O.5pa CMOS 2.4 GHz RF-Switch for

Wireless CommunicationsMekanand P., Prawatrungruang P., and Eungdamrong D.School of Communications, Instrumentations & Control

Sirindhorn International Institute of Technology, Thammasat University, ThailandPhone: (+662) 501-3505 Ext. 1808, FAX: (+662) 501-3505 Ext. 1801

Abstract - In RF transceiver, the RF switch is used to select D.D.D.X.X.Xthe operation for an antenna between working as a receiver or

transmitter. Each mode of operation generates some loss due tothe transistors used in the switching operations. Therefore it isessential to design a switch that has minimal impact on the M M

signal. This paper proposes a 2.4 GHz CMOS T/R ..-....(Transmit/Receive) Switch with low insertion loss and excellentcontrol voltage. Simulation results of the switch designdemonstrate an insertion loss of 1.102 dB for receiving mode .-...-.-.-.;and 1.085 dB for transmitting mode. Both modes can operateusing a control voltage of only 1.2 V. In addition, the switch ....D.Dexhibits a high 1 dB compression point (P1dB) of 25.33 dBm. I. ..-

Keywords - CMOS, T/R Switch, RF transceiver

1. Introduction

There has been a growing demand for RF switches as timedivision duplexing (TDD) is increasingly used for wirelessapplications such as IEEE 802.11 systems [1]. As a result,many different technologies exist for implementing theswitching circuits. Traditionally, PIN diodes have dominateddue to their excellent small signal performance and linearity[2]. However, power consumption and size becomes veryimportant issues for PIN diode [3]. As a result, topics relatedto the solution of this problem are becoming a popularresearch field. As CMOS technologies continue to evolvetowards lower supply voltages [4], CMOS RF switches are agreat promising solution for the low-cost and low-power [5]requirements of modern wireless circuits and are graduallyreplacing more and more PIN diodes and MESFETs in RFsystems [6]. CMOS technology can be use to operatewireless systems up to 5.0 GHz [7] and they are attractive forportable devices [5].By applying CMOS Technology, a switch can be

constructed by paralleling a NMOS and a PMOS device asshown in Fig. 1. The advantage of using a CMOS switchinstead of a single channel MOS switch is the dynamicrange. This dynamic range in the ON state is greatlyincreased, which allows a full signal-swing [4].The goal of this paper is to propose a fully functional,

highly integrated, 2.4 GHz CMOS switch with low insertionloss, low control voltage, and high PldB.

Fig.1: The CMOS used in the simulation.

2. Switch Design and Operations

The switch design is asymmetric to accommodate thedifferent linearity and isolation requirements in its operationmodes [7]. The switch can operates in either the transmitmode with power transmitted from the Power Amplifier (PA)to the antenna, or the receive mode with the power deliveredfrom the antenna to the Low Noise Amplifier (LNA) [8] asshown in Fig.2.

LNA

Antenna 1 Rx

T/R Switch

Fig.2: The T/R Switch is located in the front-end of the radiotransceiver.

The transceiver has a LNA that provides a 50Q inputimpedance [9]. Recent publications show a trend ofintegrating the LNA, PA, and T/R Switch on the same chip[10]. In addition, studies have shown that it is feasible to

Feb. 17-20, 2008 ICACT 2008ISBN 978-89-5519-136-3 -447-

implement RF single-pole double-throw (SPDT) switches in0.5im CMOS [11].

3. Single Channel T/R Switch

One problem that occurs in the front-end of a radiotransceiver is how to switch antenna's modes of operation.This is usually solved with a T/R switch. The switch selectseither transmit or receive mode by changing the polarity ofthe control voltage located at the transistor's gate [1]. Whenthe switch acts as a receiver, transistors M2 and M3 areswitched on. On the contrary, when the switch acts as atransmitter, transistors MI and M4 are switched on. Whiletransistors MI and M2 play the major role for switchingoperations, M3 and M4 are employed in order to improve theisolation [5] as shown in Fig.3.

