A CMOS LC-tank VCO and phase shifting network for direct conversion receiver of IEEE 802.11a

Mi-Young Lee*, Yong-Hun Kim*, Chan-Young Jeong*, Changsik Yoo*, and Jin-Su Park**

*Department of Electronics and Computer Engineering, Hanyang University, Seoul 133-791, Korea

*aphro95@ihanyang.ac.kr

**Samsung Advanced Institute of Technology (SAIT) and Samsung Electronics, Kiheung, Korea

Abstract – A CMOS LC-tank VCO and phase-shifting network are described for a direct-conversion receiver of IEEE 802.11a. The direct-conversion receiver is configured to have sub-harmonic mixing structure which requires multi-phase local oscillator (LO) signals spaced by 45°. A poly-phase filter with 3- and 4-stage RC networks generates multi-phase LO signals whose amplitude imbalance is removed by cascaded limiting amplifiers. The VCO and phase-shifting network were implemented with a 6-metal, 1-poly 0.18-µm CMOS technology. The measured results show the VCO can be tuned from 2.0GHz to 2.7GHz which is sufficient to cover the lower and mid-band of IEEE 802.11a with sub-harmonic mixing direct-conversion architecture. The phase noise of the VCO is -119.68dBc/Hz at 1MHz offset. The phase-shifting network provides multi-phase LO signals with less than 0.75° phase error. The total power consumption of VCO and phase-shifting network is 61mW under 1.8V supply voltage.

Keywords : LC-tank VCO, poly-phase filter, direct-conversion, sub-harmonic mixing, 802.11a, CMOS, wireless LAN

I. INTRDUCTION

Wireless local area network (WLAN) operating at unlicensed bands has been rapidly gaining its acceptance in the communication infrastructure both in home and office. While the maximum data rate of IEEE 802.11b operating at 2.4GHz band is 11Mbps, 802.11a and 802.11g operating at 5GHz and 2.4GHz band, respectively provides 54Mbps maximum data rate with orthogonal frequency division multiplexing (OFDM) technology [1]. Although both 2.4GHz and 5GHz bands are unlicensed ones, 5GHz band is relatively cleaner than 2.4GHz band which is crowded with various RF systems. Therefore, it is thought that 802.11a will have better chance to dominate the market.

The key requirement on a WLAN transceiver to be accepted in the market is low power consumption and low cost. Therefore, the research efforts are mainly towards a

fully integrated transceiver mostly in CMOS. Among various receiver architectures, direct-conversion structure provides most highly integrated solution with minimum number of external components, enabling a low-cost and low-power transceiver system [2-4].

This paper presents a CMOS LC-tank voltage controlled oscillator (VCO) and phase shifting network generating multi-phase local oscillator (LO) signals spaced by 45o for sub-harmonic mixing direct conversion receiver of IEEE 802.11a. The measured results of the LC-tank VCO and phase shifting network implemented in a 0.18µm CMOS technology are given.

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Fig. 1. Direct-conversion receiver for IEEE 802.11a. II. RECEIVER ARCHITECTURE The receiver is configured as shown in Fig. 1 among

which the current design includes an LC-tank VCO and phase-shifting network composed of poly-phase filter (PPF) and limiting amplifiers. The other building blocks

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are being designed and will soon be integrated in a single CMOS chip. For highest level of integration and thereby low-power and low-cost system, direct-conversion receiver architecture has been chosen.

Direct-conversion receiver, however, suffers from various practical problems such as DC-offset, I/Q mismatch, second-order distortion, LO leakage, and 1/f noise [5]. Among them, DC-offset is the most critical problem because the gain before the baseband chain is only about 20-30dB and thus a small DC-offset can easily over-ride the wanted signal. To eliminate the DC-offset due to LO leakage, sub-harmonic mixing architecture can be used with which the LO frequency is half of the RF frequency [6]. Another advantage of sub-harmonic mixing is the low-power consumption in frequency synthesizer due to the halved LO frequency.

For quadrature demodulation, the direct-conversion receiver shown in Fig. 1 with sub-harmonic mixer requires multi-phase LO signals spaced by 45o which are generated by a phase-shifting network composed of a PPF and amplitude limiting amplifiers which remove the amplitude imbalance in the outputs of PPF.

The low-noise amplifier (LNA) has two gain modes to alleviate the linearity requirements on the subsequent stages. The remaining DC-offset is cancelled by 7-bit digital-to-analog converters. The channel selection filtering is performed at baseband by active-RC filter. The programmable gain amplifier (PGA) provides input to analog-to-digital converter. The gain control code GAIN<0:7>, cut-off frequency setting code FCUT<0:7>, and DC-offset canceling code DCOFSTI/Q<0:6> are all from baseband modem [7].

III. LC-TANK VCO AND PHASE-

SHIFTING NETWORK Fig. 2 shows the circuit diagram of the CMOS LC-tank

VCO. Since the VCO is designed to generate LO signals for sub-harmonic mixing direct-conversion receiver of IEEE 802.11a, the oscillation frequency is set to the half of the RF frequency of 802.11a.

For minimum phase noise with a given power consumption (the bias current of the VCO core is 2.3mA), both nMOS and pMOS cross-coupled negative transconductance stages are employed. Due to the lack of differential inductor in the library we used, two single-ended spiral inductors are used instead to have balanced voltage swing on the differential outputs.

For wide frequency tuning range with smaller phase noise (smaller VCO gain), a two-bit binary weighted switched capacitor array is used. When the control signal VCN1 (VCN2) is high, the capacitor is connected to

ground and thus the effective tank capacitance gets larger. Otherwise, the capacitor is floating and the effective capacitance gets smaller because the on-resistance of pMOS transistor is set to be substantially high. The oscillation frequency is finely controlled by a MOS varactor diode.

