The load capacitance is the capacitance required for the crystal to oscillate normally. In other words, the frequency of the crystal is measured under the load capacitance it provides, which can ensure the error of the frequency value to the utmost. It can also ensure errors such as temperature drift. The load capacitance value of the crystal oscillator is a known number and has been fixed at the time of shipment. The two capacitors on the crystal oscillator of the MCU are the external capacitors of the crystal oscillator, which are respectively connected to the two pins of the crystal oscillator and the capacitance to the ground, generally in the tens of skins. When selecting the external capacitor, the value is selected according to the crystal oscillator requirements provided by the crystal oscillator manufacturer. Generally, the external capacitor is used to make the equivalent capacitance at both ends of the crystal oscillator equal to or close to the load capacitance. In the case of high requirements, the capacitance to ground of the ic input should also be considered. Then, based on the determined load capacitance, the external capacitor affects the resonant frequency and output amplitude of the crystal.
Load capacitance Each crystal will have parameters such as: how much stability is the PPN load capacitance PF, etc. When the crystal is connected to the oscillating circuit, the capacitance introduced by the oscillating circuit does not meet the capacity requirement of the crystal load capacitor The frequency of the circuit will be different from the frequency marked by the crystal.
So how do you choose an external capacitor?
The load capacitance of the crystal oscillator = [(Cd * Cg) / (Cd + Cg)] + Cic + △ C where Cd, Cg are respectively connected to the two pins of the crystal and the capacitance to the ground, Cic (integrated circuit internal capacitance) +△C (capacitance on PCB) The empirical value is 3 to 5pf. The values ​​of the two capacitors are the same, or the phase difference is not large. If the phase difference is too large, the resonance imbalance is easily caused, and it is easy to cause the vibration to stop or simply not vibrate. Generally, the capacitance connected to both ends of the crystal is twice the required load capacitance. This is close to the load capacitance. For example, if the load capacitance is 15pf, the two sides are connected to 27pf.
From the equivalent circuit of the quartz crystal resonator, it has two resonance frequencies, that is, (1) when the L, C, and R branches undergo series resonance, its equivalent impedance is the smallest (equal to R). The series oscillating frequency is expressed by fs. The quartz crystal is purely resistive to the series oscillating frequency fs. (2) When the frequency is higher than fs, the L, C, and R branches are inductive and can be combined with the capacitor C. Parallel resonance occurs, and its parallel frequency is represented by fd.
According to the equivalent circuit of the quartz crystal, its reactance-frequency characteristic curve can be qualitatively drawn. It can be seen that when the frequency is lower than the series resonance frequency fs or the frequency is higher than the parallel resonance frequency fd, the quartz crystal is capacitive. Only in fs
Within the scope of the license, the lower the C1, C2 value, the better. A large C value is beneficial to the stability of the oscillator, but it will increase the start-up time. The choice of crystals is very important in low-power designs, especially for systems with sleep wake-up, often using low voltages for low power consumption. Since the low supply voltage reduces the excitation power supplied to the crystal, the crystal starts to oscillate slowly or does not start at all. This phenomenon is not particularly noticeable at power-on reset. When the power is turned on, the circuit has enough disturbance and it is easy to establish oscillation. When the sleep wakes up, the disturbance of the circuit is much smaller than when the power is on, and the start-up becomes very difficult. In the oscillating circuit, the crystal can neither be over-excited (easy to vibrate to higher harmonics) nor under-excited (not easy to oscillate). The choice of crystal should consider the following elements: resonant frequency, load capacitance, excitation power, temperature characteristics, long-term stability. In other words, the crystal reliability work is not only affected by external capacitors. The choice of external capacitor should be selected according to the value of the datasheet provided by the crystal supplier. Within the permissible range, the lower the external capacitor value, the better. The larger value of the capacitor is beneficial to the stability of the oscillator, but it will increase the start-up time. Some crystal oscillator recommended circuits even require a series resistor RS, which is generally used to prevent the crystal oscillator from being overdriven. Excessively driving the crystal will gradually reduce the contact plating of the crystal, which will cause the frequency to rise, causing frequency shift and accelerating aging.
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. 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 capacitor 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 capacitance can be appropriately increased. If the peak of the sinusoidal waveform is flattened at both ends of the trough and the waveform becomes square, the crystal oscillator is excessively driven. 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.