.. ....

_ ..... -!

4. Proposed CMOS T/R Switch

The proposed TR Switch involves the use of four CMOS.When the switch acts as a transmitter, CMOS 1 and CMOS 4are switched on, and the signal makes its way to the antennaas shown in Fig.5. On the other hand, when the switch actsas a receiver, CMOS 2 and CMOS 3 are switched on, and thesignal received from the antenna makes its way to thereceiver.

In order to increase the linearity of the switch, the gate isoften biased using a large resistor [12]. Furthermore, thisswitch also integrates two bypass capacitors to allow DCbiasing of the receiver and transmitter nodes of the switch[11]. Drain and source DC biasing reduce the insertion lossby reducing the source/drain to body junction capacitanceand RF signal coupled to the substrate [6]. In addition,power handling capability can be improved [ 12].

:vi

Fig.3: Single channel T/R Switch.

Referring to Fig.4, the channel resistance, RC1 and RC2,and the substrate resistance, RB1 and RB2, are the mainsources of loss in the transistor. The source and drainparasitic junction capacitors, CSB and CDB, can also lead tosignificant losses [11], especially as the frequency ofoperation increases. The linearity of the MOSFET switch islimited for large signal swings due to conductivitymodulation caused by a changing gate-source (Vgs) anddrain-source (Vds) voltage for a large signal input.

SOUJRCE GATE1 DRAIN

Fig.5: Proposed CMOS T/R Switch.

5. Simulation Results

In order to have a power efficient transceiver withoutdegrading its sensitivity, the losses in the switch must be low[9]. Main characteristics of an RF switch are insertion lossand PldB [11]. The insertion loss is defined as the ratiobetween the power available from source and the powerdelivered to load and its formula is shown in (1) [4]:

Insertion Loss (dB) = -20 logl0 ( Ot)AKn

(1)

The waveforms shown in Fig.6 and Fig.7 are the resultsfrom the single channel T/R switch acting as a receiver andtransmitter, respectively. The input signal has amplitude of 1tV and frequency of 2.4 GHz. The insertion loss is 1.881 dBand 1.872 dB for receiver and transmitter mode respectively.

Fig.4: The NMOS.

Feb. 17-20, 2008 ICACT 2008

T RB1 RB2

ISBN 978-89-5519-136-3 -448-

.utpIt

Fig.6: Simulated input and output waveform of the single channel T/RSwitch acting as a receiver.

Fig.9: Simulated input and output waveform of the CMOS T/R Switchacting as a transmitter.

The PldB is the point at which the input level at which thesmall signal gain has dropped by 1 dB. At this point, theintermodulation products begin to emerge as a seriousproblem [13]. In Fig.10, the Pin versus Pout graph isshown. CMOS RF switches can provide higher PldB at highfrequencies [6]. The results provide a high PldB of 25.33dBm.

40

tput

20-

Fig.7: Simulated input and output waveform of the single channel T/RSwitch acting as a transmitter.

Using the same input signal for the CMOS T/R Switch, theinsertion loss is 1.102 dB and 1.085 dB for receiver andtransmitter mode, respectively. The control voltage of 1.2 Vis chosen for optimum insertion loss and PldB, as well as forlong-term reliability [12]. The waveforms for each modeare shown in Fig.8 and Fig.9.

-40--40 40

Pin

Fig.10: Pin versus Pout.

; tp"t

-C3T --------- -r--------- --- -- -- ----r----- ----- -r--- --S- ----------I

Fig.8: Simulated input and output waveform of the CMOS T/R Switchacting as a receiver.

Table 1 compares the performance of the single channelT/R Switch with the proposed CMOS T/R Switch. Using theCMOS T/R Switch, the insertion loss in both the receiver andtransmitter modes reduces greatly to only 1.102 dB and1.085 dB respectively. Single channel T/R Switch exhibits ahigher loss of 1.881 dB and 1.872 dB for receiver mode andtransmitter mode respectively. In addition, the controlvoltages at the CMOS transistor's gates are operational atonly 1.2 V, making it favorable to implement in portabledevices. Single channel T/R Switch uses a higher controlvoltage of 5.0 V.

