Simple Crystal Activity Tester

Check and match crystals for your home-brew projects.

J. Frank Brumbaugh KB4ZGC RO. Box 30 - c/o Defendini Salinas PR 00751-0030

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uarlz crystals are found in practical!) everj piece of electronics used by today's ama-lams who build iheir own equipment find themselves using crystals in many projects.

Mans low power transmitters and double conversion receivers require crystals for their stable oscillators. Crystal ladder filters used in receiver IF circuits must have crystals closely matched in frequency to ensure steep skirts.

Crystals are available from a variety of sources. Most hams, especially older hams, usually have spare crystals in their junk boxes. 1 hese arc often available for very low prices at hamfest flea markets. Most such crystals are in the lower ham bands, white some are multiplier types for higher frequency bands,

Most mail order parts (lealers advertise long lists of inexpensive surplus microprocessor crystals, ideal for ladder fillers in receiver IF strips. Many, too, are suitable for use in ho me-brew test equipment such as capacity and inductance measuring instruments.

Regardless of its intended use. a crystal must be active. That is, it must start oscillating immediately when power is applied. It musí oscillate on a single frequency , and that frequency must be stable. The more active a crystal is, the better it will perform, whether i\s an oscillator or in a filter.

In addition to being of good quality, the several crystals used in receiver IF filters must all be matched to the same frequency within tight tolerances. Purchasing a set o I four. six. or more frequency-matched crystals from a manufacturer is extremely expensive. Buying a handful of surplus microprocessor crystals for about a dollar each and matching them yourself will cost only a fraction of the price of a commercial set,

The simple instrument described here enables easy checking of crystal activity. Used with a frequency counter, crystals can readily be matched in frequency to within a few hertz. It covers the important range of below 1 MHz to slightly above 13 MH/_ The crystals used by hams in oscillators and filters fall within this range of frequencies.

Two versions of this instrument are illustrated: a simple and a deluxe. The only difference is in the method of indicating crystal activity. In the simple version, the brightness of an LED indicates activity level; in the deluxe version, a meter indicates activity level. In its simple version this instrument can he constructed in under one hour for less than five dollars, and probably at no cost at all when using junk box parts. The deluxe version will cost little more.

Circuit description

Fig. 1 shows the schematic version for the simple version; Fig, 2 illustrates the changes required for the deluxe version. Please refer to this illustration for the following discussion.

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