CN112119588A - An N-way Filter with Improved Out-of-Band Suppression - Google Patents

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 (510‑1, 510‑2,..., 510‑N) is turned on, connect the input port of the filter to a low-pass filter circuit; and when the second switch circuit (512-1, 512-2, . . . , 512-N) is turned on, connecting the output port of the filter to the low-pass filter circuit. The first switching circuits of the multiple paths are sequentially activated, and the second switching circuits of the multiple paths are sequentially activated.

Description

An N-way Filter with Improved Out-of-Band Suppression

technical field

The present application relates to electronic circuits, in particular to N-channel filter circuits.

Background technique

Wireless receivers typically include one or more filters for performing front-end band selection to suppress out-of-band interferers. Traditionally, bandpass filtering is performed by one or more off-chip 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 not tunable. Therefore, in response to the desire to achieve cheaper tunable filtering, there has been a recent 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-way filter includes 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 the input signal into a baseband signal; a low-pass filter circuit that filters the baseband signal to generate a filtered baseband signal; an output modulator that converts the filtered baseband signal The signal is upconverted to the original frequency band of the input signal. At any given time, a low-pass filter circuit is connected between the input and the output by a single path. The low-pass filtering performed on the baseband signal, once upconverted, is converted to band-pass filtering. The center frequency of the filter is determined by the mixing frequency. N-way filters have been shown to provide bandpass filters with high Q-factors and wide center frequency tuning ranges.

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

The embodiments described below are provided by way of example only, and are not limiting of implementations that address any or all of the disadvantages of known N-way filters.

SUMMARY OF THE INVENTION

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

An N-way filter is described here, wherein each path includes a low-pass filter circuit and a first switching circuit, wherein when the first switching circuit is activated, the first switching circuit connects the input port of the filter to the low-pass filter circuit; a second switch circuit, wherein when the second switch circuit is activated, the second switch circuit connects the output port of the filter to the low-pass filter circuit. The first switching circuits are activated in sequence, and the second switching circuits are 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; and multiple paths, wherein each path includes: a low-pass a filter circuit; a first switch circuit, wherein when the first switch circuit is activated, the first switch circuit connects an input port to the low-pass filter circuit; a second switch circuit, wherein when the second switch circuit is activated The second switching circuit connects the output port to the low pass filter circuit when the circuit is activated. The first switching circuits of the plurality of paths are activated in 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 conventional N-way filters. When the N-way filter of the first aspect is used in the feedback path of an amplifier (eg, a low noise amplifier (low Noise Amplifier, LNA) for short), out-of-band rejection is effectively improved.

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

The second switching circuit of the at least one path may include 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 an 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 for implementing the filter and/or reducing the load on 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 circuit for generating the control signal, since the same control signal can be used to activate the first switching circuit and the second switching circuit of the same path.

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

This allows the control signal used to activate the first switching circuit to be reused. The control signal used to activate the first switching circuit of the first path can also be used to activate the second switching circuit of the fourth path, and the control signal used to activate the first switching circuit of the second path can also be used to activate the first path. the second switching circuit, and so on.

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

The first switching circuit of one path is periodically activated, the input signal is down-converted into a baseband signal, and the baseband signal is converted into a filtered baseband signal through the low-pass filter circuit of the path.

A second switching circuit of one path is periodically activated 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, including: an amplifier; and the N-way filter according to the first aspect in a feedback path of the amplifier.

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

The output port of the N-way filter may be coupled to the input port of the amplifier, and the low-pass filter circuit may be coupled to the output port of the amplifier.

The filter circuit may also include matched resistors in the 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 an input signal to a plurality of low-pass filtering circuits through a first switching circuit associated with the low-pass filtering circuit; The connected second switching circuit sequentially outputs the signals generated by the plurality of low-pass filter circuits, thereby generating a filtered output signal.

It will be obvious to the skilled person that the above-mentioned features can be combined as appropriate. Also, these features may be combined with any aspect of the examples described herein.

Description of drawings

Various examples are now described in detail in conjunction with the accompanying drawings, in which:

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

FIG. 2 is a schematic diagram illustrating example control signals for the N-way filter in FIG. 1 .

