NOTCH FILTER

Abstract: A notch filter includes a quadrature coupler (26) having first, second, third, and fourth terminals (28,30,32,34), the first terminal being a signal input terminal and the fourth terminal being a signal output terminal. A first tuned surface acoustic wave (SAW) device (10), coupled to the second terminal, receives the input signal with 0° phase shift, and a second tuned SAW device (10), coupled to the third terminal, receives the input signal with a 90° phase shift. Each SAW device has primarily reactive characteristics at signal frequencies higher or lower than the notch region and primarily resistive characteristics across the notch region. Each tuned surface acoustic wave device comprises a SAW impedance element (12) having an input admittance which is substantially constant in both magnitude and phase over a predetermined band width across the notch region and an inductor (14) tuning the SAW element. The quadrature coupler preferably comprises four inductors (36,38,40,42) which are serially connected to form a continuous circuit. A respective one of first to fourth terminals is coupled to a junction between a respective pair of the serially-connected inductors, and a respective capacitance (52) is coupled between each of the terminals and a ground potential.

Description: This invention relates to a notch filter and in particular to a notch filter which has excellent rejection at the notch frequency and has low insertion loss across a very wide pass band. A notch filter is a device which is generally presumed to pass all frequencies except for a very narrow band which is "notched out". This type of filter is also sometimes called a band elimination filter. In practice, the notch filter will have only a limited band for passing signals but typically this band width is several octaves wide. These filters are widely used to eliminate discrete frequency regions where unwanted carriers appear from adjacent channels or other sources. In addition, they are used to deny access to certain signals or alternatively to remove interference in some signals. Typically, it is desired that a notch filter shall have the lowest possible insertion loss with no ripple in the pass region, and as much rejection as possible across the notch region. Notch filters are generally implemented with inductors and capacitors and can take one of many forms. However, they are prone to temperature drift and have poor Q at UHF frequencies. The net result is that notch filters implemented using LC components tend to drift off the desired frequency and in many cases the notch width is too broad, or the filter loss is too high, or both. For frequencies above approximately 150MHz, notch circuits are generally not utilised because of these problems. Surface acoustic wave (SAW) devices inherently have a much higher Q than LC filter elements and they generally exhibit much superior temperature stability as compared with LC elements. Hence, it is desirable to implement notch filters using SAW devices particularly in the UHF band. A difficulty in implementing a notch filter with SAW devices stems from the fact that SAW devices are of themselves band pass elements in their normal configuration in which one transducer launches a wave and a second transducer receives the wave. Hence, to produce a notch characteristic, the devices must be used in conjunction with some other circuit which converts the band pass region of the SAW to a band reject region and vice versa. One such notch filter is disclosed in US-A-4 577 168 in which a first circuit incorporates a single-port, single-phase, unidirectional surface acoustic wave transducer (SPUDT) as the impedance element coupled to and parallel with an inductor the value of which is chosen to resonate with the static capacitance of the transducer at the centre frequency of the notch. This means that this first parallel circuit is primarily reactive, except at the notch frequency where the circuit is primarily resistive in nature. By placing a resistance in parallel with the first parallel circuit to form a second parallel circuit and choosing a value of the resistance substantially equal to the resistive value of the first parallel circuit at the centre of the notch frequency, and by placing a phase reversal transformer in series with either the resistance or the first parallel circuit, the total circuit provides equal and opposite cancellation of signals at the notch frequency but allows all other frequencies to pass, thus creating a notch filter. This circuit functions well as a notch filter but has an inherent problem. In the prior art circuit, the SAW device is being used as a transmission element. In order to have maximum rejection of the signal frequency in the notch, the parallel inductor-impedance element circuit must become resistive and be equal in value to the resistor forming the second parallel circuit. The resistor forming the second parallel circuit must be chosen to