A high performance wideband interference cancellation circuit based on active all-pass filter

Abstract

The application belongs to the technical field of radio frequency microwave integrated circuits, and particularly relates to a high-performance wideband interference cancellation circuit based on an active all-pass filter. The interference cancellation circuit adopts a cascaded mixed low-noise first-order active all-pass filter and a second-order active all-pass filter with a delay peak technology, cooperates with a Hilbert transform equalization architecture, and realizes multi-tap wideband deep self-interference cancellation in a radio frequency domain and a baseband domain at the cost of lower area and power consumption; the first-order active all-pass filter adopts a mixed structure combining a transconductance amplifier and a passive resistance capacitor, realizes low noise and high cascading property, the second-order active all-pass filter adopts a delay peak technology based on an active inductor, improves a delay bandwidth product by more than 4 times, realizes nanosecond-level large delay and flat frequency response, thereby supporting a wide radio frequency working range; meanwhile, a down-mixer shared by the radio frequency domain and the baseband domain is removed, and dynamic power consumption is reduced.

Description

Technical Field

[0001] This invention belongs to the field of radio frequency microwave integrated circuit technology, and specifically relates to a high-performance broadband interference cancellation circuit based on an active all-pass filter. Background Technology

[0002] With the rapid growth in demand for wireless communication data and the increasing congestion of spectrum resources, full-duplex communication technology on the same frequency has attracted widespread attention due to its potential to double spectrum efficiency. Compared with traditional half-duplex communication, full-duplex receivers face strong self-interference signals from the local transmitter, whose power is typically more than 100 dB higher than the receiver's noise floor, posing a significant challenge to self-interference cancellation.

[0003] Existing self-interference cancellation technologies encompass multiple levels, including antenna interface isolation, RF / analog domain cancellation, and digital domain cancellation. Antenna interface isolation provides 20–40 dB of isolation, while digital domain cancellation offers approximately 40 dB of cancellation. Therefore, the RF / analog domain requires a cancellation level exceeding 30 dB. However, most existing RF cancellers still limit their cancellation bandwidth to within 100 MHz when achieving cancellation levels exceeding 30 dB, making it difficult to meet the broadband modulation requirements of modern communication systems such as 5G and Wi-Fi 6 / 7.

[0004] Vector synthesis-based cancellers, due to the frequency flatness of their phase response, are only suitable for narrowband cancellation. Frequency-domain equalizers achieve wider cancellation bandwidth by dividing the bandwidth into multiple sub-bands, but require multiple parallel taps, leading to increased dynamic power consumption and chip area, while also severely degrading the receiver noise figure. Time-domain equalizers use taps with different weights and delays to simulate self-interference channels, but traditional RC all-pass filter delay units, limited by the delay-bandwidth product, can only provide a delay of 250–350 ps at a 2 GHz carrier frequency, making it difficult to achieve nanosecond-level delays.

[0005] To reduce the demand for high delay-bandwidth products, Hilbert transform equalization architectures achieve equivalent RF delay by up-converting the baseband delay, thus reducing the delay bandwidth requirement from the carrier frequency to the baseband signal bandwidth. However, existing Hilbert transform equalizers based on low-pass filters limit the cancellation bandwidth due to their narrowband frequency response characteristics. Therefore, developing self-interference cancellation circuits with wideband flat delay response, low noise characteristics, and miniaturization remains of great significance. Summary of the Invention

[0006] The purpose of this invention is to provide a high-performance broadband interference cancellation circuit based on an active all-pass filter, which features broadband flat delay response, low noise characteristics, and miniaturization.

[0007] The high-performance broadband interference cancellation circuit based on an active all-pass filter provided by this invention has the following structure: Figure 1 As shown, it mainly consists of radio frequency domain interference cancellation circuits and baseband domain interference cancellation circuits, and uses a Hilbert transform equalization architecture (HTE) to achieve wideband self-interference cancellation, wherein:

[0008] (1) The radio frequency domain interference cancellation circuit includes: an up / down mixer, a low-pass filter (LPF), a first tap cancellation circuit, and a second tap cancellation circuit, wherein:

[0009] The downmixer is used to mix the reference radio frequency signal coupled from the transmitter and downmix it to the baseband.

