Filter

Abstract

A circuit comprising: a passive reactive component; and an active circuit, the active circuit arranged to increase the ac voltage difference across the reactive component by changing the current at an input to the reactive component and the current at an output of the reactive component by equal and opposite amounts. By increasing the current on one side of the resonant circuit and decreasing the current on the other side of the resonant circuit, the amount of current flowing through the resonant circuit is increased and thus the ac voltage difference across the inductor of the LC resonant circuit is increased. The Q of an inductor (the ratio of its imaginary to real impedance) is increased. In a filter, the improved Q provides a sharp, high rejection notch and faster pass-band to stop-band roll-off, thus improving the frequency response of the circuit.

Claims

1. A differential circuit having a first positive arm and a second negative arm; wherein the first positive arm comprises at least one first reactive component and the second negative arm comprise at least one second reactive component; further comprising: an active circuit, the active circuit arranged to increase the ac voltage difference across the first reactive component by changing the current at an input to the first reactive component and the current at an output of the first reactive component by equal and opposite amounts, and the active circuit arranged to increase the ac voltage difference across the second reactive component by changing the current at an input to the second reactive component and the current at an output of the second reactive component by equal and opposite amounts; wherein the active circuit comprises: a first current source arranged to draw current through a first amplifying element from the output of the first reactive component and arranged to draw current through a second amplifying element from the input of the first reactive component, the first amplifying element being driven by the input to the first reactive component and the second amplifying element being driven by the input of the second reactive component; and a second current source arranged to draw current through a third amplifying element from the output of the second reactive component and arranged to draw current through a fourth amplifying element from the input of the second reactive component, the third amplifying element being driven by the input of the second reactive component and the fourth amplifying element being driven by the input of the first reactive component.

2. A differential circuit as claimed in claim 1, wherein the first reactive component is part of a first resonant circuit and wherein the second reactive component is part of a second resonant circuit.

3. A differential circuit as claimed in claim 1, wherein a third reactive component is connected between the input of the first reactive component and the input of the second reactive component; and wherein a fourth reactive component is connected between the output of the first reactive component and the output of the second reactive component.

4. A differential circuit as claimed in claim 1, wherein the differential circuit is any of: a high-pass filter, a low-pass filter or an amplifier.

5. A differential circuit as claimed in claim 1, wherein the first, second, third and fourth amplifying elements are inverting amplifiers and arranged in common-source configuration.

6. A differential circuit as claimed in claim 1, wherein the first, second, third and fourth amplifying element are FETs, each being driven by the voltage at its respective gate and with the respective current source being connected to its source.

7. An electronic circuit comprising a resonant circuit formed from an inductive component in parallel with a capacitive component; wherein the capacitive component is a varactor; wherein the electronic circuit is arranged for a differential signal and wherein the electronic circuit comprises: a first resonant circuit formed from an inductive component in parallel with a varactor; and a second resonant circuit formed from an inductive component in parallel with a varactor; and wherein the first resonant circuit is arranged for receiving a positive part of the differential signal and the second resonant circuit is arranged for receiving a negative part of the differential signal.

8. An electronic circuit as claimed in claim 7, wherein an input of the resonant circuit and an output of the resonant circuit are connected to the same dc potential.

9. An electronic circuit as claimed in claim 7, wherein the varactor is optimized for high Q.

10. An electronic circuit as claimed in claim 7, wherein an input of the resonant circuit is connected to a second inductive component and wherein an output of the resonant circuit is connected to a third inductive component.

11. An electronic circuit as claimed in claim 10, wherein the second and third inductive components connect the resonant circuit to the same dc potential.

12. An electronic circuit as claimed in claim 10, wherein the second and third inductive components connect the resonant circuit to different dc potentials.

13. An electronic circuit as claimed in claim 10, wherein the second and third inductive components are inductors or center-tapped inductors or auto-transformers.

14. An electronic circuit as claimed in claim 7, wherein an input of the first resonant circuit is connected to an input of the second resonant circuit through a second inductive component and wherein an output of the first resonant circuit is connected to an output of the second resonant circuit through a third inductive component.

15. An electronic circuit as claimed in claim 14, wherein the second and third inductive components are centre-tapped inductors or autotransformers.

