1. The common PCB material used by manufacturers is FR4, with a relative dielectric constant between 4.2 and 4.7. This dielectric constant varies with temperature and can change by as much as 20% over a temperature range of 0-70 degrees Celsius. The change in dielectric constant results in a 10% change in line delay on the PCB. Higher temperatures lead to greater delays. Additionally, the dielectric constant changes with signal frequency, decreasing as the frequency increases. For frequencies under 100 MHz, the capacitance and delay between plates can be approximated with a dielectric constant of 4.5.
2. Typically, the internal signal transmission speed on FR4 PCBs is 180 ps/inch (1 inch = 1000 mils = 2.54 cm), while surface layers generally range from 140 to 170 ps/inch.
3. On an actual PCB, capacitance can be approximated as L, R, and C in series. Capacitance exhibits a resonance point, which becomes more significant at high frequencies (beyond this resonance point). The capacitance value and behavior can vary, and different PCB manufacturers may produce varying results. This resonance point largely depends on the equivalent series inductance. For instance, a 100 nF patch capacitor on a PCB has an equivalent series inductance of about 0.5 nH, with an ESR (equivalent series resistance) of 0.1 ohms. It filters at around 24 MHz with an AC impedance of 0.1 ohms. A 1 nF patch capacitor also has an equivalent series inductance of 0.5 nH (the difference between capacitance values is minimal), with an ESR of 0.01 ohms, filtering around 200 MHz.
1. To achieve an effective filtering, we use capacitors with different capacitance values to achieve proper matching.
2. However, due to the effects of equivalent series inductance and capacitance, a resonance point between 24M and 200M may occur, where impedance is significantly higher than that of a single capacitor.
3. This is an undesired outcome. (Within the range of 24M to 200M, small capacitance on the PCB is capacitive, while large capacitance becomes inductive.)
4. The two capacitors on the PCB are effectively in parallel with LC.
5. The sum of the ESR values of these capacitors constitutes the series resistance of the LC circuit.
6. If the series resistance is zero in parallel, the impedance at the resonance point becomes infinite, leading to poor filtering performance.
7. This series resistance inversion mitigates the parallel resonance phenomenon and lowers the impedance of the LC resonator at the resonance point.
8. To address this, larger ESR capacitors can be used as needed.
9. ESR functions as the series resistance in the resonant network, helping to reduce the Q value and thus flatten the frequency characteristics.
2. Typically, the internal signal transmission speed on FR4 PCBs is 180 ps/inch (1 inch = 1000 mils = 2.54 cm), while surface layers generally range from 140 to 170 ps/inch.
3. On an actual PCB, capacitance can be approximated as L, R, and C in series. Capacitance exhibits a resonance point, which becomes more significant at high frequencies (beyond this resonance point). The capacitance value and behavior can vary, and different PCB manufacturers may produce varying results. This resonance point largely depends on the equivalent series inductance. For instance, a 100 nF patch capacitor on a PCB has an equivalent series inductance of about 0.5 nH, with an ESR (equivalent series resistance) of 0.1 ohms. It filters at around 24 MHz with an AC impedance of 0.1 ohms. A 1 nF patch capacitor also has an equivalent series inductance of 0.5 nH (the difference between capacitance values is minimal), with an ESR of 0.01 ohms, filtering around 200 MHz.
1. To achieve an effective filtering, we use capacitors with different capacitance values to achieve proper matching.
2. However, due to the effects of equivalent series inductance and capacitance, a resonance point between 24M and 200M may occur, where impedance is significantly higher than that of a single capacitor.
3. This is an undesired outcome. (Within the range of 24M to 200M, small capacitance on the PCB is capacitive, while large capacitance becomes inductive.)
4. The two capacitors on the PCB are effectively in parallel with LC.
5. The sum of the ESR values of these capacitors constitutes the series resistance of the LC circuit.
6. If the series resistance is zero in parallel, the impedance at the resonance point becomes infinite, leading to poor filtering performance.
7. This series resistance inversion mitigates the parallel resonance phenomenon and lowers the impedance of the LC resonator at the resonance point.
8. To address this, larger ESR capacitors can be used as needed.
9. ESR functions as the series resistance in the resonant network, helping to reduce the Q value and thus flatten the frequency characteristics.
