PLL Feed-Forward Circuit for Low Reference Spurs and Noise
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Solution Overview
Problem
Phase-locked loops (PLLs) suffer from noise in their output signals due to undesired alternating current (AC) components in the control signal, leading to reference spurious signals that are outside the loop bandwidth and cannot be compensated by the PLL, resulting in noisy output signals.
Innovation Solution
Incorporating a feed-forward network that filters the control signal to remove AC components, providing a band-pass filtered portion back to the PLL, which reduces noise and reference spurs in the output signal by canceling out AC components within the PLL.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the PLL uses a control signal with AC components to lock frequency, then frequency locking is achieved, but noise and reference spurs appear in the output signal
Solution Approach 1:
The control signal is segmented into two separate paths: one path carries the AC components to the VCO for frequency modulation, while the other path carries only the DC component to the VCO for stable frequency control. This segmentation allows the PLL to achieve frequency locking while eliminating noise and reference spurs caused by AC components in the control signal.
Solution Approach 2:
The AC components are extracted from the control signal using a filter circuit and routed through a separate path that bypasses the VCO input. Only the essential DC component is applied to the VCO, thereby taking out the harmful AC elements that cause noise and reference spurs while preserving the frequency locking function.
2Object-affected harmful factors
If the PLL filters out AC components from the control signal, then noise and reference spurs are reduced, but frequency locking capability is compromised
Solution Approach 1:
An intermediary circuit is introduced that separates the control signal into AC and DC components, then recombines them through different paths. The DC component directly controls the VCO frequency for stable locking, while the AC component is routed through a separate modulation path that does not introduce noise, thus maintaining frequency locking while eliminating harmful effects.
Solution Approach 2:
The traditional single-path control mechanism is replaced with a dual-path electronic control system. Instead of mechanically filtering or blocking AC components (which would compromise frequency locking), the system uses electronic signal routing to direct AC and DC components through separate paths, achieving both frequency locking and noise reduction.
3Speed
If the PLL bandwidth is increased to improve response, then frequency tracking is enhanced, but reference spurs outside the bandwidth become more prominent
Solution Approach 1:
The control signal path is segmented into frequency-tracking function (handled by AC components through separate modulation path) and frequency-stability function (handled by DC component through main control path). This allows the PLL to maintain fast frequency tracking response while preventing reference spurs from appearing in the output, as the AC components no longer directly modulate the VCO.
Data Source
AI summary
In some examples, a circuit is described. The circuit may include a voltage-controlled oscillator that may be configured to generate an output signal. The circuit may also include a control signal generation unit that may be configured to generate a control signal based on the output signal. The control signal generation unit may also be configured to provide the control signal to the voltage-controlled oscillator. The voltage-controlled oscillator and the control signal generation unit may be part of a phase-locked loop (PLL) included in the circuit. The circuit may also include a feed-forward network. The feed-forward network may be configured to provide a portion of the control signal to the voltage-controlled oscillator. The voltage-controlled oscillator may generate the output signal based on the control signal from the control signal generation unit and the portion of the control signal from the feed-forward network.