Spurious Response
Understanding Spurious Responses
Spurious signals corrupt the spectral purity of RF systems. In receivers, spurs can cause false target detections or data errors. In transmitters, spurs that fall in other users' frequency bands violate regulatory requirements. Identifying and eliminating spurs is a critical part of system integration.
Spur Sources
- Harmonics: Integer multiples of the carrier from amplifier nonlinearity.
- Mixer spurs: m x f_LO +/- n x f_IF products from the mixing process.
- Synthesizer spurs: Reference spurs, fractional spurs from PLL operation.
- Clock feedthrough: Digital clock frequencies coupling into the analog signal path.
- Power supply: Switching regulator frequencies modulating RF signals.
Spur Specifications
- Harmonics: -30 to -60 dBc typical for amplifiers.
- Non-harmonic spurs: -50 to -80 dBc for well-designed synthesizers.
- Reference spurs: -60 to -100 dBc for PLL synthesizers.
Frequently Asked Questions
What is a spurious response?
A spur is any undesired signal at the output of an RF system. Spurs come from harmonics, mixer products, synthesizer artifacts, clock coupling, and power supply noise. They are measured in dBc and must be below system requirements.
How do you find spurious signals?
Use a spectrum analyzer with sufficient dynamic range. Sweep the full output spectrum looking for discrete tones. Vary the input power, frequency, and LO to identify each spur's source. Mixer spur charts predict mixer spur frequencies.
How do you eliminate spurs?
Filtering (removes out-of-band spurs), shielding (prevents coupling), power supply filtering (removes switching freq), layout optimization (separates digital and analog), and component selection (choose mixers/synthesizers with better spur specs).