VCXO voltage controlled crystal oscillator selection knowledge

Active crystal oscillator is mainly divided into ordinary crystal oscillator, differential output crystal oscillator, temperature compensated oscillator, voltage controlled oscillator, constant temperature oscillator and other crystal oscillator, and from the use of the number of common oscillator CXO(PXO) and temperature compensated oscillator TCXO most, one represents the universal type, one represents the frequency high-precision type; The VCXO and VCXO OCXO market demand is relatively small, but from the perspective of the difficulty of selection, although the constant temperature crystal oscillator represents higher accuracy and technical requirements, many indicators are required from the design has reached the best, so the choice is relatively simple, and VCXO VCXO it involves more performance indicators. The usage and cost of different performance indicators are very different, so the correct selection of VCXO is introduced here.

First, the voltage controlled crystal oscillator is abbreviated VCXO, which works by applying a control voltage to make the oscillation frequency variable. VCXO is mainly composed of quartz resonator, varactor diode and oscillation circuit, its working principle is to change the capacitance of the varactor diode by controlling the voltage, so as to "pull" the frequency of the quartz resonator, in order to achieve the purpose of frequency modulation. VCXO is mostly used in phase lock technology, frequency negative feedback system and frequency modulation, and has been an essential key component of many electronic applications such as communication machines, mobile phones, pagers, and global positioning system (GPS).

Second, frequency stability requirements. The VCXO oscillation frequency is stable at operating temperature. When we tune the VCXO, the oscillation frequency will change; However, each frequency value that deviates from the nominal frequency is also stable over the operating temperature range. But for a given frequency, the higher the frequency stability requirement, the more difficult it is to get a wide range of traction. Good frequency stability cannot be obtained by using all-silicon MEMS. This is due to the presence of flutter noise and phase noise in silicon. VCXO uses quartz crystal, the frequency is abnormally stable, is currently the best frequency control device.

Third, the relationship between voltage tuning and frequency change. The frequency offset value of the VCXO is related to the control voltage applied to its tuning circuit. The tuning voltage corresponding to the nominal frequency of the VCXO is specified as half of the VCC (supply voltage). A VCXO with a VCC of 5V generates the center frequency when the control voltage is 2.5V. The slope of the frequency change curve of a VCXO with a control voltage of (0.5 ~ 4.5 V) is positive. In other words, when the control voltage rises from 2.5V to 4.5V, the frequency of the oscillator will increase; When the control voltage drops from 2.5V to 0.5V, the frequency of the oscillator will decrease.

Fourth. Phase noise and modulation. All oscillators have some amplitude modulation noise and some phase modulation noise. The phase noise of VCXO is influenced by the structure of oscillator circuit and quartz crystal. The modulation generated by the transient process or ripple of the VCXO power supply can also worsen its phase noise performance. Phase noise is defined in terms of the frequency offset from the center frequency, expressed in units dBc/Hz. Most VCXO devices used in PLL circuits must have good phase noise characteristics. If the application has strict requirements for the corresponding noise, the allowable range of phase noise must be specified when selecting VCXO.

Fifth. Linearity. The function relationship between VCXO oscillation frequency and control voltage is nonlinear. The well-designed VCXO, its frequency and control voltage function curve close to the line, deviation from the line range is controlled within 10%. In general, the more traction a VCXO has, the less linear it is.

Sixth, the relation between the absolute traction range and the deviation of nominal oscillation frequency. In industry, there are two ways to define VCXO traction. One method is called absolute traction Range (APR). The traction degree defined by this method takes into account all the frequency changes generated by the various VCXO related factors, and thus gives a total traction range. In short, the traction degree defined by APR is equal to the sum of the frequency shift of the VCXO relative to the nominal oscillation frequency as well as the frequency change caused by factors such as stability, voltage change, load change, and aging characteristics. The second way to define the traction degree of a VCXO only considers the frequency shift relative to the nominal oscillation frequency. This method does not take into account the effects of overall stability or aging characteristics. So when we determine the traction range, we must specify clearly.

Seventh, price factor. The VCXO uses a fundamental frequency crystal to obtain the required traction value. When the frequency exceeds 30MHz, the fundamental frequency chip is more difficult to manufacture, and as the frequency increases, the crystal becomes thinner and thinner, and the operation in the manufacturing process is more difficult. Therefore, the higher the operating frequency choice, the higher the price.

Eighth, the working voltage, the lower the working voltage, the smaller the control voltage value, so the lower control voltage is difficult to obtain a wide range of traction degree.

Output mode, VCXO can provide a variety of output modes, such as TTL, CMOS,PECL,LVDS and other output, so the selection must be clear about the output mode.