Frequently asked questions for frequency control

Frequently asked questions for frequency control Find answers to all your frequency control questions, including the differences between SPXO, VCXO, TCXO, VCTCXO and OCXO, as well as minimum ordering information.

What is the Piezoelectric Effect?

The Piezoelectric Effect is the generation of an electric charge in certain non-conducting materials, such as quartz crystals and ceramics, when they are subjected to mechanical stress (such as pressure or vibration). Crystal resonators use these properties within an oscillator circuit to generate a precise frequency with very little variation, making them useful as time-keeping devices in electronic clocks, as used in wristwatches and computers.

What is the minimum information required to order a crystal?

  • Frequency in MHz or KHz, such as 13.576MHz
  • Holder/package style, such as HC49/US SMX
  • Frequency tolerance @ 25°C (in ppm)
  • Frequency stability over operating temperature range (in ppm)
  • Operating temperature range in degrees centigrade
  • Load capacitance (in pF) or series resonance.

What is "load capacitance"?

This refers to capacitance external to the crystal, contained within the feedback loop of the oscillator circuit. If the application requires a “parallel” resonant crystal, the value of the load capacitance must be specified. If the application requires a “series” resonant crystal, load capacitance need not be specified.

What is "aging"?

This refers to the cumulative change in frequency experienced by a crystal unit over time. Principle factors affecting aging are excessive drive level, thermal effects, and stress. Circuit design incorporating low operating ambient and minimum drive level will reduce aging.

What is "frequency tolerance"?

Frequency tolerance refers to the allowable deviation from nominal, specified in ppm, at 25°C.

What is "frequency stability"?

Frequency stability refers to the allowable deviation, in ppm, over a specified temperature range. Deviation is normally referenced to the measured frequency at 25°C.

Will the crystal operate outside its specified temperature range?

Yes. If, for example, a crystal is specified over -20°C to 70°C, it may perform without any issue over -40°C to 85°C, but it is possible that it will fall outside its specified stability. This may or may not matter, depending upon the requirements of the application.

What is the difference between fundamental crystals and overtone crystals?

A fundamental crystal oscillates at the frequency determined by the dimensions of the quartz blank. Overtone crystals operate at either their 3rd, 5th or 7th multiple of the fundamental frequency.

What is the most common cut of quartz crystal?

AT cut quartz is the most common in the 1-200MHz range. AT cut refers to a family of angles with respect to the Z axis of the quartz bar. The specific angle of cut determining the frequency/temperature performance is usually selected by the crystal manufacturer. Other cuts will have a specific angle (X, Y, Z) and a specific frequency versus temperature characteristic (curve).

What is the difference between SPXO, VCXO, TCXO, VCTCXO and OCXO?

SPXO – An SPXO or clock oscillator consists of a crystal and a basic drive circuit. Since there is no form of compensation, the overall stability is essentially that of the crystal itself, typically +/-50ppm.

VCXO – A voltage-controlled crystal oscillator is used for fine adjustment of the operating frequency. The frequency of a voltage-controlled crystal oscillator can be varied by the application of a DC voltage to the control pin. The variation is typically between ±50 and ±200ppm (parts per million).

The principle reasons for using a VCXO are:

  • to adjust the output frequency to match (or perhaps be some exact multiple of) an accurate external reference
  • where the oscillator drives equipment that may generate radio-frequency interference, and adding a varying voltage to its control input can disperse the interference spectrum to make it less objectionable (spread spectrum oscillator)
  • where there is a need to modulate a frequency, or
  • as part of a PLL (phase locked loop) circuit.

TCXO – TCXOs are necessary when a level of temperature stability is required that cannot be reached by a standard XO (crystal oscillator) or VCXO (voltage controlled crystal oscillator). TCXOs are useful to an engineer because they can be used anywhere from 10 to 40 times better temperature stability than a standard VCXO with the same kind of power consumption and footprint on the board. TCXOs bridge the gap between a standard XO or VCXO and an OCXO, which are more expensive and require more power to run. The push for technology is towards lower power consumption and of course lower cost, so TCXOs offer a good mid-range solution to power and cost sensitive applications.

VCTCXO – A temperature-compensated TCXO incorporates components that allow a small degree of frequency adjustment (typically ±3 to ±20ppm) by the application of a DC voltage to the control pin. This can be used to adjust the ambient frequency to a required value. It can also be used to adjust out effects of aging.

OCXO – A crystal oven is a temperature-controlled chamber used to maintain the quartz crystal in electronic crystal oscillators at a constant temperature, to prevent changes in the frequency due to variations in ambient temperature. An oscillator of this type is known as an oven-controlled crystal oscillator. This type of oscillator achieves the highest frequency stability possible with a crystal. They are typically used to control the frequency of radio transmitters, cellular base stations, military communications equipment, and for precision frequency measurement.

What is "pulling"?

Pulling is the term used to describe a wide +/-ppm frequency change of an oscillator enabled by applying a varying DC voltage to a specific pin/pad (such as hundreds of ppm). Pulling can also be achieved by varying the load capacitance of a crystal resonator.

What is "tuning"?

Tuning is the term used to describe a small +/- frequency change enabled by applying a varying external voltage to a specific pin/pad (for example +/-3 to 30ppm).

What is the minimum information required to order an oscillator?

  • Frequency in MHz or KHz, such as 48MHz
  • Package (leaded or SMD) and dimensions
  • Stability over the specified operating temperature range (in ppm), all causes
  • Supply voltage
  • Symmetry/duty cycle
  • Operating temperature range in degrees centigrade
  • Output load
  • Output enable, tristate or none

What is the minimum information required to order a TCXO or VCTCXO?

  • Package (leaded or SMD) and dimensions
  • Frequency (MHz)
  • Supply voltage (V)
  • Symmetry/duty cycle (usually 45/55 or 40/60)
  • Operating temperature range (°C)
  • Trimming/adjustment (VCTCXO only)
  • Control voltage range (VCTCXO only)
  • Frequency stability over operating temperature range (+/-ppm)
  • Output type (HCMOS, LVDS, LVPECL etc)

What is the minimum information required to order a VCXO?

  • Package (leaded or SMD) and dimensions
  • Frequency (MHz)
  • Tuning/pulling (ppm)
  • Supply voltage (V)
  • Frequency stability (ppm)
  • Symmetry/duty cycle (usually 45/55 or 40/60)
  • Operating temperature range (°C)
  • Tristate/output enable required (yes/no)
  • Control voltage range (V)
  • Polarity
  • Output type (HCMOS, LVDS, LVPECL etc)

What is the minimum information required to order an OCXO?

  • Package (leaded or SMD) and dimensions
  • Frequency (MHz)
  • Supply voltage (V)
  • Frequency stability over operating temperature range(ppm/ppb)
  • Symmetry/duty cycle (usually 45/55 or 40/60)
  • Operating temperature range (°C)
  • Tristate/output enable required (yes/no)
  • Control voltage range (V)
  • Initial frequency accuracy (ppm)
  • Output type (HCMOS, LVDS, LVPECL etc)
  • Aging requirements
  • Trimming/adjustment

See our range of frequency control products, or contact us for more information.

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