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EXTENDED RANGE AUTOMATIC THRESHOLD SETTING CIRCUIT

IP.com Disclosure Number: IPCOM000005675D
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2001-Oct-25
Document File: 2 page(s) / 90K

Publishing Venue

Motorola

Related People

Salomon Serfaty: AUTHOR

Abstract

In a multilevel digital transmission system, the sampled signal at the receiver is compared to a set of predeter- mined thresholds in order to decide what symbols were transmitted. If the threshold levels at the receiver are fixed, the amplitude of the signal at the detector input has to be constant over the range of received power, temperature and time. This amplitude can be controlled using and AGC amplifer. The requirements for the am- plifier regarding dynamic range, stability and frequency response are not easy to meet, especially in a broad- band transmission device. A possible way to overcome the problem is to provide a measure of the difference between the actual received level and the nominal expected level. With the use of this measure, a threshold level correction is possible; however, its range is limited.

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Technical Developments Volume 7 October 1987

EXTENDED RANGE AUTOMATIC THRESHOLD SETTING CIRCUIT

by Salomon Serfaty

   In a multilevel digital transmission system, the sampled signal at the receiver is compared to a set of predeter- mined thresholds in order to decide what symbols were transmitted. If the threshold levels at the receiver are fixed, the amplitude of the signal at the detector input has to be constant over the range of received power, temperature and time. This amplitude can be controlled using and AGC amplifer. The requirements for the am- plifier regarding dynamic range, stability and frequency response are not easy to meet, especially in a broad- band transmission device. A possible way to overcome the problem is to provide a measure of the difference between the actual received level and the nominal expected level. With the use of this measure, a threshold level correction is possible; however, its range is limited.

   During each symbol interval, one of L possible levels is transmitted. Each level is represented by a voltage normally chosen to be A Id, +3d, ., +(L- 1)d. Assuming the all levels are equiprobable, thresholds are set at the receiver midway between those levels. This could be achieved in the following way: an additional threshold is added the ideal received signal level. A +l is generated if the received level is slightly above the ideal level and a -1 if it is slightly below. This bit, called the error bit (ERB), is independent of the received level.

   Assume that the received signal is 10% larger. In order to conserve the same margin for noise, we would like also to increase the threshold levels by the same 10%. (When determining the threshold levels, only the most positive threshold needs to be determined; using resistive dividers and inverting op-amp, we can generate all other threshold voltages.) In ord...