[0010] The low-pass filter is used to filter out high-frequency signals that are not needed in the baseband.

[0011] The first tap elimination circuit uses a hybrid low-noise first-order active all-pass filter to delay the down-mixed baseband signal, and then uses an amplitude modulation and phase shifting module to achieve amplitude, phase, and delay matching with the self-interference signal of the direct leakage path, in order to eliminate the high-power self-interference signal of the direct leakage path.

[0012] The second tap elimination circuit uses a second-order active all-pass filter with delay peak technology to delay the baseband signal after downmixing. Then, the amplitude, phase, and delay of the signal are matched with the self-interference signal of the reflection path through the amplitude modulation and phase shifting module, which is used to eliminate the self-interference signal of the reflection path with a large delay.

[0013] The upmixer is used to upconvert the baseband signal to the radio frequency and output it to the receiver (RX) input terminal to cancel out the self-interference signal leaked to the receiver input terminal;

[0014] (2) The baseband domain interference cancellation circuit includes: a downmixer, a low-pass filter, a third tap cancellation circuit, and a fourth tap cancellation circuit. The downmixer and low-pass filter are shared with the radio frequency domain interference cancellation module, wherein:

[0015] The downmixer is used to mix the reference radio frequency signal coupled from the transmitter and downmix it to the baseband.

[0016] The low-pass filter is used to filter out high-frequency signals that are not needed in the baseband.

[0017] The third and fourth tap elimination circuits employ second-order active all-pass filters with delay peak technology to delay the down-mixed baseband signal. The signal is then matched with the amplitude, phase, and delay of the residual self-interference signal via an amplitude modulation and phase shifting module. The output is directly injected into the input of the receiver baseband transimpedance amplifier (TIA) without going through an up-converter, for secondary elimination of the residual self-interference signal.

[0018] (3) The Hilbert transform equalization (HTE) architecture upconverts the baseband delay to the radio frequency domain and achieves amplitude, phase and delay matching of the cancellation signal and the self-interference signal in a wide frequency range through gain and phase adjustment of the orthogonal IQ path.

[0019] In this invention, the hybrid low-noise first-order active all-pass filter is one of the core innovative designs, and its structure is shown in the attached figure. Figure 2 As shown. This filter consists of positive and negative input terminals (V... in+ V in+ Active transconductance amplifier (G) m The system consists of a network of two sets of parallel passive resistors (R1 / R2) and capacitors (C1) connected between the outputs of the two transconductance amplifiers; wherein, the first resistor (R1) and the first capacitor (C1) of the resistor-capacitor network are connected in parallel and then in series with the second resistor R2, and the series node is the output node; the G at the positive input terminal... m The output node is X - It is connected to R2 of one of the resistor-capacitor networks, and the other end of R2 is the negative output terminal (V). out- ), G at the negative input terminal m The output node is X + It is connected to R2 in another resistor-capacitor network, and the other end of R2 is the positive output terminal (V). out+ The relationship R1 = 2R2 must be satisfied to achieve the following transfer function for a first-order active all-pass filter:

[0020]

[0021] Where s is a complex variable in the Laplace transform; by setting G m R1 / 8 > 1, enabling the transconductance amplifier to provide voltage gain to suppress noise introduced by the resistor. At 10 MHz, the input reference noise voltage of this all-pass filter is only 0.81 nV / √Hz.

[0022] Furthermore, the hybrid low-noise first-order active all-pass filter features high input impedance, effectively isolating the effects of the output impedance of the preceding stage and the parasitic capacitance of the input stage on the delay characteristics. Compared to a passive first-order all-pass filter, the influence coefficient of parasitic capacitance is reduced from 1 to 3 / 8, significantly reducing zero-pole mismatch and ensuring gain and delay flatness during cascading. This superior cascading characteristic allows multiple hybrid first-order active all-pass filters to be cascaded without significantly degrading the frequency response.

