CN112119588A

CN112119588A - N-path filter with improved out-of-band rejection

Info

Publication number: CN112119588A
Application number: CN201880093398.4A
Authority: CN
Inventors: 哈希姆·扎尔·霍西尼; 玛丽亚姆·沙里
Original assignee: Huawei Technologies Co Ltd
Current assignee: Huawei Technologies Co Ltd
Priority date: 2018-05-18
Filing date: 2018-05-18
Publication date: 2020-12-22
Also published as: WO2019219212A1

Abstract

An N-way filter (500) comprising an input port (502), an output port (504), and a plurality of paths (506- > 1,506-2, 506-N), wherein each path comprises: a low-pass filter circuit (508-1, 508-2.., 508-N); when the first switch circuit (510-; and connecting the output port of the filter to the low-pass filter circuit when the second switch circuit (512-1, 512-2.., 512-N) is turned on. The first switching circuits of the plurality of paths are activated in sequence, and the second switching circuits of the plurality of paths are activated in sequence.

Description

N-path filter with improved out-of-band rejection

Technical Field

The present application relates to electronic circuits, and more particularly to N-way filter circuits.

Background

A wireless receiver typically includes one or more filters for performing front-end band selection to suppress out-of-band interferers. Conventionally, band-pass filtering is performed by one or more Surface Acoustic Wave (SAW) filters and/or one or more Bulk Acoustic Wave (BAW) filters. However, in addition to being expensive to implement, SAW filters and BAW filters are also non-tunable. Therefore, in response to the desire to implement cheaper tunable filtering, there has recently emerged a trend to replace off-chip SAW filters with on-chip integrated N-way filters (which may also be referred to as channel selection filters).

The N filters comprise N identical parallel signal paths, where N is an integer greater than or equal to 2. Each path includes: an input modulator that down-converts an input signal to a baseband signal; a low pass filter circuit that filters the baseband signal to generate a filtered baseband signal; an output modulator upconverts the filtered baseband signal to an initial frequency band of the input signal. At any given time, the low pass filter circuit is connected between the input and output terminals through a single path. The low-pass filtering performed on the baseband signal is converted to band-pass filtering once upconverted. The center frequency of the filter is determined by the mixing frequency. It has been demonstrated that an N-way filter provides a bandpass filter with a high Q factor and a wide center frequency tuning range.

Although N-way filters have many advantages over SAW filters and BAW filters, conventional N-way filters have a number of disadvantages, including limited out-of-band rejection.

The embodiments described below are provided as examples only and are not limiting implementations that address any or all of the shortcomings of known N-way filters.

Disclosure of Invention

This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

Described herein are N-way filters, wherein each path comprises: a low pass filter circuit and a first switching circuit, wherein the first switching circuit connects an input port of a filter to the low pass filter circuit when the first switching circuit is activated; a second switching circuit, wherein the second switching circuit connects an output port of a filter to the low pass filter circuit when the second switching circuit is activated. The first switching circuit is activated in a sequence and the second switching circuit is activated in the same sequence.

A first aspect provides an N-way filter, comprising: an input port for receiving an input signal; an output port for outputting a filtered version of the input signal; a plurality of paths, wherein each path comprises: a low-pass filter circuit; a first switching circuit, wherein the first switching circuit connects an input port to the low pass filter circuit when the first switching circuit is activated; a second switching circuit, wherein the second switching circuit connects an output port to the low pass filter circuit when the second switching circuit is activated. The first switching circuits of the plurality of paths are activated in a sequence, and the second switching circuits are activated in the same sequence.

By having two switching circuits in each path, improved out-of-band rejection can be achieved without significantly increasing the power consumption or area of the filter compared to a conventional N-way filter. The out-of-band rejection is effectively improved when the N-pass filter of the first aspect is used in the feedback path of an Amplifier, e.g. a Low Noise Amplifier (LNA).

The first switching circuit of the at least one path may comprise at least one switch.

The second switching circuit of the at least one path may comprise at least one switch.

The at least one switch of the second switching circuit may be smaller than the at least one switch of the first switching circuit.

The at least one switch of the second switching circuit is one order of magnitude smaller than the at least one switch of the first switching circuit.

The size of the switches of the second switching circuit is reduced, thereby reducing the area of implementing the filter and/or relieving the load of downstream devices such as amplifiers.

The first switching circuit and the second switching circuit of the same path may be activated simultaneously.

This simplifies the circuitry for generating the control signal, since the same control signal can be used to activate the first and second switching circuits of the same path.

The first switching circuit and the second switching circuit of the same path may be activated at different points in time.

