Bluetooth Low Energy RF transceiver design simplifies board design

Technology News |
By Nick Flaherty

Renesas Electronics has developed two 2.4 GHz RF transceiver technologies to boost Bluetooth Low Energy (LE) designs.

Shown at ISSCC 2022 this week, the first is a matching circuit technology that covers a wide impedance range and enables the IC to match a variety of antenna and board impedances without an external impedance-matching circuit, reducing board size.

The second is a signal correction technology for locally generated reference signals that uses a small circuit to self correct inconsistencies in the circuit elements and variations in surrounding conditions without calibration.

These are also being extended to other RF standards.

Renesas developed an RF transceiver circuit prototype for Bluetooth LE in a 22nm CMOS process, reducing the circuit area including the power supply to 0.84 mm2, the world’s smallest for a device of this type. This was achieved by modifying the receiver architecture to reduce the number of inductors and making enhancements such as a low-current baseband amplifier with a small mounting area and a highly efficient class-D amplifier.

These have a power consumption of 3.6 mW and 4.1 mW during reception and transmission respectively and enable smaller size, reduced board cost, and lower power consumption, while simplifying the board design process.

Renesas developed a new variable impedance-matching circuit technology that consists of two inductors and four variable capacitors. The transmitter-side inductor and receiver-side inductor used in the matching circuit are configured in a concentric arrangement, and the mutual induction is employed to reduce signal loss and cut the effective parasitic capacitance. This both expands the variable impedance range and substantially shrinks the circuit area. A voltage standing wave ratio (VSWR), which indicates impedance mismatches, equivalent to maximum 6.8 and a variable impedance range of approximately 25 to 300 Ω have been confirmed.

For the self-correction circuit, a locally generated reference signal of roughly the same frequency as that of the wireless radio signals received via the antenna is generated internally by the RF transceiver. The signal is used to convert gigahertz-band wireless signals to low-frequency baseband signals.

The accuracy of the reference signal can be degraded by factors such as inconsistencies in the circuit elements or variations in the temperature or supply voltage. In the past, the compensation technology for phase and amplitude deviations with a calibration circuit was used to accurately generate the reference signal. This led to problems, however, because integrating such a calibration circuit required a larger chip area, higher power consumption, and increased test cost.

Renesas resolved these issues by developing a new self-IQ-phase correction circuit technology that uses reference signals of four different phases to correct each other by allowing the phase differences to cancel each other out. This self-correction circuit is much smaller and can be implemented at approximately one-twelfth the size of a conventional calibration circuit. The image signal rejection ratio, crucial to reception performance, averages 39 dB, which meets the Bluetooth standard with a comfortable margin to spare.

These technologies are applicable to different types of RF transceivers and Renesas says it is currently working on practical applications for these technologies.

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