Browse Prior Art Database

Differential Carrier Detection Circuit

IP.com Disclosure Number: IPCOM000090131D
Original Publication Date: 1969-Feb-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 3 page(s) / 46K

Publishing Venue

IBM

Related People

Nussbaumer, H: AUTHOR

Abstract

Most modems are equipped with a carrier detection circuit. This is located in the modem receiver and measures the average value of the signal received from the transmission line and makes a decision as to whether what is received is actually a modem signal or just pure line noise.

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Differential Carrier Detection Circuit

Most modems are equipped with a carrier detection circuit.

This is located in the modem receiver and measures the average value of the signal received from the transmission line and makes a decision as to whether what is received is actually a modem signal or just pure line noise.

A carrier detection circuit has a very important role in all modems which requires a synchronization pattern at the beginning of the transmission. The rise of the carrier detector output indicates to the modem receiver that a synchronization pattern is transmitted and that synchronization operations are to be started immediately.

The automatic gain control range is limited, for example, to 20db. The input signal level is coarsely adjusted by a manual attenuator so that it falls within the regulation range of the AGC. The signal level can vary in wide limits. This means that the average carrier level can be anywhere between 0db and -20db while the received noise is equal or below -20db. This is illustrated in drawing 1 in which the two extreme cases of average received signal level are represented. The AGC operates in such a way that for input signals within its 20db regulation range, output signals are constant. This means that, in the low amplitude case, the carrier level and noise level are left unchanged but that in the high amplitude case the carrier level is decreased down to -20db while the noise is left constant. This results in an effective carrier signal-to-noise ratio of 0db at the AGC output.

The circuitry in drawing 2 enables avoiding such difficulties. The signal is picked up at the output of the manual attenuator. It is filtered in bandpass filter BPF centered around the carrier frequency. The filter bandwidth is approximately 300 c/s. The signal, now limited to the carrier and a 300 c/s noise band, is then full-wave rectified by P1. The rectified s...