to the central resonator of eac transmission channel which can be switched between On' and 'off' configurations. Whe i th 'on' configuration the resonant frequenc of the associated central resonator in a transmission channel is the same as that of the input and output resonators of that channel. When in the "off configuration the resonant frequenc of the associated central resonator is remote from that of the input and output resonators of thai channel such that the central resonator is effectively shorted out.

Independent of which transmission channels are switched on or off (A transmission channel is said to be on if the tuning mechanism of the center resonator of the transmission channel is in the on configuration and off if the tuning mechanism is In the Off configuration) the odd mode admittance remains the same. For the even mode admittance however if the central resonator is shorted that component of the even mode network becomes th same as the equivalent component of the odd mode network.

For a multiplexer with a finite number of transmission channels the elements which mak up the transmission channel are identical to the elements in the infinite network. There is an important difference however, if ther are n channels th multiplexer has a bandpass of 2n i frequency, with eac transmission channel having a transmission bandpass of 2. The tuning mechanism when in the off state tunes the resonant frequency of the associated center resonator out of the bandpass of the multiplexer.

if the microwave switched multiplexer having a finite number, n, of transmission channels has a bandpass between and ¾ then the admittances of the resonators will still have the same ratios. However, the absolut values of the admittances need to be scaled such that the transmission bandpass of each transmission channel Is Δί/η. The frequencies of the resonators are set such that the transmission channels are spaced equally apart b n so together covering the multiplexer bandpass Af. Another way of saying this is that the transmissio bandpasses are substantially contiguous.

The insertion loss and return loss for such a multiplexer with ail transmission channels on is shown in figure 4(a). in figure 4(b) the insertion loss and group delay are plotted with a constant delay of 100ns across most of the multiplexer bandpass. The switched microwave ^'' multiplexer ^' has 20 transmission channels. Each transmission channel has a transmission band 10MHz. in width. The transmissio channels are equally separated from each other to cover the muitiplexer bandpass betwee 2.5 and 2.7 GHz. The muitiplexer furthe comprises additional resonant cavities connected as stubs at the common signal input and output points having resonant frequencies above and below the multiplexer band. The additional resonant cavities approximate to the behaviour inband of the missing nt1 to infinity channels. This multiplexer is not according to the inventio and is included for explanatory purposes only.

To illustrate what happens to the behaviour of such a muitiplexer when transmission channeis are switched off only channels near to the center of the muitiplexer bandpass are Gonsidered. With this simpiification a theoretical ^' response can derived by still considering the multiplexer to ave an infinite number of channels.

For one channel off

p ² 4- 1 pi

Which gives

^{Sv p)} Pip ² + 1) - |sinli(7TiB)

Hence, Sit{p) is zero at p=*/~ j and transmission loss (Siaip)) has a third order zero at p=0. This response is shown in figure 5a with group delay in figure Sb. The passband begins at p™ +/- j.

One channel on

77!

y ₀ (p)

Hence, Since the numerator has a third order zero at p-Q and the denominator has a second order zero at p * 0 then SniO) ~ 0. Also, !S ^: n(+/-j) ^' - 0 and 8 _1Ξ (+/-2]} - 0. This response is shown in figure 6(a).The grou delay is shown in figure 6b. I the passband which is between P-+/-} the group delay is below iOOns at the center of the band rising towards the band edge. The phase response therefore possesses a cubic variation relative to linear phase around the center frequency. The phase response and group delay are obtained from the fractions 1+ΥοίΡ) and 1+Y _e (p) in th denominator of equatio 3. The phase response for 1 Y ₀ (p i always linear phase (constant delay) for any switching state. However, for this case one should consider the phase of the factor np 2

Ye(p) = 1 + coth (- -)

7ψ(ρ ^Ζ + 1)

For p = jw the phase is itw/2 for linear phase and hence the phase equals this value at w-0 and w = *M. Therefore the phase is a equidistant approximation to linear phase over the full passband between w=+/-1.

Three channels off -

For three adjacent channels turned off -

From which Hence Sni ) is zero at = +/-)3 and close inspection shows that Si2 ^' (p) has a third order zero at p ~ Ό and doubted ordered zeros at p-+/-|2. This is illustrated in figure 7a with the group delay in figure ,7b.

