WO2005071833A1

WO2005071833A1 - Improvements in or relating to passive filters

Info

Publication number: WO2005071833A1
Application number: PCT/IB2005/050240
Authority: WO
Inventors: Paul A. Moore; Brian J. Minnis
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-01-23
Filing date: 2005-01-20
Publication date: 2005-08-04
Also published as: GB0401407D0

Abstract

A passive filter (PPF) comprises an input (10, 10', 12, 12') for receiving a complex representation (Vin, jVin) of an input signal, an output (14, 14', 16, 16') for a filtered signal, and first and second cascade connected passive polyphase filters (ST1,ST2, and ST1', ST2'). The circuitry of stages (ST1', ST2') of the second filter have the opposite hand to the circuitry of stages (ST1, ST2) of the first filter. The first and second filters have complementary asymmetrical frequency characteristics in which an attenuation band exists on one side of DC but not the other. The stages (ST1, ST2 and ST1', ST2') of the first and second filters may be interleaved as shown or the first and second filters may be cascaded. Capacitive coupling (CB) may be used to eliminate a signal spike on and about DC.

Description

DESCRIPTION

IMPROVEMENTS IN OR RELATING TO PASSIVE FILTERS The present invention relates to improvements in or relating to passive filters which have particular, but not exclusive, application to zero IF radio receivers.

Integrated transceivers/receivers have been available for some years. A principal driving force behind integrated receivers is using the zero IF (intermediate frequency) architecture in which channel selectivity is done using a pair of low pass filters. Realising the low pass filters on-chip as opposed to relying on the use of off-chip components has assisted in making products cheap, small and light. One way of regarding low pass filters is as bandpass filters in which DC is sat in the middle of the passband. At the low frequencies involved, these filters can be readily integrated by using any one of a number of different techniques in which active circuitry is combined with capacitors. Most processes enable capacitors to be integrated. The maximum capacitance value is limited solely by the resultant economic impact of the area required. However while such filters can enable almost any degree of selectivity to be obtained, there is a consequential price to be paid in terms of extra DC power consumption and limited dynamic range performance. Completely passive filters do not suffer from these limitations but, at the low frequencies involved, would not be able to provide any significant degree of selectivity, either. This is because inductors would be required and, in most IC processes, the value of inductors is generally limited to a few tens of nanoHenries before parasitic effects start to make losses unacceptable. Other filtering opportunities are already known to exist when real signals can be processed in complex form. For example, passive polyphase filters, originally proposed by Gingell, M. J. "Single Sideband Modulation using Sequence Asymmetric Polyphase Networks", Electrical Communication, Volume 48, Number 1-2, 1973, Pages 21 to 25. These filters are capable of creating an asymmetrical frequency response in which an attenuation band exists on one side of DC, but not on the other. US Patent Specification 6,529,100 discloses a polyphase filter having metal-insulator-semiconductor (MIS) capacitors in which the whole body of the polyphase filter is fabricated as an IC. The architecture of the polyphase filter is generally known but in using MIS capacitors, capacitors that are parasitic to the MIS capacitors are connected to the input sides of the resistors used in the filter. US Patent Specification 6,236,847 discloses a receiver and a bandpass filter arrangement comprising first and second polyphase filters. In one embodiment a bandpass characteristic is obtained by polyphase mixing an input signal using a first local oscillator. Any strong out-of-band signals in the products of mixing are suppressed using shunt capacitors. The signals are next applied to an AGC(automatic gain control) amplifier after which any unwanted products produced by the mixing process are suppressed by a low pass filter. The signals are then applied to the first polyphase filter which has an asymmetrical passband for frequencies greater than DC. The signals are then applied to a second mixer which functions as an image rejection mixer. The second local oscillator signal has a frequency such that the output from the first polyphase filter is shifted in the opposite direction, that is, the edge in the characteristic is moved down in frequency away from DC. The output from the image rejection mixer is applied to the second polyphase filter which has an asymmetrical passband for frequencies lower than DC. The overall effect is to produce a bandpass characteristic which is determined by the choice of the local oscillator frequencies and is substantially independent of component values. This specification does not disclose how a bandstop or high pass characteristic can be produced.

