8/8/2019 05560324

http://slidepdf.com/reader/full/05560324 1/2

A 78 dB dynamic range, 0.27 dB accuracy, single-stage RF-PGA using

thermometer-weighted and binary-weighted transconductors for SAW-lessWCDMA/LTE transmitters

Masakazu Mizokami, Yoshikazu Furuta, Takaya Maruyama and Hisayasu Sato

Renesas Technology Corp., Hyogo, Japan

Email:mizokami.masakazu@renesas.com

Abstract

A single-stage RF programmable gain amplifier (RF-PGA)in 65-nm CMOS is presented. The RF-PGA consists of thermometer-weighted transconductors and binary-weightedtransconductors with an R-2R ladder. The transmitter

prototype with the single-stage RF-PGA achieves 78 dBdynamic range, 0.27 dB accuracy in 1dB step at 1950 MHz.The measured transmitter noise in RX band is -160.4 dBc/Hz.The ACLR and EVM with LTE modulated signal (BW=20MHz) are -40 dBc and 3.4 %, respectively.Keywords: binary-weighted, thermometer-weighted, RF-PGA,

transmitter, WCDMA, LTE, SAW-less

Introduction

A low cost RFIC for multi-standard wireless systems withsmall PCB area has become a technical issue as the supporting

bands increase. A SAW-less WCDMA transmitter has beenintensively researched to meet this demand [1]-[3]. Withoutinter-stage TX SAW filter attenuation, a WCDMA/LTEtransmitter must output much lower RX band noise such as-160 dBc/Hz.

One of the RF key building blocks for the transmitter is aPGA where low noise property and wide dynamic range withhigh accuracy in gain control are required. In 3GPP standard[4], power control performance is regulated as 74 dB range, 1dB step with +/-0.5 dB step accuracy. The previous works usemultiple PGA stages [5] or multiple modulators [3]. Thefine/coarse gain control approach degrades gain controllinearity and monotonic gain controllability and results ininferior noise characteristics as the input swing of final PGA isrecursively decreased with the fine gain control. The multiplemodulator architecture with multiple mixers or baseband PGAsubstantially has carrier leakage problem at low gain becauseeach modulator has different device mismatch.In this work, a single-stage RF-PGA is developed in order to

meet low noise performance and monotonic gain

controllability. The proposed RF-PGA utilizesthermometer-weighted and binary-weighted transconductorsfor PGA control, which allows a wide dynamic range with highresolution in gain control.

Concept of RF-PGA operation

Taking gain range of HPA and some margins to compensate process and temperature variation into account, a PGA in RFICis required around 75 dB gain control range. The RFIC gaincontrol step is set to be 0.25 dB to ensure +/-0.5 dB gain stepaccuracy.

Figure 1 shows the concept of the proposed RF-PGA for 78dB range and 0.25 dB-step gain control. The RF-PGA employseighteen binary-weighted “unit” amplifiers which are digitallyactivated by the control bits, b<18:1>.

The thirteen binary-weighted amplifiers manage 78 dBdynamic range with 6 dB gain step, and the five

binary-weighted amplifiers control 5.75 dB range with 0.25 dBgain step. The detailed PGA control scheme is as the follows.Suppose the PGA gain changes from -72 dB to -66.25 dB, thesix control bits, b<6:1> are selected. The MSB bit, b<6> is setto be 1, and the five LSB bits, b<5:1> are controlled to be 0 or 1 to realize a logarithmic gain control by approximately 0.25dB, such as b<6:1>=(1,0,0,0,0,0) for -72 dB,

b<6:1>=(1,0,0,0,0,1) for -71.73 dB,…, b<6:1>=(1,0,0,1,1,0)for -70.51 dB,…, b<6:1>=(1,1,1,1,1,0) for -66.26 dB, asshown in Fig. 1. For next 6 dB gain range, the control bits areshifted by 1 bit higher and the same control manner is adopted.Such gain control scheme realizes 78 dB range, 0.25 dB-step,PGA function. Since the unit amplifiers are used repeatedlyover a range of 78 dB, this architecture is favorable to obtain

precise and monotonic gain controllability.

