How do I calculate the loop bandwidth of a PLL for optimal phase noise performance?
PLL Loop Bandwidth Optimization
The PLL loop bandwidth determines the boundary between the two noise sources in the system: the crystal reference (excellent close-in noise, poor far-out noise after N multiplication) and the VCO (poor close-in noise, excellent far-out noise). The optimal bandwidth places this boundary at the frequency where both noise contributions are equal, minimizing the total integrated phase noise across all offset frequencies.
| Parameter | Passive Diode | Active FET | Subharmonic |
|---|---|---|---|
| Conversion Loss/Gain | 5-9 dB loss | 0-10 dB gain | 8-12 dB loss |
| LO Drive Level | +7 to +17 dBm | -5 to +5 dBm | +5 to +13 dBm |
| IP3 (typical) | +15 to +30 dBm | +5 to +20 dBm | +10 to +20 dBm |
| Noise Figure | 5-9 dB (= conv. loss) | 8-15 dB | 9-14 dB |
| LO-RF Isolation | 25-45 dB | 15-35 dB | 20-40 dB |
- 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
Frequently Asked Questions
How do I measure the loop bandwidth?
Apply a small frequency step to the reference and measure the VCO output settling time. The loop bandwidth is approximately 0.35/(settling time to within 10%). Alternatively, measure the closed-loop phase noise transfer function with a signal source analyzer; the -3 dB point of the transfer function is the loop bandwidth.
What phase margin is acceptable?
Minimum 45° for adequate damping (no ringing). Preferred: 55-65° for well-damped response. Above 70°: the loop is over-damped and slower than necessary. Below 40°: the loop rings on frequency steps and has peaking in the phase noise transfer function.
Can I change the loop bandwidth dynamically?
Some PLL chips support programmable charge pump current, which changes the loop bandwidth without changing the filter components. This allows wide bandwidth for fast locking during frequency changes and narrow bandwidth for best phase noise during steady-state operation.