FIG. 3 is a circuit diagram of a filter circuit, wherein the filter circuit includes an amplifier and the N-way filter shown in FIG. 1 in the feedback path of the amplifier.

FIG. 4 is a circuit diagram of a filter circuit, wherein the filter circuit includes two parallel N-channel filters shown in FIG. 1 .

Figure 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, wherein the filter circuit includes an amplifier and the N-way filter shown in FIG. 5 in the feedback path of the amplifier.

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

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

FIG. 9 is a flowchart of an example of an input signal filtering method.

Various examples are shown in the accompanying drawings. Skilled artisans will appreciate that the element boundaries shown in the figures (eg, boxes, groups of boxes, or other shapes) represent examples of boundaries. In some examples, one element may be designed as multiple elements, or multiple elements may be designed as one element. Where appropriate, common reference numerals are used in the drawings to refer to similar features.

Detailed ways

The following descriptions are provided to enable those skilled in the art to make and use the present 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 in this paper is an N-way filter with improved out-of-band rejection compared to the traditional N-way filter. Conventional N-way filters typically include a single switching circuit in each path that connects the input and output ports to corresponding low-pass filter circuits (ie, a single switching circuit as input and output modulators). Improved out-of-band rejection can be achieved by having two switching circuits in each path. Specifically, each path includes: a first switching circuit, connecting the input port of the filter to the corresponding low-pass filtering circuit; and a second switching circuit, connecting the output port of the filter to the corresponding low-pass filtering circuit. Compared to conventional N-way filters, N-way filters with two switching circuits in each path have been shown to significantly improve out-of-band rejection 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 increases the Out-of-band suppression.

In order to more clearly explain the N-way filter with improved out-of-band rejection, reference is first made to FIG. 1 , which shows an example of an N-way filter 100 . The N-way filter 100 includes 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. But for those skilled in the art, it is obvious that this is only an example, and other values of N may also be used. Each path 106-i includes a low-pass filter circuit (R+Ci) and a switching circuit (Si) connected in series. In the example shown in FIG. 1, each low-pass filter circuit consists of a capacitor Ci and a resistor R (representing the resistance between the input signal source and the input port 102). However, it is obvious to those skilled in the art that this is only an example of a low-pass filter circuit, and other low-pass filter circuits can also be used. In the example shown in FIG. 1, each switching circuit includes a single switch Si. However, in other examples, the one or more switching circuits may include multiple 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 (ie, 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) also connects 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 to down-convert the input signal to a baseband signal. A low pass filter circuit (R+Ci) then filters the baseband signal to generate a filtered baseband signal, and the same switching circuit (Si) upconverts the filtered baseband signal to the original frequency band of the input signal. The input signal is provided to output port 104 . The components of the low-pass filter circuit (eg, C1, C2, ..., CN) are selected to provide the desired channel filtering based on the input signal bandwidth.

Typically, the switching circuits (S1, S2, ..., SN) are activated in sequence such that only one switching circuit (Si) is activated at a time, and each switching circuit (S1, S2, ..., SN) are active for the same duration. For example, the switching circuits may be activated in the order of S1, S2, ..., SN. In some cases, each switching circuit (Si) is activated by a corresponding control signal (Pi). Specifically, the control signal P1 controls the activation of the first switching circuit (S1), the control signal P2 controls the 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) of the N-way filter 100 of FIG. 1 . where each control signal represents a phase-shifted version of the LO signal. Specifically, the i-th control signal Pi represents a ((i-1)*360/N) degree phase-shifted version of the LO signal. For example, with N=4, there will 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 LO period. This results in each switching circuit (Si) being activated for 1/N of the LO period (T_LO). The LO signal can be set to the center frequency of the input signal, making the N-way filter 100 of Figure 1 well suited for direct conversion receivers.

The low-pass filtering performed on the baseband signal through the plurality of paths 106-1 to 106-N is converted to band-pass filtering once up-converted.

Each switching circuit (Si) has an "on-resistance" (r) when activated. The on-resistance (r) of each switching circuit (Si) limits the out-of-band rejection of the N-way filter 100 since the on-resistance (r) of each switching circuit (Si) is in the signal path and current flows through it ability. Specifically, the out-of-band gain is about r/(R+r) due to potential splitter effects.

