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Monday, September 18, 2017

Toshiba pushes 5GHz RF CMOS receiver under 1V

By Nick Flaherty at

Toshiba has developed a low voltage 5GHz receiver for IEEE802.11ax wireless LAN, targetting the Internet of Things.

802.11ax has initially been aimed at the enterprise, using the MIMO antenna structures to provide four times more performance. But the continued growth in the Internet of Things requires high-speed communications in environments increasingly crowded with numerous wireless LAN devices. 
This requires CMOS devices operating with supply voltages lower than 1.0V. Unlike digital circuits, the performance of the RF CMOS receivers, degrades drastically as voltage falls, and new circuit technology that can operate below 1.0V is required to overcome this problem.

Toshiba's research labs have developed three new technologies for low voltage RF receivers.
The first is a variable linearity RF amplifier (RFAMP). The signal amplitude is affected by power supply voltage so the linearity of RFAMPs decreases as voltage falls. Conventional RFAMP uses variable resistance to improve the linearity, but internal DC voltage and amplifier performance are also changed by adjusting the resistor value. In order to avoid these problems, the new RFAMP simultaneously employs two input paths: a high lnearity path and an internal voltage adjustment path. The RFAMP can adjust linearity without any change in the internal DC voltage.

The second is a low noise frequency converter. Conventional frequency converters use a DC current source to improve the performance of conversion switches in low voltage designs, but this additional current source degrades performance with low frequency noise. A new frequency converter design suppresses low frequency noise by moving the DC current source to the RF side of the conversion switches. Low frequency noise is up-converted to the RF frequency, which is separated from the desired low frequency signal. The result is sufficient switch performance without noise degradation.

The third is a current adder opamp. This amplifies the converted low frequency signal to level a sufficient high to input the analogue signal into the digital converter. As the maximum output signal level is limited by the supply voltage, the opamp is forced to reduce its operating voltage range. The new design suppresses the DC current source from the output stage with a high speed differential current mirror, and provides larger output voltage even in a low voltage environment. .

Integrating these three technologies into a 5GHz wireless LAN receiver has allowed the labs to reach the level of performance essential for next-generation wireless LAN.

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