High frequency chokes
Than the choke differs from the inductance coil? Practically anything! However it has specific features, differently for such element would not invent a special name.
The choke's main destination – to create high AC resistance ( blocking – "choking" from it) and to pass lost-free DC. Impedance of the coil inductance is defined under the formula:
XL = 2πƒL
From the formula it is visible that the choke should have a greatest possible inductance, the it above, the better it "choking" AC. However on high frequencies there are some features. Many radio fans in the constructions as chokes apply the multilayer coils with the big inductance, by a principle – "you can't have too much of a good thing". However in that case very much parasitic capacitance increase. These capacitances in a combination to high inductance of the choke transform it into the nonuniform chain of oscillating circuits with resonances quite often getting to working range or even below it, when the choke is converted to a condenser! Often these parasitic oscillating circuits, interacting with device active elements, lead to self-excitation and generally break its operation. And in some cases on some frequencies these oscillating contours are equal to one parallel LC circuit, in this case impedance of choke reaches to zero.
The above frequency, the these bad effects are more strongly shown, hence with growth of frequency inductance of the choke should decrease. In practice this inductance select from account that its inductive resistance on the order exceeded resistance of elements parallel which it is connected on high frequency. There is one more criterion – total length of a wire by which the choke is wound should not exceed a quarter of wavelength of the high frequency of operating range.
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