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AN-802 Crystal-Measuring Oscillator Negative Resistance

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AN-802 Crystal-Measuring Oscillator Negative Resistance Crystal - Measuring Oscillator Negative AN-802 Resistance APPLICATION NOTE Introduction Negative resistance models the required gain needed from the active network to sustain stable oscillations. The negative resistance model is shown in Figure 1. The crystal is represented by a reactance term Xm and a motional resistance term Rm. The active network is represented by the reactance term Xosc and the Resistance term -Rneg where Rneg is the negative resistance of the inverter amplifier. One process used as a means to easily evaluate the negative resistance characteristics and oscillation allowance of an oscillator circuits is the method of adding a resistor to the hot terminal of the crystal unit and observing whether it can oscillate (examining the negative resistance). The oscillator circuit capacity can be examined by changing the value of the added resistance. Figure 1. Negative Resistance Model Measuring Negative Resistance of the Active Network The steps for measuring the negative resistance of the active network are as follows: 1) Compute crystal parameters Co, Cs, Ls, Rs and CL using a Network Analyzer such as 250B/C by Saunders and Associated Inc. 2) Solder crystal on X1 and X2 pins of the IDT clock IC and mount the crystal load capacitors next to the device pins (as shown in Figure 2). 3) Adjust the load capacitance to get the output frequency error to nearly 0ppm (account for contribution due to stray capacitance on the PCB board). 4) Connect a series resistor Rx (as shown in Figure 2) and keep increasing its value until oscillations stop occurring on powering up the device. 5) The negative resistance of the active network is then equal to Rx + Rs. AN-802 REVISION B 03/12/14 1 ©2014 Integrated Device Technology, Inc. AN-802 CRYSTAL - MEASURING OSCILLATOR NEGATIVE RESISTANCE Figure 2. Active Network AN-802 REVISION B 03/12/14 2 AN-802 CRYSTAL - MEASURING OSCILLATOR NEGATIVE RESISTANCE Corporate Headquarters Sales Tech Support 6024 Silver Creek Valley Road 1-800-345-7015 or email: [email protected] San Jose, CA 95138 USA 408-284-8200 480-763-2056 Fax: 408-284-2775 www.IDT.com DISCLAIMER Integrated Device Technology, Inc. (IDT) and its subsidiaries reserve the right to modify the products and/or specifications described herein at any time and at IDT’s sole discretion. All information in this document, including descriptions of product features and performance, is subject to change without notice. Performance specifications and the operating parameters of the described products are determined in the independent state and are not guaranteed to perform the same way when installed in customer products. The information contained herein is provided without representation or warranty of any kind, whether express or implied, including, but not limited to, the suitability of IDT’s products for any particular purpose, an implied warranty of merchantability, or non-infringement of the intellectual property rights of others. This document is presented only as a guide and does not convey any license under intellectual property rights of IDT or any third parties. IDT’s products are not intended for use in applications involving extreme environmental conditions or in life support systems or similar devices where the failure or malfunction of an IDT product can be reasonably expected to signifi- cantly affect the health or safety of users. Anyone using an IDT product in such a manner does so at their own risk, absent an express, written agreement by IDT. Integrated Device Technology, IDT and the IDT logo are registered trademarks of IDT. Product specification subject to change without notice. Other trademarks and service marks used herein, including protected names, logos and designs, are the property of IDT or their respective third party owners. Copyright 2014. 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