The present invention relates to a microwave switched multiplexer. More particularly, but not exclusively, the present invention reiaies to a microwave switched multiplexer comprising a plurality of signal channels, each signal channel comprising an input resonator, a center resonator an then a switch, the two outputs of whic are connected to transmit and receive channels, each of the transmit and receive channels comprising an output resonator, the two output resonators have the same admittances, the ratios of the admittances of the input, center and output resonators being in the ratio π/2;4χ%:7τ/2, where x is in the range 0.9 to 1.1. in a further aspect the present invention relates to a mobile telecommunications device suc as a mobile phone including such a multiplexer.

There are an increasing number of geographic specific microwave frequency band Used by mobile telecommunications systems, if a mobile telecommunications handset is to be used tn a piurality of different geographical areas then the handset must to be able to selectively transmit and receive microwave signals in a plurality of different microwave frequency bands. The mobile handset can identify the transmit and receive signal bands used by the telecommunications system covering its current location and then transmit and receive in those bands.

Conventional handsets typically comprise a plurality of diplexers, each corresponding to a frequency band, The antenna or transmiiter is connected to the appropriate diplexer (or diplexers) if the handset wishes to transmit or receive in a particular signal band.

Suc an arrangement has a number of drawbacks. Perhaps the most significant of these drawbacks is that interband aggregation between adjacent frequency bands (to produce a single band of larger width in frequency) is almost impossible to implement. The frequency selective diplexers have increasing group delay and loss at the center point between bands. Hence, if two diplexers adjacent in frequency ar switched on to produce an aggregate band increased loss nd group delay occurs a the center of the aggregate band,

One method used to overcome this problem is to power divide between alternate channels and recombine at the output. However, this technique increases the overall loss by 6dB. Also, ..trie rapid change in group delay at the crossover frequency make phase tracking of devices (a necessity in interferometer systems) extremely difficult

The present Invention seek to o vercome the problems of the prior a rt.

Accordingly, in a first aspect, the present invention provides a microwave switched multiplexer having a bandpass between frequencies ft and Af-f fa, th m ultiplexer comprisi n g n signal channels, wher n>1 , each signal channel having a signal bandpass at a center frequency within &f, the center frequencies of the signal channels being equally spaced apart by Δΐ/η; each signal ^■■ channel com prising

(a) a switch having first, second and third ports, the switch being adapted to be switched between a transmit position in which the first port is connected to the second port, a receive position in which the first port is connected to the third port and an off position in which the first port is not connected to either second or third ports;

{b} a common line extending from an antenna end to the first port, the common line comprising an input resonator and a center resonator connected together in cascade, the center resonator being coupled between the input resonator and the first port; ^■ (c) a transmit line extending between the second port and a transmit end, the transmit line comprising an output resonator, the output resonator being connected in cascade with the input and center resonators when the switch is in the transmit position;

(d) a receive line extfendfrtg between the third port and a receive end, the receive line comprising an output resonator, the output resonator being connected in cascade with the input and center resonators when the switch is in the receive position; the admittances of the two output resonators being equal; the admittance of the input resonator, center resonator and output resonator being in the ratio

where x is in the range 0.9 to 1.1

Wherein for at least two signal channels adjacent in frequency the antenna ends are connected to a common antenna node and at least one of (a) the receive ends are connected to a common receive node or (b) the transmit ends are connected to a common transmit node; the admittances of the input, center and output resonators having absolute values such that for every signal channel when the switch for that channel is in the transmit or receive position and the switches for all other channels are in the off position, the width of the signal bandpass of the signal channel is substantially Af/n

Preferably for the at least two signal channels the receive ends are connected to a common receive node and the transmit ends are connected to a common transmit node. Preferably for every signal channel the antenna ends are connected to a common antenna node, the transmit ends are connected to a common transmit node and the rece ive e nds a re con nected to a oommon rece ive node .

Preferabiy x is in the range 0.95 to 1.05, more preferably 0.97 to 1.03, more preferabiy 0.99 to 1.01

Preferably at least one of input, center and outpuf resonators is an FBAR or SAW resonator.

Preferably the antenna node is connected to an antenna.

