RF Front-End in Software-Defined Radio (SDR)
I. Structure of the RF Front-End in SDR
The primary function of the RF front-end in SDR is to filter out unwanted signals as much as possible, transform RF signals to match the frequency and level requirements of the ADC, and ensure minimal noise introduction, strong signal adaptability, wide operating bandwidth, and large dynamic range.
Typically, the RF front-end adopts one of three structures:
Superheterodyne Structure with Multiple Frequency Conversions
Direct-Conversion Zero-IF Structure
Non-Frequency Conversion Structure
The superheterodyne and zero-IF structures are suitable for wideband intermediate frequency (IF) sampling SDR systems, while the non-frequency conversion structure is used for RF low-pass or RF band-pass sampling SDR systems.
1. Superheterodyne Structure with Multiple Frequency Conversions
This structure involves multiple stages of frequency conversion to gradually shift the signal to a lower IF for processing. It offers high selectivity and sensitivity but requires complex hardware.
2. Direct-Conversion Zero-IF Structure
In this structure, the received signal is directly converted to baseband (zero IF) in a single frequency shift. During transmission, the baseband signal is directly upconverted to RF. It simplifies hardware but may suffer from DC offset and flicker noise.
3. Non-Frequency Conversion Structure
In this approach, the received signal is filtered to extract the desired RF signal, amplified, and directly sampled without frequency conversion. Low-pass sampling uses a low-pass filter, while band-pass sampling employs a band-pass tracking filter.
II. Components of the RF Front-End
The RF front-end primarily consists of filters, amplifiers, mixers, local oscillators, and power amplifiers.
1. Filter
RF filters pre-filter signals to improve isolation between receiving and transmitting modules. Common types include LC filters, crystal filters, ceramic filters, and mechanical resonators. Their performance is compared based on operating frequency range, Q factor, and application scenarios.
2. Mixer
Mixers perform frequency addition or subtraction to achieve frequency translation. Any nonlinear device (e.g., diodes) can serve as a mixer. Mixers are categorized into passive and active types. Frequency mixing can be achieved through:
- Linear Multiplier Operation: Outputs sum and difference frequencies.
- Nonlinear Device Operation: Generates multiple combination frequencies besides sum and difference frequencies.
- Switching Mode: Treats a high-amplitude local oscillator signal as a switching signal to sample the input, producing sum and difference frequencies.
3. Local Oscillator (LO)
The LO must exhibit high spectral purity, fast switching speed, and fine frequency resolution. Key performance metrics include frequency range, minimum frequency step, settling time, phase noise, and harmonic distortion.
Phase Noise: A critical parameter describing short-term stability, representing power relative to the carrier at a specified offset (e.g., dBc/Hz).
Implementation: Phase-locked loop (PLL) frequency synthesizers are commonly used, allowing frequency generation by adjusting division ratios (N) or reference frequencies.
4. Amplifier
SDR requires linear amplifiers to avoid nonlinear distortion. Broadband amplifiers often use feedforward techniques to improve spurious performance and feedback techniques to enhance stability and bandwidth.
5. Power Amplifier (PA)
The output power of a PA ranges from milliwatts to kilowatts, depending on communication distance. Design focuses on efficiency and linearity. The peak-to-average power ratio (PAPR) is a key factor affecting linearity requirements:
PAR=10log(Ppeak/Pav) (unit: dB)
Linearity can be achieved by biasing transistors in Class A, AB, B, or C modes:
Class A: Transistor conducts for the entire signal cycle (360° conduction angle).
Class B: Conducts for half the cycle (180° conduction angle).
Class C: Conducts for less than half the cycle (<180° conduction angle).
Class AB: Conducts between Class A and Class B (>180° conduction angle).