Feedforward Gain Correction for Fast Stable AM Signal Control

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Solution Overview

Problem

Conventional automatic gain control (AGC) circuits in amplitude modulation systems face challenges in achieving fast response without oscillation or distortion, limited by the need for loop stability and strong filtering, which restricts the selection of filter bandwidth and order.

Innovation Solution

A feedforward automatic gain control circuit that uses a low pass filter to derive the level of attenuation from the dc reference level and a multiplier to normalize the signal strength, allowing for a freely selectable filter and variable gain adjustment, enabling independent audio volume output regardless of signal strength.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If feedback techniques are used for AGC control, then loop stability is maintained, but response speed is reduced and distortion increases

Engineering Contradiction:

Improveloop stabilityVSAvoidresponse speed

Core Design Contradiction:

ReliabilityVSSpeed

Solution Approach 1:

The patent inverts the conventional feedback approach by using feedforward control. Instead of measuring the output and adjusting the input based on error signals, the system directly controls the gain based on the reference level and desired output level, eliminating the stability-speed tradeoff inherent in feedback systems.

Inventive Principle:

Principle #13The other way round (Inversion)

Solution Approach 2:

While the overall control is feedforward, the patent incorporates feedback elements in the form of monitoring the actual output level and adjusting the gain control accordingly. This hybrid approach maintains stability while improving response speed by using feedback only for fine-tuning rather than primary control.

Inventive Principle:

Principle #23Feedback

2Object-generated harmful factors

If strong filtering is applied in the gain control loop, then distortion is reduced, but filter bandwidth and order selection are restricted

Engineering Contradiction:

ImprovedistortionVSAvoidfilter bandwidth selection

Core Design Contradiction:

Object-generated harmful factorsVSAdaptability or versatility

Solution Approach 1:

The patent segments the filtering function into separate stages: a first filter processes the reference level signal, and a second filter processes the output level signal. This segmentation allows each filter to be optimized independently, reducing overall distortion while maintaining flexibility in bandwidth and order selection for each stage.

Inventive Principle:

Principle #1Segmentation

3Manufacturing precision

If higher order filtering is used, then signal quality improves, but system complexity increases

Engineering Contradiction:

Improvesignal qualityVSAvoidsystem complexity

Core Design Contradiction:

Manufacturing precisionVSDevice complexity

Solution Approach 1:

The patent moves the filtering complexity to the frequency domain by implementing filters in the I and Q channels separately before combining them. This dimensional approach allows higher order filtering to be achieved through parallel processing rather than sequential stages, improving signal quality without proportionally increasing system complexity.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

Data Source

PatentUS8559903B2Feedforward automatic gain correction

Publication Date: 2013.10.15 NXP BV

AI summary

An automatic gain control circuit is provided for an input signal in the form of a dc reference level and a superposed amplitude modulated ac data signal. A feedforward AGC loop has a low pass filter for deriving the level of attenuation from the attenuated dc reference level. A multiplier value (G) is based on the reciprocal of the level of attenuation (α) and this multiplier enables an output signal to be generated comprising a constant multiple (DG) of the input signal.