Preferably the antenna node comprises an antenna resonator.

P referabiy the ratio of the admittance of the antenna resonator to the inp ut reson ator is

where y is in the range 0.5 to 1.5

Preferably y is in the range 0.8 to 1.2, more preferably 0.9 to 1.1 , more preferably 0,96 to 1.05.

Preferabi the transmi node is connected to a transmitter, the transmitter being adapted to provide a microwave signal between frequencies ft and ¾ Preferably the transmit node comprises a transmitter resonator, the ratio of th admittances of the transmitter resonator to the output resonator being yi t where y s in the range 0.5 to 1.5, preferably 0.8 to 1.2, more preferably 0.9 to 1.1 , more preferably 0.95 to 1.05.

Preferably the receive node comprises a receiver resonator, the rati of the admittance of the receiver resonator to the output resonator being

¾¾

where y is in the range 0.5 to 1.5, preferably 0.5 to 1.2, more preferably 0.9 to 1.1 , more preferably 0.95 to 1.05.

Preferably the microwave switched resonator further comprises a controller connected to th switches for switch ing the switches between states.

in a further aspect of the invention there is provided a mobile tefecommuhieattons device comprising at least one microwave switched multiplexer as claime in any one of claims 1 to 13.

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

Figure 1 shows a simplified representation of a known: microwav switched multiplexer; Figure 2 shows an equivalent circuit for Yo(p) of a transmission line-

Figure 3 shows an equivalent circuit for Ye(p) of a iransmission iine;

Figure 4 shows an equivalent circuit for a combination of Yo(pJ and Yefp) of a transmission line;

Figure 5a shows insertion loss and return loss for a 20 channel multiplexer not according to the invention wit all channels switched on;;

Figure 5b shows the insertion loss and group delay for the mulfiplexer of figure 5a;

Figure 6a shows the insertion loss and return loss for the multiplexer of figure 5a with one channel switched off;

Figure 6 shows the group delay for the multiplexer of figure 5a with one channel switched off;

Figure 7a shows the insertion loss and return loss for the multiplexer of figure 5a wit one channel switched on;

Figure 7b shows the grou delay for the multiplexer of figure 5a with one channel switched on; Figure 8a shows the insertion ioss and return loss for the multiplexer of figure 5a with three channels switched off;

Figure 8b shows the group delay for the multiplexer of figure 5a with three channel switched off;

Figure 9a shows the insertion loss and return loss for the multiplexer of figur 5a wit three channels switched on;

Figure 9» shows the group deiay for the multiplexer of figure 5a with three channels switched on;

Figure 10a shows the insertion loss and return loss for the multiplexer of figure 5a with two channels switched off ;

Figure 10b shows the group dela for the multiplexer of figure 5a with two channels switched off;

Figure 1 a shows the insertion loss and return loss for the multiplexer of figure 5a with two channels switched on;

Figur 1 b shows the group deiay for the multiplexer of figure 5a with two channels switched on;

Figure i¾a shows the insertion loss and return ioss for the mutttpiexer of figure 5a wit four channels switched off; Figure 1¾b shows the group delay for the multiplexer of figure 5a with four channels switched off;

Figure 13a shows the insertion loss and return loss for the multiplexer of figure 5a with four channels switched on;

Figure 13b shows the group delay for the multiptexer of figure 5a with four channels switched on;

Figure 14a shows the insertion loss and return ioss fo the multiptexer of figure 5a with all channels switched on and finite Q.

Figure 14b shows the group delay for the muliip lexer of figure 5a and the resonators having finite Q.

Figure 15 shows a single signal channei of a microwave switched multiplexer according to the invention;

Figure 6 shows an embodiment of a microwave switched multiplexer according to the invention;

Figure 17 shows in schematic form the signal channels of a microwave switched multiplexer according t the invention; Figures 18a and 18b show the signal channels of figure 17 configured to produce transmit anil receive frequency bands;

Figures 19a and 19b show a further embodiment of a microwave switched multiplexer according to the invention;

Figure 20 shows insertion loss for a multiplexer of figures 19a and 9b configured to have a transmission band and a receive band;

Figure 21 shows the insertion loss for a multiplexer of figures 19a and 19 configured to have a plurality of transmission and receive bands;

Figure 22 shows a further embodiment of a microwave switched multiplexer according to the invention; and,

Figure 23 shows insertion toss and return foss for a signal channel in schematic form.