One technique to address this problem may be to incorporate the N-way filter 100 into the feedback path of a gain component, such as a low-noise amplifier (LNA). Referring now to FIG. 3 , FIG. 3 shows an example of a filter circuit 300 , wherein the filter circuit 300 includes an amplifier 302 and the N-way filter described in 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 end of the amplifier 302, and the other side of the low-pass filter circuit (C1, C2, ..., CN) (ie, The low pass filter circuit is not coupled to the side of the corresponding switching circuit (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 reduce the gain factor due to the Miller effect.

Furthermore, in this configuration, the gain factor of the amplifier 302 is improved for out-of-band rejection 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 generally around 20dB. While higher gain can be achieved, it becomes 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 the limiting factor for out-of-band rejection.

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

Another technique to solve this problem is to use two N-way 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 used to control the switching circuit of the N-way filter. Therefore, N-way filters with different center frequencies will use different LO signals to control the switching circuit. Referring now to FIG. 3, FIG. 3 shows an example of a filter circuit 400, wherein the filter circuit 400 includes a first N-way filter 402 having a first center frequency (eg, the N-way filter 100 described in FIG. 1 ). ) and a second N-way filter 404 (eg, the N-way filter 400 described in FIG. 1 ) having a different second center frequency. Then, the difference between the outputs of the two N-way filters 402 and 404 is calculated as the output (V_OUT) of the filtering circuit 400 .

When there is only a slight difference between the second frequency and the first frequency, the phases (φ ₁ and φ ₂ ) of the outputs V_OUT1 and V_OUT2 of the two N-way filters approximately satisfy φ ₁ =−φ ₂ and are located within the passband of the filters . Therefore, due to the subtraction, they are added up, thereby increasing the gain of the passband. Conversely, for frequencies far from the passband region of the filter, the outputs of the two N-way filters are almost in phase, satisfying φ ₁ =φ ₂ . This will cancel each other out in subtraction, increasing out-of-band rejection. This technique will be discussed in Milad Darvishi, Ronan van der Zee, Eric A.M. Klumperink, Bram Nauta. Widely Tunable Fourth-Order Switched Gm–C Bandpass Filter Based on _N -Way Filters, IEEE Transactions on Solid State Circuits, 47 ( 12), 3105-3119. However, this technique requires a more complex circuit to generate all the control signals for the switching circuit, which increases the power consumption of such filter circuits compared to conventional N-way filters. Furthermore, since there are two N-way filters, the area to implement such a filter circuit is much larger than that of a conventional N-way filter. Such filters also require complex calibration mechanisms to optimize their performance.

Therefore, what is described here is an N-way filter with improved out-of-band rejection relative to a conventional N-way filter, but still has similar power consumption and area requirements as a conventional N-way filter. Out-of-band rejection is improved by adding an additional switching circuit in each path of the N-way filter, where the switching circuit connects the corresponding low-pass filter circuit to the output port of the filter. Additional switching circuits in the path are 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, when the additional switching circuit is activated (ie closed), substantially no current flows through the additional switching circuit. Therefore, the on-resistance of the additional switching circuit does not limit the level of out-of-band rejection.

Referring then to FIG. 5, an example of an N-way filter 500 is shown in FIG. 5, 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 two. 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 consists of a capacitor Ci and a resistor R (representing the resistance between the input signal source and the input port 502). However, it is obvious to those skilled in the art that this is only an example of a low-pass filter circuit, and other low-pass filter circuits can also be used. In the example shown in FIG. 5, each of the first switching circuit 510-i and the second switching circuit 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 (eg, C1, C2, ..., CN) are selected to provide the desired channel filtering based on the input signal bandwidth.

Each of the first switching circuits 510-1 to 510-N is located between the input port 502 and the corresponding low-pass filter circuits 508-1 to 508-N. When the first switching circuit 510-i in path i is activated (ie, 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 filter circuit 508-i. The low pass filter circuit 508-i then generates a filtered baseband signal from the received baseband signal.

Each of the second switching circuits 512-1 to 512-N is located between the output port 504 and the corresponding low-pass filter circuit 508-1 to 508-N. When the second switching circuit 512-i in path i is activated (ie, 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 circuits 512-1 to 512-N is connected to A wiring 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 the path i is periodically activated, and the corresponding low-pass filtering circuit generates The filtered baseband signal is upconverted and the upconverted signal is provided to output port 504 . In this arrangement, when the second switching circuit 512-i of path i is activated (ie, closed), substantially no current will flow through the second switching circuit 512-i, so the second switching circuit 512- The on-resistance of i does not limit the level of out-of-band rejection.

