A 5-GHz LC VCO with digital AAC and AFBS for 2.4 GHz ZigBee transceiver applications

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Abstract

A 5 GHz LC VCO (voltage-controlled oscillator) with automatic amplitude control (AAC) and automatic frequency-band selection (AFBS) for 2.4 GHz ZigBee transceivers is presented. Instead of continuous feedback loop, an alternative amplitude calibration scheme is proposed in this paper to alleviate the deficiencies inherent in the conventional approach. It helps to keep the VCO at optimum amplitude to avoid saturation of the cross-coupled transistors and therefore stabilizes the phase noise performance over process, voltage and temperature variations. For the ZigBee application with 16 frequency channels, a coarse tuning loop is added in this work to implement the frequency-band selection using the AFBS mechanism. The VCO core and the digital AAC, AFBS modules have been fully integrated in a 2.4 GHz ZigBee transceiver which was fabricated in a 0.18 μm RF-CMOS technology. The current consumption is 4.7 mA at 4.85 GHz with 1.8 V power supply and a chip area of about 0.285 mm2 is occupied. The VCO is capable of operating from 4.67 GHz to 5.18 GHz and the measured phase-noise level is –120 dBc/Hz at 1 MHz offset from a 4.85 GHz carrier. The tuning sensitivity KVCO of the VCO is about 78 MHz/V with 0.9 V control voltage.

Introduction

The ZigBee/IEEE802.15.4 which is an industry standard for low-cost, low-power and low-rate wireless applications has been studied widely and deeply for several years. It assigns three unlicensed industrial, scientific and medical (ISM) operation frequency bands: 815 MHz, 915 MHz and 2.4 GHz respectively [1], [2], [3], [4]. Especially, 2.4 GHz is the most attractive band since it is available throughout the world. The transceiver for 2.4 GHz ZigBee generally adopts Low-IF receiver scheme to avoid sensitivity reduction due to flicker noise and direct-conversion scheme for transmitter to save chip area and power consumption. As an indispensable and key building block, the frequency synthesizer provides local oscillator (LO) signals to both the receiver and transmitter paths for mixing with the carrier. The VCO (Voltage-controlled oscillator) is the most critical block in the synthesizer and operates at the highest frequency which is about twice of the channel frequency in the transceiver. In terms of phase noise and tuning range, the VCO performance determines basic performance characteristics of a transceiver [5], [6], [7].

For the popular current-biased LC-VCO, the oscillation amplitude varies over its tuning range and cause detrimental variations in the phase noise performance over frequency. Meanwhile, steady-state oscillation amplitude set the amplitude to a predefined level so the operation of the frequency dividers in the PLL (Phase-Locked Loop) is not compromised. The amplitude variation also has a significant impact on neighboring system blocks, such as a mixer where the conversion gain would vary if the VCO amplitude changes widely, or a prescaler that interfaces to the VCO [5], [8], [9], [10]. It can be concluded that providing a way to control the oscillation amplitude is highly desirable. Conventional methods for controlling the amplitude of the VCO output are generally employing an analog AAC scheme where oscillation amplitude is controlled by a continuous-time feedback loop and have been successfully demonstrated in references [11], [12], [13]. However, the crucial and effective role in sustaining the oscillation amplitude comes at the cost of added complexity and a noise penalty due to the presence of additional noise contributors that feed back to the oscillator. This work proposes an alternative digital AAC scheme to adjust the oscillation amplitude of the VCO.

The PHY of the unlicensed 2.4 GHz band defines sixteen frequency channels ranging from 2.4 GHz to 2.4835 GHz with channel spacing 5 MHz [14], [15]. The RF (Radio Frequency) of channel k is given by:Fchannel,k=[2405+5(k11)]MHzwhere k=11,12,…,26. The base-band frequency of the ZigBee receiver is chosen as 2 MHz in this design, and therefore, the LO (local oscillator) frequency provided for the receiver should be 2 MHz lower than the channel center frequency for a low-IF scheme. E.g. if the channel number is 11, the channel center frequency Fchannel,k is 2.405 GHz, and then, a 2.403 GHz LO is required to realize the 2 MHz intermediate frequency(IF). Since the VCO operates at twice the carrier frequency to avoid the leakage of the oscillator, 4.806 GHz should be generated by the VCO. The LO of channel k can be given by:FLO,k=[4806+10(k11)]MHz

For the sixteen oscillation frequencies ranging from 4.806 GHz to 4.956 GHz, it is wisdom and necessary to provide an automatic frequency band selection (AFBS) scheme to the multi-bands VCO targeting for the ZigBee applications.

Section snippets

VCO with digital AAC

Although ring- or relaxation-type oscillators can be found in some applications, their poor phase noise performance, larger power consumption and chip area compared to the differential LC (Inductor and Capacitor) VCOs disqualify them in most RF applications. In the topologies of LC-VCOs, the complementary cross-coupled VCO has better noise performance than NMOS-only cross-coupled VCO [5], [8]. Conventional analog AAC in LC-VCOs adopts an analog negative feedback loop to tune the tail current of

AFBS mechanism

Conventional integer-N PLL consists of a Phase Frequency Detector (PFD), a Charge Pump (CP), a Loop Filter (LPF), a VCO (Voltage Control Oscillator), a divide-by-2 and an integer-N divider as shown in Fig. 6. The PFD compares the reference frequency fref generated by dividing the crystal oscillation (XOSC) frequency against the divided frequency fdiv. The CP receives the digital pulse output of PFD and converts it into analog current. The LPF which introduces zeros and poles to stabilize the

Experimental results

The proposed LC-VCO and the digital AAC, AFBS modules have been fully integrated in a 2.4 GHz ZigBee transceiver which was fabricated in a 0.18 μm RF-CMOS technology using a single poly layer, four metal layers and metal-insulator-metal (MIM) capacitors. The die microphotograph is shown in Fig. 9. The VCO core occupies an area of 0.7 mm×0.4 mm and the digital AAC, AFBS modules occupy an area of about 0.12mm×0.04 mm. The power consumption of the VCO is 4.7 mA at 4.85 GHz from a 1.8 V power supply. With

Conclusions

In this paper, a fully integrated differential LC voltage-controlled oscillator with digital AAC and AFBS, tunable within a 4.67–5.18 GHz frequency range is presented. The amplitude calibration technique is used to stabilize performance across the wide band of operation. The AAC scheme proposed in this work consumes negligible power and area without degrading the performance of the VCO. For the ZigBee application with 16 frequency channel, a digital AFBS is adopted to realize the coarse tuning

Acknowledgment

The authors acknowledge Jin-zhi Wu, Chao Fan for the layout support. Also, we thank Zhan Jing, Fu-chuang Shen and Fang-juan Wang for some block designs and the chip test. This project is supported by the Fundamental Research Fund for the Central Universities of China and the Technology and Engineering (Product group) Project of Shaanxi Province, China (Grant no. 2014KTCQ01-06).

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