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Glossary Of Terms (Oscillators)

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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 +25C.

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.

Graph showing the typical ageing performance of a Hy-Q oscillator measured at +25C over a period of 1 year.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 25C. Hysteresis is expressed as 1/2 the frequency difference divided by the nominal: if the difference is 0.5 Hz and the frequency at 25C 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 25C 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).

The square waveform outputSine 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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