What are the main characteristics of crystal oscillators?

Crystal oscillator main characteristic index
Crystal oscillator main characteristic index

✦ Output characteristics.
✦ Power supply characteristics.
✦ Power-on features.
✦ FM characteristics.
✦ Aging properties.
✦ Temperature characteristics.
✦ Load characteristics.
✦ Spectral characteristics.
✦ Phase noise.
✦ Form Factor.

Input power characteristics

Power supply voltage: the rated voltage required for normal operation. Generally it is DC voltage. Commonly expressed as Vcc. The unit is: V (volt).

Initial current: only applicable to the current provided by the power supply when the OCXO is first powered on. It is also the operating current of OCXO. Commonly used Ion representation. The unit is: A (ampere).

Steady-state current: After the operation is stable, a sustainable and stable current provided by the power supply is required. Commonly expressed by Iss. The unit is: A (ampere).

Voltage characteristics: When the power supply voltage changes within a specified range, the ratio of the change in output frequency to the frequency under the nominal power supply voltage state. Voltage characteristics are also called voltage fluctuation rate, or power supply voltage fluctuation rate.

Output characteristics

Nominal frequency: the desired value (Fn) of the product output frequency. The unit is commonly used in MHz.

Frequency accuracy: refers to the difference between the measured frequency (F) and the nominal frequency (Fn) output under normal operating conditions at room temperature. Take relative values. That is: the difference between the actual measured value and the nominal value, divided by the quotient of the nominal frequency. (F-Fn)/Fn. Use "ppm" or "ppb" to measure.

The frequency accuracy regulations of OCXO are generally divided into two indicators: "factory accuracy" and "accuracy that can be maintained within a certain period of time."

Output waveform: Generally divided into square wave (TTL/CMOS/LVPECL/LVDS/HCSL, etc.), sine wave and clipped sine wave.

The square wave specifies the high and low levels, waveform swing, duty cycle, rise/fall time; the sine wave specifies the peak-to-peak value (or effective value) under the specified load, measured in V or dbm.

Load capacity: The ability of the output to load the load. For TTL square wave output, the usual load capacity is 5 or 10TTL; for CMOS square wave output, usually 15PF (TYP), 30PF and 50PF; for differential output types, there are corresponding resistor networks, forming 50Ω or 100Ω, etc. effective impedance.

Power-on features (OCXO only)

Start-up current characteristics:

  The startup current of OCXO is a variable related to time and ambient temperature. At normal temperature, the current when just turned on is the initial current Ion. After a certain period of time, it decreases to a stable state, and the stable current after the decrease is the stable current Iss. The power-on characteristics reflect the changing state of the current over time.

Power-on frequency characteristics:

  The frequency of OCXO when turned on is far away from the nominal frequency. As the power-on time increases, it gradually approaches the nominal frequency. The turn-on frequency characteristics describe this change process. The general requirement is to meet a certain frequency difference within a specified time. There are 2 ways to describe:

(1) Take the frequency at a certain time after power-on (such as 5 minutes) and compare it with the frequency at another specified time (such as 1 hour). Take its relative value. Use "ppb" to measure.

(2) Take the frequency at a certain time after power-on (such as 5 minutes) and compare it with the nominal frequency. Take its relative value.

Temperature characteristics

Operating temperature range : refers to the temperature range that must ensure the frequency change requirements. Such as -40~85℃; -20~70℃, etc.

Operating temperature range: The temperature range in which the product can work, but the frequency change index requirements are not guaranteed.

Storage temperature range: the allowed storage range of the product. The product can still function normally after long-term storage within this temperature range.

Frequency-temperature deviation : It is the frequency change range requirement after the temperature changes. Generally, the frequency at 25°C is used as a reference to examine the frequency change within the temperature range and obtain the relative value. Use "ppb" or "ppm" to measure.

Crystal oscillator output waveform type

TTL square wave CMOS square wave sine wave (quasi sine wave) 
differential output
PECL (LVPECL) LVDS HCSL

Aging characteristics

Daily aging: The rate of change of frequency within a day. Generally, the average value of 7 days is used for description. Use "ppb/day" to measure.

Monthly Aging: The amount of frequency change within a month. Generally, the average value within one month is used to describe it. Use "ppb/month" to measure.

Aging: The amount of change in frequency within a year. Use "ppb/year" to measure. (Generally, it is taken as 100 times the daily aging, but it is related to the initial aging time).

Long-term aging prediction: aging conditions in a few years or decades. Generally, the mathematical formula of the aging curve is fitted based on the exponential law of aging characteristics, and then the aging index is calculated using the differential method at a certain time in the future, or the aging index within a certain period. Aging is generally calculated using this method.

SPXO/VCXO/TCXO are generally measured by age.

Voltage control characteristics

For oscillators with voltage control, the control voltage and pulling capability need to be defined.

Control voltage: For VCXO, VCTCXO and OCXO with voltage control, a control voltage input pin is specially defined. VCXO and VCTCXO usually use pin 1# as the control voltage input pin. The center value of the control voltage is usually 50% of the power supply voltage Vdd, and the variation range is usually 0~Vdd or (0+10%Vdd)~(Vdd-10%Vdd ).

Traction capacity: Traction capacity is divided into relative traction capacity and absolute traction capacity (APR). The former refers to the deviation between the frequency when the control voltage is adjusted to the limit and the frequency of the central control voltage. The latter is the residual value after subtracting the total frequency difference from the above deviation.

Example: A certain VCXO Vdd is 3.3V, the total frequency difference is ±50PPM, the control voltage is 1.65±1.35V, the frequency difference between the control voltage at 0.3V, 3.0V and the frequency at 1.65V is -115PPM, +120PPM respectively , then its relative traction capacity is ±115PPM, and its absolute traction capacity (APR) is ±(115-50)=±65PPM.

Linearity: Under ideal conditions, as the control voltage is adjusted from low to high, the change in frequency also increases linearly. However, in reality, it is often not absolutely linear, and the modulation curve has a certain degree of curvature. The deviation ratio between the actual modulation curve and the ideal modulation straight line is called linearity. Usually the maximum is defined as ±10%, and when the requirements are high, ±5% is defined.

Phase noise and jitter

Phase noise measured in the frequency domain is a true measure of short-term stability. The measurement range is usually 1Hz to 10MHz from the center frequency. The frequency obtained by using fundamental frequency and overtone chips usually has very low phase noise. After using PLL, the phase noise will become worse. However, current technological development can already make it close to the level of fundamental frequency and overtone mode.

Jitter is related to phase noise and is measured in the time domain. Usually expressed using effective value (RMS) and peak-to-peak (peak-to-peak), the unit is pS or fS. In fields such as communication networks, wireless data transmission, ATM, SONET, etc., the requirements for jitter and phase noise are often very high.