**1 Introduction**
WiMAX, short for World Interoperability for Microwave Access, is a global standard for wireless broadband access. It is also known as the IEEE 802.16 standard or the Broadband Wireless Access (BWA) standard. WiMAX is a wireless metropolitan area network technology designed to operate in microwave and millimeter wave bands. The SMI7035, a dual-band single-chip transceiver from Sierra Monolithics, is ideal for low-cost customer premise equipment (CPE) used in BWA applications and supports WiMAX-based products. Its advanced design makes it a reliable choice for modern wireless communication systems.
**2 Key Performance Features [1]**
◠Operating frequency range: 3.3–3.8 GHz or 2.3–2.7 GHz.
â— High receive sensitivity of -100 dBm with a gain control range of at least 75 dB.
â— Transmitter gain control range of up to 55 dB.
â— Dual-band operation with support for both fractional and integer frequency synthesizers.
â— Integrated frequency synthesizers for fast tuning and precise signal generation.
◠Fractional frequency synthesizer: 125 kHz step size, less than 90 μs switching time.
◠Integer frequency synthesizer: 2 MHz step size, less than 90 μs switching time.
â— SPI interface for microprocessor control, allowing flexible configuration of transmission and reception frequencies.
â— Operates on +5V and +3.3V power supplies.
â— Low power consumption of 1.73 watts.
â— Built-in Received Signal Strength Indication (RSSI) for signal monitoring.
◠Compact 48-pin QFN package, measuring just 7 × 7 mm.
**3 Chip Internal Structure**
The SMI7035 chip is composed of several key functional blocks, including the receiver, transmitter, signal strength indicator circuit, and frequency synthesizer. These components work together to provide high-performance wireless communication capabilities.
**3.1 Receiver Section**
The receiver uses a quadrature down-conversion architecture. The RF signal enters through the RX_RF pin, passes through a low-noise amplifier, and is filtered by a bandpass filter. It then goes through an image-rejection mixer using a local oscillator signal from the fractional frequency synthesizer. This generates a high intermediate frequency (IF) signal, which is further processed by an off-chip surface acoustic wave (SAW) filter. After another stage of mixing and filtering, the signal is amplified, controlled for gain, and output through the RX_IF_P and RX_IF_N pins.
**3.2 Transmitter Section**
Transmitted data enters the chip via the TX_IF_P and TX_IF_N pins. After amplification, it is mixed with a local oscillator signal from the integer frequency synthesizer and filtered by an off-chip SAW filter. The resulting IF signal is then mixed again using a local oscillator from the fractional frequency synthesizer, generating the final RF signal. After gain control and amplification, the signal is modulated and output through the TX_RF pin to the antenna.
**3.3 Received Signal Strength Indication Circuit (RSSI)**
The RSSI circuit provides a DC voltage that corresponds to the strength of the received signal. This voltage is proportional to the signal level and can be used for fault detection, channel selection, and handshaking during communication.
**3.4 Frequency Synthesizer**
The frequency synthesizer consists of two main parts: the integer frequency synthesizer and the fractional frequency synthesizer.
**3.4.1 Integer Frequency Synthesizer (Based on Charge Pump PLL)**
The integer frequency synthesizer operates based on a phase-locked loop (PLL) with a charge pump. When no input signal is present, the voltage-controlled oscillator (VCO) runs at its natural frequency. Upon receiving an input signal, the phase detector compares the input signal's phase with the VCO's output phase, generating an error signal. This signal controls the charge pump, adjusting the loop filter’s voltage to bring the VCO into lock. Since the frequency divider is an integer, this system is called an integer frequency synthesizer. The output frequency must be an integer multiple of the reference frequency, limiting the minimum frequency step to the reference frequency.
**3.4.2 ∑-△ Modulation Fractional Frequency Synthesizer [4-5]**
The fractional frequency synthesizer allows non-integer multiples of the reference frequency, offering finer frequency resolution. In this design, N represents the integer part of the division ratio, while F represents the fractional part. The system achieves fractional division by alternating between dividing by N and N+1 over a number of reference cycles. A digital ∑-△ modulator helps reduce quantization noise, improving overall performance. This approach enables faster switching times, better phase noise characteristics, and more flexible frequency tuning compared to traditional integer synthesizers.
Fiber Optic Box,Fiber Optics Box,Fiber Optic Boxes,Fiber Optic Junction Box
Cixi Dani Plastic Products Co.,Ltd , https://www.danifiberoptic.com