Commonly Used Crystal Frequencies
Crystal oscillator circuits are often designed around relatively few standard frequencies, such as 3.579545 MHz, 4.433619 MHz, 10 MHz, 11.0592 MHz, 14.318182 MHz, 17.734475 MHz, 20 MHz, 33.33 MHz, and 40 MHz. The popularity of the 3.579545 MHz crystals is due to low cost since they are used for NTSC color television receivers. Using frequency dividers, frequency multipliers and phase locked loop circuits, it is practical to derive a wide range of frequencies from one reference frequency. 14.318182 MHz (four times 3.579545 MHz) is used in computer video displays to generate a bitmapped video display for NTSC color monitors, such as the CGA used with the original IBM PC. (The IBM PC used 14.318182 MHz, divided by three, as its 4.77 MHz clock source, using one crystal for two purposes.) The 4.433619 MHz and 17.734475 MHz values are used in PAL color television equipment and devices intended to produce PAL signals. 11.059 MHz is popular due to it cleanly dividing standard RS232 baudrates (300 being a factor).
Crystals can be manufactured for oscillation over a wide range of frequencies, from a few kilohertz up to several hundred megahertz. Many applications call for a crystal oscillator frequency conveniently related to some other desired frequency, so hundreds of standard crystal frequencies are made in large quantities and stocked by electronics distributors.
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