[0023] In this invention, the high-delay-bandwidth product second-order active all-pass filter with peak delay technology is another core innovative design, and its structure is shown in the attached figure. Figure 3As shown. This all-pass filter consists of two pseudo-differential conventional first-order active all-pass filters and a junction (X) connected between the two filters at the intermediate node. + X - The active inductance between the five pseudo-differential transconductance units (G) constitutes the first-order active all-pass filter. m3 ~ G m7 It consists of an input node (V) and a variable capacitor C2, wherein: in+ V in- ) Connect G m3 With G m4 Input terminal, G m3 The output terminal is connected to the output node (V out+ V out- ), G m7 The load of this node is determined by shorting the input and output; G m4 The output is connected to the intermediate node (X). + X - ), G m5 The input and output are shorted together to serve as the load of this node; C2 is connected across X as a differential capacitor. + With X - Between, X + With X - The node's signal is transmitted through G m6 Output to the output node of the filter (V) out+ V out- The active inductor consists of two pseudo-differential transconductance units (G). m1 G m2 It consists of a variable capacitor C1 and a variable capacitor C1, and its equivalent inductance is L=4C1 / (G m1 G m2 ), where: G m1 The positive / negative outputs are respectively related to G m2 The positive / negative inputs are connected, and G m2 The positive / negative outputs are respectively related to G m1 The negative / positive inputs are connected, and capacitor C1 is connected across G. m1 Between the positive and negative output nodes. The design must satisfy G. m6 =2G m3 =2G m4 =2G m5 =2G m7 =2G m To achieve the frequency response of a second-order all-pass filter.

[0024] Active inductor technology avoids the use of large-area passive inductors at the microhenry level, allowing the circuit to be integrated in standard CMOS processes. The transfer function of the second-order active all-pass filter is:

[0025]

[0026] in, Let Q be the natural frequency and Q be the quality factor; the quality factor Q and the delay can be obtained. The expressions are as follows:

[0027]

[0028] This invention innovatively introduces a delay peaking technique to further extend the effective delay-bandwidth product. By adjusting the quality factor to Q > 1 / √3, a peak is introduced into the delay response, which can compensate for the delay attenuation caused by the poles of the preceding stage in cascading. After adopting the delay peaking technique, cascaded first-order and second-order active all-pass filters can achieve a flat delay of 7ns within a 151MHz bandwidth.

[0029] This invention's interference cancellation circuit employs a cascaded hybrid low-noise first-order active all-pass filter and a second-order active all-pass filter with delay peaking technology, combined with a Hilbert transform equalization architecture, to achieve multi-tap self-interference cancellation in both the RF and baseband domains. The first-order active all-pass filter uses a hybrid structure combining transconductance amplifiers and passive resistors and capacitors to achieve low noise (0.81nV / √Hz input reference noise voltage) and high cascadability. The second-order active all-pass filter uses delay peaking technology based on active inductors, increasing the delay-bandwidth product by more than four times, achieving nanosecond-level large delay and a flat frequency response. At the architecture level, the RF and baseband cancellation circuits share a downconverter, further saving dynamic power consumption. The cancellation circuit operates at frequencies covering 0.85~5GHz, achieving self-interference cancellation depths of 39.7dB / 37.1dB at modulation bandwidths of 80MHz / 120MHz, with noise figure degradation of only 1.2~2.1dB. Attached Figure Description

[0030] Figure 1 This is a block diagram of the high-performance broadband interference cancellation circuit based on an active all-pass filter of the present invention.

[0031] Figure 2 This is a circuit diagram of the hybrid low-noise first-order active all-pass filter of the present invention.

[0032] Figure 3 This is a circuit diagram of the second-order active all-pass filter with delay peak technology of the present invention.

[0033] Figure 4 This is a performance comparison chart of passive, traditional active, and hybrid first-order all-pass filters.

[0034] Figure 5 The diagram shows a comparison of the delay-bandwidth product (a) and its extension (b).

[0035] Figure 6 This is a diagram illustrating the specific implementation of the interference cancellation circuit in a full-duplex receiver.

