WO1995004383A1

WO1995004383A1 - Switched bandstop filter

Info

Publication number: WO1995004383A1
Application number: PCT/GB1994/001655
Authority: WO
Inventors: John David Rhodes
Original assignee: Filtronic Comtek Plc
Priority date: 1993-07-29
Filing date: 1994-07-29
Publication date: 1995-02-09
Also published as: GB9315644D0; GB2280559A

Abstract

A switched bandstop filter arrangement comprises a bandstop filter (formed by resonators 1 to 5) having a bandpass filter operatively connectable in parallel therewith via switching means incorporated in the bandstop filter, e.g. switched S1, S2 which couple the first and last resonators (1, 5).

Description

Switched Bandstop Filter

This invention relates to a switched bandstop microwave filter arrangement.

Often wideband receivers are designed with high sensitivity to detect low power signals. However, large power signals, which may vary in frequency, can severely affect the performance of the receiver, owing to non-linear effects in the low noise pre-a plifier stages. It is therefore important rapidly to detect large power signals and switch in a bandstop filter to reject them, whilst retaining the maximum possible bandwidth with high sensitivity. In order to meet this requirement, several switched bandstop filters are cascaded in a manner which preserves the sensitivity and dynamic range of the receiver, but enables high power signals at different frequencies to be rejected. A bandstop filter may comprise a cascade of resonators decoupled from a main forward path, and phased to produce the required passband and stopband characteristics. A switched bandstop filter can be achieved by connecting a bandstop filter in parallel with the main signal path via bypass switches, as shown in Figure 1. However, the switches contribute loss at all frequencies, both in their on and off states, and have to pass all of the high power signals. Several such arrangements must be cascaded in order to meet normal requirements, resulting in a significant increase in loss from the switches and reduction in sensitivity, and also a reduction in dynamic range owing to intermodulation products created in the switches from two or more high power signals of different frequencies.

The object of this invention is to provide a switched bandstop filter arrangement which overcomes the above difficulties.

In accordance with this invention, there is provided a switched bandstop filter arrangement which comprises a bandstop filter having a bandpass filter operatively connectable in parallel therewith via switching means incorporated in the bandpass filter.

In addition to overcoming the difficulties noted above, this arrangement has the further advantage that substantially all energy outside the bandstop rejection band flows through the bandstop filter and not the switching means, thus maintaining the low loss and dynamic range when several such arrangements are cascaded. By implication, the arrangement requires at least one bandpass resonator at each of the input and output of the bandpass filter, before respective switches: but if there are two or more resonators before the first switch, then the reflection of two or more resonators at the input and output of the bandstop could create discriminator-type effects in the bandstop characteristic. Thus, the switches in the bandpass filter are preferably located after the first resonator and before the last resonator, respectively.

A good bandstop characteristic is desired when the switches of the bandpass filter are off, and for the bandpass and bandstop filters in parallel to produce an all-pass network when the switches are on. Thus, the overall selectivity of the bandpass filter is not critical and there is no necessity for any resonators in addition to those at the input and output of the bandpass filter.

Accordingly, the switched bandstop filter arrangement may comprise a cascade or succession of resonators decoupled from a main through path (and thus forming the bandstop filter) and a switching means for coupling the first resonator to the last resonator (to form the bandpass filter) .

Embodiments of this invention will now be described by way of examples only and with reference to the accompanying drawings, in which:

FIGURE 1 is a diagram of a bandstop filter with by-pass switches, not in accordance with this invention;

FIGURE 2 is a diagram of an arrangement in accordance with this invention, comprising a bandstop filter with by-pass bandpass filter incorporating switching means;

FIGURE 3 is a diagram of an arrangement in accordance with this invention, comprising a bandstop filter with switched coupling between its first and last resonators;

FIGURE 4 is a representation of a switched bandstop filter arrangement in accordance with this invention, for the even degree case;

FIGURE 5 is a representation of an even-mode network for a 4th degree arrangement;

FIGURE 6 is a representation of the switched bandstop filter arrangement for the arbitrary even degree case;

FIGURE 7 is a representation of an even-mode network for an even degree arrangement;

FIGURE 8 is a representation of a basic lowpass section; FIGURE 9 is a representation of a lowpass section with finite transmission zeros;

FIGURE 10 is a diagram showing computed return loss and insertion loss characteristics of a 6th degree switched bandstop filter arrangement of this invention; FIGURE 11 is a similar diagram in respect of an 8th degree arrangement;

FIGURE 12 is a similar degree arrangement having transmission zeros at specific points;

FIGURE 13 is a representation of an odd degree arrangement in accordance with this invention;

FIGURE 14 is a representation of an even-mode network for a 5th degree arrangement;