This allows the control signal for activating the first switching circuit to be used again. The control signal for activating the first switching circuit of the first path may also be used to activate the second switching circuit of the fourth path, the control signal for activating the first switching circuit of the second path may also be used to activate the second switching circuit of the first path, and so on.

The low pass filter circuit of at least one path includes a capacitor.

The method comprises the steps of periodically activating a first switching circuit of a path, down-converting the input signal into a baseband signal, and converting the baseband signal into a filtered baseband signal through a low-pass filter circuit of the path.

And periodically activating a second switching circuit of one path to up-convert the filtered baseband signal into a signal in the same frequency band as the input signal.

The input signal may be a radio frequency signal.

A second aspect provides a filter circuit comprising: an amplifier; the N-way filter according to the first aspect in the feedback path of the amplifier.

In such a filter circuit, the filter rejection is not limited by the on-resistance of the switching circuit, nor by the bandwidth of the amplifier, thereby providing and improving out-of-band rejection. Furthermore, the size of the low-pass filter device can be smaller due to the miller effect, allowing the overall size of the circuit to be reduced.

An output port of the N-way filter may be coupled to an input port of the amplifier and a low pass filter circuit may be coupled to an output port of the amplifier.

The filter circuit may further comprise a matched resistor in a second feedback path of the amplifier.

The amplifier is a low noise amplifier.

A third aspect provides an input signal filtering method, comprising: sequentially connecting the input signal to a plurality of low-pass filter circuits through a first switching circuit associated with the low-pass filter circuits; sequentially outputting the signals generated by the plurality of low-pass filter circuits through a second switching circuit associated with the low-pass filter circuit to generate a filtered output signal.

The above features may be combined as appropriate, as will be apparent to the skilled person. Also, these features may be combined with any aspect of the examples described herein.

Drawings

Various examples are now described in detail, with reference to the drawings, in which:

fig. 1 is a circuit diagram of an N-way filter.

Fig. 2 is a schematic diagram illustrating exemplary control signals for the N-way filter of fig. 1.

Fig. 3 is a circuit diagram of a filter circuit including an amplifier and the N-way filter of fig. 1 in the feedback path of the amplifier.

Fig. 4 is a circuit diagram of a filter circuit comprising two parallel N-way filters of fig. 1.

Fig. 5 is a circuit diagram of an example of an improved N-way filter.

Fig. 6 is a circuit diagram of an example of a filter circuit including an amplifier and the N-way filter of fig. 5 in the feedback path of the amplifier.

Fig. 7 is a graph of input and output transfer functions of an amplifier and the filter circuit described in fig. 3.

Fig. 8 is a graph of the input and output transfer functions of the filter circuit described in fig. 3 and 6.

Fig. 9 is a flow chart of an example of a method of filtering an input signal.

Various examples are shown in the drawings. The skilled artisan will appreciate that the element boundaries (e.g., boxes, groups of boxes, or other shapes) shown in the figures represent examples of boundaries. In some examples, one element may be designed as multiple elements, or multiple elements may be designed as one element. Common reference numerals are used in the figures to indicate similar features as appropriate.

Detailed Description

The following description is presented to enable any person skilled in the art to make and use the invention. The present invention is not limited to the embodiments described herein, and various modifications to the disclosed embodiments will be apparent to those skilled in the art. The embodiments are described by way of example only.

Described herein are N-way filters that improve out-of-band rejection compared to conventional N-way filters. Conventional N-way filters typically include a single switching circuit in each path that connects the input and output ports to respective low-pass filter circuits (i.e., a single switching circuit as an input modulator and an output modulator). Improved out-of-band rejection can be achieved by placing two switching circuits in each path. Specifically, each path includes: a first switching circuit connecting an input port of the filter to a corresponding low-pass filter circuit; and the second switching circuit is used for connecting the output port of the filter to the corresponding low-pass filter circuit. It has been demonstrated that an N-way filter with two switching circuits in each path can significantly improve out-of-band rejection compared to a conventional N-way filter without significantly increasing the power consumption or area of the filter. It has been demonstrated that an N-way filter with two switching circuits in each path effectively improves out-of-band rejection when used in the feedback path of an Amplifier, such as a Low Noise Amplifier (LNA).

To more clearly explain the N-way filter that improves out-of-band rejection, referring first to fig. 1, fig. 1 shows an example of an N-way filter 100. The N-way filter 100 comprises an input port 102 for receiving an input signal (V _ IN) and an output port 104 for outputting a filtered version of the input signal (V _ OUT). IN some cases, the input signal (V _ IN) and the output signal (V _ OUT) are Radio Frequency (RF) signals. But IN other cases, the input signal (V _ IN) and the output signal (V _ OUT) may be IN different frequency bands.

