frequency multiplier (electronic circuit). An electronic circuit whose output has a frequency that is a multiple of the frequency of the input signal. Frequency doublers and triplers are commonly used frequency multipliers.
frequency synthesizer (electronic circuit). An electronic circuit used to produce alternating current having an accurately controlled frequency. One type of frequency synthesizer uses a crystal-controlled oscillator to produce a single accurate frequency. Frequency multipliers and dividers change the crystal frequency into the required frequencies.
freshening charge (battery servicing). A constant-voltage charge given to dry-charged batteries to get them ready for installation. Most lead-acid batteries are shipped from the manufacturer in a dry-charged state. The battery was charged at the factory, the electrolyte was drained, and the cells were sealed.
When the battery is ready to be put into service, electrolyte having the proper specific gravity is poured into the cells, and the battery is given a constant-voltage freshening charge. The battery is considered ready for service when the specific gravity of the electrolyte does not change in three hours of charging.
fretting. A type of mechanical damage to the surface of an object. Fretting is caused by one object rubbing against another and wearing part of it away.
fretting corrosion. A form of corrosion between two surfaces which have a slight amount of relative motion between them. The protective oxide coating that forms on aluminum or magnesium alloys is rubbed away by the movement of the parts. New oxides must be continually formed to replace those that are worn away and these oxides act as an abrasive to further damage the metal.