How to choose capacitance in crystal oscillator circuit?

1. Brief introduction of capacitor
Capacitance, also known as “capacitance”, refers to the charge storage under a given potential difference. It is recorded as C, and the international unit is farad (f). Generally speaking, the charge will move under the force in the electric field. When there is a medium between the conductors, it will hinder the charge movement and make the charge accumulate on the conductor, resulting in the accumulation and storage of charge. The amount of stored charge is called capacitance. Capacitance refers to the ability to hold an electric field. Any electrostatic field is composed of many capacitors. If there is an electrostatic field, there will be capacitance. Capacitance is described by electrostatic field.
2. Several standards of crystal oscillator capacitance selection
(1) Within the allowable range, the lower the values of C1 and C2, the better.
(2) The larger C value is beneficial to the stability of the oscillator, but it will increase the starting time.
(3) The value of C2 should be greater than the value of C1, so that when the power is on, the start-up of the crystal oscillator can be accelerated.
3. How to choose capacitor C1C2 in crystal oscillator circuit
(1) Because each crystal oscillator has its own characteristics, it is best to select external components according to the value provided by the manufacturer.
(2) Within the permitted range, the lower the value of C1 and C2, the better. The larger C value is beneficial to the stability of the oscillator, but it will increase the starting time.
(3) The value of C2 should be greater than that of C1, which can accelerate the start-up of crystal oscillator when it is powered on.
In the application of quartz crystal resonator and ceramic resonator, we need to pay attention to the selection 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’s recommended load capacitance, frequency deviation, etc. In the actual circuit, the oscillograph can also be used to observe the oscillation waveform to determine whether the oscillator works in the best state. When the oscillograph observes the oscillation waveform, the oscillograph probe with a bandwidth of more than 100MHz should be selected to observe the osco pin (oscillator output). This kind of probe has high input impedance, small capacitive reactance and relatively small influence on the oscillation waveform.
(because there is a capacitance of 10-20pf on the probe, the oscillation waveform can be obtained by appropriately reducing the capacitance at the osco pin). A good oscillation waveform should be a beautiful sine wave, and the peak to peak value should be 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 reduced appropriately. On the contrary, if the peak to peak value is close to the supply voltage and the oscillation waveform is distorted, the load capacitance can be increased appropriately. It is easy to cause the oscillator to stop when the oscillograph is used to detect the osci (oscillator input) pin. The reason is that the impedance of some probes is too small to be tested directly, and the method of series capacitance can be used to test.
For example, 4MHz quartz crystal resonators are commonly used, and the external load capacitance recommended by manufacturers is about 10-30pf. If the center value is 15pF, then the series equivalent capacitance of C1 and C2 is 15pF. At the same time, considering that the distributed capacitance of the circuit board, the pin capacitance of the chip and the parasitic capacitance of the crystal itself will affect the total capacitance, the actual configuration of C1 and C2 can be about 20-15pf each. It is better to use ceramic chip capacitor for C1 and C2.
4. Matters needing attention in selecting crystal oscillator capacitor
(1) In the selection, need to understand the key policy of this type of oscillator, such as equivalent resistance.
(2) But in the practice circuit, we can also check the oscillation waveform through the oscilloscope to determine whether the oscillator is working in the best situation.
(3) Of course, when the oscillograph inquires the oscillation waveform, the oscillograph probe with more than 100MHz bandwidth should be selected for the inquired osco pin (oscillator output). This kind of probe has high input impedance, small capacitive reactance and relatively small influence on the oscillation waveform.
(4) Since there is usually a capacitance of 10-20pf on the probe, the oscillation waveform closer to the practice can be obtained by appropriately reducing the capacitance at the osco pin.