ALC Feedforward Modulation for Sub-Sample Pulse Leveling
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Solution Overview
Problem
Existing automatic level control (ALC) loops in signal generators struggle to effectively level pulse modulation below the sample rate of the feedback path, leading to distortion and requiring high-bandwidth analog-to-digital converters (ADCs).
Innovation Solution
The implementation of a calibrated feedforward path that emulates the frequency response of the feedback loop, allowing for the adjustment of errors determined in the feedback path according to the emulated frequency response, thereby enabling pulse modulation leveling below the sample rate of the feedback path.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional feedback path is used in the ALC loop, then the output power can be controlled at the sample rate of the feedback path, but pulse modulation below the sample rate cannot be effectively leveled, leading to distortion
Solution Approach 1:
The patent applies preliminary action by calculating and applying feedforward modulation to the modulation input signal before the feedback loop processes the output signal. This feedforward path pre-compensates for expected errors based on the known frequency response characteristics of the feedback path, enabling the system to correct pulse modulation errors even when the pulse width is narrower than the feedback sample period. The feedforward calculation uses the modulation input signal and emulated frequency response to determine corrective modulation in advance.
2Manufacturing precision
If a high-bandwidth ADC is used to achieve pulse modulation leveling below the sample rate, then distortion is reduced, but the cost and complexity of the system increases
Solution Approach 1:
The patent applies copying by creating an emulated frequency response that replicates the characteristics of the feedback path. This emulated response is used in the feedforward calculation to predict and correct errors without requiring the ADC to actually operate at the higher bandwidth needed to directly measure such rapid variations. The emulated frequency response acts as a virtual model that allows the system to compensate for errors that would otherwise require high-speed sampling to detect and correct.
3Device complexity
If the feedback path sample rate is reduced to use lower rate ADCs, then cost is reduced, but pulse modulation below the sample rate cannot be leveled
Solution Approach 1:
The patent introduces an intermediary feedforward path that mediates between the modulation input signal and the feedback loop. This feedforward path calculates corrective modulation based on the emulated frequency response and applies it to the modulation input, effectively bridging the gap between low-rate ADC sampling and high-precision pulse modulation leveling. The intermediary feedforward calculation allows the system to achieve accurate leveling of narrow pulses without requiring the ADC to sample at rates higher than the pulse width.
Data Source
AI summary
An automatic level control (ALC) circuit includes an input configured to receive a radio frequency (RF) input signal, a level control circuit coupled to the input and configured to control a level of the RF input signal in response to a feedback signal, and an output coupled to the level control circuit and configured to output a level-controlled RF output signal. The ALC circuit further includes a feedback path coupled to the output and configured to sample the level-controlled RF output signal to determine an error detected in the level-controlled RF output signal, and to generate the feedback signal in accordance with the error. The ALC circuit still further includes a feedforward path coupled to the input and configured to emulate a frequency response in the feedback path and to adjust the error determined in the feedback path according to the emulated frequency response.