The N-way filter also includes N identical signal paths 106-1 to 106-N, where N is an integer greater than or equal to 2 and common values of N are 4 and 8. It will be obvious to those skilled in the art that this is merely an example and that other values of N may be used. Each path 106-i comprises a low-pass filter circuit (R + Ci) and a switching circuit (Si) in series. In the example shown in fig. 1, each low-pass filter circuit is composed of a capacitor Ci and a resistor R (representing the resistance between the input signal source and the input port 102). However, it will be apparent to those skilled in the art that this is merely an example of a low pass filter circuit and that other low pass filter circuits may be used. In the example shown in fig. 1, each switching circuit comprises a single switch Si. However, in other examples, the one or more switching circuits may include a plurality of switches.

The switching circuit (Si) of path i is located between the input port and the corresponding low-pass filter circuit. When the switching circuit (Si) is activated (i.e., closed), the input port 102 is connected to the corresponding low-pass filter circuit (R + Ci). Since the input port 102 and the output port 104 are shorted in the N-way filter 100 of fig. 1, activating the switching circuit (Si) will also connect the output port 104 to the corresponding low pass filter circuit (R + Ci). In this configuration, the switching circuit (Si) of path i is periodically activated, down-converting the input signal to a baseband signal. A low-pass filtering circuit (R + Ci) then filters the baseband signal, generating a filtered baseband signal, and the same switching circuit (Si) up-converts the filtered baseband signal to the initial frequency band of the input signal. The input signal is provided to the output port 104. The components of the low pass filter circuit (e.g., C1, C2, … …, CN) are selected to provide the desired channel filtering depending on the input signal bandwidth.

Typically, the switching circuits (S1, S2, … …, SN) are activated in a sequence such that only one switching circuit (Si) is active at a time and each switching circuit (S1, S2, … …, SN) is active for the same duration. For example, the switching circuit may be activated in the order of S1, S2, … …, SN. In some cases, each switching circuit (Si) is activated by a respective control signal (Pi). Specifically, the control signal P1 controls activation of the first switching circuit (S1), the control signal P2 controls activation of the second switching circuit (S2), and so on. In some cases, the control signal is based on a Local Oscillator (LO) signal. Fig. 2 shows an example of a set of control signals (P1, P2, … …, PN) for the N-way filter 100 of fig. 1. Wherein each control signal represents a phase-shifted version of the LO signal. In particular, the ith control signal Pi represents an ((i-1) × 360/N) degree phase shifted version of the LO signal. For example, in the case of N-4, there would be four control signals (P1, P2, P3, and P4), where P1 represents a 0-degree phase-shifted version of the LO signal, P2 represents a 90-degree phase-shifted version of the LO signal, P3 represents a 180-degree phase-shifted version of the LO signal, and P4 represents a 270-degree phase-shifted version of the LO signal. In this example, each control signal has a duty cycle equal to 1/N of the period of the LO. This results in each switching circuit (Si) being activated in 1/N of the LO period (T _ LO). The LO signal may be set to the center frequency of the input signal, making the N-way filter 100 of fig. 1 well suited for direct conversion receivers.

The low pass filtering performed on the baseband signal over the multiple paths 106-1 to 106-N translates into band pass filtering once upconverted.

Each switching circuit (Si) has an "on-resistance" (r) when activated. Since the on-resistance (r) of each switching circuit (Si) is located in the signal path and there is current passing through, the on-resistance (r) of each switching circuit (Si) limits the out-of-band rejection capability of the N-way filter 100. In particular, the out-of-band gain is approximately R/(R + R) due to potential splitter effects.

One technique to solve this problem may be to incorporate N-way filter 100 into the feedback path of the gain component, such as a low-noise amplifier (LNA). Referring now to fig. 3, fig. 3 illustrates an example of a filter circuit 300, wherein the filter circuit 300 includes an amplifier 302 and the N-way filter of fig. 1 in the feedback path of the amplifier 302. Specifically, in the example of fig. 3, the output port 104 of the N-way filter 100 is coupled to the input of the amplifier 302, and the other side of the low-pass filter circuits (C1, C2, … …, CN) (i.e., the side of the low-pass filter circuits not coupled to the respective switching circuits (Si)) is coupled to the output of the amplifier 304. This allows the size of the low pass filter circuit capacitors (C1, C2, … …, CN) to be reduced by the gain factor due to the miller effect.

Furthermore, in this configuration, the out-of-band rejection improves the gain factor of the amplifier 302 relative to the N-way filter 100 described in fig. 1. This is because the input signal is amplified at the desired frequency. This configuration is most effective in improving out-of-band rejection when the amplifier has substantial gain. The maximum gain of the LNA is typically around 20 dB. Although higher gains can be achieved, they become increasingly difficult to achieve at higher frequencies, often at the expense of higher power consumption. Furthermore, the on-resistance (r) of the switching circuit remains a limiting factor for the out-of-band rejection capability.

