Browse Prior Art Database

SECONDARY CHANNEL DIGITAL MODULATOR AND DEMODULATOR

IP.com Disclosure Number: IPCOM000025987D
Original Publication Date: 1989-Jun-30
Included in the Prior Art Database: 2004-Apr-04
Document File: 4 page(s) / 163K

Publishing Venue

Xerox Disclosure Journal

Abstract

A simple way to superimpose a slow control signal on a high speed datastream with pulse width modulation, and a simple way to recover the control signal after the data has been transmitted over a communication link, like an optical fiber, coaxial cable link, microwave channel or other mans of communications. It is assumed that both slow and fast datastreams are encoded with manchester encoding, an encoding scheme that has no energy at the low frequency end of the spectrum. Also assumed i ; the fact that the signaling rate of the high speed signal is much higher than that of the slow signal. A typical system might have a slow signaling rate of 9600 baud and a high rate of 1 to 1000 Megabaud.

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XEROX DISCLOSURE JOURNAL

fo FAST DATA RATE fl = SLOW DATA RATE FIG. I

Volume 14 Number 3 May/June 1989 163

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SECONDARY CHANNEL DIGITAL MODULATOR AND DEMODULATOR (Cont'd)

A simple way to superimpose a slow control signal on a high speed datastream with pulse width modulation, and a simple way to recover the control signal after the data has been transmitted over a communication link, like an optical fiber, coaxial cable link, microwave channel or other mans of communications. It is assumed that both slow and fast datastreams are encoded with manchester encoding, an encoding scheme that has no energy at the low frequency end of the spectrum. Also assumed i ; the fact that the signaling rate of the high speed signal is much higher than that of the slow signal. A typical system might have a slow signaling rate of 9600 baud and a high rate of 1 to 1000 Megabaud.

The figure shows a schematic diagram of the modulator and the demodulator.

ECL circuits are used as an example of implementation. The system takes advantage of the fact that the manchester encoded signal has a zero DC component. In fact, the energy of the signal is concentrated in the frequency spectrum bounded by the signaling frequency and double the signal frequency. The slow signal is modulated in the high speed signal by slightly modifying the pulsewidth of the signal. When the slow signal is LOW, the pulsewidth will be slightly reduced and when the slow signal is HIGH the pulse width will be increased slightly. The variation of pulsewidth will be small enough as not to affect the subsequent demodulation of the high speed data ( for example, less than 10% variation). At the receiver end of the transmission channel the high speed data can be recovered as if no slow modulation was present. The slow data can be recovered by assing the signal through a low pass filter with a cut-off frequency that lies rl etween the high speed signaling rate and double the low speed signaling rate. When the two signaling rates are far apart in speed, as in example of 9600 and lMbaud, a simple first order filter with an FC of 20KHz will be satisfactory. Note that the signal strength of the slow signal after separation will be much smaller than that of the fast signal. The signal strength is proportional to the pulsewidth variation imposed by the modulator and the high speed data contents. The signal strength can ea...