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Time Derivation Method for Clocking a Correlation Detector Receiver for the Decoding of Diffuse Infrared Data

IP.com Disclosure Number: IPCOM000040108D
Original Publication Date: 1987-Sep-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 3 page(s) / 61K

Publishing Venue

IBM

Related People

Bustamante, CM: AUTHOR [+2]

Abstract

A technique is described whereby an infrared receiving circuit provides accurate clocking to a correlation detector, so as to significantly increase the distance sensitivity during infrared transmission of data. The circuit is unique in that the number of required transmitting diodes and receiving photodiodes is reduced as compared to the prior art. The concept, described herein, utilizes pulse position modulation (PPM) encoding, whereby one pulse is used within a field of allowable time slots, so that each time slot encodes a different data value. To encode n bits, 2n time slots are required. Fig. 1 shows an example of a PPM nibble "11 01" in a four-slot PPM. The PPM method of encoding has as its main advantage a low duty cycle, so as to reduce the operational temperature of the transmitting diodes.

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Time Derivation Method for Clocking a Correlation Detector Receiver for the Decoding of Diffuse Infrared Data

A technique is described whereby an infrared receiving circuit provides accurate clocking to a correlation detector, so as to significantly increase the distance sensitivity during infrared transmission of data. The circuit is unique in that the number of required transmitting diodes and receiving photodiodes is reduced as compared to the prior art. The concept, described herein, utilizes pulse position modulation (PPM) encoding, whereby one pulse is used within a field of allowable time slots, so that each time slot encodes a different data value. To encode n bits, 2n time slots are required. Fig. 1 shows an example of a PPM nibble "11 01" in a four-slot PPM. The PPM method of encoding has as its main advantage a low duty cycle, so as to reduce the operational temperature of the transmitting diodes. Since the duty cycle is reduced, the signal strength of the diode can be increased. The technique utilizes a 16-slot PPM which encodes four bits of data (a nibble). The circuit provides an accurate method of defining the time slots of PPM encoded data. The topology of the circuit allows the generation of precision clocks which break up one time slot into four 12.5 nanosecond slices.

The following describes the crystal references used by the gate arrays in the control clocking circuitry. Crystals used have frequency drifts of no greater than .0001%. As a result, a typical 80 megahertz crystal will have a maximum variation of: 80 MHz * (1-6 + 1-6) = 160 hertz This will provide a crystal with a range of from
80.000160 MHz to 79.999840 MHz for which the crystal periods will vary from 12.499975 nanoseconds to 12.500025 nanoseconds. Therefore, during each cycle, any two crystals can only vary from each other by
.00005 nanosecond.

For a data packet of one Kbytes, the maximum sized packet, the amount of transmitter-crystal to receiver-crystal drift that can occur is as follows: bytes nibbles slots cycles 1024 [[[[[[ * 2 [[[[[[[ * 16 [[[[[[ * 40 [[[[[[ = 1,310,720 crystal packet byte nibble slot cycles/packet The maximum resultant time skew between the transmitter and receiver is: nanoseconds 1,310,720 cycles * .00005 [[[[[[[[[[[ 65.53 nanoseconds cycle Due to transmitting diode switching speed limitations, each slot time is limited to 500 nanoseconds. Therefore, a 65.53-nanosecond drift within a slot time of 500 nanoseconds can cause an error of 13.1%. However, this will not degrade performance since all time slots experience about the same skew. Within the boundaries of one encode, the maximum amount of skew which can occur is only 16 * .00005 nanosecond = .0008 nanosecond. Therefore, by phase locking the transmitter and receiver clocking circuits at the beginning of the packet, the largest variation which can occur between the transmitter and receiver at the end of a packet is 65 nanoseconds. To obta...