Design Considerations of Reconfigurable CMOS Mixers for Multi-standard Communication Receiver Systems

Received Aug 11, 2018 Revised Oct 12 , 2018 Accepted Oct 26, 2018 This paper has been carried out the study of reconfigurable wide-band mixers that can do the frequency conversion and gain variation standards with low noise and high linearity used in multi-mode and multi-standard receivers. Over the last few years reconfigurability has become very popular in adopting technology to meet the wideband wireless communication specifications that is compatible with multi-standards like GPS (1.57 GHz), WLAN (2.4 GHz-5.9 GHz), Bluetooth (2.402–2.483 GHz) and ZigBee (0.784-0.915 GHz) in low power consumption environment. The reconfigurability can be achieved between low and high band modes through power switching in RF frequency mixers. It can be achieved by flipping the input RF signal between gate and source terminal of input transistor and altering the trans-impedance stage output. With the concept of reconfigurable transistor pair with open and short circuit stubs, one can not only find the configurable gain with center frequencies 7.355, 8.65, 11.35 and 12.65 GHz but also with high power efficiency. Tow Thomas Bi-Quad Topology other than the traditional current commuting technique for the second order trans-impedance amplifier stage, works as a current mode filter over a tunable bandwidth. The active Gilbert mixers are used widely in most of communication system, due to its significance gain, perfect isolation, and linearity in response.


INTRODUCTION
The highly growth in wireless satellite communication, optical communication and military application with internet platform has made tremendous demand for low cost, low power and high performance multimedia, multi-standard transceivers. The system having Wi-Fi, Ultra Wideband, Bluetooth, ZigBee, Internet of things (IOT) enabled platform and cognitive interface with sophisticated performance, endless connectivity, demands the reconfigurable smart mixers. The mixer, a major building block in transmitter/receiver, affects the overall performance of the communication system, having three port nonlinear circuit is used to convert the radiofrequency signal up or down to an intermediate frequency signal. A high frequency mixer can reduce the gain, while a high linearity mixer can increase the overall system linearity and its dynamic range [1]. If a mixer increase the frequency to fRF+fLO; then the mixer is an upconvertor mixer, on the other hand if the frequency is decreased to fRF-fLO; the mixer is called a downconvertor mixer, where the fRF andfLO are the frequencies of incoming signal and local oscillator. While analyzing the performance of a mixer, the various specifications like conversion gain, noise figure, flicker noise (1/f), linearity, 1dB compression point and interception point are taken for consideration. CMOS device circuits have now been scaled down to reduce the channel length for achieving the sharpened radio frequency performance, but with extra flicker noise [2]. The flicker noise is reduced by using PMOS switching technique, to alter the height and width of noise pulse [3]. By emplacing the currents to input stage and switching stage individually other than a traditional methods of providing similar current to both stages, leads to optimization of gain and total noise contribution including flicker noise [4]. A passive mixer for television tuner operating at 48 to 860 MHz can achieve the gain of 19.5 dB with zero flicker noise [5].
To design the radio frequency reconfigurable wireless communication system, researchers have introduced the RF transceiver with mixer having flexible frequency bandwidth and conversion gain variation. A CMOS quadrature demodulator receiver provides the reconfigurability in terms of frequency variation at wide range applicable for GSM/DCS/CDMA cellular standards in multimode communication system [6]. Also an inductor-less variable range receiver have been introduced to palliate the effect of jammer on different bands signals [7]. The variable mixer with double balanced Gilbert cell [8] and CMOS folded switching mixer [9] working in 3-5 GHZ band can produce an adjustable transition gain for multiband OFDM (MB-OFDM) UWB standard. Most of proposed reconfigurable mixers have shown gain variant conversion and bandwidth tuning through current variation, load tuning etc.Recent work on variable gain conversion mixersmakes it possible for designing reconfigurable system to approach the gain controllability [10][11][12][13].