Three channels on -

For th ee adjacent channels turned on~ And

6(p ⁴ - 3p ^z ~ 12)

np(p ⁱ + l)(p ^z + 4)(p ² + 9)

Which gives

Close inspection show that Sn(p) has single ordered zeros at p-0, p=+/-j3 but double ordered zeros at p - ^· + -}. This response is shown in figure 8a. From Equation 15 it can be readily deduced that the phase response is a equidistant approximation to linear phase and equal at p=0, +/-J. +/-j2, +/-j3 hence, equidistant over the entire passband. The corresponding group delay is shown in figure 8b.

To deal with the even degree cases (ie the multiplexer having an even number of transmission channels having transmission bands arranged symmetrically about the center of the muitipiexer passband) it is easier to perform the transformation p→p+j which produces symmetricai responses around p~0. For example for the second degree case one has Two channels off -

$ ( ) ^~ ¾ρίρ ^{2 +} W ⁺ ¾ + sinhCrcpXp ³ ~ 2)

Where Sni ) is zero at p~+/-j2 and Si _? (p) has a single ordered zero at p~0 and double ordered zeros at =+H as shown in figures 9a and 9b.

Two channels on-

Hence St i(p has a third order zero at p=0 a nd zeros at ρ==* ^| as shown in figure 10a. The phase response of S-i2( ) is an equidistant approximation to linear phase at p=0, +/- ,+/~]2 and the corresponding group delay is shown in figure 10b.

Plots pr also shown in figures 1 a, 1 1b, 12a and 12b for four channels off and on .

if finite dissipation loss (i finit Q) is introduced into the resonators of the transmission channels then due io the good return loss and approximately constant delay the overall loss in the respective transmission channel passbands will be ver flat- his is illustrated in figures 13a and 13b.

Whilst the above described microwave switched rnuliipiexer clearly has highly desirabl it is not possible to effectively impedance match to the multiplexer across the bandpass so resulting in unacceptable loss. The microwave switehed multiplexer according to the invention | similar to that described above. As before the switched multiplexer comprises a finite number of transmission channels each comprising input, center and output resonators connected in cascade. The admittances and resonant frequencies of these resonators ar as previously described. However, the input resonators of each transmission channel are coupled to a common input microwave resonant waveguide. The input microwave resonant waveguide received a microwave signal from an input microwave signal line. In contrast to the switched multiplexer described with reference to figures 4(a) to 13(b) the input microwave signal line is not directly coupled to the transmission channel but only through the microwave resonant waveguide. Similarly, the output resonators of each transmission channel are coupled to a common output microwave resonant waveguide. The signal from the switched multiplexer is received via an output microwave signal line inserted into the output microwave resonant waveguide. These common input and output microwave resonant waveguides compensate for ail of the infinite number of transmission channels that are in the permanently off (odd mode) state and which are absent from the multiplexer having a finite number of transmission channels. These input and output microwave resonant waveguides have fixed admittance values which ar a functio of the number of active channels. Th values of the admittances are obtained by ensuring that the mid band delay is correct and a match occurs at the band edge frequencies of the entire switched multiplexer in the odd state.

In practice this requirement can be realised by ensuring that the admittances of the input microwave resonant Waveguide, input resonators, center resonators and output resonators are in the ratio y . π ι Ax I . πι

Where y is in the range 0,5 to 1 ,5 and x is in the rang 0.9 to 1.1. Preferably y is in the range 0.8 to 1.2, more preferably in th range 0.9 to 1.1 , more preferably 0.95 to 1.05. preferably is in the range 0.95 to 1.05, more preferably 0.97 to 1.03, more preferably 0.99 to 1.01. Shown in figure 14(a) is the insertion loss and return loss for a microwave switched multiplexer according to the invention with ail channels switched on. in figure 14(b) grou delay is plotted showing constant delay across most of the multiplexer bandpass. As described above the microwave switched multiplexer is similar to that employed fo figures 4(a) and 4(b) except it now includes the input and output microwav resonant waveguides. In thi particular example tlie admittances are set such that x ~ 1 and y - 1.05.

Figures 15(a) and 15(b) are equivalent plots to figures 14(a) and 14(b) but with one transmission channel of th microwave switched multiplexer switched off.

