## Glossary Of Terms
(Oscillators)
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link to download *Glossary of Terms (Oscillators)* in PDF.
For further explanations of SPXO, VCXO, TCXO and VCTCXO see Oscillator
Manufacturing.
**Acceleration, 2g tip-over: **the ability of the oscillator to maintain its
frequency as it changes orientation with respect to gravity (MIL-PRF-55310D, 4.8.18.2).
**Adjustment range: **See mechanical frequency adjustment or voltage frequency
adjustment (MIL-PRF-55310D, 4.8.7).
**Allan deviation: **A measure of the short term
frequency stability of the oscillator made in the time domain. It is calculated and
presented as the value of frequency variance over a specified time period (square of the
standard deviation). It is a useful measurement for industries where accurate time
intervals are required (e.g. global positioning, measurement of relativistic effects,
etc.) (MIL-PRF-55310D, 4.8.17.5).
**Allan variance: **See Allen
deviation.
**Ageing: **See frequency ageing.
**Calibration tolerance:** The allowable offset in ppm from nominal
frequency usually referenced to +25°C.
**Centre frequency: **A term describing the frequency at the centre of the tuning
range. This may differ from the nominal frequency in many designs.
**Clipped sine wave: **An output voltage format achieved by driving the output
circuit into clipping to achieve a rectangular wave form. Some applications use this
format to reduce EMI in the product. However, this technique becomes less reliable as
frequency increases and the rise/fall times may be too great at any frequency to make a
useful clock oscillator for modern technology.
**CMOS: **See HCMOS
**Control voltage:** A voltage applied to control the frequency of VCXOs
(MIL-PRF-55310D, 4.8.3.3).
**Control voltage range:** The range of voltage over which the tuning of the
oscillator is valid.
**Coupling: **Description of the connection between the oscillator and the
external circuit. The choices are AC or DC.
**dB: **See decibel.
**dBm: **Decibels referenced to 1 milliwatt, it must be specified with load
impedance (usually 50 ohms).
**Decibel: **A measure of relative amplitude of a sine wave.
**Design Frequency: **See nominal
frequency
**Deviation Linearity: **See linearity.
**Deviation slope polarity: **The slope of
the change in frequency with respect to the control voltage. A positive polarity indicates
the frequency increases when the control voltage becomes more positive (MIL-PRF-565310D,
4.8.31.4).
**Direct compensation: **A technique of building crystal oscillators in which the
circuit components affecting the crystal load reactance are selected to have temperature
characteristics such that the crystal is re-tuned over temperature to reduce the effects
of temperature on the frequency of the oscillator.
**Duty cycle: **A term with identical meaning to symmetry. Use of the term
"symmetry" is preferred to avoid confusion with other parameters. See symmetry
(MIL-PRF-55310D, 6.4.7).
**Fanout: **A specification of the number of TTL or HCMOS loads to which the
oscillator will be subjected.
**Frequency: **The repetition rate of the signal output from the oscillator. It is
measured in repetitions (cycles) per second or Hertz (Hz).
**Frequency adjustment range: **the range over which the oscillators frequency may
be adjusted above and below its nominal frequency. This may be achieved either by a
mechanical trimmer or via a tuning voltage. Ranges are usually specified in ppm.
**
Frequency ageing: **This is the
ability of the oscillator to maintain its design frequency over a specified time period.
This is usually specified as a maximum variation of plus and minus parts per million over
a specified time period or over a specified time period for a specified number of those
time periods (e.g. 10 ppm/10 years or 1 ppm per year for 10 years)
(MIL-PRF-55310D,4.8.35).
**Frequency stability: **The ability of the oscillator to maintain its design
frequency with changing environmental and electrical conditions. Parameters which may
affect stability include temperature, time, supply voltage, load impedance, humidity, and
vibration.
**Frequency load tolerance: **A specification of the allowable change in frequency
when the load impedance is changed from the minimum to the maximum specified values
(MIL_PRF_55310D, 4.8.15).
**Frequency voltage tolerance: **A
specification of the allowable change in frequency due to changes in supply voltage from
the lower to the upper limit of specified supply voltage range (MIL-PRF-55310D, 4.8.14).
