SRF And PRF And Their Relation To RF Capacitor Applications

By Johanson Technology Inc.

Capacitors exhibit both series and parallel resonant frequencies. There is a frequency for a capacitor with a given physical size/construction and a given capacitance value at which the component looks like an inductive impedance. Indeed, above this frequency one can have essentially a "DC blocking inductor." This frequency is called the Series Resonant Frequency (SRF). The magnitude of the transmission impedance the capacitor presents will be extremely low from the bottom frequency end determined by the capacitance value, right through the SRF until one approaches the first Parallel Resonant Frequency (PRF). The first PRF is frequently called simply the PRF, although in reality there is a second PRF, third PRF, and so on. A rough rule of thumb puts the PRF at twice the frequency as the SRF. The magnitude of the transmission impedance dip differs significantly at each PRF frequency, and with the various values of capacitors.

Exactly at the SRF, one will have the lowest possible impedance, and therefore, a capacitor is commonly used for narrowband RF bypass applications by choosing a capacitor whose SRF is at the required bypass frequency. In other words, at the SRF, the capacitive impedance of the device equals the parasitic inductive impedance of the device. The user is only left with the ESR (no reactive component) at that frequency.

For applications where low impedance is desired, but it is unimportant if the capacitor looks either capacitive or inductive, a capacitor could be used up to a frequency close to the PRF. Above the SRF, the user essentially has a "DC blocking inductor." At the PRF, the transmission impedance goes relatively high, and the capacitor is virtually useless around this frequency. There is a "trick" to effectively extend the usable frequency range in this application...

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