A 3.0 V Front-End GaAs Receiver MMIC for the CDMA/AMPS Dual Mode Cellular Phones

Transcription

1 Journal of the Korean Physical Society, Vol. 33, No., November 1998, pp. S333 S337 A 3.0 V Front-End GaAs Receiver MMIC for the CDMA/AMPS Dual Mode Cellular Phones Kwang-Jun Youn, Jae-Kyoung Mun, Min-Gun Kim, Eung-Ki Oh, Chang-Seok Lee, Jae-Jin Lee, Kwang-Eui Pyun and Hyung-Moo Park Semiconductor Technology Divisionm, Electronics and Telecommunications Research Institute, Taejon The front-end GaAs receiver MMIC has been developed using ETRI GaAs MMIC foundry for CDMA/AMPS dual mode cellular phones. It consists of gain controllable LNA, double balanced resistive ring mixer, CDMA/AMPS mode selective IF amplifier, and LO buffered amplifier. For successful communication with high quality in any weak signal areas, we have designed to ensure high linearity and low noise figure characteristics using the negative feedback gain control technique of LNA. At 3.0 V supply voltage and frequency range of MHz, developed Rx MMIC showed a low NF of 2.0 db and high IIP 3 of 7.5 dbm, gain of 25/20 db at CDMA/AMPS mode. We can improve IIP 3 by 7dB using gain control of LNA and achieved low current consumption of 39 ma/34 ma in the CDMA/AMPS mode. It satisfied the IS-95 specification which is the standard for CDMA systems and showed excellent talk quality in the commercial dual mode cellular phones. I. INTRODUCTION As the number of cellular phone subscibers are increasing rapidly, the digital cellular phone using CDMA access technology has been widly used to increase the number of users in the limited frequency bands. The CDMA technology was known to have the capacity of six times that of analog cellular system by allowing the use of the same frequency in every cell [1]. As a trend, the CDMA/AMPS dual mode phone has been used as a transition phase from analog to digital system and supply voltage is decreasing to reduce the weight of hand-held phones [2,3]. The receiver MMIC is critical component in the RF part to convert weak RF signal received through antenna to IF signal with low noise amplification. It consists of low noise amplifier, frequency conversion mixer with local oscillator (LO) buffered amplifier, and intermediate frequency (IF) amplifier. There have been several reports on the Rx MMIC with low noise figure and low power dissipation operating in the CDMA/AMPS dual mode phone [4,5]. The critical issues in the analog system were the reduction of noise figure and dissipation power. Besides theses issues, high linearity is an important feature in the CDMA system due to the interference with adjacent channels. As many users are allocated at the same frequency and the same time, there were interferences and channel background noise caused by adjacent channels. The worst case that occurred was the detection of weak signal in the presence of adjacent channels with Tel: ; Fax: stronger signals. The front-end receiver IC should have high linearity and low noise figure for successful talk in any signal condition. Linearity can be expressed by input 3-rd order intercept point (IIP 3 ). Several techniques have been known to improve linearity such as back-off from saturation power, feedback, predistortion, feedforward, etc. Back-off technique has the drawback of low efficiency and predistortion and feedforward are difficult to adopt to hand-held phone due to their complex structure. Therefore, feedback is suitable to apply in LNA due to its simple structure. We improved the IIP 3 by controlling gain maintaining constant output 1 db compression power (P 1 db ). We achieved low noise figure at high gain mode and high linearity at low gain mode. In this work, we analyzed the relation between linearity and gain according to gain control of LNA. This paper reports the test results for CDMA standard (IS-95), which is the standard for cellular systems using CDMA mobile station test set. II. CIRCUIT DESIGN Fig. 1 shows a schematic circuit of Rx MMIC. The Rx MMIC consists of the gain controllable LNA, double balanced resistive ring FET mixer and LO buffer amplifier with TxLO signals, and CDMA/AMPS mode selective IF amplifier. A received RF signal with the frequency band of MHz is amplified in LNA and downconverted to IF frequency with 85 MHz using LO signal with MHz in mixer, and then amplified to CDMA or -S333-