**Harmonic distortion:** The levels of harmonic distortion are measured
with respect to the fundamental signal and are specified in dB.
**HCMOS: **An output signal format defined as a rectangular wave
capable of driving a multiple of CMOS loads. It has a positive signal peak greater than
90% of the supply voltage and a negative signal peak less than 10% of the supply voltage.
The number of loads must be specified. Note that the "H" in HCMOS stands for
high speed and does not affect the output voltage specification (MIL-PRF-55310D, 3.6.20).
**Hysteresis: **When applied to a crystal oscillator, this refers to the difference
in frequency at a given temperature or control voltage depending on how the set of
conditions was approached. For example, as temperature increases to 25°
C the frequency will be different than as the temperature decreases to 25°C. Hysteresis is expressed as ± 1/2 the
frequency difference divided by the nominal: if the difference is 0.5 Hz and the frequency
at 25°C is 10.000000 MHz then the hysteresis is ±2.5 x 10^{-8} (MIL-PRF-55310D,6.4.21).
**Indirect compensation: **A technique for temperature compensation in which a
temperature sensitive circuit external to the oscillator is used to modify the crystal
load reactance with temperature so that oscillator tuning is modified to reduce or
eliminate the effects of temperature on the frequency of the oscillator.
**Initial frequency accuracy: **A specification of the allowable range of
frequencies within which the oscillator will operate at the time of shipment from the
manufacturer and at a temperature of 25°C unless otherwise
specified. This applies only to oscillators that are of fixed frequency (no ability to
tune) (MIL-PRF-55310D).
**Input impedance: **The load represented by input connections on the oscillator
used for voltage control or tri-state control (MIL-PRF-55310D, 4.8.30).
**Jitter: **A measure of the short term frequency stability of the oscillator. It
applies only to rectangular wave forms. It is measured as the uncertainty in the location
of one edge of the signal with respect to other edges. It is usually specified in units of
time (nano-seconds or pico-seconds), but may also be specified in degrees. This
measurement has particular application to the digital communications industry.
**Linearity: **A term that applies only to VCXO’s and
TCVCXO’s. It describes the maximum frequency error in following a straight line curve
fitted to the actual frequencies measured at different control voltages within the control
voltage range. This error is measured in percent according to the maximum deviation from
the curve as a percentage of the total tuning range (MIL-PRF-55310D, 4.8.31.5).
**Linearity error: **See Linearity.
**Load: **See Load impedance.
**Load impedance: **The characteristics of the circuit
connected to the output. Normally this specification will be used as part of the
performance specification.
**Load stability: **The ability of the oscillator to maintain its design frequency
over a range in load impedance. This is usually specified as a maximum variation of plus
and minus parts per million over a specified percentage change in load impedance.
**Long term frequency stability: **An oscillator will change frequency over time due
to internal and environmental changes This is often referred to as frequency drift.
Causative factors include ageing of components, temperature and humidity, electrical
environment, and shock or vibration. Long term frequency stability is a design criterion
or set of criteria that specify the ability of the oscillator to maintain its design
frequency over time. Long term stability measured in terms of seconds to years.
**Mechanical frequency adjustment:** The range in parts per million, both above and
below the centre frequency to which the oscillator can be tuned by an internal or external
trimmer (MIL-PRF-55310D, 4.8.7.1).
**Modulation: **A term referring to the ability of a VCXO to be voltage controlled
using a time-varying signal.
**Modulation bandwidth: **See Modulation frequency response.
**Modulation frequency response: **The
range of frequencies over which the modulation signal produces frequency variations of the
same magnitude of Hz/volt. It can be measured by observing the sidebands produced by the
modulation to be of fixed amplitude within 3 dB (MIL-PRF-55310D, 4.8.31.7).
**Modulation range:** The point at which the frequency deviation of the oscillators
turning decreases to 70.7% of its d.c. value while being modulated.
**Nominal frequency: **The frequency for which the
oscillator is designed and to which it is tuned.
**Non-harmonic distortion (Spurious
responses):** Unwanted non-harmonic or spurious responses are found in the output
signal of an oscillator. Their content is specified in dBc.
**OCXO: **Oven controlled crystal oscillator. A crystal oscillator maintained,
during operation, at a fixed temperature so that frequency changes due to ambient
temperature changes do not affect its performance.