RECONFIGURABLE MIXER DESIGN TECHNIQUES
The communication systems with multimode and multi-standard configurability are designed for reconfiguration between active and passive mode with gain and frequency with low noise occurrence. The system gives the high conversion gain, low noise figure, moderate linearity but high power consumption in active mode, while in passive mode low conversion gain, low flicker noise with better linearity and low power consumption.
The Nisha Gupta et al [15][16], proposed a reconfigurable down conversion mixer with adjustable gain and bandwidth through current distribution between input and switching stages, and altering the load output. A fully differential RF input impedance of 50 Ω is usually considered in order to eliminate the common mode noise and second harmonics. It is achieved by converting RF input voltage into RF current, to get commutated with switches stage and then passing on to first order low pass filtering circuit shown in Figure 1. The higher IIP3 with same bias current and high band transconductance (gm) can be achieve to work from 0.6-5 GHz by using multi-gated input pair of transistors. To minimize the current and parasitic capacitance to a least possible value at the transconductance output, thereby increasing the output impedance to a high level; so that reducing the overall noise, the common mode voltage is set to VDD/2. The switching stage changes the circuit configuration to work in active or passive mode by flipping the load. In passive mode, the mixer is simply combination of four transistors working as degeneration resistor and switches to increase the linearity, while capacitor CC to suppress the noise at higher frequency [17]. In active mode configuration, the common source topology in double balanced Gilbert cell architecture provide the current, flowing through Mn1 and Mn2 (SW 5-6) switches with different selection of bias voltage to control the parameters of transconductance stage, so as to achieve the variation in high gain and low noise figure. The high linearity is achieved by providing the zero voltage to gm stage through virtual ground at trans-impedance amplifier stage.
A two stage miller compensated OTA topology having output transconductance amplifier with feedback RF, CF is opted for obtaining output undulation at high peak and input attributed noise at low level to design a trans-impedance amplifier, that convert the input current to voltage output, serving as a load and anti-aliasing filter in passive mode. In active mode configuration, to save the power TIA is being switched off.
Ming Wang and Carlos E. Saavedra [18] discovered a down-converter Gilbert cell mixer, in which the conversion gain can be varied by doing restyle of total gate width (W) of transistor devices other than the traditional variable gain amplifier and attenuator. In this architecture three transistors in left side and three transistors in right side are used in RF transconductance stage of the mixer in parallel to accomplish two disengage tasks of gain curtailment and frequency transformation simultaneously, which lead to reduce the chip area. This reconfigurable mixer shown in Figure 2 can provide the conversion gain from 9 dB consuming 2.4 mW to 24 dB with 18 mW, power consumption. The conversion gain of mixer is varied in accordance with transistor gate-width variation having different combination of switches in ON/OFF condition. Zhangf Liu, Jing Wang proposed a reconfigurable mixer [19] compromise of double balanced Gilbert cell mixer and reconfigurable matching network which is being used in USA GPS and China Beidou receiver system. This RF integrated circuit a single chip circuit is capable of achieving the reconfiguration function of GNSS receiver. The mixer shown in Figure 3 is designed on 0.18 micrometer CMOS process. The reconfigurable matching network has software definable MOS switches SW1, SW2 SW3 and SW4 shown in Figure 3. When the switches SW1,SW2, SW3and SW4 are in the different combination of "on" and "off" conditionwith L1, L2, C1 and C2 series and parallel connection, the matching network make different structure to approach the frequency configuration for GPS and Beidou.
Chiu et al. [20][21] rediscovered a self-oscillating mixer, comprises of an active mixer and oscillator, to produce the frequency transition at centre frequency 7.35, 8.65, 11.35 and 12.65 GHz with a conversion gain providing variation from 7.83dB to 18.7 dB. The self-oscillating mixer circuit is made in a reconfigurable transistor topology for a less cost, high efficient and stable bias front end receiver. Sub-harmonic self-oscillating mixer as shown in Figure 4. The circuit shown in Figure 4 consist of a band pass filter playing a role of DC block capacitor and resonator, that make oscillation between band pass filter and transistor T1, tends to a self-oscillating operation, sixth order Butterworth response low pass filter to subdue the high frequency components, local radio frequency, image and spurious signal, and dual bias network with different biased transistors T1 and T2 to achieve the frequency and gain conversion. The band pass filter is a Chebyshev BPF has a small insertion loss. The transistor T1 is biased for oscillation in class AB operation of the common emitter configuration and T2 is biased with zero base emitter voltage to serve as an auxiliary oscillator to provide the stable oscillation for T1. The capacitor at base emitter of T2 and open circuit stub working as a return path for the RF signal, make the series feedback of the common emitter oscillation of T1 to generate the local oscillator signal and its harmonics mix with the RF within T1. The IF signals are taken out from the low pass filter.
