In simple reactive circuits with almost no resistance or no resistance, the effect of fundamentally changing impedance will appear at the resonant frequency predicted by the earlier given equation. In parallel (tank) LC circuits, this means that the impedance at resonance is infinitely large. In a series resonant (also known as series variable frequency resonant) circuit, this means that the impedance at resonance is zero:
In LC circuits, the tendency of impedance to deviate to the maximum or minimum point due to increased resistance is called anti resonance. Sharp observers will notice a pattern in how resistance affects the resonance peak of a circuit:
Parallel ("tank") LC circuit:
Series connection of R and L: resonant frequency shifts downwards
R and C series: resonance frequency offset
Series resonant circuit:
R and L - parallel: resonance frequency offset
Parallel connection of R and C: resonant frequency shifts downwards
Again, this illustrates the complementarity between capacitors and inductors: how a series resistor produces an anti resonant effect equivalent to a parallel resistor with another resistor. Anti resonance is an impact that resonant circuit designers must be aware of. The equation for determining the anti resonance "displacement" is very complex.
The added resistance in LC circuits is not an academic issue. Although capacitors with negligible excess resistance can be manufactured, inductors are often plagued by a large amount of resistance due to the long length of wires used in their structure. In addition, due to a strange phenomenon called skin effect, it tends to exclude AC current from traveling through the center of the wire, thereby reducing the effective cross power of the wire, and the resistance of the wire tends to increase with increasing frequency. Cross sectional area. Therefore, inductors not only have resistance, but also resistance that varies with frequency.
It seems that the resistance of the inductor wire is not sufficient to cause the problem, and we must also deal with the "core loss" of the iron core inductor, which is manifested as additional resistance in the circuit. Due to the fact that iron is both a conductor of electricity and a conductor of magnetic flux, the change in magnetic flux generated by alternating current passing through a coil will tend to induce current (eddy current) in the iron core itself. It can be considered that this effect seems to be due to the fact that the iron core of the transformer is a secondary transformer coil that supplies power to resistive loads: the conductivity of iron metal is not perfect. Stacked iron core, good iron core design, and high-grade materials can minimize this impact, but it will never be completely eliminated.
A notable exception to the circuit resistance rule that causes resonance frequency shift is the case of series resistor inductor capacitor ("RLC") circuits. As long as all components are connected in series with each other, the resonant frequency of the circuit will not be affected by resistance.