In some cases, the filtering circuit 300 may also include an impedance matching circuit. For example, if the input port 102 is coupled to a circuit/component (e.g., an RF antenna) that presents an impedance to the filter circuit 300, the impedance matched to the filter circuit 302 may include an impedance matching circuit that presents a corresponding impedance to the component (e.g., an RF antenna). In some cases, the impedance matching circuit may be implemented as a Resistor (RF) in the feedback path of the amplifier 302, as shown in fig. 3. However, in other cases, impedance matching may not be required, or may be performed or achieved in other ways.

Another technique to solve this problem is to use two N filters with different center frequencies. As described above, the center frequency of the N-way filter is set according to the frequency of the LO signal for controlling the switching circuit of the N-way filter. Thus, N filters with different center frequencies will use different LO signals to control the switching circuit. Referring now to fig. 3, fig. 3 illustrates an example of a filtering circuit 400, wherein the filtering circuit 400 includes a first N-way filter 402 (e.g., the N-way filter 100 of fig. 1) having a first center frequency and a second N-way filter 404 (e.g., the N-way filter 400 of fig. 1) having a second, different center frequency. Then, a difference between the outputs of the two N-

way filters

402 and 404 is calculated as the output (V _ OUT) of the filter circuit 400.

The phase (phi) of the outputs V _ OUT1 and V _ OUT2 of the two N-way filters when there is only a slight difference between the second frequency and the first frequency₁And phi₂) Approximately satisfies phi₁＝-φ₂Within the pass band of the filter. Therefore, due to the subtraction, they are added up, thereby increasing the gain of the passband. Conversely, for frequencies away from the passband region of the filter, the outputs of the two N filters are nearly in phase, satisfying φ₁＝φ₂. This will cancel each other out in the subtraction, thereby increasing the out-of-band rejection. This technique will be described in Milad Darvishi, Ronan van der Zee, Erica. M. Klumperink, Bran Nauta. N-way filter based widely tunable fourth order switching G_m-C band-pass filter, solid-state circuit IEEE report, 47(12), 3105-. However, this technique requires more complex circuitry to generate all the control signals for the switching circuit, which increases the power consumption of such a filter circuit compared to a conventional N-way filter. Furthermore, since there are two N-way filters, the area to implement such a filter circuit is much larger than the area to implement a conventional N-way filter. Such filters also require complex calibration mechanisms to optimize their performance.

Accordingly, described herein is an N-way filter that improves out-of-band rejection over conventional N-way filters, yet has similar power consumption and area requirements as conventional N-way filters. Out-of-band rejection is improved by adding an additional switching circuit in each path of the N-way filter, which connects the respective low-pass filter circuit to the output port of the filter. An additional switching circuit in the path is periodically activated to up-convert the filtered baseband signal generated by the corresponding low-pass filter circuit to the original frequency band of the input signal and provide the resulting up-converted signal to the output port. In this configuration, substantially no current flows through the additional switching circuit when the additional switching circuit is activated (i.e., closed). Therefore, the on-resistance of the additional switching circuit does not limit the level of out-of-band rejection.

Referring next to fig. 5, fig. 5 illustrates an example of an N-way filter 500 according to an embodiment. Similar to the N-way filter 100 described IN fig. 1, the N-way filter 500 includes an input port 502 for receiving an input signal (V _ IN), an output port 504 for outputting a filtered version of the input signal (V _ OUT), and N identical paths 506-1 to 506-N, where N is an integer greater than or equal to 2. Each path 506-i includes a low pass filter circuit 508-i, a first switching circuit 510-i, and a second switching circuit 512-i. In the example shown in fig. 5, each low-pass filter circuit 508-i is composed of a capacitor Ci and a resistor R (representing the resistance between the input signal source and the input port 502). However, it will be apparent to those skilled in the art that this is merely an example of a low pass filter circuit and that other low pass filter circuits may be used. In the example shown in FIG. 5, each of the first and second switching circuits 510-i and 512-i includes a single switch Si or SEi. However, in other examples, one or more of switching circuits 510-1 through 510-N and switching circuits 512-1 through 512-N may include more than one switch. The components of the low pass filter circuit (e.g., C1, C2, … …, CN) are selected to provide the desired channel filtering depending on the input signal bandwidth.

Each first switching circuit 510-1 to 510-N is located between the input port 502 and the corresponding low pass filter circuit 508-1 to 508-N. When the first switching circuit 510-i in path i is activated (i.e., closed), the input port 502 is connected to the corresponding low pass filter circuit 508-i. In this configuration, the first switching circuit 510-i of path i is periodically activated, down-converts the input signal to a baseband signal, and provides the baseband signal to the low pass filtering circuit 508-i. Low pass filter circuitry 508-i then generates a filtered baseband signal from the received baseband signal.