Shown in figure 1 is a known microwave switched multiplexer 1 , The multiplexer 1 comprises a plurality of dipiexers 2, one for each frequency band. Each dipiexer 2 is connected via a: signal line 3 to a port of a switch 5, The common port 6 of the switch 5 is cdnhected to an antenna 7 < Extending from each dipiexer 2 is a transmission line 8 and a receive line 9 which connect to common transmitters 10 and receivers respectively via switches 11 as shown. Only the common transmitte 10 and transmitter switch 1 are shown for clarity. In order to transmit in a particular frequency band the switch 1 to the transmitter 10 is set to connect the transmitter 10 to the appropriate dipiexer 2. Similarly, the switch 5 to the antenna 7 is set to connect the antenna 7 to the same dipiexer 2. To receive a signal the sam procedure is followed using the receiver and receiver switch.

Such known multiplexers 1 have a number of drawbacks;. Firstly, an additional dipiexer 2 is required for every frequency band that is to be covered. Secondly, as the number of diplexers increases the switches 5,11 become more complex, so degrading performance. Thirdly, in the simplified architecture shown in figure 1 onl one dipiexer 2 can be employed at any one time. If one wishes to employ a plurality of diplexers 2 simultaneously the switching circuitry becomes far more complex an again performance degrades; Finally, and perhaps most significantly, with such a microwave switched multiplexer 1 it is almost impossibl to agg egate adjacent (i frequency) bands together, if one connects to two diplexers 2 having adjacent bands the resulting aggregate band would be essentially unusable having increases loss and group dela at the cross over point of th two bands makin up the aggregat band.

The operation of a microwave switched multiplexer according to the invention may best be explained by referenc to the behaviour of a length of transmission line matched to load and source resistances (normalised to 1 Ohm). The transmission line may § viewed as a filter having no loss and constant group deiay (linear phase at ail frequencies.

Such a transmission line can be defined by a transfer matrix-

Where ^■ c ^■ - cosh (np) and s = sin h (irp) , where p is the normal ised com p!ex f req uency variable.

The reflection coefficient Sn(p) and he transmission coefficient S12CP) are defined in terms of even Y _e (p) and odd Y ₀ (p) admittances of the network as

Where from the above one has

And hence Sti(p) - 0 and S12(p) = e ^~ni

Hence, the circuit is matched at all frequencies wit a linear phase of πω (ρ=]ω).

To form the equivaieni parallel connection of circuit elements a partial fraction expansion of Y _e {p) and Y _e (p.) is performed -

Hence,

Thus, the equivalent circuit for Y ₀ (p) is as shewn in figure 2

For Y _e (p),

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

Thus, the equivalent circuit for ¾(p) is as shown in figure 3,

Combining the even and odd mode networks gives the circuit as shown in figure 4, The circuit of figure 4 whic represents the transmission fine can -alternately be viewed as a muitipiexe comprising an infinite number of signal channels equaily spaced apart in frequency. Each signal channel comprises three resonators connected together in cascade, an input resonator coupled to an input waveguide for receiving the microwave signal, an output resonator coupled to an output waveguide and a center resonator connected between the two. The resonators are connected in cascade. In each signal channel the admittances of the resonators are in the ratio 7Γ/2:4/π:ίτ 2 (arid in fact have these absolute values in this normalised model). The admittances of equivalent resonators in each signal channel are the same (ie ail input resonators have the same admittance, ail center resonators have the same admittance and all output resonators have the same admittance). The signal channels are spaced equally apart in frequency. Whilst the signai 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 signal channels has 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 some means of turning channels on and off is required, This could for example comprise a tuning mechanism connected to the central resonator of each signal channel which can be switched betwee On' and 'off' configurations. When in the 'on' configuration th resonan frequency of the associated central resonator i a signal chanhei is the same as that of the input and output resonators of that channel. When in the 'off configuration the resonant frequency of the associated central resonator is remote from that of the input and output resonators of that channel such that the central resonator is effectively shorted out,

Independent of which signal channels are switched on or off (a signal chanhei is said to be on if the tuning mechanism of the center resonator of the signal channel is in the on configuration and off if the tuning mechanism is in the off configuration) the odd mode admittance re mains the same, Fo r th e even mode admittance however if the central resonator is shorted that component of the even mode network becomes the same as the equivalent component of the odd mode network.