An object of the present invention is to provide a completely passive filter having a bandstop or high pass characteristic. According to a first aspect of the present invention there is provided a filter having an input for receiving a complex representation of an input signal and an output for a filtered signal, the filter comprising first and second cascade connected passive filters, each of the first and second filters having an asymmetrical frequency characteristic in which an attenuation band exists on one side of DC but not the other, the circuitry of the stages of the second filter having the opposite hand to the circuitry of the stages of the first filter. According to a second aspect of the present invention there is provided a receiver having a filter comprising an input for receiving a complex representation of an input signal and an output for a filtered signal, the filter including first and second cascade connected passive filters, each of the first and second filters having an asymmetrical frequency characteristic in which an attenuation band exists on one side of DC but not the other, the circuitry of the stages of the second filter having the opposite hand to the circuitry of the stages of the first filter. According to a third aspect of the present invention there is provided a transceiver comprising a transmitter, a receiver and means for coupling-out part of the transmitter output and supplying it to the receiver, the receiver having a filter for blocking in-band signals and passing out-of-band signals, the filter comprising an input for receiving a complex representation of the coupled-out part of the transmitter output, an output for a filtered signal, and first and second cascade connected passive filters, each of the first and second passive filters having an asymmetrical frequency characteristic in which an attenuation band exists on one side of DC but not the other, the circuitry of the stages of the second filter having the opposite hand to circuitry of stages of the first filter. According to a fourth aspect of the invention there is provided and integrated circuit including a filter according to the first aspect of the invention. The present invention is based on the realisation that when signals are able to be processed in complex form, filters that provide frequency responses that are symmetrical about DC can be created by cascading pairs of passive polyphase filters that are identical, apart from being of opposite handedness, as herein defined, thereby providing much more selectivity than would be obtained from known passive filters that could be realised on-chip at low frequency. In implementing a filter made in accordance with the present invention it is possible, in practice, to combine such pairs of passive polyphase filters in a greater number of ways rather than merely cascading them. The individual stages of each of the filters can be interleaved so that those responsible for the transmission zeros that are equidistant from DC are grouped together in logical pairs. The successive reversal of the handedness of each of the stages in the resultant filter has been found, contrary to expectations, not to give complicated interactions, due to the effects of loading, which may have required the insertion of buffer stages. The input impedance of a polyphase filter is not asymmetrical about DC, even though its frequency response is. Thus, the loading that each stage of the filter places on the one that precedes it does not depend on whether that stage is one providing a transmission zero on the negative or positive side of DC.

The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a schematic diagram of a 6 stage single asymmetric polyphase filter, Figure 2 is a graph of a simulated frequency response of the filter shown in Figure 1 , Figure 3 is a schematic diagram of a filter comprising two opposite handed asymmetric polyphase filters connected in cascade, Figure 4 is a schematic diagram of a filter comprising two opposite handed asymmetric filters whose stages have been interleaved, Figure 5 is a graph of a simulated frequency response of the filter shown in Figure 4, Figure 6 is a section of the simulated frequency response shown in Figure 5 which has been folded about DC and drawn on a logarithmic frequency axis, Figure 7 is a graph of a simulated frequency response of the filter shown in Figure 4 having AC coupling, Figure 8 is a section of the simulated frequency response shown in Figure 7 which has been folded about DC and drawn on a logarithmic frequency axis, and Figure 9 is a block schematic diagram of an embodiment of an arrangement for monitoring the output of a transmitter. In the drawings the same reference numerals have been used to represent corresponding features.