The low frequency noise of the bias transistor such as thermal noise and more seriously 1/f noise can be up-converted by the switching action of the nMOS transistor pair. This up-converted noise can greatly degrade the phase noise performance of the VCO. In order to minimize this, the common-source node of the nMOS transconductance pair is AC grounded by a large capacitor (5pF). The up-converted noise component is bypassed to ground.

Fig. 2. CMOS LC-tank VCO The buffered output of the LC-tank VCO in Fig. 2 is

applied to the passive RC PPF shown in Fig. 3 for the generation of multi-phase LO signals spaced by 45o. For 45o, 135o, 225o, and 315o LO signals, three-stage PPFs were used while for 0o, 90o, 180o, and 270o, one more stage is used for additional 45o phase shift.

Although the phase characteristics of PPF are accurate over the whole bandwidth with only one RC network, multiple stage RC networks are used to minimize the difference in the magnitudes of the multi-phase LO signals

VDD

BIAS

Vtune

VCN2

VCN1

25u/0.18u

60u/0.18u

360u/1u 5pF

1.545nH

Cmax=1.2pF

800fF

400fF

180u/0.18u

0.24u/2u

25u/0.18u

60u/0.18u

1.545nH

Cmax=1.2pF

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over the bandwidth. The component values of the RC networks were chosen so the RC time constant of each stage correspond to the frequencies shown in the figure. These frequencies are chosen so the amplitude mismatch among multi-phase LO signals be minimized over the whole band. The remaining mismatch in the amplitudes is eliminated by 3-stage cascaded limiting amplifiers with resistive load.

Fig. 3. Poly-phase filter generating multi-phase LO signals.

IV. EXPERIMENTAL RESULTS The LC-tank VCO and phase-shifting network have

been implemented in a 6-metal, 1-poly, 0.18µm CMOS technology whose layout is shown in Fig. 4.

The frequency tuning characteristic of the VCO is in Fig. 5 which confirms the VCO can be used in sub-harmonic mixing direct-conversion receiver of 802.11a operating at lower and mid-band. The phase noise of the VCO is measured to be -119.68dBc/Hz at 1MHz offset as shown in Fig. 6.

Because the current design has no mixer included, the accuracy of the phase-shifting network cannot be measured accurately. Instead, the output waveform of the phase-shifting network is measured with oscilloscope as shown in Fig. 7. Although the estimated value must have

some error, the phase accuracy is measured to be better than 0.75o over the whole band.

Fig. 4. Layout of the VCO and phase-shifting network.

Fig. 5. Frequency tuning characteristics of the VCO. The measured amplitudes of external multi-phase LO

signals are different which is due to the unequal parasitics such as bond-wire inductance and path length mismatch in the test setup. The duty cycle of the measured waveform is distorted which may result in direct feedthrough of low-frequency noise if used in a direct-conversion mixer [5]. The duty cycle distortion was founded to be caused by the bonding wire inductance connecting internal differential pair type buffer to external load resistor. The HSPICE simulation including all layout parasitics has shown the amplitudes and duty cycle of the internal multi-phase LO signals are all equalized by limiting amplifiers over the whole band.

The total power consumption of the LC-tank VCO and phase-shifting network including limiting amplifier is 61mW under 1.8V supply voltage.

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Fig. 6. Phase noise of the VCO.

Fig. 7. Measured waveform of multi-phase LO signal.

V. CONCLUSION

A CMOS LC-tank VCO and phase-shifting network are described for a direct-conversion receiver of IEEE 802.11a. The direct-conversion receiver has sub-harmonic mixing structure to remove the DC-offset due to LO leakage which requires multi-phase LO signals spaced by 45o. A poly-phase filter passive RC networks generates multi-phase LO signals whose amplitude imbalance is removed by cascaded limiting amplifiers. The VCO and phase-shifting network were implemented with a 6-metal, 1-poly 0.18µm CMOS technology. The measured results show the VCO can be tuned from 2.0GHz to 2.7GHz which is sufficient to cover the lower and mid-band of 802.11a with sub-harmonic mixing direct-conversion architecture. The phase noise of the VCO is -119.68dBc/Hz at 1MHz offset. The phase-shifting network provides multi-phase LO signals with less than

0.75o phase error. The total power consumption is 61mW under 1.8V supply voltage.

ACKNOWLEDGEMENT

This work was supported by the Center for Advanced

Transceiver System (CATS), Seoul National University and Human Resource Development Project for IT SoC Key Architect funded by the Ministry of Information and Communication, Korea. The CAD tools were provided by IDEC.

REFERENCES

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[4] P. Zhang, T. Nguyen, C. Lam, D. Gambetta, T. Soorapanth, B. Cheng, S. Hart, I. Sever, T. Bourdi, A. Tham, and B. Razavi, “A 5GHz direct conversion CMOS transceiver,” IEEE J. Solid-State Circuits, vol. 38, pp. 2232-2238, Dec. 2003.

[5] B. Razavi, RF Microelectronics, Prentice-Hall [6] H. Yoshida, T. Kato, T. Toyoda, I. Seto, R. Fujimoto, T.

Kimura, O. Watanabe, T. Arai, T. Itakura, H. Tsurumi, “Fully differential direct conversion receiver for W-CDMA using an active harmonic mixer,” IEEE RF IC Symposium, Dig. Tech. Papers, pp. 395-398, 2003.

[7] C. Yoo, Technical report on CMOS direct-conversion receiver for wireless LAN, Center for Advanced Transceiver System, Aug. 2004.

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