have a value equal to 2Z o , where Z o is the input load, so that the insertion loss in the pass band is flat. This value, 2Z o , for the resistance causes the circuit in the notch region to be perfectly matched to the source, Z o . Hence there is total absorption of the power across the notch region. However, at frequencies in the pass band, the parallel inductor-impedance element circuit becomes substantially purely reactive or an open circuit. This means that the signal is passing only through the resistance forming the second parallel circuit. However, that resistance is now twice the value of the resistance of the input load, thus creating a mismatch. Due to the mismatch there is a reflection loss, and there is a dissipation loss in the resistance, resulting in a net insertion loss of GdB in the filter. This insertion loss cannot be eliminated with this type of circuit because the notch filter circuit is actually the equivalent of two parallel resistors at the notch frequency, and in order obtain a flat response in the pass band, they must be double the value of the input load. In the band pass region, the notch filter consists only of the resistance of the second parallel circuit, which, alone, is twice the value of the input load. The reflections from the notch circuit in the pass band and the minimum GdB insertion loss are both undesirable. Other notch filters in which the SAW device or devices are also used as transmission elements are disclosed in the document US-A-4 126 837. It is an object of the present invention to provide an improved notch filter for rejection of a predetermined band of signal frequencies from an input signal and having a minimum of insertion loss across a wide pass band region. According to one aspect of the invention there is provided a notch filter for rejecting a predetermined band of signal frequencies from an input signal, characterised by a quadrature coupler having first, second, third, and fourth terminals, the first terminal being a signal input terminal and the fourth terminal being a signal output terminal; a first tuned surface acoustic wave (SAW) device coupled to the second terminal for receiving the input signal with 0° phase shift, the first SAW device having primarily reactive characteristics at signal frequencies higher of lower than a notch region, and primarily resistive characteristics across the notch region; and a second tuned SAW device coupled to the third terminal for receiving the input signal with a 90° phase shift, the second SAW device having primarily reactive characteristics at signal frequencies higher or lower than the notch region and primarily resistive characteristics across the notch region; whereby no output signal is present on the fourth terminal across the notch region but an output signal is present in a band pass region. According to another aspect of the invention there is provided a method of forming a notch filter for rejecting a predetermined band of signal frequencies from an input signal, characterised by the steps of forming a quadrature coupler having first, second, third and fourth terminals, the first terminal being a signal input terminal and the fourth terminal being an output terminal; coupling a first tuned surface acoustic wave device to the second terminal for receiving the input signal with a 0° phase shift, the first tuned device having primarily reactive characteristics at signal frequencies higher or lower than a notch region and primarily resistive characteristics across the notch region; and coupling a second tuned surface acoustic wave device to the third terminal for receiving the input signal with a 90° phase shift, the second tuned device having primarily reactive characteristics at signal frequencies higher or lower then the notch region and primarily resistive characteristics across the notch region such that substantially no output signal is present on the fourth terminal across the notch region but an output signal is present in a band pass region. The notch filter operates as follows. The parallel inductor SAW circuit is coupled as a load to the second terminal for receiving the input signal with a 0° phase shift. The load has a primarily reactive characteristic at a signal frequency higher or lower than the notch frequency band width (in the pass band) and primarily resistive characteristics across the notch region as described in the prior art. A second load is coupled to the third coupler terminal for receiving the input with a 90° phase shift. This load is also primarily reactive at signal frequencies in the pass band and is primarily resistive across the notch region. When an input signal is applied to the input terminal at a frequency in the notch region, both of the loads coupled to the second and third terminals are primarily resistive and matched to the source impedance and simply absorb the load, thus providing substantially no output in creating the notch. However, in the band pass region a