FIGURE 15 is a representation of an odd-mode network for the 5th degree arrangement; FIGURE 16 is a representation of the switched bandstop filter arrangement for the arbitrary odd degree case;

FIGURE 17 is a diagram showing computed return loss and insertion loss characteristics of a 5th degree switched bandstop filter arrangement of this invention; FIGURE 18 is a similar diagram in respect of a 7th degree arrangement;

FIGURE 19 is a diagrammatic plan view of an experimental switched bandstop filter arrangement of suspended substrate stripline form; FIGURE 20 is a diagram showing the measured response of the experimental device of Figure 19 in its all-pass state; and

FIGURE 21 is a diagram showing the measured response of the experimental device in its switched-bandstop state. Referring to Figure 2 of the drawings, there is shown a. switched bandstop filter arrangement in accordance with this invention, comprising a bandstop filter and a bandpass filter which incorporates switching means: the switching means in the bandpass filter are operable to connect the bandpass filter in parallel with the bandstop filter. An example of this arrangement is shown in Figure 3 ; this example comprises a cascade of five resonators 1 to 5 decoupled from the main through path (and thus forming a bandstop filter) , and switches SI and S2 for coupling the first resonator 1 to the last resonator 5 (to form a bandpass filter) .

Figure 4 is a representation of the nth degree arrangement of Figure 3 (i.e. a cascade of n resonators) where n is even. All impedance inverters have a characteristic admittance of unity value, except the inverter in the coupling between the first and last resonators which has a characteristic admittance value of -1 as shown. The network is electrically symmetrical, this being a necessary requirement for an "all-pass" condition when the first and last resonators are coupled.

Consider the case of n=4. By forming the even mode admittance, the network shown in Figure 5 is produced where all values are admittances, giving:

Ye 1

CaP⁺J

-JC_χp c_xp-j C₂P+j

-jC₁C₂p² +2 C₁p-j C₁C₂p² + ( C₁ -C₂ ) jp+1

C_λC₂p² + 2 C_χjp+l

(1) C_χC₂p- + ( C_λ-C₂) jp + 1

Now the odd mode admittance Yo = Ye* and for an 'all-pass' response:

Yo Ye = 1 ie: Ye Ye* = 1 (2) and hence from (1) :

ie: C, = 3C₍ (3)

Normalising C, = 1, consider the general nth degree case from the network shown in Figure 6 where the transfer matrix of the remaining networks in the input and output sides respectively are:

A B C D and D B C A (4)

and due to reciprocity AD-BC = 1.

If these are direct connections i.e.: A = D = l B = C = 0, then we have the solution for n = 4 where the capacitors combine to give a value of 3.

The even mode network is shown in Figure 7 for the all- pass case. The input admittance of part of the network Y* as shown is:

y, = C + D(2p+j) A + B(2p+j)

Ap + C + (Bp + D) (2p+j) (5) A + B (2p+j)

Thus:

ye = - P A + B(2p+j)

P-J Ap + C +(Bp + D) p+j)

Ap + B(3p² + 1) +Dp + j[-A(l+p²) -pJ3(l+2p²) -pC-2p²D] (6) Ap² + pB(2p² + l) +pC+D(2p² + l) +j [-Ap-p²B-C-pD]

and for the all-pass condition Ye Ye* = 1, therefore:

Ap²+pS(2p²+l) +pC+__5(2p² + l) = A(l+p²) +pB(l+2p²) +pC+2p²D (7) and :

Ap+p²B+C+pD = Ap+B ( 3p² +1 ) +Dp (8)

hence from (7)

D = A (9) and from (8)

C = (2p² + 1)B (10)

It will be noted that this is a particularly simple relationship.

The transfer matrix now becomes:

A B B ( l +2p²) (11)

This matrix may be represented in terms of image parameters as:

cosh γ Z_xsinh γ Y_jSinh γ cosh γ (12)

where y = cosh^"1(A) and:

Such a section of arbitrary degree may always be decomposed into a cascade of basic sections of degree two with the same image impedance.

The second degree section with transmission zeros at infinity is shown in Figure 8 where:

Ye = Cp + jK, Yo = Ye *

and: = Ye Yo = C '2-pr_.2 K² (14)

Hence:

K = 1, C = X∑ (15)

for the correct image admittance.

This now enables the explicit formulas to be given for the general case shown in Figure 4 for n > 6 as: = 1, C, = 1 + X∑, Cn = 2 + 2

2

and: (16)

_τ = 2*/2 r = 3 - (- - 1)

Now any network section with an image admittance as given in equation (13) will meet the overall requirements with respect to the 'all-pass' condition. As an example, the third order section is shown in Figure 9 where:

Ye = pCo , Yo = (Co + 2CXp + —≤*-- (17)

L_±p

YeYo = -^≥ + CoiCo + 2C_l)p² = 1 + 2p² (18)

2CO = L_χ, Cθ(Cθ + 2C_X) = 2 (19)

If:

then: CO = 2- »?

thus allowing real frequency transmission zeros to be realised in the switched state.