[0036] Figure 7 This is a spectrum diagram of the baseband output interference signal before and after the elimination circuit is turned on when the circulator is terminated at 50 ohms in a specific embodiment of the present invention.

[0037] Figure 8 The results show the interference cancellation depth (a) of the interference cancellation circuit in the broadband range and the noise figure of the receiver module before and after activation (b) in a specific embodiment of the present invention. Detailed Implementation

[0038] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0039] As attached Figure 1 The diagram shown is a block diagram of the high-performance broadband interference cancellation circuit based on an active all-pass filter proposed in this invention. The circuit mainly consists of a radio frequency domain interference cancellation module and a baseband domain interference cancellation module.

[0040] The specific implementation of the radio frequency domain interference cancellation module includes: a downmixer, an upmixer, a low-pass filter, a delay unit, an amplitude modulation unit, and a phase shifting unit. The input reference radio frequency self-interference signal is first downmixed to baseband by the downmixer. The downmixer employs a four-phase 25% duty cycle non-overlapping clock controlled passive mixer structure, featuring low on-resistance and low noise characteristics. The downmixed baseband IQ signal first passes through a low-pass filter to remove unwanted high-frequency signals from the baseband. Then, it is processed by the delay unit, phase shifting unit, and amplitude modulation unit to achieve delay, phase, and amplitude matching with the self-interference signal leaking to the receiver. Finally, it is upmixed back to the radio frequency domain by the upmixer and injected into the receiver input to cancel the self-interference.

[0041] The radio frequency domain cancellation includes a first tap and a second tap, wherein:

[0042] The first tap delay unit employs a hybrid low-noise first-order active all-pass filter, as shown in the attached diagram. Figure 2 As shown, its single-ended input reference noise voltage is:

[0043]

[0044] in, The thermal noise figure of the MOSFET. This is the flicker noise of the transistor; by setting G mR1 / 8 > 1 can suppress noise introduced by the resistor. As a tradeoff between noise and linearity, the voltage gain of this invention is set to 5dB. At a frequency of 10MHz, the input reference noise voltage of this all-pass filter is only 0.81nV / √Hz, which is 7dB and 14dB lower than that of a passive first-order all-pass filter and a conventional active first-order all-pass filter, respectively, as shown in the attached figure. Figure 4 As shown in (a). In this embodiment, the all-pass filter uses a 6-bit controllable C1 capacitor array to achieve delay adjustment, with a delay range of 0.3~4ns and a delay resolution of 0.05ns, to meet the requirements of broadband depth cancellation.

[0045] When considering the output impedance R of the preamplifier s and the parasitic capacitance C of the input stage par When affected by the effect of the hybrid first-order all-pass filter, the transfer function becomes:

[0046]

[0047] Ignoring the effects of the second-order term, compared to a passive first-order all-pass filter, the parasitic capacitance C par The coefficient in the first-order term is reduced from 1 to 3 / 8, significantly reducing the zero-pole mismatch. Furthermore, due to G... m Due to its high input impedance characteristics, the effect of the preamp output impedance on the transfer function is almost negligible. (Appendix) Figure 4 The noise performance and frequency response of passive, traditional active, and hybrid first-order all-pass filters were compared. The hybrid first-order all-pass filter showed the best noise performance and flat gain and group delay response.

[0048] The second tap delay unit employs a high-delay-bandwidth product second-order active all-pass filter with delay peak technology, as shown in the attached figure. Figure 3 As shown. To prevent the transistor from entering the linear region, G m1 G m2 G employs a high differential impedance but low common-mode impedance m Technical implementation, other G m The unit employs a current-reused inverter-like structure to improve linearity. In this embodiment, a 6-bit controllable C1 capacitor and a 5-bit controllable C2 capacitor are used, with a delay range of 1~7ns and a delay resolution of 0.1ns. According to equation (3), the Q value is determined by the ratio of C2 to C1, while the delay... The size is determined solely by C1, therefore this second-order active all-pass filter supports independent adjustment of the Q value and delay. For example... Figure 5As shown in (a), a second-order all-pass filter theoretically provides a delay-bandwidth product that is 4 times greater than that of a first-order all-pass filter and 8 times greater than that of a low-pass filter. This invention innovatively introduces a delay peaking technique to further extend the effective delay-bandwidth product. The maximum flat group delay response can be obtained when the quality factor Q = 1 / √3, but by setting Q > 1 / √3, a peak can be introduced into the delay response to compensate for the delay attenuation caused by the poles of the preceding stage in the cascade. (See attached diagram) Figure 5 As shown in (b), after adopting the delay peak technique, the cascaded first-order and second-order active all-pass filters can achieve a flat delay of 7ns within a 151MHz bandwidth, and the effective delay-bandwidth product is more than 6 times that of a single-stage first-order all-pass filter and more than 12 times that of a first-order low-pass filter.