different operation occurs. At the second port or terminal which is receiving the input signal with a 0° phase shift, the load, in the band pass region, is now primarily reactive. Hence the input signal is reflected. It is reflected back to the input terminal with a 0° phase shift and to the fourth or output terminal with a 90° phase shift. The load which is coupled to the third port or terminal is also primarily reactive and thus reflects the 90° phase shifted signal back to the input terminal shifted the 90° so that it is again 0° at the input terminal. However, it is coupled directed (with 0° phase shift) to the fourth or output terminal and remains the 90° phase shifted signal, thereby adding with the 90° phase shifted signal being reflected from the second terminal. Hence the input signal has been transferred to the output terminal in the band pass region with very little insertion loss and yet provides high signal rejection across the notch region and therefore the device has all of the characteristics of a good notch filter. An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which Fig. 1 is a diagrammatic representation of a prior art notch filter configuration; Fig. 2 is a diagrammatic representation of a prior art quadrature coupled; Fig. 3 is a diagrammatic representation of a notch filter configuration of the present invention utilising a quadrature coupler; Fig. 4 is a diagrammatic representation of the notch filter of the present invention utilising inductor and capacitor components to form a quadrature coupler; and Fig. 5 is a graph illustrating the frequency response of the present notch filter over a very broad frequency range. A schematic representation of the prior art notch filter set forth in US-A-4 577 168 is illustrated in Fig. 1. The filter comprises a first circuit 10 including a packaged, single-phase, unidirectional transducer impedance element 12 coupled as a single-port device in parallel with and tuned by an inductor 14. A second circuit is connected in parallel with the first circuit 10 and comprises a phase-reversing transformer 16 and a series-connected resistor 18. A frequency source 20 applies a signal through an input load or resistor 22 to the parallel arrangement of the first and second circuits. The output load resistor 24 is coupled to the output of the first and second parallel circuits. The wide band frequency characteristic of this parallel SAW-inductor circuit can be understood by recognising that, except for a small frequency range near resonance, the single-phase, unidirectional transducer 12 is only a finite Q capacitor. If the transducer 12 is simply regarded as a capacitor with a finite Q, the circuit will have all-pass characteristics. For optimum performance, if the bridging resistor 18 is chosen to have a resistance value that is twice the resistance value of the input load resistance 22 and the output load resistance 24 (which are of nominally equal value), the transmission characteristic of the complete circuit is nominally flat at all frequencies in the pass band. In the neighbourhood of the notch frequency or the notch region, the inductor 14 is turned to and resonates with the static capacitance of the transducer 12. This resonant condition existing between the inductor 14 and the transducer 12 causes the first or upper parallel circuit 10 to have primarily resistive characteristics in the neighbourhood of the notch frequency because the inductor 14 reactance and the SAW capacitive reactance cancel. The transmission of the signal frequencies is then a balance of current flowing in the upper resonant circuit 10 and the second or lower circuit comprising the phase reversal transformer 16 and the resistor 18. If the resistor 18 is chosen to be approximately equal to the equivalent resistance of the finite Q resonant circuit 10 at resonance, the phase reversal transformer will cause the balanced signals from the first and second parallel circuits to cancel at the output. Hence, in the notch region no signal is transmitted from the input source 20 to the output load 24. At all frequencies other than in the notch region, the transmission from the signal source 20 to the output load 24 is the sum of the frequency-dependent coupling through the parallel resonant circuit 10, which has variation in both magnitude and phase, and the frequency-independent coupling through the reversal transformer 16 and the resistor 80. If the source and load resistors 22 and 24 are chosen to have a value equal to one-half of the bridging resistor 18, the magnitude and phase of the two contributions (parallel circuit 10 and resistor 18) are such that the magnitude of the transmission is approximately flat at all frequencies other than in the notch band. In other words no signal passes through the parallel circuit 10 in the pass band because it becomes essentially a high reactance. It passes only through the resistor 18 and then the transmission is substantially flat. Assuming