Typical return loss and insertion loss characteristics are shown in Figures 10, 11, 12 for the cases of n = 6 and 8 and the case of n = 8 where there are finite transmissions zeros in each half of the network at

W = _2

^ 3

In all three cases the selectivity characteristics are very good considering the fact that the elements have been determined purely from the 'all-pass' condition. In Figure 12, where there is freedom of choice on the location of the finite transmission zero, a further significant increase in selectivity is achieved.

The general nth degree prototype network, where n is odd, is shown in Figure 13: again a symmetrical network is assumed. The case of n = 5 will be considered first. Forming the even mode admittance, the network shown in Figure 14 is produced, where:

C₂p

Ye = ( 21 )

C₂p C₂C,p^z + 2

C₂p

and for the odd mode shown in Figure 15: ₌ C_tC₂p- + 1

Yo = C_λp 1

(22) C₂p C₂p

For the all-pass condition YeYo = 1, therefore:

and: (23)

C, = -X

Normalising C, = 1, then:

C₂ = 1, C₃ = 2 (24) To analyse the nth degree case consider the network shown in Figure 16, where for the even mode network:

C + P

P 2

1 2 A + -^B

Thus:

Ye = ^{2A +} P^B (26) p (A+D) +— B+C 2

Similarly for the odd mode network:

Yl = £ ₊ = P^β+2D (27)

2 B 2B

and: Yo - p 2B pB+2D

___ B(2+p²) +2pD (28) pB+2D

For the all-pass condition YeYo = 1, therefore:

2A + pB = 2D +pB and: (29)

p(A+D) +P-B+2C = B(2+p²) +2pD 2

Hence:

D = A and:

C = 5(1 21 (30) 4

The image admittance is:

ι = 1 + ^pi (31)

4

and for the basic section shown in Figure 8:

K = 1, C = x (32)

thus providing the explicit formulas for the nth degree case shown in Figure 13 as:

and: (33) C π+i = 2 2

Therefore, as in the even degree case, an explicit formula can readily be obtained for sections with finite transmission zeros. However, the characteristics for the switched case are not as attractive as the even degree solution. For n = 3, we have in fact the classical maximally flat passband with element values of 1, 2, 1. For n = 5 and above however an undesirable zero is produced in S_u(p) and is illustrated by the response for n = 5 in Figure 17 and n = 7 in Figure 18. The ripple level for n = 5 is acceptable, but for n = 7 and above, there is a large ripple in the return loss. Preferably therefore, with n odd, n = 5 or less.

In order to test the above results, a normal SSS (Suspended Substrate Stripline) bandstop filter was designed with a 50Ω through line and capacitively coupled open circuited shunt resonators, as shown in Figure 19: the bandpass path was simply a length of line capacitively coupling the first and last resonators as shown. For test purposes the PIN diodes shown were not incorporated and the coupling line was removed for the bandstop response case.

Figure 20 shows the all-pass case response for the arrangement of Figure 19. The level of return loss out of band was due to the interface with the connectors and the loss in- band is the dissipation loss in the resonators. The bandstop response is shown in Figure 21 and is very close to the theory apart from the asymmetry of the response due to the fact that the distance between the resonators was slightly greater than 90° at the centre frequency.

In the above description, explicit formulas have been developed for a switched bandstop filter which may be transformed into an all-pass device by simply coupling the first resonator to the last resonator. This arrangement meets all of the system requirements for the front end of receivers where attenuation is required for large signals without compromising the sensitivity or dynamic range of the receiver.

The explicit solution has been shown for an odd degree ladder network of arbitrary degree which, when bridged by a single capacitor, becomes an all-pass network. Further the explicit solution has been shown for an even degree ladder network of arbitrary degree which, when bridged by a second degree symmetrical ladder structure, becomes an all-pass networ .

The experimental device of Figure 19, designed in SSS to protect receivers from X-band radars, showed excellent agreement with theoretical predictions.

Claims

1) A switched bandstop filter arrangement, comprising a bandstop filter (1-5) having a bandpass filter operatively connectable in parallel therewith via switching means (S1,S2) incorporated in the bandpass filter.

2) A switched bandstop filter as claimed in claim 1, in which the switching means comprises switches (S1,S2) located after a first resonator (1) and before a final resonator (5) of the bandpass filter.

3) A switched bandstop filter as claimed in claim 1, comprising a succession of resonators (1-5) decoupled from a main through path and arranged to form said bandstop filter, and switching means (S1,S2) for coupling the first and last resonators (1,5) of said succession to form said bandpass filter.