For a multiplexer with a finite number of signal channels the elements which make up the signal channels are identical to the elements in the infinite network. There is an important difference however, if ther are n channels the multiplexer has a bandpass of 2n in frequency, with each signal channel having a signai 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 of signal channels has a bandpass &f between fi and ¾ then the admittances of the resonators will still have the same ratios. However, the absolute values of the adrnittances need to be scaled such that the signal bandpass of each signal channel is Af/n, The frequencies of the resonators are set such that the signal channels are spaced equally apart by Δί/π so together covering the multiplexer bandpas M.

The insertion loss and return loss for such a multiplexer with all signal channels on is shown in figure 5(a). In figure 5(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 signal channels. Each signal channel has a signal bandpass iOMHz in width. The signal channels are equally separated from eac other to cover the multiplexer bandpass between 2.5 and 2.7 GHz, The multiplexe further 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 h+ to infinity channels. This multiplexer is not according to the inventio and is included for explanatory purposes only. It is important to note that there is no change in los or group delay at the cross over point from on channel to the next.

To illustrate what happens to the behaviour of such a multiplexer when signal channels are switche off only channels hear to the center of the multiplexer bandpass are considered. With this simplification a theoretical response can be derived by still considering the multiplexer to have an infinite number of channels.

For one channel off Yo(p)∞ cothCy)

which gives

Hence, Sfi^p is zero at ^+Z- j and transmission loss (Sia(p¾ has a third order zero at p=0. This response is shown in figure 6a with grou dela in figur 6b. The passband begins at p ~ +/- j.

One channel oh -

Hence,

¾(p) JrpCp ² + 1)— sinh(irp

Since the numerator has a third order ro at p~0 and the denominator has a second order zero at p - Q then Sn{0) = 0. Also, Sni*/-|) = 0 and Si2(+/-2j) - 0. This response is shown in figure 7(a).The group delay is shown in figure 7b. In the passband which is between P=+/-j th group delay is below 100ns at the center of the band rislhg 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 ar obtained from the fractions 1+Yo(p) and 1+Ye(p) in the denominator of equation 3, The phase response for 1+Υ _α {ρ) is always linear phase (constant delay) for any switching state. However, for this case one should consider the phase of the factor

For p - jw the phase is JEW/2 for linear phase and hence the phase equals this value at w^O and Therefore the phase is an equidistant approximation to linear phase over the full passband between w=+/-1.

Three channels off -

For three adjacent channels turned off ·

¾Q?) = coth(¾!

* ^{(p) = tan +} π p((p»? ² + + * ^' fpp --m mmm · -···m m ²⁷ - * t- nj CP mm + RY * n

From which

Hence Sn(p) is zero at p - */-j3 and close inspection shows that Sisi ) has a third order zero at p = 0 and doubled ordered zeros at p=*/-j≥. This is illustrated in figure 8a with the group delay in figure 8b.

Three channels on For three adjacent channels turned on-

ΫΜ = coth and

6(p ^4■■ — ^■ 3p

¾ ) ^ coth(p) - ·

πρϊρ ² + l)(p ² + 4)(p

Which gives

Close inspection shows that .Sn(p has single ordered zeros at p==0, p=+/-j3 but double ordered zeros at p = +/-]. This response is shown in figure 9a¾ rora Equation 15 it can be readily deduced that the phase response is an equidistant approximation to linear phase and equal at p=0, +/-J. +/-j2, +ί-β ^' hence, equidistant over the entire passband. The corresponding group delay is shown in figure 9b,

To deal with the even degree cases (ie the multiplexer having an even number of signal : channels having signal bands arranged symmetrica !iy about the center of the multiplexer 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