that the resistances of the upper parallel circuit 10 and the resistor 18 are equal in the notch region, the value of the parallel combination will be one-half the value of either the resistance 18 or the equivalent resistant circuit 10 alone. This is the case because parallel resistors, if of equal value, have an equivalent total value equal to ½ the value of either of the resistors. Again, the resistor 18 forming the second parallel circuit must be chosen to have a value equal to twice the value of the input load resistor 22 (Z o ) so that the insertion loss in the pass band region is flat. This value 2Z o causes the circuit in the notch region to be perfectly matched to the source Z o . Hence there is a total absorption of power across the notch region. However, at frequencies in the pass band region, the parallel tuned circuit 10 becomes essentially reactive or an open circuit. The signal therefore passes only through the resistor 18. The resistor 18 is twice the value of the input load 22, and so creates a mismatch. Because of this mismatch there is a reflection loss and there is a dissipation loss in the resistor 18, resulting in a net insertion loss of approximately GdB in the filter. The prior art circuit of Fig. 1 functions well to provide the notch filter as disclosed in US-A-4 577 168. However, it has inherent disadvantages as explained above. The present invention allows the use of a quadrature coupler in conjunction with a SAW device to obtain a notch filter that has a low insertion loss and a wide pass band, with good impedance matching across the entire range. It is also small in size and capable of handling high power. Transmission line quadrature couplers are well known in the art and one is illustrated schematically in Fig. 2. The quadrature coupler 26 has a first terminal 28, second and third terminals 30 and 32 and a fourth terminal 34. If an input signal is applied to the terminal 28, it is transmitted directly to the second terminal 30 through a coupling circuit with 0° phase shift. It is also coupled to the third terminal 32 through a coupling circuit with a 90° phase shift. In like manner, if a signal is applied at the second terminal 30 it will be coupled directly to the first terminal 28 through a coupling circuit with a 0° phase shift and to the fourth terminal 34 through coupling circuit 42 with a 90° phase shift. Each of the other third and fourth terminals 32 and 34 function in a similar manner. As indicated previously, quadrature couplers are well-known in the art. They can be acquired as shelf items. They are capable of handling extremely high frequencies and can also handle low frequencies. They vary in size and cost with the frequency they are required to handle. As the frequency decreases, the couplers become larger and more expensive. The coupling features of the quadrature coupler as described in relation to Fig. 2 can be utilised in a novel manner with surface acoustic wave devices to form an excellent notch filter. Consider now the operation of the circuit illustrated in Fig. 3, particularly in the frequency range of the notch. It can be seen in Fig. 3 that a respective tuned load circuit 10, comprising a SAW device 12 and an inductor 14, is coupled to each of the third and fourth terminals 30 and 32 of the quadrature coupler 26. While the inductor 14 is shown in parallel in Fig. 3, other tuning such as series coupled inductors could be used. It will be recalled from the discussion earlier that, in the notch region, the reactance of the inductor 14 is adjusted to equal the static capacitive reactance of the SAW device 12, thereby providing an essentially resistive circuit as the parallel circuit 10. The input signal on the first terminal 28 will then be conducted directly to the second terminal 30 with no phase shift and will see an essentially resistive load formed by the parallel circuit 10 in the notch region. Therefore the energy will simply be dissipated in the resistive circuit 10 only across the notch frequency region. In like manner, the input signal on the first terminal 28 will be transferred to the third terminal 32 with a phase shift of 90° but will also be dissipated in an essentially resistive parallel circuit 10, formed by the SAW device 12 and the tuned inductor 14, only across the notch frequency region. Hence, all of the energy at the notch frequency is absorbed by the tuned circuit loads 10 coupled to the second and third terminals 30 and 32. However, the operation of the circuit is considerably different in the pass band region. Consider again an input signal applied to the first terminal 28. This signal is again transmitted to the second terminal 30 with no phase shift. However, at this point, because of the pass band frequency, the parallel circuit 10 is essentially reactive. Hence, the signal arriving at the terminal 30 is reflected back from the terminal 30 and is transmitted directly to the input terminal 28 with 0° phase shift and to the output