Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 63

SMT capacitors have a different model from resistors. To a first order, their parasitic effects tend to be all in series, as shown in the model of Figure 1.40. The series inductance is due to primarily to the package size and is similar to that of a resistor. The series resistance is due to the manufacturing characteristics of the capacitor and thus cannot be easily estimated. If an SMT capacitor is used in a resonant structure, this resistance will have the principal effect on the Q of the resonator. However, its effect is typically small in most wideband applications, where the capacitor is used as a series DC blocking capacitor or a shunt RF bypass capacitor. This is because the series inductance will dominate the series resistance in these use cases and cause the impedance of the capacitor to rise with frequency (rather than go to zero). At high frequencies, there may also exist a parasitic shunt capacitance across the entire package, which may cause the impedance to fall again.

Figure 1.40 Model of an SMT capacitor.

The case where the series resistance is of consequence is when the capacitor is used in a turned circuit, where the package inductance may be subsumed in the resonating inductor and thus at resonance the series resistance adds to a degradation of the Q of the capacitor. With careful design, the capacitance value may be compensated for by the including the effects of the series inductance; this effect is to make the capacitor look larger than its prescribed value. In fact, where the reactance of the parasitic inductance equals the reactance of the capacitance, the effective value of capacitance goes to infinite and the series impedance becomes just the parasitic resistance. So, for characterizing capacitors for use in tuned circuits, one must really assess their value near the frequency on which they will be having the most effect on a circuit. Consider a one‐pole filter, where the cutoff of the filter starts to occur when the reactance of the capacitor reaches 50 Ω. In many cases, the inductance is quite significant and already altering the effective value of the capacitor. Thus, it is important to evaluate the effective capacitance near this point. A good rule of thumb is to evaluate a capacitor where the reactance is j50 Ω.

A further characteristic of capacitors that is significant is the internal assembly structure. Capacitors are typically formed by a set of interleaved parallel plates with alternate plates connected at each end to the terminals. The plates can be parallel to or vertical to the PC board. For some cases, the capacitor body itself can form a dielectric resonator at high frequency, but below that the capacitor can act as a single, large conductive block on a PC board trace, typically resulting in a model that might best be considered a transmission line of somewhat lower impedance than the mounting line.

Capacitors used as bypass capacitors have an additional parasitic effect from the series inductance of the ground via, and from the pad above the ground via.