A Reconfigurable Analog Baseband for Multistandard Wireless Receivers in 22-nm CMOS

This paper presents a low noise, and high linearity reconfigurable receiver (RX) analog baseband (ABB) with tunable bandwidth (BW) and gain for multi-standard applications. The designed ABB consists of a programmable gain amplifier (PGA) and a second-order active RC low-pass filter (LPF) with cutoff frequency range from 0.7 MHz–10 MHz, whereas the gain could be tuned between 0 dB and 49 dB. The proposed ABB is implemented in 22 nm CMOS process. The post-simulation results show that the current consumption is 3.36 mA from 1 V supply and the area occupies 571×328 μm 2. The ABB achieves 42.8 dBm in-band third-order harmonic intercept point (IIP3). The spurious-free dynamic range (SFDR) and in-band total harmonic distortion (THD) is 83.2 dBc and is -83.1 dB, respectively. The input referred in-band integrated noise (IRN) is 144.8 μVrms . A digital DCOC is used to calibrate the output DC level.


Introduction
With the rapid development of wireless communication technology, communication system standards have evolved from Global System for Mobile Communications (GSM) to New Radio (NR) encompasses Bluetooth, Global Positioning System (GPS), Wideband Code Division Multiple Access (W-CDMA), Wireless Local Area Network (WLAN), Long-Term Evolution (LTE) and so on [1].It is particularly important for supporting multi-standard communications in an RF front-end circuit, and different communication standards make RF front-end circuit design complex.Table 1 summarizes the performance requirements of ABB in some multi-standard wireless receivers, and it can be seen that there are differences in performance requirements for different communication standards [2]-[4].
Heterodyne receivers, low IF receivers and zero IF receivers are commonly used receiver architecture.In zero-IF and low-IF receiver architecture, the signal power level used for analog-to-digital conversion is about 0 dBm, and the required gain range from the antenna to the ADC input is approximately 0-75dB, where most of the gain is provided by ABB module.The zero IF receiver used in this paper is shown in figure 1.Since the input signal power affects the performance of the ADC, the signal power arriving at the ADC input needs to be amplified to a suitable level to ensure that the ADC can work stably and efficiently [5]- [6].
As a key component of a wireless receiver, analog base-band, in addition to its gain and bandwidth, noise, linearity and power consumption are also critical design metrics.There are trade-offs between these parameters, which require careful consideration during design.Filters are classified into active and passive filters according to the existence of active devices.Active filters are more widely used in integrated circuits because they avoid the use of large inductors and are easy to integrate.Active filters are further categorized into active RC filters and Gm-C filters.The op-amp of the active RC filter is in closed-loop, with better linearity and dynamic range, while the op-amp of the Gm-C filter operates in an open-loop state, providing the advantage of large bandwidth and low power consumption [7].The type of PGA includes Gilbert structure, variable transconductance based structure, feedback resistor/ capacitor structure and so on, among which the feedback capacitor/resistor structure has better linearity and dynamic range [8]- [9].The ABB used in this paper contains a feedback resistor/capacitor structure PGA and a low pass active RC filter.
This paper is organized as follows.The Section 2 is the ABB architecture and circuit design.The post-simulation results are presented in Section 3. Finally, the conclusion is given in Section 4.

ABB Topologies
There are two traditional ABB topologies which are shown in figure 2(a) and (b), namely, LPF-first ABB and PGA-first ABB.In the LPF-first ABB topology, the advantage of this topology is that the linearity and the output swing requirements of the LPF are moderate, and the out-of-band signal rejection of the LPF reduces the linearity requirements of the post-stage PGA.The disadvantage is that LPF has low gain and poor noise performance.At the front end of the ABB, the LPF will deteriorate its noise performance.In the PGA-first ABB topology.The topology requires moderate linearity of the PGA and the high gain of the PGA can optimize the noise performance of the ABB.However, the disadvantage is that the linearity of the LPF largely determines the linearity of the ABB [1].
In the conventional ABB topology shown in figure 2(b), the PGA is responsible for switching the gain, while the bandwidth is changed by the LPF.The bandwidth of the PGA is larger than that of the LPF, which also leads to higher power consumption of the PGA.In this paper, the PGA and LPF form a third-order Chebyshev filter shown in figure 3.At the same time the PGA and LPF achieve tunable gain and bandwidth, which increases the order of the ABB while reducing the power consumption of the PGA.The LPF is a Thomas-Tow biquad structure which consists of a lossy integrator and a lossless integrator.The ABB gain and bandwidth can be reconfigured by altering the resistors and capacitors surrounding the op-amps.The transfer function of the PGA can be expressed as (1), where  1 and  0_ represent the gain and cutoff frequency of the PGA.The transfer function of the LPF can be expressed as (3), where  2 ,  0_ and  represent the gain, cutoff frequency and quality factor of the LPF, respectively [10]. (1) (3)

