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Continued growth in the Internet of Things requires high-speed communications in environments increasingly crowded with numerous wireless LAN devices. IEEE 802.11ax, the next-generation wireless LAN, will be four times faster than its predecessor, even in crowded environments.
However, high speed digital signal processing with low power dissipation requires adoption of a faster CMOS devices, operating with supply voltages lower than 1.0V. Unlike digital circuits, the performance of CMOS analog circuits, such as RF2 CMOS receivers, degrades drastically as voltage falls. New circuit technology that can operate below 1.0V is required to overcome this problem.
TDSC has developed three new technologies for low voltage RF receivers.
The first is a variable linearity RF amplifier (RFAMP). Since signal amplitude is affected by power supply voltage, 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, Toshiba’s new RFAMP simultaneously employs two input paths: a high linearity 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. A frequency converter switches a received RF signal into a low frequency signal to input into the analog to digital 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.
Toshiba’s new frequency converter 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. The OPAMP amplifies the converted low frequency signal to level a sufficient high to input the analog signal into the digital converter. Since the maximum output signal level is limited by the supply voltage, OPAMP is forced to reduce its operating voltage range. The new OPAMP suppresses the DC current source from the output stage with a high speed differential current mirror3, and provides larger output voltage even in a low voltage environment.
Integrating these three technologies into a 5GHz wireless LAN receiver has allowed TDSC to secure the level of performance essential for next-generation wireless LAN.
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