Laleh Rabieirad and Saeed Mohammadi [22] presented a programmable in dual mode having three stages distributed amplifier/mixer consist of transistor switches, low loss transmission line and RF transistors. These are combined to achieve functional reconfigurability using 1 bit programmable signal. The circuit shown in Figure 5 provides a gain of 8 dB in 3-8 GHz bandwidth in distributed amplifier mode, while in distributed mixer mode; the average conversion gain is 4 DB in a 1-16 GHZ bandwidth. The CMOS transistor switches are used for low insertion loss in on state and isolation in off state, which are employed to couple and decouple RF transmission lines from the signal path. The RF block is a two incorporated cascode cell having two transistors constructing a differential pair to operate with same signal amplitude and phase in amplifier mode. In distributed mixer mode these transistors functional companion as a switch pair with transconductance stage.
Mirko Pasca et al. [23][24][25][26] presented a highly linear, low power 12 dBm IIP3 reconfigurable mixer for impulse radio ultra wideband receiver. This mixer are being used in various applications like, sensor networking and short range communication system because of low power consumption and high operating frequency. This down-conversion mixer is design to reconfigured for different channels, in low band (Channel 3 consists4.4928 GHz carrier with 499.2 MHz channel bandwidth), in high band (Channel 9 consists7.9872 GHz carrier with 499.2 MHz channel bandwidth and Channel 11 consists, 7.9872GHz carrier with 1.331 GHz channel bandwidth). The high linearity is achieved using derivative superposition method and source degeneration input stage as shown in Figure 6. The mixer consists of a Gilbert switching stage and source degeneration RF input stage to reduce voltage drop across load resistors, achieving high output signal swing and high linearity.
The transistors M1, M2, M3, M4 and resistors R5, R6, R7, R8 and ON-OFF switches M14, M15 are used to make source degenerated input stage with CMOS amplifier to achieve high gain and linearity acquiring second order distortion cancellation. To improve third-order intercept point (IIP3) performance by cancelling order non-linear current, the transistors M1 and M3 are biased in weak inversion region [27][28][29]. To emphasize conversion gain and to cut-down parasitic capacitances, the inductor L1 is placed between RF input stage, limiting load resistors. The transistors M12, M13, M16, and M17 enable/disable capacitances C9 and C10 for the modification of resonance frequency that can be operated in lower or upper band in infraredultra wide band (IR-UWB). The resistors R1, R2, R3 and R4 provide parallel switchable connection to acquire

CONCLUSION
This paper has described the study of wideband multimode mixers that can be used to perform the multiple task of frequency conversion and gain control simultaneously presented by various researchers. To enhance the trans-conductance's efficiency and reducing the 1/f flicker noise, a differential complimentary pair of transistor was used. Multiband sub-harmonic self-oscillating mixer was designed to achieve high performance conversion gain using reconfigurable transistor pair for microwave and millimeter baseband circuits. In RF trans-conductor stage, the size effective gate width of the devices can be reconfigured by placing parallel transistors and switches to change conversion gain. Derivative superposition method with source degeneration input stage can achieve high gain with different channel bandwidth for sensor network and high operating frequency system. To acquire the high linearity, source degeneration and tuning techniques with a little contribution of noise figure has been introduced in Gilbert cell mixer. Finally we conclude that, a fully differentially pair of transistors at the RF input to eliminate the second harmonics by converting RF input voltage into RF current to get commutated with switches stage for the reconfiguration strategies and design techniques in a very sophisticated manner for wireless applications.