2 -S334- Journal of the Korean Physical Society, Vol. 33, No., November 1998 Fig. 1. A schematic circuit of Rx MMIC. AMPS mode in IFA according to mode selection signal. The Rx MMIC has to have low noise figure less than 3 db and high IIP 3 greater than 10 dbm to operate correctly for low input signal of 105 dbm and high input signal of 25 dbm. An off-chip BPF is used to reject image signals. Applying 3.0 V (CDMA) and 0 V (AMPS) determine the mode in the IFA as a control bias. We used ETRI GaAs MMIC library which consists of a 0.5 um gate-length ion implanted GaAs MESFET, MIM capacitor, spiral inductor, and thin film resistor to design Rx MMIC. Sensitivity analysis and design centering were performed to test circuit topology and to enhance chip yield considering process variation. 1. Gain Controllable LNA Noise figure of LNA is very important because noise figure of front-end receiver is almost determined by the LNA. Also, high IIP 3 is required to improve dynamic range at high input signal. IIP 3 can be represented as follows, OIP 3 = Pout + IMD 3 /2 (1) IIP 3 = OIP 3 Gain (2) Fig. 2. Linearity (IIP 3) variation of LNA according to gain control voltage. Fig. 3. Microphotograph of the fabricated Rx MMIC. IMD 3 represents the third order intermodulation distortion. As we can see in Eq. (1), IIP 3 is directly related to gain and it can be improved by gain control. LNA was designed to have maximum gain at weak input signal for low noise figure and minimum gain at strong input signal for high IIP 3. LNA consist of a cascaded three stage LNA to meet 23.5 db maximum gain. The gate widths of the first and second, and third-stage are 200 um, 400 um, and 600 um, respectively. Gate of the first-stage FET is matched by one off-chip inductor to have a low noise figure and inductive series feedback at the source was used to provide simultaneous noise and gain matching. The first-stage FET is operated at 20 % Idss for low noise operation and the second and third-stage FETs are operated at 50 % Idss for high gain and linearity. Feedback circuits consisted by common drain FET and two series FET were used at the second and third-stage for gain control and stability. The gain is also controlled by gate controlled FET resistor at the interstage between the second and third stage for small VSWR change during gain control. The gain is controlled by 14 db by adjusting control voltage from 0V to 3 V. Fig. 2 shows the variation of gain and IIP 3 of LNA according to gain control voltage. When control voltage varies from 0V to 3.0 V, the gain varies from 23.5 db to 9.5 db and IIP 3 increases from 14 dbm to 7 dbm. The noise figure is

3 A 3.0 V Front-End GaAs Receiver MMIC for the CDMA/AMPS Dual Mode... - Kwang-Jun Youn et al. -S335- IF amplifier was designed using differential amplifier to amplify downconverted IF signal with CDMA/AMPS dual mode output. CDMA mode has dual complementary output and AMPS mode has single output. Output mode is selected using mode selection circuit. CDMA mode is selected by Vmode=3 V and AMPS mode is selected by Vmode=0 V. External reactive loads as a CDMA/AMPS output load are used to enhance gain. Output impedances of CDMA/AMPS mode are 1000 ohm and 850 ohm, respectively. The gains of the IFA in the CDMA/AMPS mode are 22 db/17 db and currents are 11 ma/6 ma, respectively. Fig. 4. Frequency dependence of measured conversion gain and NF. 4. LO Buffer Amplifier LO buffer amplifier provides amplified LO signal to the Rx and Tx mixer for pumping of mixer circuit. The input stage receiving LO input signal is actively matched using common drain structure and divided to complementary signal through active balun. Buffered LO output for Rx mixer is always provided but it for Tx mixer can be disabled to reduce battery consumption when upconverter doesn t operate. The output power of buffered LO output is 5 dbm. III. FABRICATION Fig. 5. Gain control and output power characteristics of the Rx MMIC in the CDMA mode 1.8 db at the frequency band of MHz. 2. Mixer For low intermodulation characteristics, double balanced resistive ring FET mixer was designed. All four corners of the ring FETs were virtual grounds for the LO signal and the IF connection points are virtual ground for the RF signal. The RF, LO, and IF signals were inherently isolated due to these virtual grounds. Careful design was performed to balance RF and LO signals to achieve low intermodulation. RF port was actively matched to 50 ohm and active balun was used to provide complementary signal connected to ring FETs. The drain and source of ring FET were biased to the same bias and the gate of ring FETs were biased near pinch voltage to increase conversion gain using voltage divider circuit. 3. IF Amplifier The designed receiver MMIC was fabricated using GaAs MMIC foundry composed by GaAs MESFET with 0.5 um gate length, MIM capacitor, spiral inductor, and NiCr/ion implanted resistors. A MESFET channel was formed by rapid thermal annealing for 30 seconds at 900 C after Si + ion implantation with a dose of cm 2 on undoped 3 semi-insulating GaAs substrates. The sheet capacitance of MIM capacitor was pf/µm 2 and the sheet resistance of NiCr and ion implanted resistor were 20 Ω/sq. and 1000 Ω/sq., respectively. Total 12 masks were used to fabricate receiver MMIC and it s size is 1.5 mm 2.5 mm. The MMIC chip was packaged in the plastic 24-pin SSOP to cut down packaging cost. Microphotograph of the fabricated MMIC chip is shown in Fig. 3. IV. PERFORMANCE EVALUATION Packaged MMIC chip was mounted on RF evaluation board designed to test noise and RF characteristics. Frequency dependence of measured conversion gain and noise figure at maximum gain are shown in Fig. 4. Rx frequency range is MHz and LO frequency range is MHz for IF frequency of 85 MHz. At CDMA mode, it shows noise figure of 2.0 db, conversion gain of 25 db, and current consumption of 39 ma.