Furthermore, 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 switching of the first switching circuits 510-1 to 510-N switch. In some cases, the switches of the second switching circuit may be an 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 (length, L for short) and a width (width, W for short) of the switch. For example, the size of a switch can be defined by the area of the switch, which is equal to the product of length and 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 can be reduced to any size as long as the total noise caused is acceptable. Generally, the lower the frequency, the smaller the switch can be. The size of the switches of the second switching circuit is reduced, thereby reducing the area for realizing the filter and/or reducing the load of downstream devices such as amplifiers connected to the output port 502 of the N-way filter 500 .

Additionally, separating input port 502 from output port 504 may enable additional filtering at output port 504, thereby protecting any components (eg, amplifiers) connected to 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 activated at a time, and each of the first switching circuits 510-1 to 510-N is activated for the same period 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 of the first switching circuits 510-1 to 510-N is activated by a corresponding control signal, wherein the control signal is a phase-shifted version of a local oscillator (LO) signal, each The control signal has 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 shown in FIG. 2, respectively. 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, with N=4, there will 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 just an example, the control signals of the first switching circuits 510-1 to 510-N may represent different phase shifts of the LO signal, however, typically spaced 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 according to Sequential activation of 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 simultaneously activated, and the first switching circuit 510-2 and the second switching circuit 512-2 of the second path are activated simultaneously are activated at the same time, and so on. This may be advantageous where the circuit coupled to the output port 504 of the N-way filter 500 provides matched impedance to the input port 502 . This also simplifies the circuit for generating the control signal, since the same control signal can 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 can be used to activate the first switching circuit 510-1 and the second switching circuit 512-1 of the first path, and the control signal P2 can be used to activate the first switching of the second path circuit 510-2 and second switching circuit 512-2, and so on.

Table 1

path Phase Offset of the Control Signal of the First Switch Circuit Phase shift of the second switch circuit control signal 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 time point. In particular, the second switching circuit may be activated by a control signal that is phase-shifted by a predetermined amount relative to the control signal for activating the respective first switching circuit. For example, a second switching circuit in a path may be activated by a control signal that is phase shifted by 90 degrees relative to the control signal used to activate the corresponding first switching circuit. When N equals 4, this may result in the control signals of the first switching circuit and the second switching circuit being phase shifted versions of the LO shown in Table 2.

Table 2

path Phase Offset of the Control Signal of the First Switch Circuit Phase shift of the second switch circuit control signal 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 for the phases of their control signals to differ by a factor of 360/N. This allows the control signal used to activate the first switching circuit to be reused. For example, in Table 2, N=4, so 360/4=90, and the control signals of the first switching circuit and the second switching circuit of the same path have a phase difference of 90 degrees. In this way, the control signal used to activate the first switching circuit of the first path can also be used to activate the second switching circuit of the fourth path, and the control signal used to activate the first switching circuit of the second path can also be used to activate the first switching circuit of the second path. A second switching circuit of a path, and so on.

The N-channel filter 500 shown in FIG. 5 provides a filter, which greatly improves the out-of-band suppression compared with the N-channel filter such as the N-channel filter 100 shown in FIG. 1 and does not require additional power consumption. There is little impact on area, little impact on noise, no additional complex circuitry, and no need for fine-tuning.

In some cases, filters that further improve out-of-band rejection can be implemented by placing the N-way filter described herein in the feedback path of the amplifier. This combines the out-of-band rejection improvement achieved by the N-way filter 500 described in FIG. 5 and 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 in 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 to 508 -N are electrically coupled to the output end of the amplifier 602 . In some cases, the amplifier 602 may be a low noise amplifier (LNA). In this filter circuit 600 , because the filter suppression is not limited by the on-resistance of the switching circuit, nor is it limited by the bandwidth of the amplifier, the out-of-band suppression is improved compared to the filter circuit 300 described in FIG. 3 . Also, due to the Miller effect, the size of the low-pass filter circuit components (eg, C1, C2, . . . , CN) can be smaller.