Each second switching circuit 512-1 to 512-N is located between the output port 504 and a corresponding low pass filter circuit 508-1 to 508-N. When the second switching circuit 512-i in path i is activated (i.e., closed), the output port 504 is connected to the corresponding low pass filter circuit 508-i. In the example shown in fig. 5, one side of each of the second switching circuits 512-1 to 512-N is connected to the output port 504, and the other side of the second switching circuit 512-1 to 512-N is connected to the trace or line between the corresponding first switching circuit and the corresponding low pass filter circuit. When the second switching circuits 512-1 to 512-N are located between the output port and the corresponding low-pass filtering circuit, the second switching circuit 512-i in path i is periodically activated to up-convert the filtered baseband signal generated by the corresponding low-pass filtering circuit and provide the up-converted signal to the output port 504. In this arrangement, when the second switching circuit 512-i of path i is activated (i.e., closed), substantially no current will flow through the second switching circuit 512-i, and thus the on-resistance of the second switching circuit 512-i does not limit the level of out-of-band rejection.

Further, since substantially no current flows through the second switching circuits 512-1 to 512-N, the switches of the second switching circuits 512-1 to 512-N may be smaller than the switches of the first switching circuits 510-1 to 510-N. In some cases, the switches of the second switching circuit may be one order of magnitude smaller than the switches of the first switching circuit. The size of the switch may be a physical size of the switch, and may be defined by a length (L) and a width (W) of the switch. For example, the size of the switch may be defined by the area of the switch, equal to the product of the length and the width (LxW). In some cases, the minimum size of the switches of the second switching circuit may be limited by the maximum acceptable noise. For example, the switches of the second switching circuit may be reduced to any size as long as the resulting total noise is acceptable. Generally, the lower the frequency, the smaller the switch may be. The switches of the second switching circuit are reduced in size to reduce the area of the implemented filter and/or to relieve the load on downstream devices such as amplifiers connected to the output port 502 of the N-way filter 500.

Furthermore, separating the input port 502 from the output port 504 may enable additional filtering at the output port 504, thereby protecting any components (e.g., amplifiers) connected to the output port 504 from any unwanted sources of interference.

The first switching circuits 510-1 to 510-N are activated in sequence such that only one switching circuit is active at a time and each first switching circuit 510-1 to 510-N is active for the same length of time. For example, the first switching circuit may be activated in the order of 510-1,510-2, … …, 510-N. In some cases, each first switching circuit 510-1 to 510-N is activated by a respective control signal, which is a phase-shifted version of a local oscillator signal (LO), each control signal having a duty cycle equal to 1/N of the LO period (T _ LO). For example, the first switching circuits 510-1 to 510-N may be controlled by the example control signals P0 to PN respectively shown in FIG. 2. As mentioned above, in the example shown in fig. 2, the i-th control signal Pi represents a ((i-1) × 360/N) degree phase shifted version of the LO signal. For example, in the case of N-4, there would be four control signals (P1, P2, P3, and P4), where P1 represents a 0-degree phase-shifted version of the LO signal, P2 represents a 90-degree phase-shifted version of the LO signal, P3 represents a 180-degree phase-shifted version of the LO signal, and P4 represents a 270-degree phase-shifted version of the LO signal. It will be clear to those skilled in the art that this is only an example and that the control signals of the first switching circuits 510-1 to 510-N may represent different phase shifts of the LO signal, but are typically spaced apart by 360/N degrees.

The second switching circuits 512-1 to 512-N are activated in the same order as the corresponding first switching circuits 510-1 to 510-N. For example, if the first switching circuits 510-1 to 510-N are activated in the order of 510-1,510-2, … …,510-N, the second switching circuits 512-1 to 512-N are activated in the order of 512-1,512-2, … …, 512-N.

In some cases, the first switching circuit 510-i and the second switching circuit 512-i of the same path i are activated simultaneously. Specifically, in these cases, the first switching circuit 510-1 and the second switching circuit 512-1 of the first path are activated simultaneously, the first switching circuit 510-2 and the second switching circuit 512-2 of the second path are activated simultaneously, and so on. This may be advantageous where a circuit coupled to the output port 504 of the N-way filter 500 provides a matched impedance to the input port 502. This may also simplify the circuitry for generating the control signals, since the same control signals may be used to activate the first switching circuit 510-i and the second switching circuit 512-i of the same path i. For example, as shown in Table 1, the control signal P1 of FIG. 2 may be used to activate the first switching circuit 510-1 and the second switching circuit 512-1 of the first path, the control signal P2 may be used to activate the first switching circuit 510-2 and the second switching circuit 512-2 of the second path, and so on.

