This is because the impedance transformer line only has the correct electrical length of λ/4 at one specific frequency. Filters incorporating λ/4 inverters are only suitable for narrow band applications. Inverters are classified as K-inverters or J-inverters depending on whether they are inverting a series impedance or a shunt admittance. Equally, for bandpass filters, a two-resonator-kind (resonators and anti-resonators) filter can be converted to a one-resonator-kind. The most common use for inverters is to convert a 2-element-kind LC filter design such as a ladder network into a one-element-kind filter. There are many such circuits and the term does not necessarily imply a λ/4 transformer. Inverter is a general term for the class of circuits that have the function of inverting an impedance. Impedance inverters are not to be confused with the more common meaning of power inverter for a device that has the inverse function of a rectifier. The device can be used as a component in a filter, and in this application it is sometimes known as an inverter because it produces the mathematical inverse of an impedance. Likewise, an open circuit can be transformed into a short circuit. Feeding in the DC via a λ/4 transformer will transform the short circuit into an open circuit which has no effect on the signals on the line. An ideal DC voltage source has zero impedance, that is, it presents a short circuit and it is not useful to connect a short circuit directly across the line. Īnother application is when DC power needs to be fed into a transmission line, which may be necessary to power an active device connected to the line, such as a switching transistor or a varactor diode for instance. It is especially useful for making an inductor out of a capacitor, since designers have a preference for the latter.
The lumped-element low-pass filter (top) can be converted to a design that eliminates the inductors and contains capacitors only by the use of J-inverters, resulting in a mixed lumped-element and distributed-element design.Īt radio frequencies of upper VHF or higher up to microwave frequencies one quarter wavelength is conveniently short enough to incorporate the component within many products, but not so small that it cannot be manufactured using normal engineering tolerances, and it is at these frequencies where the device is most often encountered.
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