16. An electronic circuit as claimed in claim 15, wherein the centre taps of the second and third inductive components are connected to the same dc potential.

17. An electronic circuit as claimed in claim 7, wherein the electronic circuit is an elliptic filter.

18. An electronic circuit as claimed in claim 7, further comprising an active circuit, the active circuit arranged to increase the ac voltage difference across the first resonant circuit by changing the current at an input to the first resonant circuit and the current at an output of the first resonant circuit by equal and opposite amounts, and the active circuit arranged to increase the ac voltage difference across the second resonant circuit by changing the current at an input to the second resonant circuit and the current at an output of the second resonant circuit by equal and opposite amounts.

19. An electronic circuit as claimed in claim 7, further comprising an active circuit, the active circuit comprising: a first current source arranged to draw current through a first amplifying element from the output of the first resonant circuit and arranged to draw current through a second amplifying element from the input of the first resonant circuit, the first amplifying element being driven by the input to the first resonant circuit and the second amplifying element being driven by the input of the second resonant circuit; and a second current source arranged to draw current through a third amplifying element from the output of the second resonant circuit and arranged to draw current through a fourth amplifying element from the input of the second resonant circuit, the third amplifying element being driven by the input of the second resonant circuit and the fourth amplifying element being driven by the input of the first resonant circuit.

20. An electronic circuit as claimed in claim 19, wherein the first, second, third and fourth amplifying elements are inverting amplifiers and arranged in common-source configuration.

21. An electronic circuit as claimed in claim 19, wherein the first, second, third and fourth amplifying elements are FETs, each being driven by the voltage at its respective gate and with the respective current source being connected to its source.

22. A differential circuit having a first positive arm and a second negative arm; wherein the first positive arm comprises at least one first reactive component and the second negative arm comprise at least one second reactive component; further comprising: an active circuit, the active circuit arranged to increase the ac voltage difference across the first reactive component by changing the current at an input to the first reactive component and the current at an output of the first reactive component by equal and opposite amounts, and the active circuit arranged to increase the ac voltage difference across the second reactive component by changing the current at an input to the second reactive component and the current at an output of the second reactive component by equal and opposite amounts; wherein a third reactive component is connected between the input of the first reactive component and the input of the second reactive component; and wherein a fourth reactive component is connected between the output of the first reactive component and the output of the second reactive component.

Description

(1) Certain preferred embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

(2) FIG. 1 shows a basic block diagram of a direct sampling receiver front-end suitable for wideband signal processing;

(3) FIG. 2 shows a single-ended high-pass filter according to an embodiment of the invention, employing a single harmonic trap;

(4) FIG. 3 shows a differential high-pass filter according to an embodiment of the invention, employing a single harmonic trap;

(5) FIG. 4 shows a differential high-pass filter according to another embodiment of the invention, employing a double harmonic trap;

(6) FIG. 5 shows the forward transmission coefficient (S.sub.21) of the high-pass filter with and without active Q-boosting; and

(7) FIG. 6 shows the input reflection coefficient (S.sub.11) of the high-pass filter with and without active Q-boosting; and

(8) FIG. 7 shows the S.sub.21 for single and double harmonic trap high-pass filters;

(9) FIG. 8 shows the S.sub.21 of the high-pass filter (with and without active Q-boosting) when combined with a low-noise amplifier (LNA); and

(10) FIG. 9 illustrates a prior art negative-g.sub.m technique.

(11) FIG. 1 depicts a typical direct sampling receiver front-end 100 for a wideband receiver operating for example in the 6 to 8.5 GHz band. Antenna 1 receives a RF signal and passes it to high-pass filter 2 which rejects signals below about 6 GHz, with a high rejection notch at around 5.1 to 5.8 GHz (although it will be appreciated that these numbers are provided purely by way of example). The output of high-pass filter 2 feeds to the input of low-noise amplifier 3 which provides gain for the signal of interest across the operating band of 6 to 8.5 GHz. The output of low-noise amplifier 3 is then fed to an analogue-to-digital converter (ADC) 4 that finally digitises the RF signal.

(12) FIG. 2. shows a high-pass filter 200 according to an embodiment of the invention. The passive filter core 210 is depicted as a single-ended filter, operating on a signal part (signal path) RF.sub.i.