TABLE 1

Route of travel	Phase shift of control signal of first switch circuit	Phase shift of control signal of second switch circuit
			1	0	0
2	90	90
			3	180	180
4	270	270

However, in other cases, the first switching circuit 510-i and the second switching circuit 512-i of the same path i are not activated at the same point in time. In particular, the second switching circuit may be activated by phase shifting the control signal by a predetermined amount relative to the control signal used to activate the respective first switching circuit. For example, the second switching circuit in the path may be activated by a control signal that is phase shifted by 90 degrees with respect to the control signal used to activate the respective first switching circuit. When N equals 4, this may result in the control signals of the first and second switching circuits being phase shifted versions of the LO shown in table 2.

TABLE 2

Route of travel	Phase shift of control signal of first switch circuit	Phase shift of control signal of second switch circuit
			1	0	90
2	90	180
			3	180	270
4	270	0

When the first switching circuit 510-i and the second switching circuit 512-i of the same path i are not activated at the same time, it may be advantageous that the phases of their control signals differ by a factor of 360/N. This allows the control signal for activating the first switching circuit to be used again. For example, in table 2, since N is 4, 360/4 is 90, and the control signals of the first switching circuit and the second switching circuit on the same path have a phase difference of 90 degrees. This is so that the control signal for activating the first switching circuit of the first path may also be used for activating the second switching circuit of the fourth path, the control signal for activating the first switching circuit of the second path may also be used for activating the second switching circuit of the first path, and so on.

The N-way filter 500 illustrated in fig. 5 provides a filter that has greatly improved out-of-band rejection over the N-way filters such as the N-way filter 100 illustrated in fig. 1, does not require additional power consumption, has little impact on area, has little impact on noise, does not require additional complex circuitry, and does not require fine tuning.

In some cases, by placing the N-way filter described herein in the feedback path of the amplifier, a filter that further improves out-of-band rejection may be achieved. This combines the out-of-band rejection improvement achieved by the N-way filter 500 described in figure 5 with the out-of-band rejection improvement achieved by placing the N-way filter in the feedback path of the amplifier.

Referring now to fig. 6, fig. 6 shows an example of a filter circuit 600, wherein the filter circuit 600 includes an amplifier 602 and the N-way filter 500 of fig. 5 in the feedback path of the amplifier 602. Specifically, in the example of FIG. 6, the output port 504 of the N-way filter 500 is electrically coupled to the input port of the amplifier and the low pass filters 508-1 through 508-N are electrically coupled to the output of the amplifier 602. In some cases, the amplifier 602 may be a Low Noise Amplifier (LNA). In such a filter circuit 600, out-of-band rejection is improved over the filter circuit 300 described in fig. 3, because the filter rejection is not limited by the on-resistance of the switching circuit, nor by the bandwidth of the amplifier. Also, the size of the low pass filter circuit components (e.g., C1, C2, … …, CN) may be smaller due to the miller effect.

Furthermore, as described above, since there is no direct short connection between the input port and the input of the amplifier 602, additional filtering is performed at the input of the amplifier 602. (i.e., a higher level of out-of-band rejection at the input of the amplifier 602) to protect the amplifier 602 from any unwanted large interferer. This means that the amplifier 602 will not be limited by the full power of the unwanted interferer and therefore may not require large signal processing capabilities. This in turn reduces the risk of signal compression within the amplifier 602.

The out-of-band linearity of filter circuit 600 is also greatly improved relative to filter circuit 300 depicted in fig. 3. With regard to the example transfer functions that describe the above, reference is made to fig. 7 and 8.

Like the filter circuit 300 described in fig. 3, the filter circuit 600 described in fig. 6 may also include an impedance matching circuit. For example, if the input port 502 of the N-way filter 500 is coupled to a circuit/component (e.g., an RF antenna) that presents an impedance to the filter circuit 600, the impedance matched to the filter circuit 600 may include an impedance matching circuit that presents a corresponding impedance to the circuit/component (e.g., the antenna). In some cases, as shown in fig. 6, the impedance matching circuit may be implemented as a Resistor (RF) in the feedback path of the amplifier 602. However, in other cases, impedance matching may not be required, or may be performed or achieved in other ways.

