Radio and Electronics (DED Philippinen, 66 p.) 
8. PASSIVE COMPONENTS 
8.4. COMBINATION OF PASSIVE COMPONENTS 
8.4.3. TUNED CIRCUIT CONNECTED TO AN ACVOLTAGE 

For radio technology tuned circuits are mainly means to filter out a special frequency from a spectrum of frequencies. Therefore radio technicians would wish it would be possible to construct tuned circuits with a graph as shown in fig. 83. A tuned circuit with such a characteristics gle frequency. But a tuned circuit in reality will never have such a characteristics. Its characteristics will always be much smoother.
fig. 83
But different combinations of different inductors and capacitors, will show different characteristics. And it is now necessary to differentiate them.
To find out the quality of a tuned circuit mostly we use a connection like shown in fig. 84. If we vary the frequency of the acsource we will measure changing voltages at the voltmeter. This can be understood very easy if we look at the circuit as a series connection of a resistor and a tuned circuit of parallel type.
fig. 84
Recollecting the overall impedance of the parallel type we can easily predict that the current at resonant frequency will be minimum and therefore the voltage V at the Voltmeter will be maximum. If we plot these values, we find also for the resonant frequency certain resistance in the tuned circuit. This resistance is called the RESONANT RESISTANCE.
fig. 85
fig. 86
If we imagine we would connect additional resistances in parallel and we would repeat the same experiment, we can predict too, that the graphs get as flatter as lower the parallel resistance gets. A flatter graph shows that the circuit is less able to filter. The QUALITY of tuned circuits can be calculated by the following formulas:
parallel Tuned circuit: _{} series Tuned circuit: _{} 