Easy Crystal Impedance Checker Circuit

Posted on Sep 9, 2012

On occasion, microprocessors/microcomputers and microprocessor crystals just aren`t compatible with each other. Many microprocessor data sheets specify maximum values for a crystal`s equivalent series resistance (Rs) that aren`t met by some crystals advertised for microprocessor/ microcomputer use. As a result, a crystal with an Rs value greater than the maximum specified for the chip might cause problems, such as a balky or even inoperative clock oscillator. To tackle this problem, a suspected crystal can be given a quick check for Rs with a simple test setup that consists of a sweep generator, oscilloscope, and three resistors (see the figure). When the frequency source is brought to the crystal`s frequency, output 2 will maximize.

Easy Crystal Impedance Checker Circuit
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If it exceeds the amplitude of output 1, the crystal`s Rs value will be less than the Rs reference resistor`s value. If it doesn`t exceed output l`s amplitude, the crystal`s Rs value is too large.

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