Fig. 1 Concept of 78 dB-gain range, 0.25 dB-step gain control

Circuit implementation of RF-PGA

Figure 2 illustrates the block diagram of the fabricatedsingle-stage RF-PGA. To address the proposed 78 dB dynamicrange and 0.25 dB step gain control scheme, the eighteen

binary-weighted “unit” amplifiers are split into the fourteenLSB amplifiers and the four MSB amplifiers. The fourteenLSB amplifiers consist of the R-2R weighted transconductorsin order to realize fourteen binary-weighted transconductors.On the other hand, the four MSB amplifiers are decoded to thefifteen thermometer-weighted transconductors in order toensure the maximum output power of 3 dBm as well as todecrease the current consumption depending on the output

power.

978-1-4244-7641-1/10/$26.00 ©2010 IEEE 2010 Symposium on VLSI Circuits/Technical Digest of Technical Papers 13

8/8/2019 05560324

http://slidepdf.com/reader/full/05560324 2/2

Fig. 2 78 dB range, 0.25 dB step single-stage RF-PGA

Fig. 3 Unit transconductor

The unit transconductor in Fig. 2 is illustrated in Fig. 3. Thetransconductor uses the simple cascode-type amplifier, wherethe input transistors, M1 and M2 use 1.2-V transistors to obtainhigh input/output isolation and the cascaded transistors, M3and M4 use 2.8-V transistors to avoid reliability problem.

Measurement results

The RF-PGA was fabricated in 65-nm CMOS process. Figure4 shows a die micrograph of three RF-PGAs for multiple bandsupports and the other transmitter blocks which consists of anupconverter, TX-PLL and LO buffer. The size of the RF-PGAis 0.23 mm 2. Figure 5 shows the measured result of output

power and 1dB step gain control characteristics at 1950 MHz.It confirms that the RF-PGA produces good monotoniccontrollability with the introduced PGA concept. The RF-PGAachieved 78 dB dynamic range. 1 dB step accuracy wassatisfied with 0.27 dB in 74.6 dB range. Table 1 showsmeasurement summary. A transmitter noise of -160.4 dBc/Hzat 190 MHz offset was obtained under 0 dBm output power.The low transmitter noise indicates that the transmitter with the

proposed RF-PGA is allowed to eliminate SAW filters intransmit system. Current consumption at 0 dBm output was36.8 mA and that at -24 dBm output was 12.9 mA from 2.8 Vsupply. OP1dB was 8 dBm. Figure 6 shows measured resultsof ACLR and EVM when the transmitter modulates LTEsignal of 20 MHz bandwidth. ACLR and EVM were -40 dBcand 3.4 %, respectively.

References

[1] T. Sowlati, et al . “Single-Chip Multiband WCDMA/HSDPA/HSUPA/EGPRS Transceiver with DiversityReceiver and 3G DigRF Interface without SAW filters inTransmitter / 3G Receiver Paths,” ISSCC, Dig. Tech. Papers,

pp. 116-118, Dec. 2009.[2] X. He, et al . “A 45nm Low-Power SAW-less WCDMA

Transmit Modulator Using Direct Quadrature VoltageModulation,” ISSCC, Dig. Tech. Papers, pp. 120-122, Dec.2009.

[3] C. Jones, et al. “Direct-Conversion WCDMA Transmitter with -163dBc/Hz Noise at 190MHz Offset,” ISSCC, Dig.Tech. Papers, pp. 336-338, Dec. 2007.

[4] 3GPP TS 36.101 V8.5.1, “User Equipment (UE) radiotransmission and reception (FDD)”

[5] T. W. Kim et al. “Low Power 60 dB Gain Range with 0.25 dBResolution CMOS RF Programmable Gain Amplifier for Dual-band DAB/T-DMB Tuner IC,” ASSCC, pp.133-136,

Nov. 2005.

Fig. 4 Die micrograph of the transmitter

Fig. 5 Measured result of 1dB-step gain control

Table 1 Measurement summary of the transmitter Item Unit This work

Max. output power dBm 3.4OP1dB dBm 8.0

Gain dynamic range dB 78Gain step dB 0.25

1dB step accuracy dB 0.27Output noise at 190MHz offset dBc/Hz -160.4

Current at 0 dBm output mA 36.8Current at -24 dBm output mA 12.9

ACLR@BW=20MHz dBc -40.0EVM@BW=20MHz %rms 3.4

Fig. 6 Measured results of ACLR and EVM

978-1-4244-7641-1/10/$26.00 ©2010 IEEE 2010 Symposium on VLSI Circuits/Technical Digest of Technical Papers 13