Variable Resisror and Capacitor
The variable resistor arrays  1 and  3 are shown in figure 4 and the variable resistor arrays  2 ,  4 ,  5 and  6 are shown in figure 5.The bandwidth is determined by  2 after the capacitor  1 is fixed in the PGA.In the case where  2 is determined, the gain for different bandwidth is changed by  1 .In the LPF, the bandwidth is given by  5 and  6 when capacitors  2 and  3 are fixed.

Operational Amplifier
Being the core module in PGA and LPF, op-amp is critical to PGA and LPF performance, especially its gain and gain bandwidth product (GBW).The higher the gain of the op-amp, the smaller the closedloop error of the PGA and LPF.The GBW of the op-amp has a significant impact on the linearity of the PGA and LPF, and we generally expect the GBW to be greater than 15 times the cut-off frequency of the PGA and LPF.High GBW and stability means that it consumes more power.It is a great challenge to design an op-amp with low power consumption, high GBW and good stability.
A power and GBW switchable op-amp used in this study is shown in figure 7. The op-amp incorporates an op-amp core, a bias circuit and a common-mode feedback (CMFB) circuit.The op-amp core is a two-stage class A/AB structure. 0 ~3 form a simple differential pair as the first stage. 4 ~5 form as the current mirror stage and  6 ~7 form as the second stage with class AB operation. 3 and  3 are employed as Miller compensation, whereas  1 ,  1 and  2 are engaged in tuning the phase margin.
The CMFB is utilized to stabilize the output DC voltage of the op-amp core.CMFB compares the common mode level of   and   with   to change the DC voltage at OPA and OPB to stabilize the output DC voltage of op-amp core around   as the output DC voltage   and   changes.Transistors  22 and  23 are designed to control the level of bias current with 3 modes of operation, mode1 for  22 and  23 off, mode2 for  22 or  23 on, and mode3 for both  22 and  23 on.The AC response of the opamp under different modes is shown in figure 8 at a supply voltage of 1 V and a load capacitance of 2 pF.The performance summary of the op-amp is shown in Table 2.

Simulation Resuilt
The design of the ABB is implemented in a standard 22 nm CMOS process.The layout of the ABB is shown in figure 10 and the area occupies 571×328  2 including I and Q branches.The post-simulation results of ABB are discussed below.

Frequency Response
Figure 11 shows the gain simulation results of the ABB with 10 MHz bandwidth.Combining the gain of PGA and LPF by digital control bits, the gain dynamic range of the ABB covers 0 dB to 49 dB. 1 dB gain step from 0 dB to 34 dB, followed by 36/37/38/39/40/43/45/49 dB. Figure 12 shows the gain step error, which is less than 0.5 dB.The bandwidth simulation results of ABB circuit at gain of 0 dB are shown in figure 13 with bandwidth covering 0.7/1.5/2.5/5/7.5/10MHz.The gain in passband is relatively flat and attenuates at a rate of 60 dB per decade in the stopband for different gains and bandwidths.

Linearity Performance
The IIP3 simulation results of ABB are shown in figure 14.The IIP3 is 42.89 dBm with two-tone signal frequency frf1 and frf2 of 3MHz and 4MHz respectively.The input signal power is -10 dBm, gain of 6 dB and bandwidth of 10 MHz.

FFT Simulation
The FFT simulation results of the ABB are shown in figure 14.The   of the output signal is 600 mV and figure15(b) shows the spectrum.The SNR, SFDR, and THD is 89.5 dB, 83.2 dBc and -83.1 dB respectively.

DCOC Simulation
The DCOC simulation results are shown in figure 17.The output DC voltages  1 and  1 show opposite trends under the influence of CMFB, and the average value of both stays around the common mode voltage.DC calibration range and accuracy are adjusted by changing the digital control bits. ABB

Figure 10 .
Figure 10.The layout of proposed ABB.