Feb. 17-20, 2008 ICACT 2008ISBN 978-89-5519-136-3 -449-

Table 1: Single Channel T/R Switch VS CMOS T/R Switch

Single Channel T/R CMOS T/R SwitchSwitch

Insertion Loss 1.881 dB 1.102 dB(receiver mode)

InsertionLoss 1.872dB 1.085 dB(transmitter mode)

Control Voltage 5.0 V 1.2 V

6. Conclusion

A 2.4 GHz CMOS T/R Switch was designed with lowinsertion loss, low control voltage, and high PldB for radiotransceiver compatible with wireless 802.11 systems. Theswitch exhibits insertion loss of 1.102 dB and 1.085 dB forreceiver and transmitter modes, respectively. In addition, thecontrol voltage uses only 1.2 V to switch between differentmodes and it also exhibits a high PldB of 25.33 dBm. Fromthis design, it can demonstrate that T/R switches at radiofrequency can be realized with CMOS for betterperformance. Furthermore, the switch can be integrated intoMultiple Input and Multiple Output (MIMO) systems thatoperate at 2.4 GHz and 5.0 GHz frequencies.

REFERENCES

[1] Chang-Ho Lee, Bhaskar Banerjee, and Joy Laskar, "Novel T/RSwitch Architectures for MIMO applications", IEEE, 2004.

[2] Skyworks Solutions Inc., Application note, APN1002, "Design withPIN Diodes."

[3] D. Kelly, "Integrating Next Gen CMOS Designs in GSM Front-Ends," Wireless Design & Development, pp. 18-22, Sept. 2004.

[4] A. Galhardo, J. Goes, and N. Paulino, "Novel LinearizationTechnique for Low-Distortion High-Swing CMOS Switches withImproved Reliability", IEEE, 2006.

[5] Robert H. Caverly and Gerald Hiller, "A Silicon CMOS MonolithicRF and Microwave Switching Element".

[6] Jonghoo Park and Zhenqiang Ma, "A 15 GHz CMOS RF SwitchEmploying Large-Signal Impedance Matching", IEEE, 2006.

[7] Niranjan Talwalkar, C Patrick Yue, and S. Simon Wong,"Integrated CMOS Transmit-Receive Switch Using LC-TunedSubstrate Bias for 2.4-GHz and 5.2-GHz Applications.

[8] Niranjan Talwalkar, C. Patrick Yue, and S. Simon Wong, "AnIntegrated 5.2GHz CMOS T/R Switch with LC-tuned SubstrateBias", Stanford University, Stanford, CA, IEEE, 2003.

[9] J.P. Carmo, P.M. Mendes, C. Couto, and J.H. Coreia, "A 2.4-GHzRF CMOS transceiver for wiles sensor applications".

[10] D. Su, M. Zargari, P. Yue, S. Rabii, D. Weber, B. Kaczynski, S.Mehta, K. Singh, S. Mendis, and B. Wooley, "A 5 GHz CMOStransceiver for IEEE 802.11a wireless LAN," in IEEE Int. Solid-State Circuit Conf. Dig. Tech. Papers, Feb. 2002, pp.92-93.

[11] Feng-Juan Huang and Kenneth K. 0, "Single-Pole Double-ThrowCMOS Switches for 900-MHz and 2.4-GHz Applications on p-Silicon Substrates", IEEE, 2004.

[12] Feng-Juan Huang and Kenneth K. 0, "A 0.5-pm T/R switch for900-MHz wireless applications," IEEE J. Solid-State Circuits, vol.36. Pp. 486-492, Mar. 2001.

[13] Joseph J. Carr, "RF Circuit Design", McGraw Hill, 2001.

Feb. 17-20, 2008 ICACT 2008ISBN 978-89-5519-136-3 -450-