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Browse Prior Art Database

SYNTHESIZED CLOCK GENERATOR FOR COMMUNICATION DEVICE

IP.com Disclosure Number: IPCOM000007054D
Original Publication Date: 1993-Oct-01
Included in the Prior Art Database: 2002-Feb-21
Document File: 2 page(s) / 142K

Publishing Venue

Motorola

Related People

King F. Lee: AUTHOR [+4]

Abstract

The new generation of communication equip- ment usually employs one or more microprocessors for controlling and signalling functions. The microprocessor requires a master clock signal for proper operation. This master clock for the micro- processor is usually a full digital level signal with frequency in the order of several to tens of MHz. The harmonics ofthis microprocessor clock and the sub-multiples of the clock frequency from the microprocessor's input and output ports can easily interfere with other part of the more sensitive circuitries in the communication equipment. For example, harmonics of the microprocessor clock can fall within the passband of one of the channels of a transceiver and interfere with the normal transceiver operation.

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0 M MO-LA

INC. Technical Developments Volume 20 October 1993

SYNTHESIZED CLOCK GENERATOR FOR COMMUNICATION DEVICE

by King F. Lee, Gordon W. James, Kenneth A. Hansen and Gary A. Ku&man

  The new generation of communication equip- ment usually employs one or more microprocessors for controlling and signalling functions. The microprocessor requires a master clock signal for proper operation. This master clock for the micro- processor is usually a full digital level signal with frequency in the order of several to tens of MHz. The harmonics ofthis microprocessor clock and the sub-multiples of the clock frequency from the microprocessor's input and output ports can easily interfere with other part of the more sensitive circuitries in the communication equipment. For example, harmonics of the microprocessor clock can fall within the passband of one of the channels of a transceiver and interfere with the normal transceiver operation.

  One solution is to use a "switchable" crystal oscil- lator to generate the required microprocessor clock. By switching in additional reactive element(s) in a conventional crystal oscillator, the frequency (and the harmonics) of the oscillator can be shihed thus

avoiding the interference problems in some cases. The problems with this switchable oscillator approach are:
a) It requires extra components, at least a transistor for the switch and an additional reactive compo- nent, which add to the reliability concerns.
b) The amount of frequency shift is not well con- trolled over make tolerance and temperature.
c) Only very limited number of alternate hequencies can be generated by expanding the switching idea due to the added complexity and Q degradation horn the external components. This limitation may

leave some interference unavoidable.

  An integrated frequency synthesizer to generate the clock signal for the microprocessor will be able to overcome the above problems with the switched crystal oscillator approach.

Frequency synthesizers had previously been designed to provide the system clock for the newer

0 Matorola.

IX 1993

generation of microprocessors and micro-controllers such as MC68332. Its application is mainly aimed at using the Frequency synthesis technique to gener- ate the microprocessor clock signal, which is nor- mally in the order of several to tenths of MHz from a very low cost crystal such as a 32.768KHz watch crystal. The fixed crystal frequency and the relatively large frequency step of the MC68332 implementa- tion proves to be ineffective in eliminating interfer- ence in the communication equipment.

  The proposed system is shown in Figure 1. The input to the microprocessor clock synthesizer is a stable low level sinusoidal signal (Sl) usually availa- ble from other parts of the communication equip- ment such as the reference oscillator of the trans- ceiver. This low level (Sl) signal is first amplified by an amplifier to the proper logic level internal to the integrated circuit. This amplified...