PANASONIC TX28MDT4F
Service Manual

Philips Semiconductors

Circulators and Isolators, unique passive devices

Application Note AN98035

isoductor: three inner conductors coming from the three ports cross under an angle of 120 degrees and are connected to the outer conductors at the other end. They are isolated from each other where they cross each other. Between the inner and outer conductors there are two ferrite disks, magnetized by a permanent magnetic field. An rf current in inner conductor 1 generates in the ferrite disks an rf magnetic field perpendicular to the plane of the loops, which it makes together with the outer conductors. Due to the properties of the ferrite the rf magnetization is not in the same direction, as we can see in the Polder tensor page 3. The angle between the rf magnetic field and the rf magnetization depends on the frequency and the permanent magnetic field. If we adjust the permanent magnetic field so that the rf magnetization is parallel to the inner conductor 3, then loop 3 is not induced but only loop 2: an excitation of inner conductor 1 will be coupled to inner conductor 2 but not to inner conductor 3, inner conductor 3 is decoupled. If we look from outside into one port of the isoductor, we see an inductance parallel to a resistor (see figure 22). The simplest way to make a ciculator out of this isoductor is given in figure 23: We bring the inductance of the isoductor into resonance by a parallel capacitor and match the whole to the line by a series capacitor. With Fig.22: Equivalent circuit of an isoductor more complicated networks we can build broadband circulators with a band-width of up to one octave. Lumped element circulators can be made for frequencies between 30 MHz and about 2 GHz, but normally they are used between 50 and 500 MHz.

Fig. 23: Small band lumped element circulator

1998 Mar 23

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