(13) The passive filter core 210 is a fifth-order elliptic LC ladder filter with a resonant circuit 211 as one of its reactive elements. The other four reactive elements (to make up a fifth-order filter) are capacitor C.sub.1, inductor L.sub.2, inductor L.sub.3 and capacitor C.sub.2. The resonant circuit 211 comprises an inductor L.sub.1 and a capacitor C.sub.3 in parallel.

(14) The capacitor C.sub.3 of resonant circuit 211 is connected between nodes A and B. As can be seen in FIG. 2, nodes A and B are both connected to ac ground nodes. Node A is connected through inductor L.sub.2 and node B is connected through inductor L.sub.3. The signal passing from RF.sub.i to RF.sub.0 is in phase at nodes A and B.

(15) The active block 220 is a single-ended to differential Q-boosting circuit that enhances the Q of the inductive components (L.sub.1, L.sub.2 and L.sub.3) of the circuit so as to improve the notch rejection of the filter and provide improved (steeper) pass-band and stop-band roll-off. The active block 220 has a single-ended input and provides differential output (both inverting and non-inverting outputs). The single-ended input is connected to node A on the input side of the resonant circuit 211 so that this node A drives the active block 220. The differential outputs (which are out of phase) are connected at nodes A and B (where the signal path is in phase), i.e. either side of the resonant circuit 211. Providing the differential outputs either side of the reactive components of the resonant circuit 211 increases the current (and thus the voltage) on one side and decreases it on the other side, thus increasing the voltage drop across the components (and hence the Q of the components), while retaining stability of the circuit. The change in current due to the active circuit on one side of the resonant circuit 211 is equal and opposite to the change in current due to the active circuit on the other side of the resonant circuit 211 so that the energy added on one side is removed on the other side, thus providing circuit stability. It will be appreciated that the input of active block 220 could alternatively be driven by node B instead of node A and the differential inputs could be applied the opposite way round (with the inverting output connected at A and the non-inverting output connected at B).

(16) The Q of an inductor is defined as the ratio of its imaginary to real impedance. The active circuit is arranged to increase the ratio of the imaginary part to the real part of the impedance of said inductor. The quality factor is also defined as the ratio of the voltage drop appearing across the inductor or capacitor to the supply/bias voltage. An increase in voltage drop seen across the inductor, means an increase in its Q.

(17) FIG. 3 shows a high-pass filter 300 according to an embodiment of the invention. The circuit 300 is divided into two parts with the upper part being the passive filter core 310 and the lower part being the active circuit 320.

(18) The passive filter core 310 is depicted as a differential filter, operating on a positive signal part RF.sub.i, + and a negative signal part R.sub.i, . Although a differential filter is shown and described here, it will be appreciated that half of this circuit can be used as a single-ended filter as shown in FIG. 2.

(19) Each half of the passive filter core 310 is a fifth-order elliptic LC ladder filter with a resonant circuit (311 for the positive signal half and 312 for the negative signal half) as one of its reactive elements. The other four reactive elements (to make up a fifth-order filter) are capacitor C.sub.1, center-tapped inductor L.sub.2 (or more generally, just an inductor), center-tapped inductor L.sub.3 (or more generally, just an inductor) and capacitor C.sub.2.

(20) Each resonant circuit 311, 312 comprises an inductor L.sub.1 and a varactor V.sub.1 in parallel. The varactor V.sub.1 is used in place of a more traditional capacitor due to its high Q, capacitance density and robustness with respect to process variations, voltage variations and temperature variations (collectively referred to as PVT variations). The varactor V.sub.1 has a tolerance better than or comparable to metal-insulator-metal (MIM) capacitors, but without requiring the more expensive MIM fabrication process. Metal-oxide-metal (MOM) capacitors are cheaper to construct than MIM capacitors, but suffer typically 15% PVT variation which adversely affects the filter performance on account of variability of notch frequency under PVT variations.