**Operable temperature: **The temperature range over which the oscillator may be
operated without damage. The oscillator is not required to perform within specification
over this range. To avoid confusion this parameter is normally not specified and will
never be specified when its values are the same as operating temperature.
**Operating current: **See supply
current
**Operating temperature: **The temperature range over which the oscillator
will operate within its specifications.
**Operating voltage:** See supply
voltage
**Output impedance: **A specification that allows calculation of the change
in output amplitude with changing load impedance (MIL-PRF-55310D, 4.8.26).
**Output power: **The output power calculated from the rms output voltage
and the load impedance or measured directly. It is usually specified in dBm into a 50 ohm
load (MIL-PRF-55310D, 4.8.21). See *Dealing with dBm* for
further information.
**Output voltage: **The specified output voltage magnitude and waveform. It may be
specified several different ways: TTL output, HCMOS output, 1 volt +/- 25% Sine wave
output, or as output power (MIL-PRF-55310D, 4.8.21).
**Overall frequency accuracy: **The maximum permissible frequency deviation of the
oscillator due to all combinations of specified operating and environmental parameters
within a specified period of time. This applies to fixed frequency oscillators.
(MIL-PRF-55310D, 6.4.33).
**Overall frequency stability: **The maximum permissible frequency deviation of the
oscillator due to all combinations of specified operating and environmental parameters
within a specified period of time and at a fixed tuned frequency. This applies to
oscillators which may be tuned (MIL-PRF-55310D, 6.4.33).
**Overshoot: **A characteristic of circuits employing high
performance rectangular wave techniques. Overshoot is the tendency of the rising and
falling edges of the signal to have a damped oscillation superimposed upon them causing
the voltage to momentarily rise above the normal peak signal voltage by as much as several
volts.
**Package: **A definition of the style of enclosure in which the oscillator will be
packaged. This may be done by drawing or may be selected from a set of standard packages.
The package includes all dimensions and connection definitions.
**Parts per million: **A measure of frequency
compared to the centre frequency of the oscillator. It is defined as one Hertz per
Megahertz of the oscillator frequency.
**Period: **The time required for one** **repetition (cycle) of the output
waveform measured in seconds or fractions of seconds. It is mathematically identical to
the inverse of frequency (1/frequency).
**Phase locked loop: **A circuit in which a crystal
oscillator is used as a time base for a voltage controlled oscillator maintaining
frequency and phase lock between the two oscillators. This is a useful technique for
achieving a frequency programmable crystal oscillator and output frequencies that are out
of the range of a crystal manufactured to achieve certain performance specifications.
However, short term frequency stability is one or two orders of magnitude worse than for a
true crystal oscillator.
**Phase noise: **A measure of the short term frequency instability of the
oscillator. It is measured as the single sideband noise, resulting from changes in
frequency, in decibels below the amplitude at the operating frequency of the oscillator
using a 1 Hz bandwidth (dBc/Hz) at two or more frequency displacements from the operating
frequency of the oscillator. This measurement has particular application to the analog
communications industry. There are three primary sources of this instability, steady-state
(MIL-PRF-55310D, 4.8.17.1), random vibration (MIL-PRF-55310D, 4.8.17.2), and acoustic
(MIL-PRF-55310D, 4.8.17.3).
**PLL: **See Phase locked loop.
**ppm: **See Parts per million.
**Rectangular wave: **An output waveform which is characterised by having
short rise and fall times and stable at one of two flat or fixed amplitudes for most of
each cycle. It may be specified as TTL or HCMOS output.
**Ringing: **See Overshoot.
**Rise/fall times: **A measure of the sharpness of the edges of a rectangular wave
form. It is usually specified in units of time (nano-seconds) from 10% to 90% of the rated
output voltage (MIL-PRF-55310D, 6.4).
**Shock: **A specification referring to the ability of the oscillator to withstand,
without performance degradation, a non-repetitive acceleration. A specification is given
for three mutually perpendicular axes (MIL-PRF-55310D, 4.8.41).
