crystal oscillator
The crystal oscillator is equivalent to the human heart, it can beat, and the whole system is "alive".
Crystal oscillators commonly have active crystal oscillators and passive crystal oscillators.
Active crystal oscillators are more expensive, but require less peripheral circuits, and can work with only one power supply.
The passive crystal oscillator is cheap, and the matching circuit is more complicated.
The passive crystal oscillator is used for analysis. The following is the passive parameter information. The frequency FL, load capacitance CL, frequency offset, excitation power DL, and resonance Rr are all key parameters.
The application of passive crystal oscillator is generally shown in the figure:
Design of Passive Crystal Oscillator
1. Setting of RF value
In most cases, the feedback resistor RF is embedded in the oscillator circuit. Its role is to make the inverter function equivalent to an amplifier by introducing feedback. The added feedback resistor between Vin and Vout biases the amplifier at Vout = Vin, forcing the inverter to operate in the linear region (shaded area in the figure). An inverter operating in the linear region is equivalent to an inverting amplifier, which causes the crystal oscillator to start oscillating.
2. Calculation of capacitors CL1 and CL2
CL1 and CL2 are generally equal, and can be estimated from the load capacitance CL given in the crystal oscillator specification. By adjusting the capacitance of CL1/CL2, the CL value reaches the calibration value of the crystal oscillator. Generally, it is relatively easy to know the appropriate value of CL1/CL2 through the crystal oscillator matching test.
Estimated as follows:
3. Driving power
Drive Level describes the power consumption of the crystal oscillator. The power consumption of the crystal oscillator must be limited within a certain range, otherwise the quartz crystal may not work properly due to excessive mechanical vibration. The maximum drive level is usually given by the crystal oscillator manufacturer in milliwatts. When this value is exceeded, the crystal oscillator will be damaged. The drive level can be calculated by the following formula:
4. Calculation of external current limiting resistor REXT
This resistor and CL2 form a low-pass filter to ensure that the oscillator starts at the fundamental frequency, not at other higher harmonic frequencies (avoid the 3rd, 5th, and 7th harmonic frequencies).
An external resistor, Rext, limits the drive strength to prevent the crystal from being overdriven. If the value of RExt is too small, too much power may be borne by the crystal oscillator. If the RExt value is too large, the oscillator will not work properly. A large resistance RExt can reduce the excitation power, but at the same time -R will also change.
Generally, after the crystal oscillator circuit is designed, the prototype needs to be sent to the crystal oscillator supplier for crystal oscillator matching test. Through the test, the supplier will give the most appropriate crystal oscillator circuit parameters.