(21) The varactor V.sub.1 of resonant circuit 311 is connected between nodes A and B. As can be seen in FIG. 3, nodes A and B are both connected to V.sub.DD and are in phase on the signal path. Node A is connected through inductor L.sub.2 and node B is connected through inductor L.sub.3. As there is no (or insignificantly small) voltage drop across these inductors, both sides of varactor V.sub.1 are held at V.sub.DD and therefore there is zero dc bias applied to varactor V.sub.1. This has the significant advantage of keeping the capacitance of the varactor constant, providing a well-defined capacitance value that is least susceptible to PVT variations.

(22) The inductors L.sub.2 and L.sub.3 are centre-tapped inductors exploiting mutual coupling in order to reduce chip area. Preferably L.sub.2 and L.sub.3 are identical components so as to ensure that any voltage losses across them are also identical.

(23) As well as being part of the fifth-order filter, capacitors C.sub.1 and C.sub.2 dc isolate the filter core and the V.sub.DD connection from the antenna input and downstream processing. This is particularly beneficial in relation to the active Q-boosting circuit described further below as C.sub.1 and C.sub.2 guarantee self-biasing (at V.sub.DD) of the differential amplifiers in the active Q-boosting circuit.

(24) Inductors L.sub.1 in the two resonant circuits can be replaced with a bifilar (transformer) to exploit mutual coupling and further reduce chip area.

(25) The negative (inverting) signal half of the filter is identical to the positive (non-inverting) signal half described above, except that the varactor V.sub.1 of second resonant circuit 312 is connected between nodes C and D which are connected to V.sub.DD through the center-tapped inductors L.sub.2 and L.sub.3, respectively.

(26) The active circuit 320 is a Q-boosting circuit that enhances the Q of the inductive components of the circuit so as to improve the notch rejection of the filter and provide improved (steeper) pass-band and stop-band roll-off.

(27) Previous efforts to provide Q-boosting across inductors have involved providing a negative resistance in parallel with the inductor so as to effectively reduce the series resistance of the inductor. However such arrangements over PVT variations may cause the filter to become unstable.

(28) The active circuit 320 comprises two differential amplifiers. A first differential amplifier 321 is connected across the first resonant circuit 311 via nodes A and B. The second differential amplifier 322 is connected across the second resonant circuit 312 via nodes C and D. First differential amplifier pair 321 comprises amplifying elements M.sub.1 and M.sub.2 (here in the form of MOSFETs, self-biased at V.sub.DD to operate in the saturation region).

(29) The gate of amplifying element M.sub.1 is connected to (and therefore driven by) node A, i.e. the input of the first resonant circuit 311. The gate of amplifying element M.sub.2 is connected to (and therefore driven by) node C, i.e. the input of the second resonant circuit 312. As the two gates are driven by out of phase signals (being taken from opposite differential signals), amplifying elements M.sub.1 and M.sub.2 operate in anti-phase. The sources of both amplifying elements M.sub.1 and M.sub.2 are connected together and to current source which draws current (0.51*I.sub.1) through each of the amplifying elements M.sub.1 and M.sub.2. Any change in the input signals causes one amplifying element to draw more current and the other amplifying element to draw equivalently less current. Accordingly, any current injected at A is drawn off at B and vice versa.

(30) To describe the operation of first differential amplifier 321 by way of example, an increase in signal voltage at node A is mirrored by a corresponding signal voltage drop at node C. As the voltage at node A rises, the amplification of amplifying element M.sub.1 is increased, resulting in more current draw at node B and a corresponding decrease in voltage at node B. Simultaneously, as the voltage at node C drops, the amplification of amplifying element M.sub.2 is reduced, resulting in less current draw at node A and a corresponding increase in voltage at node A.

(31) The voltage rise at node A is followed by the voltage drop at node B which keeps the circuit operation perfectly stable. The voltage rise at A and drop at B causes a large change in the voltage across inductor L.sub.1, thus greatly increasing the Q of inductor L.sub.1 which in turn produces a marked improvement in the notch rejection of the filter and the pass-band and stop-band roll-off. The current change at node A is equal and opposite to the current change at node B.

(32) The operation of second differential amplifier pair 322 is identical, but applied across nodes C and D to cause a corresponding increase in the Q of L.sub.1 of the second resonant circuit 312.