Where Sii(p) is zero at p~+/¾2 and Si2(p) has a single ordered zero at p=0 and double ordered zeros at p-+/~j as shown in figures 10a and 10b. Two channels on-

S _n (p) ¾p(p ^; l)(p ² -i- 4) + sinh(¼pXp ² - 2)

½(P) ~ ~2(p ² - 2)cosft ² (¾¾

Hence Sni ) has a third order zer at p=0 and zeros at p~+/~j as shown in figure 11 a. The phase response of Si2( ) is an equidistant approximation M* linear phase at p~Q, +/-j,+/-j2 and the corresponding group delay is shown in figure Ti b.

Plots are also shown in figures 3* 12 b, 13a and 13b for four channels off and on.

If finite dissipation loss (fe finite Q) is introduced into the resonators of the signal channels then due to the good return loss and approximately constant delay the overall loss in the respectiv signal channel passbands will be very fiat. This is illustrated in figures 14a and 14b.

The above is included as theoretical background to help explain the behaviour of the microwave switched multiplexer 21 according to the invention.

A single signal channel 20 of a microwave switched multiplexer 21 according to the invention is shown in figure 15. The signal channel 20 comprises a switch 22 having first 23, second 24 and third 25 ports. The switch 22 can be switched between thre positions. In the first position (the transmit position) the first and second ports 23,24 are connected together such that a signal received at the first port 23 is passed to the second port 24 and vice versa. In the second position (the receive position) the first and third ports 23,25 are connected together such that a signal received at the first port 23 is passed to the third port 25: and vice versa. In the third positio (the off position) none of trie ports 23,24,25 are connected together, The switch is typically a <3aAs, CMOS or MEMS switch.

Extending frdhi the first port 23 to an antenna end 26 is a common iine 27. The common fine 27 comprises an input resonato 28 and a center resonato 29 connected together in cascade. The center resonator 29 is coupled between the input resonator 28 and the first port 23. The resonators 28,29 are typically thin film bulk acoustic resonators (FBAR resonators) or surface acoustic wave resonators (SAW resonators). Such resonators 28,29 are highly compact and so suitable for use in mobile handsets. Th invention is not limited to such resonators and Othe resonators such as cavity resonators are possible.

Extending from the secon port 24 of the switch 22 to a transmit end 30 is a transmit line 31. The transmit line 31 comprises an output resonator 32. When the switch 22 is in the transmit position the output resonator 32 is connected in cascade with the input and center resonators 28,29, When the switch 22 Is not in the transmit position the output resonator 32 is not connected to the input and center resonators 28,29. Again, the output resonator is preferably an FBAR resonator or SAW resonator althoug other types of resonator are possible.

Extending from the third port 25 to a receive end 33 is a receive line 34, The receive line 34 comprises an output resonato 32. When the switch 22 is in the receive positio the output resonator 32 is connected in cascade with the input and center resonators 28,29. When the switch 22 is not in the receive position the output resonator is not connected to the input and center resonators 28,29, Again, th output resonator 32 is preferably an FBAR resonator or SAW resonator although other types of resonator are possible.

The two output resonators 32 are preferably identical. In particular the have th same admittance as each other. The ratio of the admittances of the input resonator 28, center resonator 29 and output resonator 32 is π}2:4χίπ πϊ2 wit x in the range 0.9 to 1.1 , more preferabl 0.95 to 1.05, more preferably 0.97 to 1.03, more preferably 0.99 to 1 .01. Preferably x is as Close to 1 as possible. All resonators 28,29,32 within a single channel 20 have the same resonant frequency related to the center frequency for the signal bandpass for that channel 20,

Shown fn figure 16 is a microwave switched multiplexer 21 according to the invention having n signal channels 20. Each signal channel 20 is show as a block with the antenna 26, receive 33 and transmit 30 ends extending from the block 20. The microwave switched multiplexe 21 has a bandpass M between frequencies f< and f _2l with Δί = fi-fa- Each signal channel 20 has a signal bandpass at a center frequenc within Δ|. Eac signal channel 20 has a different center frequency. For each signal channel 20 the resonant frequencies of the input, center and output resonators 28,29,32 are set such that the center frequencies of the signal channels 20 ar equally spaced apart by Af/n, such that they cover the entire bandpass of the microwave switched multiplexer 21 ,