Figure 16(a) and 1.6(b) are equivalent plots to figures 14(a) and 14(b) with three transmission channels on,

Figures 17(a) and 17(b) are equivalent to figures 14(a) and 14(b) with all channels on except the resonators have finite Q, As before the behaviour of the microwave switched resonator is substantially independent of Q value.

As can be seen from figures 14(a) to 17(b) inclusion of the input and output microwave resonant waveguides in the microwave switched multiplexer has little effect on the desirable properties of the microwave switched multiplexer. The important difference however is that it is now possible to match to the microwave switched multiplexer according to th invention across the microwave bandpass with negligible loss.

Shown in figure 18 is a specific embodiment of a microwave switched multiplexer ^" !, according to the invention in cross section. The microwave switched multiplexer 1 has a bandpass Δί between frequencies fi and f ₂ - itli M = fi-f ₃ . he microwave switched multiplexer 1 comprises an input microwave resonant waveguide 2 and an Output microwave resonant waveguide 3. Coupled to the Input and output microwave resonant waveguides 2,3 are three transmission channels 4. Each transmission channel 4 comprises an input microwave resonator 5, an output microwave resonator 6 and a center microwave resonator 7. Each microwave resonator 5,6,7 comprises an electrically conducting resonator cavity 8 comprising first and second end faces and a side wait 9 extending therebetween. The cavites 8 are shown in plan view and so only the side wall 9 of Circular cross section is shown. Arranged in each cavity 8 is a circular dielectric member 10. Each dielectric member 10 is spaced apart from the first and second end faces of its associated cavity 8. The length of each cavity 8 is defined by the distance bet fe n its end faces. The width of each cavity 8 in a plane normal to the length is at least four times the length.

Extending through the center of each dielectric member 10 is a small aperture 11. Associated with each center resonator 7 is a tuning mechanism 12, The tuning mechanism 12 comprises a finger 13 which can be inserted into the aperture 1 1 of the dielectric member 10 of the center resonator 7. When the finger 13 is in the aperture 11 the center resonator 7 (and the associated transmission channel 4) is in the off configuration with the center resonator 7 effectively being shorted for more generally having a resonant frequency out of the passband of the multiplexer 1 ). When the finger 13 is not in the aperture 11 the center resonator 7 and the associated transmission channel 4 is in the o configuration. In the on configuration all of the resonators 5,6,7 in a transmission channel 4 have the same resonant frequency.

The input resonator 5 is coupled to the input microwave resonant waveguide 2 by means of an aperture 14 in the side wall 9 of its cavity 8 as shown. The output resonator 6 is coupled to the output microwave resonant waveguide 3 by means of an aperture 15 in the side wail 9 of its cavity 8 as shown. Similarly, the center resonator 7 is coupled to the input and output resonators 5, by means of apertures 16 in the side wail 9 of its resonator cavity 8. Each transmission channel 4 is defined b a transmission ^ center frequency. The center frequency of a transmission channel 4 is determined by the resonant frequency of the resonators 5,6,7 (in the on state) in that transmission channel 4- The resonators 5,6 _* 7 may be tuneabl (by known means such as dispiacement of the dielectric member or a metal rod within the resonator cavity 8) or may have a fixed frequency.

The center frequencies of the transmission channels 4 are equally spaced apart. Generally, f the bandpass of the multiplexer 1 is and there are n transmissio channels 4 they are spaced apart in frequency by Δί/η,

Considering one transmission channel 4 again, as explained above the admittances of the resonators 5,6,7 are designed to be substantially in the ratio input-center;output n(2 4fii niZWi ttst this ratio of admittances is ideal in a practical device a more general range of admittance ratios will still provide an acceptable performance. More generally the ratio of admittances is in the rang /2:χ4/#:π-/2 with X in the range 0.9 to 1.1 , more preferably 0,95 to 1.05, more preferably 0.97 to 1.03, more preferably 0.99 to 1.01 , more preferably 1.

The absolute values of the admittances of the resonators 5,6,7 within a transmission channel 4 are set so that the transmission bandpass of the transmissio channel 4 is equal to the separation between the center frequencies of the transmission channels , or in other words that th transmissio bandpasses are substantially contiguous. The transmission channels 4 interact and so the transmission bandpass of a channel is measure with that transmission channel 4 in the on stat and all other transmissipn channels 4 in the off state.

