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FSK MODULATOR FOR SCA PAGING

IP.com Disclosure Number: IPCOM000005526D
Original Publication Date: 1984-Apr-01
Included in the Prior Art Database: 2001-Oct-11
Document File: 2 page(s) / 86K

Publishing Venue

Motorola

Abstract

This technique provides crystal controlled frequency stability with direct coupled data input to allow any pattern of data input without tilt in the modulation.

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@ MOTOROLA Technical Developments April 1984

FSK MODULATOR FOR SCA PAGING

This technique provides crystal controlled frequency stability with direct coupled data input to allow any pattern of data input without tilt in the modulation.

   To produce the binary FSK signal, an oscillator, which may be crystal controlled, supplies a clocking signal to a programmable counter. The oscillator frequency is selected such that frequency division by some integer, N, yields the upper output frequency and division by N + 1 yields the lower frequency.

   It is desirable to have a square wave output from the counter to facilitate output filtering. This can be accomplished by using a counter which produces twice the desired output frequency followed by a flip- flop which divides by two and produces a square wave output. For example, for an output frequency of 87 KHz with * 200 Hz deviation, one would use an oscillator frequency of 22.4448 MHz, then divide by 187 or 168 to yield frequencies of 134.4 KHz or 133.6 KHz, then divide by two to yield the output frequencies, 87.2 KHz or 66.8 KHz.

This circuit produces an output which is approximately a square wave in a different way. Using the same example as above, a crystal frequency of 11.2224 MHz would be used. For an output frequency of
66.8 KHz the counter would be programmed to count an 84-84-84-84.. sequence to produce successive half cycles of the output waveform - an overall frequency division of 168. For an output frequency of 67.2 KHz the counter would be programmed to count 84-83-84-83.. sequence for successive half cycles of the output - an overall frequency division of 167. Note that both the oscillator frequency and the number of counts are cut in half - a factor...