Browse Prior Art Database

Sigma-Delta Automatic Gain Control

IP.com Disclosure Number: IPCOM000037339D
Original Publication Date: 1989-Dec-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 3 page(s) / 55K

Publishing Venue

IBM

Related People

Schulz, RA: AUTHOR

Abstract

There are many ways to accomplish automatic gain control (AGC), but most of them suffer from the problem that the transient response of the AGC changes with input signal level. The purpose of this disclosure is to describe a method of performing AGC which does not have the signal level dependency, and yet is simple to implement.

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Sigma-Delta Automatic Gain Control

There are many ways to accomplish automatic gain control (AGC), but most of them suffer from the problem that the transient response of the AGC changes with input signal level. The purpose of this disclosure is to describe a method of performing AGC which does not have the signal level dependency, and yet is simple to implement.

A method for accomplishing AGC which has a transient response that is independent of the signal level is shown in Fig. 1. This technique is probably not new. The input signal is first converted to a slowly varying DC level by performing a root mean square calculation, or perhaps full wave rectifying the input and filtering the result. In either case, a signal representative of the average magnitude of the input level is obtained. This signal is filtered heavily to obtain essentially a DC level. The time constant of this filter is typically 0.5 second.

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The divider circuit performs the calculation Ref/@ein*@ which is the absolute value of the complement of ein. This signal is then applied as one input of the multiplier. The original input signal ein is applied as the second multiplier input, so that the product becomes einxRef/@ein*@. The output signal is seen to be the desired function. As long as the input signal is near its long-time average, the output signal will have the same shape as the input, but it will be attenuated so as to have a long-term average level near Ref.

A rapid shift of the average input signal level will cause @ein*@ to move to a new level according to the transient response of the low pass filter at the output of the absolute magnitude circuit. Note that this time constant does not depend on the signal level, but only on the linear low pass filter itself. Sigma-Delta AGC

There are various ways to implement the divider circuit noted in Fig. 1. These include multipliers in the feedback circuit of a high gain amplifier and analog-to-digital converters (ADCs...