This article describes the non-mational behavior of high-frequency capacitors and helps you choose a suitable capacitance and inductor to match networks, DC modules, crystals, and power supply.
In addition to the expected capacitance, all capacitors also include parasitic resistance, parasitic capacitance, and parasitic inductance. The theoretical model of a typical capacitor is shown below.
Capacitor model
C is a capacitor for a capacitor. Due to the reactance (XC) and parasitic capacitance (CP) caused by the capacitor (CP), the reactance (XCP) is respectively
As can be seen from the equation, the reactance of the capacitor decreases as the frequency increases. CP is a very low parasitic capacitance. Therefore, the reactance of the element is very high at low frequencies. Since the component is parallel with the main capacitor, the CP has no effect on low frequencies. The current change of the capacitor causes the magnetic field change around the capacitor, wherein a portion of the conductor is induced, the EMF is introduced, and the resistance of the antifact current results in parasitic inductance. This parasitic inductance is
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The reactance of parasitic inductance increases frequency. Typically, the capacitor is very small in the capacitor; so, the XL can be ignored at low frequencies. R is a valid series resistance of the capacitor. It is usually a very low value.
The effective impedance of the capacitor is
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At low frequencies, XCP is very high and effective impedance is
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At low frequencies, the circuit is mainly capacitive. XEFF is almost the same as XC. However, as the frequency increases, the XC continues to decrease, and XL continues to increase. Finally, at a certain frequency, XL is equal to XC, the resistance of the capacitor is equal to R. This frequency is the series resonance frequency (SRF) of the capacitor.
When the capacitor is selected, make sure the SRF is much higher than the operating frequency. This ensures that the reactance of the capacitor is dominant because the capacitance value is published, and the effective reactance will not be lowered by parasitic inductance.
When selecting a decoupling capacitor, it is preferable to select a value having an SRF decoupling that is close to the noise frequency. This ensures noise to seek a low impedance ground path.
At higher frequencies, the reactance XCP becomes equal to the reactance of the other reactance arm (most of which is equal to XL). At this frequency, the capacitor performs like an open circuit.
This frequency is a parallel resonant frequency. Avoid using capacitors under parallel resonant frequencies.
Capacitor quality factor
The quality factor (q) of the capacitor (C) is the ratio of the reactance of the capacitor (f) and its resistance (R).
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High Q capacitors have less unwanted resistors. Make sure to use a high Q value capacitor at the operating frequency of the RF circuit; otherwise, the RF energy is wasted as thermal energy in the resistance of the capacitor.
Capacitor recommended
▪ For components that match the network, only C0G / NP0 capacitors can be used. This ensures that the matching network does not change over the entire temperature range.
▪ For crystalline loads, only C0G / NP0 capacitors can be used. This ensures that the clock timing and RF frequencies will not change over the entire temperature range.
▪ For matching networks, select capacitors that work normal when far below SRF.
▪ The RF circuit can only use a high Q capacitor.
▪ For decoupling capacitors, it may not require the accuracy of C0G capacitance. The X5R or X7R capacitor is usually used (depending on the temperature range). Use low ESR capacitance
Effective decoupling
▪ For the decoupling capacitor, the component value of the SRF is selected at the noise frequency.
▪ It is recommended to use smaller components (0402 or 0201) because their parasitic reactors are smaller.
▪ When the DC module is added to the matching RF trace, it is preferable to use the SRF approach the operating frequency and the ESR low capacitance, because the capacitance of the SRF is effective
The reactance becomes zero. So it does not change the impedance match.
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