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Ultrawideband (UWB) communications has been in use for military and various niche applications for more than 20 years.1 But in February 2002, the United States’ Federal Communications Commission (FCC) allocated a generous 7500 MHz of bandwidth from 3.1 to 10.6 GHz for unlicensed use of UWB devices, opening up a potentially large market. UWB technology can provide many benefits for commercial applications, including operation at low power and low cost at high data rates and with reduced interference. The low-voltage and low-power operation of UWB devices enable mobile wireless communications transceivers to be reduced in size and weight,2 although the use of low supply voltages presents challenges for some of the analog circuit portions of UWB designs.3 The frequency mixer, for example, is a critical part of UWB applications.4 It must translate incoming RF signals to an intermediate-frequency (IF) range that can be handled by data converters.5

Double-balanced Gilbert-cell mixers are among the more popular of active mixer types, owing to their low noise, high gain, high port-to-port isolation, and simplicity. These technology-independent mixers can be realized in a number of different semiconductor processes—among them, the use of silicon CMOS or bipolar processes. Still, designing low-voltage, low-power Gilbert-cell mixers at RF/microwave frequencies is challenging due to the stacking of transistors between supply and ground. Also, the mixer’s transistors typically operate in the saturation region which increases current dissipation. A new approach is clearly needed to create a low-power mixer suitable for UWB requirements.

Some low-voltage, low-power mixer topologies using CMOS technology have been proposed in recent years.6-11 A low-voltage topology using inductor-capacitor (LC) tanks was reported in ref. 6. This approach allows for low-power operation, but the inductor occupies a large area on the chip (1.5 x 1.1 mm2) and yields narrowband operation (2.4 GHz). In ref. 7, a modified Gilbert-type mixer achieved low power consumption using triode operation and shunt capacitors at the local-oscillator (LO) switching stage but, again, this was a narrowband circuit. In ref. 8, DC power consumption was held to 500 μW by operating the mixer’s CMOS transistors in the subthreshold region. But because the bias current is so low, the noise performance was poor, with a double-sideband (DSB) noise figure (NF) of 18.3 dB.

On-chip transformers can also be used to lower the supply voltage in mixers, and such an approach was adopted in ref. 9 with low DC power consumption. However, the mixer’s 3-dB bandwidth was relatively narrow (2.1 to 3.0 GHz) because of the bandwidth limitation of the transformer.

Mixers using a Gilbert folded topology or with a switched-transconductance topology10,11 are good choices for low-voltage applications. That said, the current consumption is generally twice that of a conventional Gilbert cell mixer, making the power consumption similar to that of a conventional Gilbert cell mixer. Current bleeding can improve the mixer noise figure, since less current will flow at the LO switching stage.12 Since parasitic capacitances are increased by a bleeding circuit, however, it indirectly generates more noise. To eliminate the effects of the parasitic capacitance, resonating inductors could be used, although this would increase chip size while decreasing bandwidth.13

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