Furthermore, as described above, since there is no direct short connection between the input port and the input of amplifier 602, additional filtering is performed at the input of amplifier 602. (ie, there is a higher level of out-of-band rejection at the input of amplifier 602), thereby protecting amplifier 602 from any unwanted large sources of interference. This means that the amplifier 602 will not be subject to the full power limitation of unwanted interferers, and thus may not require large signal processing capabilities. This in turn reduces the risk of signal compression within amplifier 602 .

Compared with the filter circuit 300 described in FIG. 3 , the out-of-band linearity of the filter circuit 600 is also greatly improved. For example transfer functions describing the above, reference is made to FIGS. 7 and 8 .

Similar to the filter circuit 300 shown in FIG. 3 , the filter circuit 600 shown 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 (eg, an RF antenna) that contributes an impedance to the filter circuit 600, the impedance matched to the filter circuit 600 may include an impedance matching circuit that provides an impedance to the circuit/component (eg, an RF antenna). For example, an antenna) brings corresponding impedance. In some cases, as shown in FIG. 6 , the impedance matching circuit may be implemented as a resistor (RF) in the feedback path of amplifier 602 . However, in other cases, impedance matching may not be required, or impedance matching may be performed or achieved by other means.

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

Referring now to FIGS. 7 and 8 , FIGS. 7 and 8 illustrate the 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 FIG. 6. The filter circuit 600 includes the N-way filter 500 shown in FIG. 5 in the feedback loop of the LNA, and the filter circuit 300 in FIG. 3 includes the N-way filter 100 described in FIG. 1 in the feedback loop of the LNA. In these examples, the center frequency of the N-path filter is 2GHz (ie, the switching circuit of the N-path filter is controlled by an LO signal having a frequency of 2GHz).

FIG. 7 shows the input transfer function 702 and output transfer function 704 of the LNA without N-way filters in the feedback loop, and the input of the LNA with N-way filters in the feedback loop (ie, the circuit 300 of FIG. 3 ). Transfer function 706 and output transfer function 708. 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 input transfer function 702 and output transfer function 704, when there are no N-way filters in the feedback loop, the input to the LNA typically matches the original input signal, and the LNA outputs an amplified version of the input signal. Although it can be seen that typically more gain is applied to frequencies below the center frequency (eg, 2 GHz) 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 (ie, resulting in the circuit 300 described in FIG. 3), both the input and output of the LNA are relative to the The initial input signal at frequencies above and below the center frequency is attenuated. Specifically, although the inputs and outputs of the circuit 300 depicted in FIG. 3 may have substantially the same gain as the inputs and outputs of an LNA at the center frequency without an N-way filter in the feedback loop. That is, the input of the LNA without the N-way filter in the feedback loop has a gain of 70.719mdB at the center frequency, and the input of the circuit 300 described in FIG. 3 has a gain of 359.82mdB at the center frequency; there is no N-way filter in the feedback loop. The output of the LNA has a gain of 18.057dB at the center frequency, and the output of the circuit 300 of FIG. 3 has a gain of 13.809dB at the center frequency. Both 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 from Figure 7, the transfer functions 706 and 708 are asymmetrical with respect to the center frequency. Specifically, for the input and output of the LNA, the gain of frequencies above the center frequency generally increases with frequency, and the gain of frequencies above the center frequency generally decreases with frequency. This is caused by the bandwidth of the LNA.

FIG. 8 shows the input transfer function 706 and output transfer function 708 of an LNA with an N-way filter in the feedback loop (ie, the circuit 300 described in FIG. 3 ), and the feedback loop (ie, the circuit 300 described in FIG. 6 ) The circuit 600) has an improved input transfer function 802 and an output transfer function 804 of the N-way filter 500 described in FIG. 5 . As can be seen in FIG. 8, with respect to the input and output of the LNA in the circuit 600 of FIG. 6, the attenuation of out-of-band frequencies (ie, out-of-band rejection) is significantly improved relative to the input and output of the LNA in the circuit 300 of FIG. 3 . It can also be seen that for the input and output of the LNA in the circuit 600 depicted in Figure 6, the attenuation of frequencies on either side of the center frequency does not increase or decrease in the same way as when an N-way filter is used in the feedback path. Thus, the out-of-band linearity of the circuit 600 of FIG. 6 is improved relative to the circuit 300 of FIG. 3 . Correspondingly, when the modified N-way filter is used in the feedback loop of the LNA, out-of-band rejection is not limited in the same way by the LNA bandwidth on either side of the center frequency.