Referring to Figure 1 , the polyphase filter comprises 6 cascaded stages ST1 to ST6 having identical architectures but with each stage having components of different values. The architecture of the filter comprises four rows A to D of six series connected resistors Ri to Re. In each stage capacitors Ci to Cβ, respectively connect the input side of the resistor in the row A to the output side of the resistor in row B, the input side of the resistor in the row B to the output side of the resistor in row C, the input side of the resistor in the row C to the output side of the resistor in row D, and the input side of the resistor in the row D to the output side of the resistor in row A. Real input signals V_in are applied to inputs 10, 10' of the rows A and C, respectively, and imaginary signals jV_in are applied to inputs 12, 12' of the rows B and D, respectively. Real outputs V_out are derived from outputs 14, 14' of the rows A and C, respectively, and imaginary outputs jV_0Ut are derived from outputs 16, 16' of the rows B and D, respectively. The values of the resistors Ri to R₆ and the capacitors Ci to Cβ are selected to give respective minima in the simulated frequency response curve shown in Figure 2, to be discussed later. By way of example the following table gives component values of the stages ST1 to ST6 and the frequency for the respective minima.

Referring now to the simulated frequency response shown in Figure 2 it will be noted that the characteristic is asymmetrical and an attenuation band exists on one side of DC but not the other. For convenience of reference the respective minima have been identified by the stage number. A filter such as that shown in Figure 1 can be used for providing some of the adjacent channel selectivity in low IF receivers. Figure 3 illustrates an embodiment of a filter PPF made in accordance with the present invention. This filter PPF comprises first and second oppositely handed polyphase filters F1 , F2 connected in cascade with the outputs of the first filter F1 being connected directly to the inputs of the second filter F2. By opposite handedness is meant that within each stage, ST1' to ST4', of the second filter F2, the capacitors Ci to C₆ respectively connect the output side of the resistor in the row A to the input side of the resistor in row B, the output side of the resistor in the row B to the input side of the resistor in row C, the output side of the resistor in the row C to the input side of the resistor in row D, and the output side of the resistor in the row D to the input side of the resistor in row A. Apart from the opposite handedness, the component values of the corresponding stages of the filters are identical. In the interests of brevity the first filter F1 will not be described because it is essentially a 4 stage version of that shown in Figure 1. The architecture of the second filter F2 comprises four rows A to D of four series connected resistors Ri to R . In each stage ST1' to ST4', capacitors Ci to C , respectively connect the output side of the resistor in the row A to the input side of the resistor in row B, the output side of the resistor in the row B to the input side of the resistor in row C, the output side of the resistor in the row C to the input side of the resistor in row D, and the output side of the resistor in the row D to the input side of the resistor in row A. Real input signals V_in are applied to inputs 10, 10' of the rows A and C, respectively of the first filter F1 , and imaginary signals jVj_n are applied to inputs 12, 12' of the rows B and D, respectively of the first filter F1. Real outputs V_out are derived from outputs 14, 14' of the rows A and C, respectively of the second filter F2, and imaginary outputs jV_0Ut are derived from outputs 16, 16' of the rows B and D, respectively of the second filter F2. The frequency responses of first and second filters F1 , F2 mirror each other in that the minima due to the stages ST1 and ST1 ' have the same numerical value but opposite sign and the same applies to the stages ST2, ST2'; ST3, ST3'; and ST4, ST4' so that when the frequency spectra are folded over at DC, the corresponding minima overlie each other to give the appearance of a high pass filter having a spike at DC. The spike can be removed by AC coupling any two of the stages in each of the rows A, B, C and D. Purely for the purposes of illustration capacitors C_B, shown in broken lines, are optionally inserted into the inputs 10, 10', 12, 12' or alternatively into the outputs 14,14', 16, 16'. The effect of AC coupling is illustrated in Figure 7 to be described later. Figure 4 illustrates a variant of the cascaded filter PPF shown in Figure

3. In this embodiment each filter F1 , F2 comprises two stages, respectively ST1 , ST2 and ST1 ', ST2', the architecture of the latter being opposite handed to the former but otherwise they are similar. In that respect the resistors Rι=R₂ and R₃=R₄ and the capacitors Cι=C₂ and C₃=C . The stages of the filters have been interleaved so that they are ordered ST1 , ST1 ', ST2 and ST2'. However they may be ordered differently for example ST1 , ST2', ST1 ' and ST2 or ST2, ST1', ST2' and ST1. Quadrature related inputs V_in and jV_in are applied to 10, 10' and 12, 12', respectively. The input impedance of a polyphase filter is not asymmetrical about DC, even though its frequency response is. Thus, the loading that each stage of the filter places on the one that precedes it does not depend on whether that stage is one providing a transmission zero on the negative or positive side of DC. AC coupling capacitors C_B may be coupled into the filter input, filter output or in between stages of the filter. An example of the resistance and capacitance values are tabulated below:

The simulated frequency response shown in Figure 5 illustrates the contributions of the asymmetric response of the stages ST1 , ST2 of the filter F1 and of the stages ST1', ST2' of the filter F2. Without having any AC coupling there is a peak 18 at DC so that the filter can be regarded as a very narrow bandpass filter in which DC is sat in the middle of the passband. Figure 6 shows a portion of the simulated frequency response shown in Figure 5 folded about DC and drawn on a logarithmic frequency axis. When represented in this manner the portion of the simulated frequency response can be regarded as a bandstop filter. Figure 7 illustrates in full lines a simulated frequency response of a filter made in accordance with the present invention modified by the addition of AC coupling in the polyphase filter. The peak 18 at DC shown in Figure 5 has now been eliminated and the filter's frequency response has now assumed that of a high pass filter or a band stop filter in which DC is sat in the middle of the stopband 22. For the sake of comparison broken lines 20 show the response of an AC coupling (without the polyphase filter) chosen to follow most closely the out-of-band response of the polyphase filter (with AC coupling) as an example of the best which can be achieved if polyphase filtering did not exist. Figure 8 shows a portion of the simulated frequency response shown in Figure 7 folded about DC and drawn on a logarithmic axis. The response produced by AC coupling alone, that is, without the polyphase filter, is indicated by the broken lines 20. Compared to AC coupling alone the frequency response shows that the polyphase filter with the addition of AC coupling cuts-off over a wider band of frequencies and is able to do this by having a specified attenuation which is not possible with AC coupling alone. Figure 9 illustrates an application of the filter PPF as a bandstop filter in an arrangement for determining spurious out-of band transmissions generated by a transmitter 28. The transmitter 28 comprises a source 30 of digital signals, for example a cellular telephone baseband modem, having outputs respectively connected to first and second digital-to-analogue converters (DACs) 32, 34. Outputs of the first and second DACs 32, 34 are coupled to first inputs of first and second mixers 36, 38, respectively. Quadrature related outputs I and Q of a frequency source 40 are respectively connected to second inputs of the mixers 36, 38. Outputs of the frequency mixers 36, 38 are combined in a summing circuit 42. An output of the summing circuit 42 is connected to a bandpass filter 44, the output of which is applied to a power amplifier 46. The amplified output is filtered in another bandpass filter 48, the output of which is coupled to an antenna 50. The signal to be propagated has the form shown in the inset drawing D1. The middle portion of the signal waveform comprises the wanted signal WS and the flanks S1 , S2 comprises spurious out-of-channel signals. If it is desired to measure the spurious out-of-channel signals it is necessary to suppress the wanted signal WS to leave the flanks S1 , S2 as shown in the inset drawing D2. In order to do this a portion of the output of the filter 48 is coupled out by a coupler 52 and supplied to a receiver 54. In the receiver 54 a signal splitter 56 supplies portions of the signal to first inputs of mixers 58, 60. A signal generator 62 supplies quadrature related local oscillator signals o and Q o to second inputs of the mixers 58, 60, respectively, which mix the signals down to a zero IF. Outputs V_ιn and jV,_n of the mixers 58, 60 are supplied to respective inputs of a filter PPF made in accordance with the present invention which functions as a bandstop filter eliminating the wanted signal WS. The flanks S1 , S2 are supplied to a stage 64 which provides an output on a terminal 66. The stage 64 comprises means, either analogue or digital, for calculating suitable metrics at the terminal 66. By way of example the stage 64 calculates the unwanted power in the flanks S1. S2. In the present specification and claims the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. Further, the word "comprising" does not exclude the presence of other elements or steps than those listed. From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design, manufacture and use of passive polyphase filters and component parts therefor and which may be used instead of or in addition to features already described herein.

