How do I identify the source of spurious signals in a receiver or transmitter?
Spur Identification Methodology
Spurious signals in RF systems can originate from many sources: LO harmonics leaking through the mixer, intermodulation products in amplifiers, power supply switching noise coupling into the signal path, digital clock harmonics radiating into sensitive circuits, and even passive intermodulation from connectors. Identifying the source requires systematic measurement and analysis.
| Parameter | Class A | Class AB | Class F/Doherty |
|---|---|---|---|
| Max Efficiency | 50% | 50-78% | 70-90% |
| Linearity | Excellent | Good | Moderate (needs DPD) |
| P1dB Backoff | 0-3 dB | 3-6 dB | 6-10 dB |
| Complexity | Low | Low | High |
| Common Use | Test, small signal | General PA | Base station, broadcast |
Compression Behavior
The first step is precise frequency measurement. Every spur frequency can be expressed as m·f1 + n·f2 + p·f3 + ... where f1, f2, f3 are the system's reference frequencies (LO, clock, signal). By solving for the integer coefficients m, n, p, you can determine which frequencies are involved and therefore which stage is responsible. Spur charts (plots of all possible mixing products versus frequency) are invaluable for this analysis.
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Efficiency Trade-offs
Power dependence reveals the order of the nonlinearity. First-order spurs (leakage) do not change with signal level. Second-order spurs grow at 2 dB/dB. Third-order spurs grow at 3 dB/dB. If a spur does not change with input signal level, it is likely LO leakage, clock coupling, or power supply noise rather than an intermodulation product.
Frequently Asked Questions
What tools are best for spur hunting?
A high-dynamic-range spectrum analyzer (>80 dB SFDR) is essential. A tracking generator or network analyzer helps identify resonances. An EMC probe kit localizes radiated coupling. A current probe on DC supply lines identifies conducted coupling.
How do I distinguish between conducted and radiated spurs?
Insert a well-shielded attenuator between stages. If the spur drops by the attenuator value, it is conducted through the signal path. If it does not change, it is coupling through radiation, ground loops, or the power supply.
What is a spur chart?
A spur chart plots all possible mixing products (m·fLO ± n·fRF for various m, n) versus input frequency. Lines where products cross the IF passband indicate potential spur frequencies. Spur charts are essential for receiver frequency planning and IF selection.