Enter your Reference Frequency (REF), VCO Frequency, N Divider, R Divider, and Phase Error to calculate your PFD Frequency — along with the Feedback Frequency, Reference Divided Frequency, Frequency Error, and Multiplication Ratio for your PLL Phase-Frequency Detector.
PFD Frequency
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Feedback Frequency
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Reference Divided Frequency
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Frequency Error
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Multiplication Ratio
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A Phase-Frequency Detector (PFD) is a key component in Phase-Locked Loops that compares the phase and frequency between a reference signal and a feedback signal from the VCO. It produces output pulses with duty cycles proportional to the phase difference, which are then converted to a control voltage by the charge pump.
The PFD frequency is calculated as the reference frequency divided by the R divider: PFD_freq = REF_freq / R. This is the comparison frequency at which the phase detector operates and determines the loop bandwidth characteristics.
The N divider (feedback divider) determines the multiplication ratio of the PLL, while the R divider (reference divider) sets the PFD frequency. The output frequency equals (N/R) × reference frequency. Proper selection of these values affects phase noise, spurious performance, and settling time.
PFD frequency directly affects loop bandwidth, phase noise, and spurious performance. Higher PFD frequencies generally provide better phase noise and faster settling times, but may increase power consumption and spurious content. It's typically chosen as a compromise between these factors.
Frequency error occurs when the VCO frequency divided by N doesn't match the reference frequency divided by R. This creates a constant phase ramp at the PFD inputs, resulting in a DC error voltage that drives the VCO to the correct frequency.
Choose divider values based on your required output frequency, desired PFD frequency, and system constraints. Higher PFD frequencies improve phase noise but may limit the maximum N value. Consider spurious requirements, power consumption, and settling time when selecting R and N values.
PFD frequency sets an upper limit for the loop bandwidth. Generally, the loop bandwidth should be 1/10th to 1/20th of the PFD frequency for stability. Higher PFD frequencies allow wider loop bandwidths, which can improve settling time and reduce VCO phase noise contribution.
No, the PFD frequency cannot exceed the reference frequency. It equals the reference frequency divided by the R divider value (R ≥ 1). If you need a higher comparison frequency, you must use a higher reference frequency or consider alternative PLL architectures.