terminal 34 with a 90° phase shift. In like manner, the signal arriving at the third terminal 32 from the input terminal 28 with a 90° phase shift also sees a reactive circuit 10 because the load at that frequency is basically a capacitive reactance, and is also reflected. It is reflected back to the input terminal 28 shifted by another 90°, so that it arrives back at the terminal 28 with a 180° phase shift with respect to the signal reflected from the second terminal 30, and they cancel. However, it is transmitted also directly to the output terminal 34 with a 0° phase shift and since it has already been shifted 90°, that original 90° phase shift now appears at the output terminal 34 and adds to the 90° phase shifted signal from the second terminal 30. Hence, the entire input signal is now present at the output terminal 34. The circuit shown in Fig. 3 therefore utilises the reflective properties of the surface acoustic wave devices. Such devices can handle large amounts of power in reflection, but not so much power in transmission such as when utilised in the prior art circuit of Fig. 1. In addition, the circuit in Fig. 3 is perfectly matched at all frequencies within the design range of the quadrature coupler. Also, no power is dissipated in each parallel circuit 10 in the pass band region because they are essentially reactive circuits. They, the loads 10, dissipate power only in the notch region because they become essentially resistive circuits. Therefore, in the circuits shown in Fig. 3 the insertion loss is very small, and in an actual circuit constructed as shown in Fig. 3 the insertion loss was only 1.5dB. Further, the circuit shown in Fig. 3 becomes a very broad band circuit and is substantially flat in the pass band region. Fig. 5 illustrates the frequency response of the present notch filter over a very broad frequency range and magnitude. As can be seen, the circuit has approximately 1.5dB insertion loss and is essentially flat in the band pass region. As indicated earlier, the circuit in Fig. 3 works well over a large frequency range such as, for example, from 700MHz to 1300MHz as shown in Fig. 5. However, also as indicated earlier, as the frequency at which the circuit operates becomes lower, the quadrature hybrid coupler must become larger in size and has a greatly increased cost. This problem can be overcome by using a low-frequency lumped element quadrature hybrid 26 shown in Fig. 4. In this case, first, second, third and fourth inductors 36, 38, 40 and 42, respectively, are serially connected to form a continuous circuit. The terminal 28 is coupled to a junction 44 between the inductors 36 and 42. The terminal 30 is coupled to a junction 46 between the inductors 36 and 38. The terminal 32 is coupled to a junction 48 between the inductors 38 and 40, and the terminal 34 is coupled to a junction 50 between the inductors 40 and 42. In addition, a respective capacitor 52 is coupled between each of the terminals 28, 30, 32 and 34 and ground potential 54. The quadrature coupler 26 in Fig. 4 operates in the same manner as the quadrature coupler 26 in Fig. 3. That is, a signal applied to the terminal 28 will be coupled to the terminal 30 with 0° phase shift and to the terminal 32 with a 90° phase shift. Each of the other terminals 30, 32 and 34 operate in similar manner. The difference between the circuit in Fig. 4 and the transmission line quadrature hybrid circuit in Fig. 3 is that the circuit in Fig. 4 does not function over as wide a band width as the transmission line quadrature coupler of Fig. 3. For instance, the circuit in Fig. 4 will function to a frequency of approximately 800MHz, at about which frequency the parasitics become noticeable and the performance is degraded. However, the circuits in Fig. 3 and Fig. 4 overlap between approximately 500MHz and 800MHz and the choice of one over the other will depend on the size of the devices, cost, performance and the like. In general, the circuit of Fig. 4 operates identically to the circuit of Fig. 3 except in a lower frequency range. A notch filter has therefore been disclosed which utilises a quadrature coupler having an input on one terminal and a first load comprising a parallel tuned circuit formed of an inductor and a SAW device coupled to the 0° phase shift terminal and a second, like tuned load, coupled to the 90° phase shift terminal. Each of these parallel tuned loads is primarily resistive in the notch region and primarily reactive in the band pass region, thereby allowing the signals to be absorbed by the resistive nature of the parallel tuned circuits in the notch region but reflecting the band pass frequencies to the output terminal in an adding relationship so that a notch circuit is obtained which has a very little insertion loss, a wide pass band and a good impedance match across the entire range of frequencies. The notch filter is also small and can handle signals of high power since it reflects those signals in the pass band.