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

Referring now to FIG. 9, an example of an input signal filtering method 900 is shown. The method 900 begins at blocks 902 and 904 . At block 902, the input signal is connected in turn to each of a plurality of low-pass filter circuits (eg, low-pass filter circuits 508-1 through 508-N). In some cases, the input signal may be connected to the low-pass filtering circuit (eg, low-pass filtering circuit 508-i) through a first switching circuit (eg, first switching circuit 510-i) associated with the low-pass filtering circuit ). At block 904, the low-pass filter circuits (eg, low-pass filter circuits 508-1 through 508-N) are sequentially connected to the output ports of the filters to generate filtered output signals. 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 circuit is connected to the output port in the same order as the low-pass filter circuit connected to the input port. In some cases, the same low-pass filter can be connected to both the input port and the output port. In other cases, the same low-pass filter can be connected to the input port and the output port at different points in time. For example, a low-pass filter can be connected to the input port during a first cycle (through a corresponding first switching circuit), and the low-pass filter can be connected to an input port during a different second cycle (through a corresponding second switching circuit) Connect to the output port. The first switching circuit may include one or more switches. The second switching circuit may include 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.

Applicants hereby disclose each individual feature described herein individually and any combination of two or more such features. With the ordinary knowledge of those skilled in the art, such features or combinations can be implemented as a whole based on this specification, regardless of whether such features or combinations of features can solve any problems disclosed herein; and do not fall within the scope of the claims cause. This application demonstrates that aspects of the invention may consist of any such individual feature or combination of features. Various modifications that may be made within the scope of the invention will be apparent to those skilled in the art in view of the foregoing description.

Claims (16)

1. an N-way filter, is characterized in that, comprises: Input port for receiving input signal; an output port for outputting the filtered version of the input signal; Multiple paths, where each path includes: Low-pass filter circuit; a first switching circuit, wherein when the first switching circuit is activated, the first switching circuit connects the input port to the low-pass filter circuit; a second switching circuit, wherein when the second switching circuit is activated, the second switching circuit connects the output port to the low-pass filter circuit; Wherein, the first switching circuits of the multiple paths are activated in sequence, and the second switching circuits of the multiple paths are activated in the same sequence. 2. The N-path filter, characterized in that the first switching circuit of at least one path includes 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, wherein, when dependent on claim 2, the at least one switch of the second switching circuit is smaller than the at least one switch of the first switching circuit switch. 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 claim 1 , wherein the first switching circuit and the second switching circuit of the same path are activated simultaneously. 7 . 7 . The N-way filter according to claim 1 , wherein the first switching circuit and the second switching circuit of the same path are activated at different time points. 8 . 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 the first switching circuit of one path is periodically activated, the input signal is down-converted to a baseband signal, and the low frequency of the path is passed through. A pass filter circuit converts the baseband signal into a filtered baseband signal. 10 . The N-way filter according to claim 9 , wherein the second switching circuit of one path is periodically activated to up-convert the filtered baseband signal to be in the same frequency band as the input signal. 11 . Signal. 11. The N-way filter according to any one of the preceding claims, wherein the input signal is a radio frequency signal. 12. A filter circuit, characterized in that it comprises: amplifier; An N-way filter according to any preceding claim in the feedback path of the amplifier. 13 . The filter circuit according to claim 12 , wherein the output port of the N-channel 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 . 14. The filter circuit of claim 12 or 13, wherein the filter circuit further comprises a matched resistor in the second feedback path of the amplifier. 15. The filter of any one of claims 12 to 14, wherein the amplifier is a low noise amplifier. 16. An input signal filtering method, characterized in that, comprising: connecting the input signal to a plurality of low-pass filter circuits in sequence through a first switching circuit associated with the low-pass filter circuit; The signals generated by the plurality of low-pass filter circuits are sequentially output through the second switching circuit associated with the low-pass filter circuit, thereby generating a filtered output signal.

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