Load capacitance Each crystal will have parameters such as: how much stability is the PPN load capacitance PF, etc. When the crystal is connected to the oscillating circuit, the capacitance introduced by the oscillating circuit does not meet the capacity requirement of the crystal load capacitor The frequency of the circuit will be different from the frequency marked by the crystal.
So how do you choose an external capacitor?
The load capacitance of the crystal oscillator = [(Cd * Cg) / (Cd + Cg)] + Cic + △ C where Cd, Cg are respectively connected to the two pins of the crystal and the capacitance to the ground, Cic (integrated circuit internal capacitance) +△C (capacitance on PCB) The empirical value is 3 to 5pf. The values ​​of the two capacitors are the same, or the phase difference is not large. If the phase difference is too large, the resonance imbalance is easily caused, and it is easy to cause the vibration to stop or simply not vibrate. Generally, the capacitance connected to both ends of the crystal is twice the required load capacitance. This is close to the load capacitance. For example, if the load capacitance is 15pf, the two sides are connected to 27pf.
From the equivalent circuit of the quartz crystal resonator, it has two resonance frequencies, that is, (1) when the L, C, and R branches undergo series resonance, its equivalent impedance is the smallest (equal to R). The series oscillating frequency is expressed by fs. The quartz crystal is purely resistive to the series oscillating frequency fs. (2) When the frequency is higher than fs, the L, C, and R branches are inductive and can be combined with the capacitor C. Parallel resonance occurs, and its parallel frequency is represented by fd.
According to the equivalent circuit of the quartz crystal, its reactance-frequency characteristic curve can be qualitatively drawn. It can be seen that when the frequency is lower than the series resonance frequency fs or the frequency is higher than the parallel resonance frequency fd, the quartz crystal is capacitive. Only in fs
Within the scope of the license, the lower the C1, C2 value, the better. A large C value is beneficial to the stability of the oscillator, but it will increase the start-up time. The choice of crystals is very important in low-power designs, especially for systems with sleep wake-up, often using low voltages for low power consumption. Since the low supply voltage reduces the excitation power supplied to the crystal, the crystal starts to oscillate slowly or does not start at all. This phenomenon is not particularly noticeable at power-on reset. When the power is turned on, the circuit has enough disturbance and it is easy to establish oscillation. When the sleep wakes up, the disturbance of the circuit is much smaller than when the power is on, and the start-up becomes very difficult. In the oscillating circuit, the crystal can neither be over-excited (easy to vibrate to higher harmonics) nor under-excited (not easy to oscillate). The choice of crystal should consider the following elements: resonant frequency, load capacitance, excitation power, temperature characteristics, long-term stability. In other words, the crystal reliability work is not only affected by external capacitors. The choice of external capacitor should be selected according to the value of the datasheet provided by the crystal supplier. Within the permissible range, the lower the external capacitor value, the better. The larger value of the capacitor is beneficial to the stability of the oscillator, but it will increase the start-up time. Some crystal oscillator recommended circuits even require a series resistor RS, which is generally used to prevent the crystal oscillator from being overdriven. Excessively driving the crystal will gradually reduce the contact plating of the crystal, which will cause the frequency to rise, causing frequency shift and accelerating aging.
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. 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 capacitor 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 capacitance can be appropriately increased. If the peak of the sinusoidal waveform is flattened at both ends of the trough and the waveform becomes square, the crystal oscillator is excessively driven. 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.
Electronic Components Transformer
Transformer is a device that USES the principle of electromagnetic induction to change the ac voltage. Its main components are primary coil, secondary coil and iron core.The main functions are: voltage transformation, current transformation, impedance transformation, isolation, voltage stabilization (magnetic saturation transformer), etc.According to use can be divided into: power transformer and special transformer (electric furnace change, rectifier, power frequency transformer, voltage regulator, mining, audio transformers, intermediate frequency transformer, high frequency transformer, impact transformer, instrument transformer, electronic transformer, reactor, transformer, etc.).The circuit symbol T is often used as the beginning of the number. Examples: T01, T201, etc.
Electronic Components Transformer, Electrical Transformer, AC Transformer, 12V Transformer
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