[0049] The baseband domain interference cancellation module and the radio frequency domain interference cancellation module share a down-mixer, and its output is directly injected into the input of the receiver's baseband transimpedance amplifier (TIA) without passing through an up-converter. The baseband domain interference cancellation includes two taps, where the delay units both employ the aforementioned second-order active all-pass filters with peak delay technology. Each tap achieves a delay range of 2–14.5 ns, with a delay step of 0.2 ns. The total delay range of the baseband domain interference cancellation module is 3–30 ns, which can meet the long-delay cancellation requirements of self-interference signals from the reflection path.

[0050] The specific implementation of the interference cancellation circuit of this invention in a full-duplex receiver is shown in the appendix. Figure 6 As shown. The receiver employs a zero-IF architecture, consisting of a wideband low-noise transconductance amplifier (LNTA), a quad-phase passive mixer, and a common-mode rejection transimpedance amplifier (TIA). A closed-loop test platform was built to verify the cancellation performance of the interference cancellation circuit. Test examples show that the cancellation circuit achieves a total cancellation of 39.7 dB for an 80 MHz signal and a total cancellation of 37.1 dB for a 120 MHz signal at a 2.6 GHz carrier frequency, as shown in the attached figure. Figure 7 As shown in the attached diagram. Leveraging the frequency agility of the Hilbert equalization architecture, this cancellation circuit can operate over a wide frequency range of 0.85~5GHz, consistently achieving a cancellation level greater than 30dB. Furthermore, due to the use of a hybrid low-noise first-order active all-pass filter as the first tap, the noise figure degradation introduced to the receiver by the interference cancellation circuit is only 1.2~2.1dB, as illustrated in the attached diagram. Figure 8 As shown.

Claims

1. A high-performance broadband interference cancellation circuit based on an active all-pass filter, characterized in that, It includes radio frequency domain interference cancellation circuits and baseband domain interference cancellation circuits, and uses a Hilbert transform equalization architecture to achieve wideband self-interference cancellation, wherein: The radio frequency domain interference cancellation circuit includes: an up / down mixer, a low-pass filter, a first tap cancellation circuit, and a second tap cancellation circuit, wherein: The downmixer is used to mix the reference radio frequency signal coupled from the transmitter and downmix it to the baseband. The low-pass filter is used to filter out high-frequency signals that are not needed in the baseband. The first tap elimination circuit uses a hybrid low-noise first-order active all-pass filter to delay the down-mixed baseband signal, and then uses an amplitude modulation and phase shifting module to achieve amplitude, phase, and delay matching with the self-interference signal of the direct leakage path, in order to eliminate the high-power self-interference signal of the direct leakage path. The second tap elimination circuit uses a second-order active all-pass filter with delay peak technology to delay the baseband signal after downmixing. Then, the amplitude, phase, and delay of the signal are matched with the self-interference signal of the reflection path through the amplitude modulation and phase shifting module, which is used to eliminate the self-interference signal of the reflection path with a large delay. The upmixer is used to upconvert the baseband signal to the radio frequency and output it to the receiver input, thereby canceling out the self-interference signal leaked to the receiver input. The baseband domain interference cancellation circuit includes: a downmixer, a low-pass filter, a third-tap cancellation circuit, and a fourth-tap cancellation circuit. The downmixer and low-pass filter are shared with the radio frequency domain interference cancellation module. The downmixer is used to mix the reference radio frequency signal coupled from the transmitter and downmix it to the baseband. The low-pass filter is used to filter out high-frequency signals that are not needed in the baseband. The third and fourth tap elimination circuits both use second-order active all-pass filters with delay peak technology to delay the baseband signal after downmixing. The signal is then matched with the amplitude, phase, and delay of the residual self-interference signal by the amplitude modulation and phase shifting modules. The output is directly injected into the input of the receiver baseband transimpedance amplifier without going through the upconverter, and is used to perform secondary elimination of the residual self-interference signal. The Hilbert transform equalization architecture upconverts the baseband delay to the radio frequency domain. Through gain and phase adjustment of the quadrature IQ path, it achieves amplitude, phase, and delay matching of the cancellation signal and the self-interference signal over a wide bandwidth.