The filter circuit 600 includes an impedance matching circuit (e.g., resistor RF) in a feedback path of the amplifier 602, and a first switching circuit and a second switching circuit in the same path of the N-way filter 500 may be simultaneously activated, so that impedance matching performed by the impedance matching circuit (e.g., resistor RF) performs impedance matching in the same manner as the impedance matching circuit in the filter circuit 300 of fig. 3. In particular, when a first switching circuit and a second switching circuit of the same path are simultaneously activated, a component (e.g., an antenna) is coupled to the input port 502, and an impedance circuit is found to be responsive to the input signal. However, in the filter circuit 300 of fig. 3, there is no direct short connection between the input port and the input terminal of the amplifier, and the input port is connected to the input terminal of the amplifier 602 through two switching circuits.

Referring now to fig. 7 and 8, fig. 7 and 8 illustrate improved out-of-band rejection achieved by the filter circuit 600 of fig. 6 relative to the out-of-band rejection achieved by the filter circuit 300 of fig. 3, wherein the filter circuit 600 of fig. 6 includes the N-way filter 500 of fig. 5 in the feedback loop of the LNA and the filter circuit 300 of fig. 3 includes the N-way filter 100 of fig. 1 in the feedback loop of the LNA. In these examples, the center frequency of the N-path filter is 2GHz (i.e., the switching circuitry of the N-path filter is controlled by an LO signal having a frequency of 2 GHz).

Fig. 7 shows the input transfer function 702 and the output transfer function 704 of an LNA without an N-way filter in the feedback loop, and the input transfer function 706 and the output transfer function 708 of an LNA with an N-way filter in the feedback loop (i.e., the circuit 300 described in fig. 3). Each

input transfer function

702 or 706 shows the magnitude (in dB) of the signal input to the LNA relative to the magnitude of the original input signal in the frequency domain. Similarly, each

output transfer function

704 or 708 shows the magnitude (in dB) of the signal output from the LNA relative to the magnitude of the original input signal in the frequency domain.

As can be seen from the input transfer function 702 and the output transfer function 704, when there is no N-way filter in the feedback loop, the input to the LNA is typically matched to the original input signal, and the LNA outputs an amplified version of the input signal. Although it can be seen that more gain is typically applied to frequencies below the center frequency (e.g., 2GHz) and less gain is applied to frequencies above the center frequency.

As can be seen from the input transfer function 706 and the output transfer function 708, when an N-way filter is placed in the feedback loop of the LNA (i.e., resulting in the circuit 300 described in fig. 3), both the input and output of the LNA are attenuated relative to the original input signal at frequencies above and below the center frequency. In particular, although the input and output of the circuit 300 depicted in FIG. 3 may have substantially the same gain as the input and output of an LNA without an N-way filter in the feedback loop at the center frequency. That is, the input to the LNA without the N-pass filter in the feedback loop has 70.719mdB gain at the center frequency, and the input to the circuit 300 depicted in fig. 3 has 359.82mdB gain at the center frequency; the output of the LNA without the N-pass filter in the feedback loop has 18.057dB gain at the center frequency and the output of the circuit 300 depicted in fig. 3 has 13.809dB gain at the center frequency. The input and output of the LNA are significantly attenuated at other frequencies. That is, when an N-way filter is used in the feedback loop of the LNA, there is significant out-of-band rejection at both the input and output. However, as can be seen in fig. 7,

transfer functions

706 and 708 are asymmetric with respect to the center frequency. In particular, for the input and output of the LNA, the gain for frequencies above the center frequency generally increases with increasing frequency and the gain for frequencies above the center frequency generally decreases with decreasing frequency. This is caused by the bandwidth of the LNA.

Fig. 8 shows the input transfer function 706 and the output transfer function 708 of an LNA having an N-way filter in a feedback loop (i.e., the circuit 300 depicted in fig. 3), and the input transfer function 802 and the output transfer function 804 of the N-way filter 500 depicted in fig. 5 with modifications in the feedback loop (i.e., the circuit 600 depicted in fig. 6). As can be seen from fig. 8, with respect to the input and output of the LNA in the circuit 600 depicted in fig. 6, the attenuation of out-of-band frequencies (i.e., out-of-band rejection) is significantly improved relative to the input and output of the LNA in the circuit 300 depicted in fig. 3. It can also be seen that the attenuation of frequencies on both sides of the center frequency does not increase or decrease in the same manner as when using an N-way filter in the feedback path for the input and output of the LNA in the circuit 600 depicted in fig. 6. Thus, the out-of-band linearity of the circuit 600 depicted in FIG. 6 is improved relative to the circuit 300 depicted in FIG. 3. Accordingly, out-of-band rejection is not limited in the same manner by the LNA bandwidth on either side of the center frequency when using the improved N-way filter in the feedback loop of the LNA.

The improved out-of-band rejection may reduce the linearity of the compression devices downstream of the LNA/filter circuit. For example, when the filter circuit is used in a wireless receiver, the LNA/filter circuit may be followed by a mixer and/or baseband circuit, and the improved out-of-band rejection may reduce the linearity and compression requirements for the mixer and baseband circuit.