Corresponding resonators 28,29,32 in different signal channels 20 have the same admittance values ie at! the input resonators 28 have the same admittance values, all the center resonators 29 have the same admittance values and all the output resonators 32 hav the sam admittance values. As the absolute values of the admittances of these resonators 28,29,32 is changed {whilst keeping the ratios of the admittances constant within the range specified above) the width of the signal bandpass of each channel 20 changes. The absolute values of these admittances are set such that the width of the signal bandpass for each signal channel 20 Is substantially Δί/η. Between them the signal bandpasses of the signal channels 20 cover the entire bandpass of the microwave switched multiplexer 21. This is shown schematically in figur "17, A alternative wa of expressing this is that the absolute values of the admittances of the input, center and output resonators 28,29,3 are set such thai the signal bandpasses for the signal channels 20 are substantially contiguous.

Each antenna nd 26 is connected to an antenna node 35. A common antenna 361s connected to the antenna node 35. Each receive end 33 is connected to a common receive node 37. A microwave receiver 38 is connected to the receive node 37. Eac transmit end 30 is connected to a common transmit node 39. A microwave transmitter 40 adapted to provide a; microwave signai within th bandpass of the microwave switched multiplexer 21 is connected to the transmit nod 9.

Connected to each of the switches 22 is a controller (hot shown). The controller prog ram maticaiiy set the position of each switch 22. By suitably setting the switches 22 ones can produce a microwave switched multiplexer 21 with any desired arrangement of transmit and receive frequency bands withi the bandpass of th microwave switched multiplexer 21 with each transmit or receiv frequency band made up of the signal bandpasses of one or more signal channels 20. This is show schematicaily in figures 18a and 18b. The bandpasses of signai channels 20 in the off state are shown dotted,

The microwave switched multiplexer 21 according to the invention has a number of advantages. Firstly, one does not require a separate diplexer for each frequency band. The multiplexer 21 com rises a fixed number of signal channels 20 which can be configured to produce any desired configuration of transmit and receive frequency bands. No complex switching is required. All that is required is a single simple switch 22 for each signai channel 20. Most significantly if on sets two (or more) adjacent signai channels 20 to be in the same state (ie eithe receiv or transmit) an aggregated band is produced having a bandpass width equal to the sum of the signal bandpass widths of the signal channels 20. Unlike in the prior art thi aggregated band has constant group delay and loss across the aggregated band, i particular there is no increase in loss and/or group delay at the crossover point of th signal bandpasses of adjacent signal channels 20 making up the aggregated band. Whilst the above embodiment qf a microwave switched multiplexer 21 according to the invention is a significant improvement over known microwave switched multiplexer it can be difficult to coupie to th rnultipiexer 21 across the entire passband without significant loss. Shown in figures 19(a) and 19(b) is an alternative embqdime t of a microwave switched multiplexer 21 according to the invention which overcomes this problem.

In this embodiment the antenna node 35 is ah antenna resonator 35. The antenna ends 26 of each of the Signal channels 20 are couple to the antenna resonator 35 such that the antenna resonator 35 is connected in cascade with each of the input resonators 28. The antenna 36 is also coupled to the antenna resonator 35. Signals pass from the signal channels 20, through the antenna resonator 35 to the ahfenna 36 (and vice versa), Again, the antenna resonator 35 Is typically a SAW resonator or FBA resonator although other types of resonaior are possible.

If the microwave switched rnultipiexer 21 had an Infinite number of signal channels 20 and infinite bandwidth the antenna resonator 35 would not be required One would be able to coupSe to the microwave switched multiplexer 21 across the entire passband with only minimal insertion loss. The microwave switched multiplexer 21 according to th invention however has only a finite number of signal channels 20 and finite bandpass. The antenna resonator 35 compensates for the absence of the infinite number of signal channels which have signal bandpasses outside the bandpass of t he m ultiplexer 21 , a [lowi ng one to cou p le to the rnu lt ip iexer 21 withou t significant loss across the bandpass of the multiplexer 21.