Figure 11 .
Figure 11.The gain simulation of proposed ABB.

Figure 12 .
Figure 12.The gain and gain step error simulation.

Figure 15 .Figure 16 .
Figure 15.The FFT simulation: (a) Output signal; (b)Spectrum.3.4.Noise PerformanceFigure16shows the noise figure simulation results of the ABB at a bandwidth of 10 MHz and different gains.The noise figure is 23.6 dB when the PGA gain at 25 dB and the LPF gain at 24 dB.With a PGA gain of 0 dB and LPF gain of 0 dB, the noise figure is 49 dB.A better noise performance of ABB can be found when the PGA has a large gain.The input referred in-band integrated noise (IRN) is 144.8   .
F and Sanchez-Sinencio E 2020 A 0.6-V power-efficient active-RC analog lowpass filter with cutoff frequency selection IEEE Transactions on Very Large Scale Integration (VLSI) Systems vol 28 no 8 pp 1757-1769 [8] MD Matteis, Pipino A, Resta F, Pezzotta A, SD Amico and Baschirotto A 2017 A 63-dB DR 22.5-MHz 21.5-dBm IIP3 fourth-order FLFB analog filter IEEE Journal of Solid-State Circuits vol 52 no 7 pp 1977-1986 [9] Jin J, Liu X and Zhou J 2019 A 0.25-dB-step, 68-dB-dynamic range analog baseband with digitally assisted DCOC and AGC for multi-standard TV applications IEEE Transactions on Circuits and Systems II: Express Briefs vol 66 no 10 pp 1623-1627 [10] Ye L, Shi C, Liao H, Huang R and Wang Y 2013 Highly power-efficient active-RC filters with wide bandwidth-range using low-gain push-pull opamps IEEE Transactions on Circuits and Systems I: Regular Papers vol 60 no 1 pp 95-107

Table 1 .
Typical specification on the ABB in a zero IF receiver for various standards [1].
The variable resistor array used for gain reconfiguration.
The gain for different bandwidth is defined by  3 , and the  is varied by changing  4 .The design of the variable capacitor array which shown in figure6is used to calibrate the bandwidth after chip fabrication to overcome the inaccuracy caused by the process variation.For simplicity, we take  1 = 2 =2 3 .The gain tunable

Table 2 .
Performance of op-amp at different modes.Caln1 does not inject current, and the CMFB causes  1 to rise.Adjusting the injected current level makes  1 and  1 return to normal.The injection current is selected by digital bits I_sel<1:0>, S<6:0> and Sn<6:0>. the DCOC of the LFP is the same as the PGA.
Small input DC offset voltages can produce large DC offsets at the outputs due to the presence of gain in the PGA and LPF.DC offset may cause the ABB unable to operate properly, so a DC offset cancellation (DCOC) is required.The DCOC used in this paper is shown in Figure9.When the PGA output DC voltage  1 rises,  1 decreases as CMFB is present.Then the DCOC injects current into Calp1 in figure3, which generates a voltage drop across  2 , causing  1 to drop.

Table 3 .
Table 3 gives the performance of this ABB and compares it with other ABBs and active RC filters.ABB performance and comparison.ABB with tunable BW and gain is implemented in this work.The designed ABB achieves a gain of 0-49 dB and bandwidth of 0.7-10 MHz.Implemented in a 22 nm CMOS technology, the ABB consumes 3.36 mA from 1 V supply and occupies 0.178  2 including I and Q branches.The in-band IIP3, SFDR, THD, and IRN of the designed ABB is 42.8 dBm, 83.2 dBc, -83.1 dB and 144.8   respectively.The output DC voltage is calibrated by using a digital DCOC.Tan Y, Liu Z, Liu J and Liao H 2019 A low power analog baseband for IoT applications in 40 nm CMOS 2019 China Semiconductor Technology International Conference (CSTIC) pp 1-3 [5] Delshadpour S 2019 A 5/10/20/40 MHz 5th order active-RC chebychev LPF for 802.11abgIF receiver in 0.18 μm CMOS technology 2019 IEEE 20th Wireless and Microwave Technology Conference (WAMICON) pp 1-4 [6] Alaybeyoglu E and Kuntman H 2017 A new method to design multi-standard analog baseband low-pass filter 2017 10th International Conference on Electrical and Electronics Engineering (ELECO) pp 1216-1220 [7] Lavalle-Aviles