How do I diagnose a frequency synthesizer that produces excessive spurious outputs?
Synthesizer Spur Diagnosis
Spurious outputs from a frequency synthesizer degrade the signal quality of any system using it: in a transmitter, spurs can violate emission masks; in a receiver, spurs can create false responses and degrade sensitivity.
- 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
Frequently Asked Questions
How do I reduce reference spurs?
Reducing reference spurs: increase the loop filter's attenuation at the reference frequency (add a third or fourth pole to the filter). This reduces the reference ripple on the VCO's tuning voltage. Improve charge pump matching: the charge pump's up and down currents must be matched within 1% to minimize the periodic error signal at f_ref. Reduce charge pump leakage: even when the PLL is locked, a small amount of charge pump current leaks through the switches, creating a periodic disturbance at f_ref. Use a charge pump with low leakage current (modern integrated PLLs have optimized charge pump designs). Add power supply filtering: reference-frequency noise on the charge pump's power supply modulates the VCO through the tuning line. Add ferrite beads and capacitive filtering on the charge pump supply.
What about phase noise vs. spurs?
Phase noise vs. spurs: phase noise is the continuous noise skirt around the carrier (measured in dBc/Hz at various offsets). Spurs are discrete spectral lines at specific frequency offsets. Both degrade system performance, but in different ways: phase noise mixes with nearby signals, creating reciprocal mixing that reduces the receiver's selectivity. Spurs create discrete false signals that can be confused with real signals or violate emission regulations. A clean synthesizer must have both low phase noise AND low spurious levels. In many systems: the spur specification is harder to meet than the phase noise specification.
What about sub-harmonic spurs?
Sub-harmonic spurs: appear at f_out/2, f_out/3, etc. (frequencies below the carrier). Typically caused by: the VCO's frequency divider in the feedback path (if the divider output leaks to the VCO output through the PCB layout or power supply). The VCO's buffer amplifier operating in a nonlinear mode (generating sub-harmonic oscillation). Fix: improve layout isolation between the divider output and the VCO output. Add filtering on the VCO output to attenuate sub-harmonics. Verify the buffer amplifier's bias point (ensure it is operating in the linear region).