RF transceivers modified for BLE with impedance tuning

Article By : Majeed Ahmad

Variable impedance matching and reference signal self-correction are the key highlights of RF transceivers remake for Bluetooth LE.

Two new 2.4 GHz RF transceivers for Bluetooth Low Energy (LE) unveiled by Renesas Electronics claim to be the world’s smallest for a device of this type with a circuit area of 0.84 mmincluding the power supply. The company credits this to the modification of the receiver architecture to reduce the number of inductors. Then there are enhancements like a low-current baseband amplifier with a small mounting area and a highly efficient class-D amplifier.

That allows the Bluetooth LE RF transceiver circuit built on a 22-nm CMOS process to offer energy efficiency with power usage of 3.6 mW and 4.1 mW during reception and transmission, respectively. Besides lower power consumption and compact form factor, the RF transceivers reduce board cost and simplify the board design process.

Source: Renesas

To accomplish these capabilities, Renesas has incorporated two technology enhancements in its RF transceivers targeted at Bluetooth LE designs. First, a matching-circuit technology covers a wide impedance range and enables the transceiver IC to match a variety of antenna and board impedances without an external impedance-matching circuit.

Second, a signal-correction technology for locally generated reference signals uses a small circuit to self correct inconsistencies in the circuit elements and variations in surrounding conditions without calibration. Below is a more detailed treatment of these technology enhancements for RF transceivers serving BLE applications.

Variable impedance matching

Currently, in a BLE design, depending on the type of antenna or board design considerations, the impedance doesn’t necessarily reach 50 Ω. So, an external matching circuit is still needed for switching between reception and transmission or impedance matching. Moreover, when using the current technology and adding a matching circuit with impedance-changing functionality, issues could arise relating to increased signal loss and inability to achieve a sufficient range of variation.

To address these issues, Renesas has developed a variable impedance-matching circuit technology that comprises 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; their mutual induction is employed to reduce signal loss and cut the effective parasitic capacitance.

That expands the variable impedance range as well as substantially shrinks the circuit area. It attains a voltage standing wave ratio (VSWR)—which indicates impedance mismatches—equivalent to maximum of 6.8 and a variable impedance range of approximately 25 Ω to 300 Ω.

Reference signal self-correction

A reference signal—locally generated signal—is generated internally by the RF transceiver. The signal is of roughly the same frequency as that of the wireless radio signals received via the antenna. It’s used to convert gigahertz-band wireless signals to low-frequency baseband signals. However, 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 wireless designs like BLE, the compensation technology for phase and amplitude deviations with a calibration circuit has been used to accurately generate the reference signal. That, however, leads to higher power consumption and increased test cost because integrating such a calibration circuit requires a larger chip area.

The new RF transceivers from Renesas resolve these issues via a 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. The self-correction circuit is much smaller and can be implemented at approximately one-twelfth the size of a conventional calibration circuit.

This article was originally published on Planet Analog.

Majeed Ahmad, Editor-in-Chief of EDN and Planet Analog, has covered the electronics design industry for more than two decades.


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