Claims

1. A filter (PPF) having an input (10, 10', 12, 12') for receiving a complex representation of an input signal and an output (14, 14', 16, 16') for a filtered signal, the filter (PPF) comprising first and second cascade connected passive filters (F1 , F2), each of the first and second filters having an asymmetrical frequency characteristic in which an attenuation band exists on one side of DC but not the other, the circuitry (R1-R4, C1-C4) of the stages (ST1'-ST4') of the second filter (F2) having the opposite hand to the circuitry (R1-R4, C1-C4) of the stages (ST1-ST4) of the first filter (F1 ).

2. A filter as claimed in claim 1 , characterised in that the first and second filters (F1, F2) are polyphase filters.

3. A filter as claimed in claim 1 or 2, characterised in that stages

(ST1-ST2, ST1'-ST2') of the first and second filters (F1, F2) are interleaved.

4. A filter as claimed in claim 3, characterised by AC coupling (C_B) provided at the input to the filter (PPF).

5. A filter as claimed in claim 3, characterised by AC coupling (C_B) provided at the output of the filter (PPF).

6. A filter as claimed in claim 3, characterised in that AC coupling (C_B') is provided between two interleaved stages (ST1 , ST1' and ST2, ST2').

7. A receiver (54) having a filter (PPF) comprising an input (10, 12) for receiving a complex representation of an input signal and an output (14, 16) for a filtered signal, the filter (PPF) including first and second cascade connected passive filters (F1 , F2), each of the first and second filters having an asymmetrical frequency characteristic in which an attenuation band exists on one side of DC but not the other, the circuitry (R1-R4, C1-C4) of the stages (ST1'-ST4') of the second filter (F2) having the opposite hand to the circuitry of the stages (ST1-ST4) of the first filter (F1 ).

8. A receiver as claimed in claim 7, characterised in that the first and second filters (F1 , F2) are polyphase filters.

9. A receiver as claimed in claim 7 or 8, characterised in that stages (ST1-ST2, ST1'-ST2') of the first and second filters (F1 , F2) are interleaved.

10. A receiver as claimed in claim 9, characterised by AC coupling

(C_B) provided at the input to the filter (PPF).

11. A receiver as claimed in claim 9, characterised by AC coupling (C_B) provided at the output of the filter (PPF).

12. A receiver as claimed in claim 9, characterised in that AC coupling (C_B) is provided between two interleaved stages (ST1-ST1 ' and ST2, ST2').

13. A transceiver (28, 54) comprising a transmitter (28), a receiver

(54), and means (52) for coupling-out part of the transmitter output and supplying it to the receiver, the receiver (54) having a filter (PPF) for blocking in-band signals and passing out-of-band signals, the filter (PPF) comprising an input (10, 12) for receiving a complex representation of the coupled-out part of the transmitter output, an output (14, 16) for a filtered signal, and first and second cascade connected passive filters (F1 , F2), each of the first and second passive filters having an asymmetrical frequency characteristic in which an attenuation band exists on one side of DC but not the other, the circuitry (R1-R4, C1-C4) of the stages (ST1 '-ST4') of the second filter (F2) having the opposite hand to the circuitry (R1-R4, C1-C4) of stages (ST1-ST4) of first filter (F1 ).

14. A transceiver as claimed in claim 14, characterised in that the first and second filters (F1 , F2) are polyphase filters.

15. A transceiver as claimed in claim 14 or 15, characterised in that stages (ST1-ST2, ST1'-ST2') of the first and second filters (F1 , F2) are interleaved.

16. A transceiver as claimed in claim 15, characterised by AC coupling (C_B) provided at the input to the filter (PPF).

17. A transceiver as claimed in claim 15, characterised by AC coupling (C_B) provided at the output of the filter (PPF).

18. A transceiver as claimed in claim 15, characterised in that AC coupling (C_B) is provided between two interleaved stages (ST1 , ST1' and ST2,

ST2').

19. An integrated circuit including the filter (PPF) as claimed in any one of claims 1 to 6.