**Short term frequency stability: **An oscillator will change frequency over time
due to internal or external causes in a temporary manner. That is, there are no permanent
frequency changes due to this type of instability. Short term stability can usually be
measured in terms of the period of the signal frequency. Causative factors include thermal
noise in active components, shock and vibration, construction parameters, and external
electrical fields (MIL-PRF-55310D, 4.8.17).
**Sine wave: **An output waveform described by a sine curve. A voltage or
decibel value and tolerance must be included to complete the specification.
**Spurious responses:** See Non-harmonic distortion.
**Square wave: **A rectangular wave output waveform having a 50% symmetry. If
specified, the tolerance to the symmetry must be specified. TTL, LSTTL, CMOS and HCMOS
outputs are described as square wave and comply to the wave form drawing at right.
**Start-up time: **The maximum time that the oscillator requires to reach stable
operation within specifications after application of the supply voltage (MIL-PRF-55310D,
4.8.29).
**Storage temperature: **The temperature range over which the oscillator may be
stored, non-powered, without damage (MIL-PRF-55310D, 4.8.47).
**Supply Current: **The maximum operating current of
the oscillator at the nominal supply voltage (MIL-PRF-55310D, 4.8.47).
**Supply voltage: **The operating voltage (or range) of
the oscillator.
**Supply voltage stability: **See Frequency voltage tolerance.
**Symmetry: **The ratio of the positive portion of the output signal to the entire
period of the output signal. This applies only to rectangular wave outputs. The voltage at
which to measure the symmetry is 1.4v for TTL and 50% of supply voltage for CMOS/HCMOS.
Other rectangular output formats should include a voltage at which to measure symmetry
(MIL-PRF-55310D, 4.8.23, 6.4.7).
**TCXO: **Temperature compensated crystal oscillator (MIL-PRF-55310D, 6.4.41).
**TCVCXO: **Temperature compensated voltage controlled crystal oscillator
(MIL-PRF-55310D,6.4.42).
**Temperature: **As applied to crystal oscillators, temperature is specified in
degrees Celsius. Normally the centre of all temperature ranges is +25 degrees Celsius.
**Temperature compensation: **A technique used to stabilise the frequency of a
crystal oscillator over a specified temperature range. The temperature range and the
limits of frequency change are specified.
**Temperature stability: **The ability of the oscillator to maintain its design
frequency over a temperature range. This is usually specified as a maximum variation of
plus and minus parts per million over the specified temperature range.
**Test load impedance: **Usually identical to the load impedance, this is the
characteristic of the external circuit connected to the oscillator during performance
testing.
**THD: **See Total
harmonic distortion
**Time stability: **See Frequency
ageing.
**Tolerance: **The allowable variation in specified parameters where the oscillator
is considered to be within operating specifications.
**Total harmonic distortion: **The remaining
signal appearing at the output of the oscillator after the signal at the operating
frequency is subtracted. It is expressed in percent of the rms value signal at the
operating frequency. This term only applies to sine wave output wave forms
(MIL-PRF-55310D, 4.8.24).
**Tri-state output: **A definition of the ability to turn the output signal on and
off. It must include a definition of the off state (logic high, logic low, high impedance,
etc.)
**Trimmer: **A mechanical device used to tune the crystal oscillator. It may be a
variable capacitor or some other suitable device.
**TTL: **An output signal format defined as a rectangular wave capable of driving a
multiple of TTL loads. It has a positive signal peak greater than +2.4 volts and a
negative signal peak less than +0.5 volts. The number of loads must be specified
(MIL-PRF-55310D, 3.6.20).
**Transfer function: **See Deviation slope polarity.
**Voltage coefficient:** The amount of frequency deviation caused by a
change in the oscillators supply voltage.
**Voltage frequency adjustment:** The range in parts per million, both above and
below the centre frequency to which the oscillator can be tuned by the control voltage.
**VCXO:** Voltage controlled crystal oscillator (MIL-PRF-55310d, 6.4.45).
**Vibration: **A specification referring to the ability of the oscillator to
withstand, without performance degradation, a repetitive acceleration. Usually a
specification is given for one or more frequency ranges MIL-PRF-55310D, 4.8.39).
**XO: **Crystal oscillator. This nomenclature is often used to refer to a crystal
oscillator of fixed frequency that is not temperature compensated (MIL-PRF-55310D, 6.4.5).
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