In the particular embodiment shown in figure 18 all of the input resonators 5 have the sam admittance. All of the output resonators 6 have the same admittance and all of the center resonators 7 have the same admittance, so producing transmission channels 4 which ail have the same transmission bandpass, merel separated equally in frequency.

The input and output mierowave resonant waveguides 2,3 have the same admittances. The admittances of the input microwave resonant waveguide 2, input resonators 5, center resonators 7 and output resonators 6 are in the ratio as previously described .

Typically the microwave signal is provided to the input microwave resonant waveguide 2 of th microwave switched multiplexer 1 by means of an input signal line 17 inserted into the input microwav resonant vvaveguide 2. The input signal line 17 is typicaliy connected to a microwave source (not shown). The microwave source can be a microwav transmitter adapted to provide a microwave signal wit a frequency in the range fi to ¾. Alternatively the microwave source may be an antenna adapted to receive a microwave signal in this range. The output from the microwave switched multiplexer 1 is received from an output signal line 18 which is inserted into the output microwave resonant waveguide 3.

Shown in figure 19 is one transmission channel 4 of an alternative embodiment of a switched microwave multiplexer 1 according to the invention. As before the transmissio channel 4 comprises an input resonato 5 coupled to a input microwave resonant waveguid 2, an output microwave resonator 6 coupled to an output microwav resonant waveguide ¾ and a center resonator 7 connected between the two. The resonators 5,6,7 are connected in cascade.

I this embodiment each resonator 5,6,7 comprises an electrically conducting resonator cavity 20 comprising first and second end faces 21 ,22 and a side wall 23 extending therebetween. Arranged within the eavity 20 is a resonator post 24 extending from one end fac 21 part way towards the other en face 22. The resonators 5,6,7 are coupfed to each other and to the microwave resonant waveguides 2,3 through apertures 25 in the side wails of their resonator cavities 20 as shown,

Connected to the center resonator 7 is a tuning mechanism 26. The tuning mechanism 26 comprises a switch 27 connected between the end of the resonator post 24 and the remote end face 22 of the resonator cavity 20. In the on configuration the switch 27 is open circuit and so the center resonator ^' 7 resonates at the same frequency as the input and output resonators 5,5. In the closed configuration the switch 27 is closed circuit so shorting the resonator post 24 and resonant cavity 20 together.

In an alternative embodiment of the invention the microwave resonators 5,6,7 of the transmission channels 4 are FBAR (thin fiim bulk acoustic resonators) or alternatively SAW (surface acoustic wave) resonators. Such resonators 5,6,7 are known in the art and accordingly are not described in detail. Such resonators 5,6,7 are more compact than the resonators described above, By employing such resonators 5,6,7 the microwave switched multiplexer 1 can be made sufficiently compact that it can be employed in mobile telecommunications devices such as mobile telephones.

As mentioned above the behaviour of the microwave switched multiplexer 1 according to th invention is substantially insensitive to the Q values of the resonators 5,6,7. Nonetheless, resonators having Q values in the range 500 to 4000, more preferably 1000 to 3000 are typically employed.

The microwav switched multiplexer 1 according to the invention possesses a number of unique properties - For any transmission channel 4 in the switched on state the phase response over the entire passband is an equidistant approximation to linear phase and hence produces a constant delay over most of the band,

For any transmission channel on the on state the passband has the maximum number of zeros in the return loss including both band edge frec uencies.

if two or more adjacent transmission channels 4 are switched on a single bandpass filter response is created which has an equidistant approximation to linear phase over the entire cpmbined passband. Furthermore, the overall retur loss function has the maximum number of zeros over the entire combined filter response.

If finite dissipation loss is introduced into the resonators 5,6,7, in the passbands, due to the good return loss and approximately constant delay thev overall loss in the respective passbands will be very flat.

Finally, it is possible to match to the microwave switched multiplexer 1 across the transmission band with very little loss, n the above description reference is made to the width of the transmission bandpass of a single transmission channeL As the transmission channels interact the width of the transmission bandpass of a transmission channel is measured wit onl one transmission chart nei switched on and all other transmission channels switched off. Figure 20 shows, in schematic form, insertion loss and return loss for a single transmission channel within and close to the edges of a transmission bandpass. As can be seen, at the center of the bandpass there Is an infinity in the return loss. At each edge of the bandpass there is a further infinity in the return loss. The width of the iransmission bandpass is the difference in frequency between these two edge infinities.