Supply Voltage

Supply voltage, specified in volts (V), is the input power required to operate the oscillator. Supply voltage powers the oscillator through the VDD pin and is sometimes referred to as VDD. Standard voltages for single-ended oscillators include 1.8, 2.5, and 3.3V. Voltages for modern differential oscillators typically range between 2.5 and 3.3V. SiTime offers oscillators that operate as low as 1.2V for regulated supply applications such as coin-cell or super-cap battery backup. The supply voltage of most SiTime oscillator families is programmable, which reduces the need for external components such as level translators or voltage regulators.

Thermal Hysteresis

Thermal hysteresis is the difference between up-cycle and down-cycle frequency over temperature characteristics and is quantified by the value of the difference at the temperature where the difference is at its maximum. Thermal hysteresis is especially important for precision oscillators such as TCXOs and OCXOs because it consumes a significant portion of the overall frequency stability budget.

The causes of thermal hysteresis are not fully understood, but can involve strain changes in the resonator mounting structure, contamination redistribution inside the package, and a thermal gradient between the temperature sensor and resonator. SiTime TCXOs have among the industry’s lowest (best) hysteresis, typically ±15 ppb over -40°C to 105°C, because of the negligible thermal lag between the oscillator and temperature sensor and the extremely low contamination levels on the order of parts-perbillion (ppb) due to wafer level encapsulation of the resonator.

Total Pull Range

See Pull Range

Tri-State

Tri-state is the high impedance output state that typically occurs when the output is shut down by disabling the output driver and no clock signal is produced.

VOH/VOL

Voltage output high/voltage output low (VOH/VOL) are the high and low voltage levels of the clock output. The diagram below shows how VOH and VOL relate to the clock waveform.

Crystal (X or XTAL)

A crystal is a passive resonator that vibrates at a fixed frequency. Crystals are used as external timing reference for semiconductor ICs with an integrated oscillator circuit (i.e., on-chip generation).

Crystal Oscillator (XO) or Oscillator

An oscillator is an active device that combines the resonator and oscillator circuit into a single package. Oscillators do not require external components to generate a clock signal. Although in some cases, power supply decoupling components and/or termination resistor(s) may be required. In some regions, XOs are referred to as OSC or SPXO (simple packaged crystal oscillator). Typical frequency stability of XOs ranges from ±10 to ±100 ppm. The minimum pin count for single-ended oscillators is three pins for power, ground, and the oscillator output. However oscillators usually have at least four pins to accommodate output enable or other control functions. Differential oscillators are usually packaged in six-pin packages. Some oscillators which include serial interface control such as I2 C are packaged in 10-pin or higher pin-count packages. Frequency stability for XOs usually ranges from ±10 ppm to ±100 ppm and they are usually offered in the following packages: 7050, 5032, 3225, 2520, and 2016.

Digitally Controlled Crystal Oscillator (DCXO) or Digitally Controlled Oscillator

A DCXO is similar to a VCXO in that both types of devices allow pulling the frequency. In some cases, DCXOs have the capability to program output frequency to a wider range beyond the limited pull range. The difference with DCXOs compared to VCXOs, is that frequency is adjusted by writing digital control words over a serial interface such as I2 C or SPI.

Digitally Controlled Temperature Compensated Crystal Oscillator (DCTCXO) or Digitally Controlled Temperature Compensated Oscillator

A DCTCXO is a TCXO that incorporates the frequency pulling and programming functionality of a DCXO.

Oven Controlled Crystal Oscillator (OCXO)

An OCXO provides temperature compensation and ovenization to maintain an almost constant temperature for the oscillator at as ambient temperature varies. These devices enclose the resonator, along with temperature-sensing and compensation circuits inside a heated enclosure. This temperature compensation and ovenization enables the OCXO to achieve very good frequency stability ranging from 0.05 ppb to 200 ppb. The typical package size of a quartz-crystal OCXO ranges from to 9.7 mm x 7.5 mm to 135 mm x 72 mm.

Temperature Compensated Crystal Oscillator (TCXO) or Temperature Compensated Oscillator

A TCXO is an oscillator that incorporates temperature compensation to compensate for the frequency vs. temperature characteristic of the resonator. This compensation enables TCXOs to achieve better frequency stability than non-compensated oscillators (XOs). Frequency stability of TCXOs ranges from ±0.05 ppm to ±5 ppm. These devices are used in applications where precision timing references are required such as high performance telecom and networking equipment

Voltage Controlled Crystal Oscillator (VCXO) or Voltage Controlled Oscillator

VCXOs incorporate a control voltage pin that controls the output frequency around the nominal frequency. The extent of frequency control is called the pull range which typically ranges from ±50 ppm to ±200 ppm but can extend to ±3200 ppm for SiTime VCXOs. VCXOs are often used in discrete jitter attenuation and clock recovery applications.

Voltage Controlled Temperature Compensated Crystal Oscillator (VCTCXO) or Voltage Controlled Temperature Compensated Oscillator

A VCTCXO is a TCXO that incorporates a control voltage pin to allow the output frequency to vary around the nominal frequency. The frequency tuning range for a VCTCXO is typically ±5 ppm to ±25 ppm. Some vendors refer to these devices as TCVCXOs.