Why is it undesirable to use ferrite magnetic cores in resonant circuits of the radio frequency path of receiving devices. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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In any handbook on radio engineering, you can find a graph of the dependence of the magnetic induction B in a ferromagnet on the magnetic field strength H, the so-called magnetization curve (Fig. 1.a).

Fig.1.a

If a demagnetized ferrite is placed in a magnetic field, then with increasing strength, the magnetic induction will increase in accordance with the OA curve. If the ferrite is pre-magnetized, then the dependence of B on H has the form of the so-called hysteresis loop. When the field strength decreases, the value of B changes in accordance with the upper part of the curve, and with an increase, in accordance with the lower one.

If the current i flowing through the coil changes in time according to a harmonic law, i.e., along a sinusoid (Fig. 1.b), then the voltage that arises at the ends of the coil is proportional to the rate of change of the magnetic flux penetrating the section of the coil.

Fig.1.b

Thus, from Fig. 1c it can be seen that the U(t) graph is very different from a sinusoid, i.e., in the voltage spectrum, in addition to the first harmonic, there are also higher harmonics. Moreover, if the hysteresis loop is symmetrical, and the current through the coil does not have a constant component u. therefore, there is no permanent bias, then the spectrum contains only odd harmonics - mainly the first, third, fifth and seventh. If, however, currents of two frequencies f1 and f2 flow simultaneously through the coil, then in the frequency spectrum of the voltage, as a result of mutual modulation, not only higher harmonics will appear, but also combination frequencies 2f1-f2, 2f2-f1, 3f1-2f2, 3f2-2f2, etc. d. Therefore, the shapes of the output and input signals are very different.

Fig.1.c

In receiving equipment, ferrite magnetic cores can be used in non-resonant current transformers and balancing devices, which must be calculated so that the magnetic fields created by currents flowing through different windings are mutually compensated.

Literature

1. Radio No. 3, 1985, p.63.

Publication: N. Bolshakov, rf.atnn.ru

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