The use of crystal oscillators is not uncommon. In previous articles, the editor also introduced crystal oscillators, such as crystal oscillator circuits and quartz crystal oscillators. In this article, the editor will introduce how to choose C1C2 capacitors in the crystal oscillator circuit.
1. Introduction to Capacitors
Capacitance, also known as “capacitance”, refers to the amount of charge storage under a given potential difference, denoted as C, and the international unit is Farad (F). Generally speaking, the electric charge will be moved by force in the electric field. When there is a medium between the conductors, the movement of the electric charge will be hindered and the electric charge will be accumulated on the conductor, resulting in the accumulation and storage of the electric charge. The amount of the electric charge stored is called the capacitance. Capacitance refers to the ability to accommodate an electric field. Any electrostatic field is composed of many capacitors. If there is an electrostatic field, there is a capacitor. The capacitance is described by the electrostatic field.
2. Several criteria for the selection of crystal capacitors
(1) Within the allowable range, the lower the C1 and C2 values, the better.
(2) Although the larger value of C is beneficial to the stability of the oscillator, it will increase the start-up time.
(3) The value of C2 should be greater than the value of C1, so that the crystal oscillator can start to vibrate faster when it is powered on.
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 values provided by the manufacturer.
(2) Within the permitted range, the lower the value of C1 and C2, the better. A larger value of C is beneficial to the stability of the oscillator, but it will increase the start-up time.
(3) The value of C2 should be greater than the value of C1, so that the crystal oscillator can start to vibrate faster when it is powered on.
In the application of quartz crystal resonators and ceramic resonators, it is necessary to pay attention to the selection of load capacitors. 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, it is also possible to observe the oscillation waveform through the oscilloscope to judge whether the oscillator is working in the best state. When the oscilloscope observes the oscillating waveform, the OSCO pin (Oscillator output) should be observed, and an oscilloscope probe with a bandwidth of more than 100MHz should be selected. This probe has high input impedance and small capacitive reactance, and has relatively little impact on the oscillating waveform.
(Because there is generally a capacitance of 10 to 20 pF on the probe, during observation, appropriately reducing the capacitance at the OSCO pin can obtain a more realistic oscillation waveform). A well-working oscillator waveform should be a nice sine wave with peak-to-peak values 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. Detecting the OSCI (Oscillator input) pin with an oscilloscope can easily cause the oscillator to stop vibrating. The reason is that some probes have low impedance and cannot be tested directly. You can use the method of series capacitance to test.
Such as the commonly used 4MHz quartz crystal resonator, usually the external load capacitance recommended by the manufacturer is about 10 ~ 30pF. If the central value is 15pF, then C1 and C2 each take 30pF to obtain the equivalent capacitance value of 15pF in series. At the same time, considering that the distributed capacitance of the circuit board, the chip pin capacitance, and the parasitic capacitance of the crystal itself will affect the total capacitance value, the actual configuration of C1 and C2 can be about 20 ~ 15pF. And it is better to use ceramic capacitors for C1 and C2.
4. Precautions for selecting crystal capacitors
(1) When selecting, it is necessary to understand the key policies of this type of oscillator, such as equivalent resistance, (the manufacturer of Excellexiang recommends load capacitance, such as frequency deviation, etc.
(2) But in the practice circuit, it is also possible to check the oscillation waveform through the oscilloscope to determine whether the oscillator is working in the best condition.
(3) Of course, when the oscilloscope is inquired about the oscillating waveform, the OSCO pin (Oscillator output) to be queried should choose an oscilloscope probe with a bandwidth of more than 100MHz. This probe has high input impedance and small capacitive reactance, and has relatively little impact on the oscillating waveform.
(4) Since there is usually a capacitance of 10 to 20 pF on the probe, when observing, properly reducing the capacitance at the OSCO pin can obtain an oscillation waveform closer to the practice.
5. How to judge whether the crystal oscillator in the circuit is overdriven
Resistor RS is often used to prevent the crystal from being overdriven. Overdriving the crystal will gradually wear down 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 values of the sine wave meet the needs of the clock input, the crystal oscillator is not overdriven; on the contrary, if the peak of the sine waveform, The ends of the valley are flattened and the waveform is squared, and the crystal is overdriven. At this time, resistor RS is needed to prevent the crystal oscillator from being overdriven. The easiest way to judge the value of the resistance RS is to connect a 5k or 10k trimmer resistor in series, and gradually increase it from 0 until the sine wave is no longer flattened. In this way, the closest resistance RS value can be found.
