Spurious
Understanding Spurious Signals
Spurious signals degrade system performance by falling on undesired frequencies where they can be mistaken for real signals (in receivers) or cause interference with other systems (in transmitters). Unlike harmonics (which occur at predictable integer multiples), spurious signals can appear at any frequency, making them harder to predict and filter.
Spurious Sources
- PLL reference spurs: Occur at offsets of the reference frequency from the carrier. Caused by charge pump leakage and mismatch.
- Mixer spurs: M x f_RF +/- N x f_LO products at various combinations of M and N.
- Fractional-N spurs: Near-integer-boundary spurs caused by sigma-delta modulator periodicity.
- Power supply coupling: Switching regulator frequencies modulate oscillators.
Specification
Spurious levels are specified in dBc (below carrier). Typical requirements: -60 dBc for general communications, -70 to -80 dBc for radar LOs, and below -90 dBc for test equipment.
Typical specifications:
General purpose: < -50 dBc
Communications: < -60 dBc
Radar LO: < -70 to -80 dBc
Test equipment: < -80 to -100 dBc
PLL reference spur frequency:
f_spur = f_carrier +/- n x f_ref (n = 1, 2, 3...)
Frequently Asked Questions
What are spurious signals?
Spurious signals (spurs) are unwanted spectral lines at frequencies other than the desired carrier and its harmonics. They are generated by nonlinear processes in oscillators, mixers, and synthesizers, and by coupling from digital circuits and power supplies.
How are spurious signals different from harmonics?
Harmonics occur at exact integer multiples of the fundamental and are predictable. Spurious signals can occur at any frequency due to mixing products, PLL leakage, substrate coupling, or power supply noise. Spurs are often harder to predict and eliminate than harmonics.
How do you reduce spurious emissions?
Improve PLL loop filter design to reduce reference spurs. Use better mixer topologies (double balanced) to suppress unwanted products. Add filtering to remove specific spurs. Improve power supply filtering and decoupling. Use shielding between circuit sections.