Referring now to fig. 9, fig. 9 illustrates an example of an input signal filtering method 900. The method 900 begins at

blocks

902 and 904. At block 902, the input signal is coupled to each of a plurality of low pass filter circuits (e.g., low pass filter circuits 508-1 to 508-N) in turn. In some cases, the input signal may be connected to a low pass filter circuit (e.g., low pass filter circuit 508-i) through a first switching circuit (e.g., first switching circuit 510-i) associated with the low pass filter circuit. The low pass filter circuits (e.g., low pass filter circuits 508-1 to 508-N) are in turn coupled to the output ports of the filters to generate filtered output signals, block 904. In some cases, the low pass filter circuit may be connected to the output port through a second switching circuit associated with the low pass filter circuit. The low-pass filter circuits are connected to the output ports in the same order as the low-pass filter circuits connected to the input ports. In some cases, the same low pass filter may be connected to both the input port and the output port. In other cases, the same low-pass filter may be connected to the input port and the output port, respectively, at different points in time. For example, the low-pass filters may be connected to the input port (via respective first switching circuits) during a first period, and the low-pass filters may be connected to the output port (via respective second switching circuits) during a second, different period. The first switching circuit may include one or more switches. The second switching circuit may comprise one or more switches. In some cases, the switches of the second switching circuit may be smaller than the switches of the first switching circuit.

The applicants hereby disclose in isolation each individual feature described herein and any combination of two or more such features. Such features or combinations of features can be implemented as a whole based on the present description, without regard to whether such features or combinations of features solve any of the problems disclosed herein, with the ordinary knowledge of a person skilled in the art; and do not contribute to the scope of the claims. The present application shows that aspects of the present invention may consist of any such individual feature or combination of features. Various modifications within the scope of the invention will be apparent to those skilled in the art in view of the foregoing description.

Claims

1. An N-way filter, comprising:

an input port for receiving an input signal;

an output port for outputting a filtered version of the input signal;

a plurality of paths, wherein each path comprises:

a low-pass filter circuit;

a first switching circuit, wherein the first switching circuit connects the input port to the low pass filter circuit when the first switching circuit is activated;

a second switching circuit, wherein the second switching circuit connects the output port to the low pass filter circuit when the second switching circuit is activated;

wherein the first switching circuits of the plurality of paths are activated in a sequence and the second switching circuits of the plurality of paths are activated in the same sequence.

2. The N-way filter wherein the first switching circuit of at least one path comprises at least one switch.

3. The N-way filter according to claim 1 or 2, wherein the second switching circuit of the at least one path comprises at least one switch.

4. The N-way filter according to claim 3, when dependent on claim 2, wherein the at least one switch of the second switching circuit is smaller than the at least one switch of the first switching circuit.

5. The N-way filter of claim 4, wherein the at least one switch of the second switching circuit is one order of magnitude smaller than the at least one switch of the first switching circuit.

6. The N-way filter according to any of the preceding claims, wherein the first switching circuit and the second switching circuit of the same path are activated simultaneously.

7. The N-way filter according to any one of claims 1 to 5, wherein the first switching circuit and the second switching circuit of the same path are activated at different points in time.

8. The N-way filter according to any one of the preceding claims, wherein the low pass filter circuit of at least one path comprises a capacitor.

9. The N-way filter according to any one of the preceding claims, wherein a first switching circuit of a path is periodically activated to down-convert the input signal to a baseband signal, which is converted to a filtered baseband signal by a low-pass filtering circuit of the path.

10. The N-way filter of claim 9, wherein the second switching circuit of one path is periodically activated to up-convert the filtered baseband signal to a signal in the same frequency band as the input signal.

11. The N-way filter according to any of the preceding claims, wherein the input signal is a radio frequency signal.

12. A filter circuit, comprising:

an amplifier;

an N-way filter as claimed in any preceding claim in the feedback path of the amplifier.

13. The filter circuit of claim 12, wherein the output port of the N-way filter is coupled to the input port of the amplifier and the low pass filter circuit is coupled to the output port of the amplifier.

14. The filter circuit of claim 12 or 13, further comprising a matched resistor in a second feedback path of the amplifier.

15. The filter of any of claims 12 to 14, wherein the amplifier is a low noise amplifier.

16. A method of filtering an input signal, comprising:

sequentially connecting the input signal to a plurality of low-pass filter circuits through a first switching circuit associated with the low-pass filter circuits;

sequentially outputting the signals generated by the plurality of low-pass filter circuits through a second switching circuit associated with the low-pass filter circuit to generate a filtered output signal.