2. The broadband interference cancellation circuit according to claim 1, characterized in that, The hybrid low-noise first-order active all-pass filter consists of positive and negative input terminals (V... in+ V in+ Active transconductance amplifier (G) m The system consists of two sets of parallel passive resistors (R1 / R2) and capacitors (C1) connected between the outputs of the two transconductance amplifiers; wherein, the first resistor (R1) and the first capacitor (C1) of the resistor-capacitor network are connected in parallel and then in series with the second resistor R2, and the series node is the output node; the G at the positive input terminal... m The output node is X - It is connected to R2 of one of the resistor-capacitor networks, and the other end of R2 is the negative output terminal (V). out- ), G at the negative input terminal m The output node is X + It is connected to R2 in another resistor-capacitor network, and the other end of R2 is the positive output terminal (V). out+ And satisfy R1=2R2 to achieve ideal all-pass filtering characteristics.

3. The broadband interference cancellation circuit according to claim 1, characterized in that, The high-delay-bandwidth product second-order active all-pass filter with delay peak technology consists of two pseudo-differential first-order active all-pass filters and a node (X) bridging the middle of the two filters. + X - The active inductance between the two elements constitutes the pseudo-differential first-order active all-pass filter, which consists of five pseudo-differential transconductance units (G). m3 ~ G m7 It consists of an input node (V) and a variable capacitor C2; wherein: the input node (V) in+ V in- ) Connect G m3 With G m4 Input terminal, G m3 The output terminal is connected to the output node (V out+ V out- ), G m7 The load of this node is determined by shorting the input and output; G m4 The output is connected to the intermediate node (X). + X - ), G m5 The input and output are shorted together to serve as the load of this node; C2 is connected across X as a differential capacitor. + With X - Between, X + With X - The node's signal is transmitted through G m6 Output to the output node of the filter (V) out+ V out- The active inductor consists of two pseudo-differential transconductance units (G). m1 G m2 It consists of a variable capacitor C1 and a variable capacitor C1, wherein: G m1 The positive and negative outputs are respectively related to G m2 The positive and negative inputs are connected, and G m2 The positive and negative outputs are respectively related to G m1 The negative and positive inputs are connected together, and capacitor C1 is connected across G. m1 Between the positive and negative output nodes; and satisfying G m6 =2G m3 =2G m4 =2G m5 =2G m7 =2G m To achieve the frequency response of a second-order all-pass filter.

4. The broadband interference cancellation circuit according to claim 2, characterized in that, The transfer function of the hybrid low-noise first-order active all-pass filter is: Set G m R1 / 8>1 enables the transconductance amplifier to provide voltage gain to suppress noise introduced by the resistor; at the same time, the all-pass filter has high input impedance characteristics, which can effectively isolate the effects of the output impedance of the preceding stage and the parasitic capacitance of the input stage on the gain and delay flatness.

5. The broadband interference cancellation circuit according to claim 3, characterized in that, The transfer function of the high-delay-bandwidth product second-order active all-pass filter with delay peak technology is: in, Let Q be the natural frequency and Q be the quality factor; obtain the quality factor Q and the delay. The expressions are as follows: By adjusting the quality factor to Q>1 / √3, a peak value is introduced into the delay response to compensate for the delay decay caused by the previous stage poles in the cascade.

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