How to choose capacitor C1C2 in crystal oscillator circuit

Capacitor introduction:

Capacitance, also known as "capacity", refers to the amount of charge stored at a given potential difference, denoted as C, and the international unit is Farad (F). Generally, the electric charge moves in the electric field. When there is a medium between the conductors, the electric charge is hindered and the electric charge accumulates on the conductor, causing the accumulated storage of the electric charge. The stored electric charge amount is called a capacitance. Capacitance refers to the ability to accommodate an electric field. Any electrostatic field is composed of many capacitors. There are capacitors in the electrostatic field. Capacitors are described by electrostatic fields.

Several major criteria for crystal oscillator selection:

(1) Within the allowable range, the lower the C1, C2 values, the better.

(2) Although the C value is large, it is beneficial to the stability of the oscillator, but it will increase the start-up time.

(3) The C2 value should be made larger than the C1 value, so that the crystal oscillator can be accelerated when the power is turned on.

How to choose capacitor C1C2 in the crystal oscillator circuit:

(1) Since each crystal has its own characteristics, it is best to select external components according to the values provided by the manufacturer.

(2) Within the scope of the license, the lower the C1 and C2 values, the better. A large C value is beneficial to the stability of the oscillator, but it will increase the start-up time.

(3) The C2 value should be made larger than the C1 value, so that the crystal oscillator can be accelerated when the power is turned on.

In the application of quartz crystal resonators and ceramic resonators, attention should be paid to the choice of load capacitance. The characteristics and quality of quartz crystal resonators and ceramic resonators produced by different manufacturers are quite different. When selecting, it is necessary to understand the key indicators of this type of oscillator, such as equivalent resistance, manufacturer recommended load capacitance, frequency deviation and so on. In the actual circuit, the oscillating waveform can also be observed by an oscilloscope to determine whether the oscillator is operating at its optimum state. When observing the oscillating waveform, the oscilloscope observes the OSCO pin (Oscillator output). It should select an oscilloscope probe with a bandwidth of 100MHz or more. This probe has high input impedance, small capacitive reactance, and relatively little influence on the oscillating waveform.

(Because there is usually a capacitance of 10 to 20 pF on the probe, it is possible to reduce the capacitance at the OSCO pin to obtain a more realistic oscillation waveform when observing). A well-functioning oscillating waveform should be a beautiful sine wave with a peak-to-peak value greater than 70% of the supply voltage. If the peak-to-peak value is less than 70%, the external load capacitance on the OSCI and OSCO pins can be appropriately reduced. Conversely, if the peak-to-peak value is close to the power supply voltage and the oscillation waveform is distorted, the load capacitance can be appropriately increased. Using an oscilloscope to detect the OSCI (Oscillator input) pin can easily cause the oscillator to stop. The reason is that some probes have a small impedance and cannot be directly tested. You can use the string capacitor method to test.

For example, a commonly used 4MHz quartz crystal resonator, the external load capacitance recommended by the manufacturer is usually about 10 to 30pF. If the center value is 15pF, then C1 and C2 each take 30pF to obtain a series equivalent capacitance of 15pF. At the same time, considering the additional distributed capacitance of the circuit board, the chip pin capacitance, the crystal's own parasitic capacitance, etc. will affect the total capacitance value, so when actually configuring C1, C2, each can take about 20 ~ 15pF. And C1, C2 use ceramic chip capacitors is better.

Note on the choice of crystal capacitor:

(1) When selecting, you need to understand the key policy of this type of oscillator, such as equivalent resistance, (Kai Yuexiang manufacturers recommend load capacitance, such as frequency deviation.

(2) However, in the internship circuit, it is also possible to check the oscillation waveform by the oscilloscope to determine whether the oscillator is operating at the optimum condition.

(3) Of course, when the oscilloscope queries the oscillating waveform, the OSCO pin (Oscillator output) to be queried should select an oscilloscope probe with a bandwidth of 100 MHz or more. The probe has high input impedance, small capacitive reactance, and relatively small influence on the oscillating waveform.

(4) Since there is usually 10 to 20 pF of capacitance on the probe, it is possible to reduce the capacitance at the OSCO pin to obtain an oscillation waveform closer to the internship during observation.

How to determine if the crystal oscillator in the circuit is overdriven:

Resistor RS is often used to prevent the crystal from being overdriven. Excessively driving the crystal will gradually reduce the contact plating of the crystal, which will cause the frequency to rise. An oscilloscope can be used to detect the OSC output pin. If a very clear sine wave is detected and the upper and lower limits of the sine wave meet the clock input requirements, the crystal oscillator is not excessively driven; conversely, if the peak of the sinusoidal waveform, The ends of the trough are flattened, and the waveform becomes square, and the crystal is driven excessively. At this time, it is necessary to use the resistor RS to prevent the crystal oscillator from being excessively driven. The easiest way to determine the magnitude of the resistor RS is to connect a 5k or 10k trimmer resistor in series, slowly increasing from 0 until the sine wave is no longer flattened. By this method, the nearest resistance RS value can be found.