(33) FIG. 4 shows another embodiment which is similar to the embodiment of FIG. 3 except that in the passive filter core 410 each of the positive and negative signal arms of the differential filter includes an additional resonant circuit. In the positive half, in addition to the first resonant circuit 411, a third resonant circuit 413 is formed by putting an inductor L.sub.4 in parallel with the previous dc blocking capacitor C.sub.1. Similarly, in the negative half, in addition to the second resonant circuit 412, a fourth resonant circuit 414 is formed by putting an inductor L.sub.4 in parallel with the previous dc blocking capacitor C.sub.1.

(34) In the active circuit 420 an additional set of differential amplifiers is required for Q boosting of the additional inductive components L.sub.4. The first differential amplifier 421 and second differential amplifier 422 are identical in operation to the first and second differential amplifiers pairs 421 and 422 of FIG. 3. However, in addition, a third differential amplifier 423 and fourth differential amplifier 424 are provided. The third differential amplifier 423 comprises amplifying elements M.sub.5 and M.sub.6 connected together at their sources to third current source I.sub.3. The fourth differential amplifier 424 comprises amplifying elements M.sub.7 and M.sub.8 connected together at their sources to fourth current source I.sub.4. The third differential amplifier 423 is connected across the third resonant circuit 413 at node A and at non-inverting input RF.sub.i, +. The gate of M.sub.5 is driven by RF.sub.i, + and the gate of M.sub.6 is driven by RF.sub.i, . The fourth differential amplifier 424 is connected across the fourth resonant circuit 414 at node C and at inverting input RF.sub.i, . The gate of M.sub.7 is driven by RF.sub.i, and the gate of M.sub.8 is driven by RF.sub.i, .

(35) In FIG. 4, a capacitor (e.g., high-Q MIM cap.) can substitute varactor V.sub.1 as shown in FIG. 2. This variation serves to demonstrate that the active circuit may achieve significant gains when a varactor V.sub.1 is not used. This would also apply to the active circuit 320 of FIG. 3 when applied to a passive core 310 with the varactor V.sub.1 replaced with a capacitor.

(36) FIG. 5 shows the filter response (forward transmission coefficient S.sub.21) for a particular setup with a single harmonic trap (i.e., one resonant circuit) placing the notch at around 5.1 GHz. Two lines are shown on the graph, the line marked with line-points shows the response of the filter with the active circuit switched OFF (effectively achieved simply by turning off the current sources I.sub.1 and I.sub.2). The solid line shows the response of the filter with the active circuit switched ON. The improvement in notch depth and roll-off steepness is readily apparent. With Q-boosting ON (active circuit ON) there is at least 20 dB rejection across the range of about 5 to 5.5 GHz, providing excellent rejection of the IEEE 802.11a wireless LAN band. There is at least 10 dB rejection at 5.8 GHz. The insertion loss in the pass-band is about 3 dB which is comparable to off the shelf components.

(37) FIG. 6 shows the input reflection coefficient S.sub.11 of the filter. At the input, the filter is differentially matched (i.e., S.sub.11<10 dB) to 100 over the passband (e.g., 6-8.5 GHz) with Q-boosting (line) enabled and without Q-boosting (line-points).

(38) FIG. 7 compares the filter responses (S.sub.21) for single (line-points) and double harmonic (line) trap elliptic high-pass filters. The double harmonic trap filter with two notches shows a steeper pass-band to stop-band roll-off, thus allowing for one notch to be placed closer to the lower edge of the passband with >20 dB rejection at around IEEE 802.11a.

(39) FIG. 5 shows the standalone frequency response of the filter. In practical set ups, the high-pass filter is generally linked to a low-noise amplifier to provide signal gain in the pass-band and out-of-band rejection. FIG. 8 shows the response of the combination of the high-pass filter evaluated in FIG. 4 together with a low-noise amplifier. The results are again shown both with and without the active Q-boosting circuit enabled. Without Q-boosting, the signal rejection is >20 dB at 5.1 GHz (i.e., lower-frequency end of IEEE 802.11a). With Q-boosting enabled, the rejection is >45 dB at 5.1 GHz. Again the roll-off improves with Q-boosting enabled. Combined with the LNA, the rejection at around 2.4 GHz (IEEE 802.11b/g) is >50 dB.

(40) It will be appreciated that many variations of the above embodiments may be made without departing from the scope of the invention which is defined by the appended claims.