Pairs of coupled tuned circuits are often used in receivers and transmitters. The effect of varying the degree of coupling between two parallel tuned circuits resonant at the same frequency is shown in Fig 2.16.
Fig 2.16. Inductively-coupled tuned circuits. Curves shown at (b) represent various frequency-response characteristics of coupled circuit shown at (a) for different degrees of coupling
When the coupling is loose, the response from one circuit to the other is as curve (I). As the coupling is increased to what is known as 'critical coupling', the output at resonance increases to curve (II); here the mutual coupling between the coils is l / Q of the inductance of either coil. