In the WLAN network, due to the low power and high frequency of the technology, the coverage area is limited, which results in most existing products having a relatively short communication range and limited bidirectional transmission and reception capabilities. To address this, many transceivers now integrate power amplifiers, improving integration and simplifying the circuit design. However, the power of these integrated amplifiers is often insufficient for long-distance communication, necessitating an external power module.
This paper focuses on the design and hardware implementation of a wireless power amplifier, aiming to increase the transmit signal power and enhance the sensitivity of received signals. By doing so, it enables more reliable and extended bidirectional communication, particularly suitable for 802.11b/g mode applications over longer distances.
Figure 1 shows the system schematic, where part of the RF input power is directed to a detection circuit. The detected RF envelope is compared with a fixed threshold to control the transmission and reception status of the signal port. If the level exceeds the threshold, the RF switch is activated, enabling the transmit channel, and vice versa.
1.1 Transmission Microstrip Line Design
The microstrip line plays a crucial role in RF circuit design, serving as a key component in the matching network and acting as a bridge between different functional modules. It is used for signal input/output or connecting circuits. Proper impedance matching between the front-end and back-end circuits minimizes power loss during transmission. The characteristic impedance of the microstrip line is set to 50Ω.
The characteristic impedance of a microstrip line depends on factors such as its width (w), the dielectric constant (ε) of the PCB material, the board thickness (H), and the copper foil thickness (T). For FR4 material, the impedance is primarily determined by the width of the line. Using the formula provided, the design parameters are calculated accordingly.
1.2 Power Amplifier Circuit Design
The power amplifier circuit is designed to boost the transmission signal and output a higher power. A double-tube balanced configuration is employed, with two identical circuits connected in parallel to increase the output power. The selected power amplifier chip, AWL6153UM7P8 from Anadigics, can achieve 25dBm at 5V DC and 54Mb/s under 802.11g mode. In simulation using Agilent ADS2004, the insertion loss is less than 3dB at 2.4GHz, and the output power reaches 27dBm, equivalent to 500mW.
1.3 Low Noise Amplifier Receiver Circuit Design
The low noise amplifier improves the receiver's sensitivity. The RFMD2373 chip is chosen for its high gain (up to 15dB), low noise figure (1.3dB), and low current consumption (10mA). It also features a first-order gain compression point of -3.5dBm and an IP3 of 9.5dBm at 2.4GHz. Adding a band-pass filter at the input helps reduce noise interference.
1.4 Detection Circuit Design
To prevent excessive voltage output when receiving strong signals, a directional coupler is used to couple a portion of the RF signal into the detection circuit. This ensures that the detection circuit does not affect the characteristic impedance of the main path. The switching control signal is derived from the processed output of the detector. The coupling line has a width of 11.8 mil, spacing of 3.9 mil, and a length of 531 mil, with a coupling degree of 13dB.
1.5 Switch Control and Power Circuit Design
The signal from the detection circuit cannot directly control the RF switch. Instead, a transistor (MBT2222) is used as a driver to manage the switching process. Additionally, a power supply circuit converts 9V to 5V to provide stable power to the amplifier module.
2. Wireless Power Amplifier Testing
2.1 S-Parameter Test
Using the Agilent E5071B vector network analyzer, the S-parameters of the power amplifier were tested. The results, shown in Figure 4, confirm the performance of the amplifier across the desired frequency range.
2.2 Communication Test
An IQVIEW test instrument was used to emit a -30dBm RF signal as the input to the power amplifier. The amplified signal was analyzed using IQSignal Vector Signal Analyzer, showing successful reception at both 54Mb/s (802.11g) and 11Mb/s (802.11b) data rates. The transmitted signal was further amplified by a self-made module (up to 13.44dBm) and tested for gain performance.
The throughput rate, representing the amount of data transferred or processed in a given time, was measured after integrating the power amplifier into the existing WLAN network. The results showed that the average throughput increased to over 20Mb/s, demonstrating the effectiveness of the amplifier in extending network performance.
3. Conclusion
The 2.4GHz WLAN wireless power amplifier developed in this study achieves a transmission power of 500mW and a receiving gain of over 10dB. It supports automatic switching between transmit and receive modes and can be directly connected to major wireless APs. With an effective working range of up to 5km, it offers a practical solution for enhancing WLAN coverage and reliability in various applications.
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