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DELAY MEASUREMENTS UTILIZING AMPLITUDE MODULATION

IP.com Disclosure Number: IPCOM000006094D
Original Publication Date: 1991-Apr-01
Included in the Prior Art Database: 2001-Dec-03
Document File: 2 page(s) / 92K

Publishing Venue

Motorola

Related People

Steven J. Goldberg: AUTHOR

Abstract

For various systems it is necessary to measure the delay that signals will experience as they transverse various communications paths. The path may involve telephone h-&ted pairs, radio frequency transmission and reception, splitters, nonanalog transmission medium (ex. digitized analog facilities), etc. The delay measurement is usually performed as a loop back type of operation. Figure 1 is a typical example.

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MOIOROLA INC. Technical Developments Volume 12 April 1991

DELAY MEASUREMENTS UTILIZING AMPLITUDE MODULATION

by Steven J. Goldberg

   For various systems it is necessary to measure the delay that signals will experience as they transverse various communications paths. The path may involve telephone h-&ted pairs, radio frequency transmission and reception, splitters, nonanalog transmission medium (ex. digitized analog facilities), etc. The delay measurement is usually performed as a loop back type of operation. Figure 1 is a typical example.

  In order to perform this measurement the source sends a known sequence. At some point in the sequence a timer is started. It is stopped when the same point in the sequence is received.

  Digital communications over telephony type medium usually uses frequency shift keying for data rates up to 2400 baud. Signals sent over these paths will however experience frequency dependent propagation times. Although bit states may be recoverable, accurate timing of the received bit stream delay may not he due the uncertainty of the exact switching points.

  Above 2400 baud phase shifting of a single frequency is often used. This avoids the different delays due to using multiple frequencies, but it is even more diffhxlt to derive a precise switching point due to the phase discontinuities.

  Amplitude modulation also has the advantage of being a single frequency. In telephony type systems it is not generally used for modems because the change from one amplitude to another is a discontinuity which generates higher frequencies. The phone lines however act as low pass filters. The result is a ringing which decays with time. Figure 2 shows this with the extreme form of amplitude modulation known as on/off keying. (The curves shown are approximations

to the actual waveforms.) The problem then is to differentiate the true end of the signal from the decaying tra...