Claims: A notch filter for rejecting a predetermined band of signal frequencies from an input signal, characterised by a quadrature coupler (26) having first, second, third, and fourth terminals (28,30,32,34), the first terminal being a signal input terminal and the fourth terminal being a signal output terminal; a first tuned surface acoustic wave (SAW) device (10) coupled to the second terminal for receiving the input signal with 0° phase shift, the first SAW device having primarily reactive characteristics at signal frequencies higher or lower than a notch region, and primarily resistive characteristics across the notch region; and a second tuned SAW device (10) coupled to the third terminal for receiving the input signal with a 90° phase shift, the second SAW device having primarily reactive characteristics at signal frequencies higher or lower than the notch region and primarily resistive characteristics across the notch region; whereby no output signal is present on the fourth terminal across the notch region but an output signal is present in a band pass region. A notch filter according to claim 1, characterised in that each tuned surface acoustic wave device (10) coupled respectively to the second and third terminals (30,32) comprises a surface acoustic wave impedance element (12) having an input admittance substantially constant in both magnitude and phase over a predetermined band width across the notch region; and an inductor (14) tuning the impedance element to form a circuit primarily resistive across the notch region and primarily reactive across the pass band region. A notch filter according to claim 2, characterised in that the impedance element (12) is a surface acoustic wave resonator. A notch filter according to claim 3, characterised in that the impedance element (12) is a single-port, single-phase, surface acoustic wave resonator. A notch filter according to claim 4, characterised in that the single-phase surface acoustic wave device (12) has an inherent static capacitance; and the value of the inductor (14) is chosen to resonate with the static capacitance at the centre of the notch region, thereby forming a substantially resistive circuit at the notch frequency. A notch filter according to any preceding claim, characterised in that the quadrature coupler (26) comprises first, second, third and fourth inductors (36,38,40,42) serially connected to form a continuous circuit; means for coupling a respective one of the first, second, third and fourth terminals (28,30,32,34) to each junction between each two serially connected inductors; and a respective capacitance (52) coupled between each of the terminals and a ground potential. A method of forming a notch filter for rejecting a predetermined band of signal frequencies from an input signal, characterised by the steps of forming a quadrature coupler (26) having first, second, third and fourth terminals (28,30,32,34), the first terminal being a signal input terminal and the fourth terminal being an output terminal; coupling a first tuned surface acoustic wave device (10) to the second terminal for receiving the input signal with a 0° phase shift, the first tuned device having primarily reactive characteristics at signal frequencies higher or lower than a notch region and primarily resistive characteristics across the notch region; and coupling a second tuned surface acoustic wave device (10) to the third terminal for receiving the input signal with a 90° phase shift, the second tuned device having primarily reactive characteristics at signal frequencies higher or lower than the notch region and primarily resistive characteristics across the notch region such that substantially no output signal is present on the fourth terminal across the notch region but an output signal is present in a band pass region. A method according to claim 7, characterised in that the step of coupling the tuned surface acoustic wave device (10) to the second and third terminals (30,32) further comprises the steps of forming surface acoustic wave impedance elements (12) having an input admittance substantially constant in both magnitude and phase over a predetermined band width across the notch region; and tuning each impedance element with a respective inductor (14) to form a circuit that is primarily resistive across the notch region and primarily reactive across the pass band region. A method according to claim 8, characterised by a step of forming the impedance element (12) of a single port, single-phase surface acoustic wave device. A method according to claim 9, characterised by a step of forming the impedance element (12) of a surface acoustic wave resonator. A method according to claim 9, characterised by the steps of forming the single phase surface acoustic wave device (10) with an inherent static capacitance; and choosing the value of the inductor (14) to resonate with the static capacitance at the centre of the notch, thereby forming a substantially resistive circuit. A method according to any one of claims 7-11, characterised in that the step of forming the quadrature coupler (26) further comprises the steps of serially connecting first, second, third and fourth inductors (36,38,40,42) to form a continuous circuit; coupling a respective one of the first, second, third and fourth terminals (28,30,32,34) to each junction between each two serially connected inductors; and coupling a respective capacitance (52) between each of the terminals and a ground potential.

Inventors: WRIGHT, PETER V
Applicants: R.F. MONOLITHICS, INC.
Application year: 1987
Main classification: H03H7
IPC codes: H03H3, G10K11, H03H9, H03H7