4 -S336- Journal of the Korean Physical Society, Vol. 33, No., November 1998 Table 1. Measured characteristics of Rx MMIC. Overall Characteristics Items Measured Results Measurement condition Cascaded Performance (CDMA Mode, Vmode=3.0 V) Conversion Gain Max db - Supply Voltage = Min db 3.0 V Noise Gain 2.0 db - RF Frequency= IIP 3@Min.Gain 7.5 dbm MHz 39 ma - LO Frequency = Cascaded Performance (AMPS Mode, Vmode=0 V) MHz Conversion Gain Max db - IF Frequency=85 MHz Min. 6.0 db Noise Figure@Max.Gain 2.0 db 34 ma Individual Characteristics Measured Results Measurement condition Max db (Vcont=0 V) - RF Frequency= Gain Min. 9.5 db (Vcont=3 V) MHz LNA NF 1.8 db 8.5 ma Mixer/IFA Conversion Gain 1.5 db -Vmode=3 V CDMA mode 16.0 ma Mixer/IFA Conversion Gain 3.5 db -Vmode=0V AMPS mode 11.0 ma Output Power 5 dbm - 5 dbm input LO to RF Rej. 25 LO buffer LO to IF Rej. 25 Amp VSWRi,o < 2:1 -single ended 14.5 ma At AMPS mode, it showed noise figure of 2.0 db, conversion gain of 20 db, and current consumption of 34 ma. Fig. 5 shows gain control and output power characteristics of packaged MMIC chip in the CDMA mode. The gain decreased from its maximum value of 25 db at Vcont=0 V as control voltage increases and its range of gain control was 14 db for the control voltage of 0 3 V. Output power increases linearly as input power increases and its input 1 db compression power was 17.5 dbm and IIP 3 is 7.5 dbm in the CDMA/AMPS mode. Packaged MMIC chip passed the test for IS-95 specification using CDMA mobile station test set. It was also loaded on the commercial CDMA/AMPS dual mode cellular phone for talk test and showed excellent call quality. Typical RF characteristics are summarized in Table 1. V. CONCLUSIONS A GaAs receiver MMIC with low noise figure and high linearity was developed for the CDMA/AMPS dual mode handphone using ETRI GaAs MMIC library. Noise figure was improved by GaAs MESFET with excellent noise characteristics and series feedback inductor connected to source of MESFET for noise and gain matching, simultaneously. Also linearity was improved by gain control using negative feedback structure from drain to gate of MESFET. Developed MMIC was packaged in the plastic SSOP-24 package to cut down packaging cost and tested using evaluation board for the measurement of noise figure and RF characteristics. At 3.0 V supply voltage, the developed MMIC showed noise figure of 2.0 db, conversion gain of 25 db in the CDMA mode and noise figure of 2.0 db, conversion gain of 20 db in the AMPS mode. Also it showed a high IIP 3 of 7.5 dbm. consumptions were 39 ma in the CDMA mode and 34 ma in the AMPS mode. These data are regarded as a superior result compared to any other published product. Packaged chip passed the IS-95 measurement using CDMA tester and showed excellent talk quality in the commercial CDMA/AMPS dual mode cellular phone. REFERENCES [1] Tia/EIA/IS-95 Interim Standard, Mobile Station-Base Station Compatibility Standards for Dual-Mode Wideband Spread Spectrum Cellular System, July (1993). [2] C. H. Kim, M. G. Kim, I. G. Hwang, C. S. Lee, J. L. Lee, E. G. Oh, J. W. Yang, C. S. Park, K. S. Yoon, K. E. Pyun and H. M. Park, A 3.3 V Front-end Receiver GaAs MMIC for the Digital/Analog Dual-mode Hand-held Phones, IEEE GaAs IC Symposium Technical Digest, October, 55 (1995).

5 A 3.0 V Front-End GaAs Receiver MMIC for the CDMA/AMPS Dual Mode... - Kwang-Jun Youn et al. -S337- [3] V. Nair, R. Vaitkus, D. Scheitlin, J. Kline and H. Swanson, Low GaAs Integrated Down Converter for Portable Communication Applications, IEEE GaAs IC Symposium Technical Digest, October, 41 (1993). [4] Y. Imai et al., Design and Performance of Low GaAs MMIC s for L-Band Front-end Applications, IEEE Trans. Microwave Theory and Tech., MTT-39, Feb. 209 (1991). [5] RF Micro-Devices Co., CDMA/FM Low Noise Amplifier/Mixer, RFMD Data Sheet, Jan. (1997).