In order to achieve this compensation the ratio of the admittance of the antenna resonator 35 to the input resonator 28 is y/n: π/2 ₍ where y is in the range 0,5 to 1,5, more preferably 0.8 to 1.2, more preferably 0.9 to 1.1. more preferably 0.95 to 1.05. Similarly, in this embodiment t e transmitter node 39 is a transmitter resonator 39 coupled in cascade with the output resonators 32 of the transmit line 31. Signals from the transmitter 40 pass though the transmitter resonator 39 to the Signal channels 20 and vice versa. The ratio of the admittance c-f the transmitter resonator 39 to the output resonator is y/n: nil, where y is in the range 0.5 to 1.5, more preferably 0.8 to 2, more preferably 0 J to 1 ,1 , more preferably 0.95 to 1,05.

Similarly, in this embodiment the receiver node 37 is a receiver resonator 37 coupled in cascade with the output resonators 32 of th receive line 34, Signals from the signal channels 20 pass though the receiver resonator 37 to the receiver 38. The ratio of the admittance of the receiver resonator 37 to the output resonator is y/n: ir/2 where y is in the range 0.5 to 1 ,5, more preferabl 0,8 to ,2, more preferably 0.9 to 1.1 , more preferably 0.95 to 1.05,

Ideally the transmitter resonator 39, receiver resonator 37 and antenna resonator 35 all have the same admittance values.

Figure 19(a) shows the transmitter resonator 39 and associated connections, Figure 9(b) shows the receiver resonator 37 and associated connections. These are not shown in the same figure for clarity.

Figure 20 shows the insertion loss as a function of frequency tor a microwave switched multiplexer 21 according to the invention as shown in figures 19a and 19b. The multiplexer 21 has twent two signal channels 20 each having signal bandpass of width 12.5ΜΗίί and each separated from th next by 12,5MHz. Th signal bandpasses of the signal channels 20 are therefore substantially contiguous and cover the bandpass of the multiplexer 21 from about 700 and 960MHz. The ratio of the admittance; of the input, center and output resonators 28;29,32 are /2:4/π:π/2. The ratio of the admittance of the antenna resonator 35 to the input resonator 28 is 1/n: ?r/2 where n Is the number of channels. Th transmitter resonator 39 and receiver resonator 37 each have the same admittance as ; the antenha resonator 35.

In this embodiment two adjacent signal channeis 20 are configured in the transmit state such that their signal bandpasses form an aggregated transmission band. Two adjacent signal channels 20 are configured in the receive state such thai their signal bandpasses form an aggregated receive band, As can be seen for both the aggregated transmit and receive bands there is no change in insertion loss at the cross over point of the bandpasses of the signal channels 20 making up the bands. Further, there is no change in group delay at this point

Figure 21 is similar to that of figure 20. The signal channeis 20 of the microwave switched multiplexer 21 are configured to provide one aggregate transmission band and two receive bands. One receive band comprises the signal bandpass of a single signal channel 20. The other receive band comprises the signal bandpasses of two adjacent signal channeis producing an aggregated receive band.

Figure 22 shows a further embodiment of a microwave switched multiplexer 21 according to the invention, i thi embodiment for a first subset 20 of the signal channels the receive ends are connected to a common receive node and the transrnf ends 30 are connected to a common transmit node 39. The antenna ends are connected to a common antenna node 35. A second subset of th signal channels 20 are configured in the same way. Both subsets are connected to the same antenna node 35. Only the common transmit nodes 39 are shown for clarity.

Throughout the above description reference is made to the width of th bandpass of a signal channel 20, As the signal channel interact this is measured with only on signal channel 20 switched on and ail other signal channels 20 switched off. Figure 23 shows* in schematic form, insertion loss and return loss for single signal channel 20 within and clos to the edges of th signal bandpass. As can be seen, at the center of the bandpass there is an infinity in the return loss. At each edge of the signal bandpass there is a further infinity in th return foss. The width of the signal bandpass is the difference in frequency between these two edge infinities.