How do you know when to use series or parallel model in a specific frequency. I do have the 4263B.
Shortly saying the conclusion first, there is no significance in the guideline (values) of the series/parallel mode selection criteria. The difference between 100 W and 1 kW for the upper limit of the series circuit mode doesn’t matter. Also, the difference between 10 kW and 100 kW for the lower limit of the parallel circuit mode doesn’t matter as well. It is because the selection criteria is a very rough guideline, and cannot be determined more precisely.
Almost all capacitors and inductors have both parallel conductance (parallel resistance) and series resistance (which is also called ESR; Equivalent Series Resistance) included in their components. When DUT exhibits a low impedance (reactance) at the measurement frequency, we assume that the series resistance is more dominant than the parallel resistance. It is reasonable to assume that the DUT’s impedance is expressed by series equivalent circuit model. On the other hand, when DUT exhibits a high impedance (reactance) at the measurement frequency, we assume that the parallel resistance is more dominant than the series resistance. It is reasonable to assume that the DUT’s impedance is expressed by parallel equivalent circuit model. But, there is NO definite value of the criteria which discriminates between the series and parallel model. There is a wide GRAY ZONE in the middle impedance range, which cannot be clearly classified into series or parallel, because the magnitudes of the series/parallel resistances are different depending on the materials and structures of the components.
For example, in case of air-core inductors (which do not have magnetic core), the coil resistance is the dominant resistance factor in the components even if the inductive impedance (reactance) is relatively large (for example, 1 kW.) Hence, the air-core inductors are usually measured in Series circuit mode. In practice, when impedance-frequency characteristic of an air-core inductor is measured with an impedance analyzer, series circuit mode is usually used over the entire sweep frequency range. The circuit mode is not changed to Parallel mode even if the impedance increases to more than 1 kW or 10 kW. But, when measuring the impedance-frequency characteristic of a choking inductor which has a large magnetic core, parallel circuit mode is often used even if the inductive impedance (reactance) is relatively low (for example, hundreds Ohms) because the large magnetic loss of the inductor’s magnetic core is expressed by the parallel resistance model.
On the other hand, there is a case that the series/parallel circuit model is determined from industrial measurement standard. For example, the ceramic capacitors below 1000 pF are measured at 1 MHz by using Parallel circuit model in accordance with industrial standards. A 1000 pF capacitor has a capacitive impedance (reactance) of 160 W at 1 MHz. Thus, in this case, the parallel circuit mode is applied to the relatively low impedance down to 160 W.
Next, we assume that a 100 pF capacitor is measured by sweeping test frequency from 1 MHz to 1 GHz in order to analyze the resonant frequency characteristic around 500 MHz. At 1 MHz, the parallel circuit mode is reasonable in the aspect of the industrial standard. But, the capacitive impedance (reactance) decreases as the frequency goes up. If the objective of measurement is to evaluate the ESR (series resistance) at the resonant frequency point, the series circuit mode should be necessarily used to measure the ESR. In such case, the series circuit mode is usually used over the entire sweep frequency range, without changing the parallel/series circuit mode, even if the industrial standards specify parallel circuit mode at 1 MHz.
As it is understood from the above discussions and measurement examples, there is no obvious criteria for the selection of series/parallel circuit mode. In some cases, a measurement instrument user or a component designer needs to define the measurement circuit mode to be used. We need to consider the measurement objective, the actual characteristics of components, industrial standards, and component manufacturers specifications in addition to the rough criteria (shown in the impedance measurement hand book and other literatures) when selecting the series or parallel measurement circuit mode. The guideline of series/parallel circuit mode shown in the literatures are not the authorized selection criteria applicable to all the components. The selection criteria shown in the documents always include a large GRAY ZONE.
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