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temperatures; temperatures near absolute zero.
crystal (frequency control device). A piece of quartz or Rochelle salt which has piezoelectric
characteristics. A piezoelectric crystal produces a voltage between two of its opposite surfaces, or faces,
when it is bent or compressed. A crystal also distorts when a voltage is placed across two of its opposite
faces.
All physical objects have a natural resonant frequency at which they vibrate most easily. A
piezoelectric crystal will, when it is placed in an electrical circuit and excited with a pulse of electrical
energy, vibrate at its resonant frequency. As the crystal vibrates, it produces an AC voltage whose
frequency is the same as the resonant frequency of the crystal.
crystal-controlled oscillator. An electronic oscillator which produces alternating current from direct
current. The frequency of the AC is determined by the resonant frequency of a piezoelectric crystal in the
oscillator circuit.
crystal diode. A form of electronic check valve. A crystal diode uses a semiconductor material which
allows electrons to pass in one direction, but prevents their flowing in the opposite direction.
A crystal diode is made of either silicon or germanium which has areas doped with impurities that
make P-areas and N-areas.
crystal earphones. Earphones in which alternating current distorts a piezoelectric crystal. The crystal is
attached to a diaphragm in such a way that, as the crystal distorts, the diaphragm produces changes in air
pressure (sound waves). The sound waves duplicate the changes in the alternating current.
crystal filter (electronic circuit). An electronic filter which opposes a certain band of frequencies, but
allows all frequencies on either side of the band to pass. A crystal filter attenuates, or decreases the value of,
AC signals whose frequencies are near the resonant frequency of the crystal in the filter circuit.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 168
crystalline (physical structure). A type of molecular structure in which the molecules of a material are
arranged in a definite pattern throughout the material. This pattern of molecular arrangement is called a
crystal lattice.
crystal microphone (aircraft radio component). A microphone used for aircraft radio transmissions in
which sound pressure waves distort a piece of piezoelectric crystal.
A diaphragm in the microphone is attached to a crystal of Rochelle salt or other piezoelectric
material in such a way that, when sound waves vibrate the diaphragm, it distorts the crystal.
When the crystal vibrates, it produces a voltage across two of its opposite faces that changes in
exactly the same way the sound pressure changes.
crystal oscillator. See crystal-controlled oscillator.
crystal oven. A device inside a piece of sensitive electronic equipment that holds the temperature of a
frequency-controlling crystal constant.
The crystal is kept at a constant temperature inside the oven to prevent change in its physical
characteristics and thus in the frequency of the alternating current it produces.
crystal transducer. A device that uses a piezoelectric crystal to change mechanical pressure or vibrations
into electrical signals. When a piezoelectric crystal is physically distorted, a potential difference is created
across its faces.
Crystal transducers are also called piezoelectric transducers.
CSD (constant-speed drive). A component used with either aircraft gas turbine or reciprocating engines to
drive AC generators. The speed of the output shaft of the CSD is held constant while the speed of its input
shaft varies.
The CSD holds the speed of the generator constant and thus the frequency of the AC it produces,
as the engine speed varies through its normal operating range.
CTAF (common traffic advisory frequency). A frequency designated for the purpose of carrying out
airport advisory practices while operating to or from an uncontrolled airport. The CTAF may be a
UNICOM, MULTICOM, FSS, or tower frequency, and it is identified in the appropriate aeronautical
publications.
CTF (centrifugal twisting force). The force acting about the longitudinal axis of a propeller blade which
tries to rotate the blade to a low-pitch angle. See CTM.
CTM (centrifugal twisting moment). The force moment, acting about the longitudinal axis of a propeller
blade, which tries to rotate the blade toward a low-pitch angle. As the engine rotates the propeller,
centrifugal force tries to flatten the blade so all of its mass rotates in the same plane.
Centrifugal twisting moment (CTM) opposes aerodynamic twisting moment (ATM), but normally
CTM is the greater. The resultant of these two twisting moments is a force on a rotating propeller that tries
　
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