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Low-power CMOS radio frequency chip design
The wireless communication market has increasingly focused on achieving low cost, low power consumption, and compact size. Short-range devices are driving the demand for RF ICs, especially through the concept of wireless sensor networks. RF transceivers (TRX) are specifically designed to minimize power usage, with low-voltage operation being a key requirement. However, this creates a challenge between maintaining high performance and reducing power consumption. In recent years, RF IC production technology has advanced rapidly, with high-speed, low-power components becoming the focus of innovation. For instance, current 0.13µm RF CMOS transistors have an fT value of up to 60 GHz, demonstrating that CMOS technology is capable of handling high-frequency signals. As a result, the industry is largely shifting toward RF CMOS for the development of low-power RF ICs.
This article explores the design of an ultra-low power CMOS radio frequency chip, focusing on the 2.4GHz IEEE 802.15.4 RF transceiver used in Zigbee and RF4CE standards. From both a circuit and system perspective, it highlights key considerations in chip design and application. These include adherence to communication standards and understanding circuit behavior. The receiver section involves receiving the 2.4GHz RF signal via an antenna, amplifying it through the LNA, then processing it through the mixer, filter, limiter, RSSI, and finally demodulating it digitally before storing the data in the RX-FIFO. On the transmit side, digital information from the TX-FIFO is modulated using VCO and two-point delta-sigma modulation, amplified by the PA, and transmitted via the antenna. Additionally, the article discusses hardware design priorities for antennas and PCBs, along with software control, to help readers understand how to implement low-power Zigbee or RF4CE networks using the A7153.
Zigbee modulation and PA design play a critical role in achieving efficient transmission. The 2.4GHz Zigbee standard uses a 250kbps DSSS data rate with Offset-QPSK and half-sine pulse shaping, which is equivalent to MSK. Unlike PSK or OFDM, MSK is a constant envelope modulation, making it more power-efficient for the PA. This helps reduce overall power consumption during transmission.
In terms of transmitter design, IQ modulation is commonly used in digital systems. It splits the data into IQ components, converts them to analog signals via DACs and pulse shaping, and then upconverts them to RF using a quadrature mixer. While this approach offers precise modulation, it requires more complex circuitry. Alternatively, since Zigbee’s MSK can be seen as a form of FSK, a VCO can be used to directly shift frequencies without mixers, simplifying the design and reducing power consumption. There are two types of VCO-based modulation: open-loop and closed-loop. Open-loop systems are simpler but suffer from frequency drift due to lack of phase locking. Closed-loop systems use delta-sigma modulation to adjust the PLL divider, ensuring stable frequency output, though they may limit bandwidth and are better suited for lower data rates. To achieve higher data rates, a two-point delta-sigma modulation technique can be applied, combining differential integration and VCO modulation for complete data transmission.
It’s important to note that the VCO’s voltage-to-frequency curve can vary with process variations, so correction circuits are often needed. A more linear VCO reduces the need for complex calibration, improving reliability and efficiency.
On the receiver side, zero-IF and low-IF architectures are popular for integrated designs. Zero-IF receivers convert RF directly to baseband, requiring only a low-pass filter, which is power-efficient. However, they face issues like DC offset and flicker noise, necessitating additional circuitry. Low-IF receivers reduce these problems by downconverting to a low intermediate frequency, but they introduce image interference, requiring image rejection filters and band-pass filters, which increase power consumption. Each architecture has its trade-offs, and the choice depends on the specific application requirements.