A 5-GHz frequency-doubling quadrature modulator with a ring-type local oscillator

Abstract

A 5-GHz direct-conversion quadrature modulator has been developed using a frequency-doubling circuit technique to integrate a local oscillator on a single silicon chip. A three-level mixer, newly designed with symmetrical topology for two local oscillator inputs, is used to apply the frequency-doubling technique, so low image component levels of the modulated signals are achieved. The modulator is implemented on a single chip using Si-bipolar process technology with a cutoff frequency of 40-GHz. The image ratio is less than -34 dBc at a 5-GHz carrier frequency. I NT RO DUCT IONThe demand for low-cost wireless LANs with high transmission rates has grown over the past few years. A 5-GHz wireless LAN system built with inexpensive silicon chips would meet this demand, and several system components, including an up/down converter [l], have already been developed. However, the frequency of the developed oscillators on silicon is 2.4-GHz at most [2]. Such systems require a highfrequency local oscillator for carrier frequency generation. However, as the local oscillation frequency becomes higher, the oscillation phase noise and the error rate increases. Thus, it is difficult to implement an oscillator as a monolithic modulator on a single silicon chip. These problems could be overcome with a monolithic quadrature modulator whose local oscillation frequency can be reduced without reducing the carrier frequency.We propose a frequency-doubling modulator with a four-stage ring-type local oscillator and a novel "threelevel mixer (multiplier)". Since the local oscillation frequency is half that of the carrier frequency, the modulator is expected to have small phase noise. The local oscillator supplies four local oscillator signals with different phases to double the local oscillation frequency and to generate quadrature carriers. On the other hand, since the mixer has symmetrical topology for two local oscillator inputs, the modulated signals have low image component levels. CIRCUIT DESIGNA frequency-doubling quadrature modulator requires accurate four-phase local oscillator signals and a three-level mixer with symmetrical topology for multiplication of the signals. Since a four-stage ringtype oscillator generates the signals accurately and is suitable for integration on a silicon chip, it is used for the modulator. The mixer with symmetrical topology was newly designed and built. As shown in the block diagram in Fig. 1, the modulator consists of a fourstage ring-type local oscillator, four buffers, two threelevel mixers, an analog adder and an output buffer with a 50R load. The oscillator generates four phase local oscillator signals, namely LO-0, , whose phases are 0, 45, 90, and 135 degrees, respectively. The signals are supplied to the two mixers through the buffers. The I-channel mixer multiplies LO-0 and LO-90 to generate a carrier with a frequency that is twice that of LO, and the carrier is modulated by a baseband signal. On the other hand, the mixer for the Q-channel mixes LO-45, LO-135 and a baseband signal. Since th...

A low-phase noise injection-locked quadrature voltage-controlled oscillator

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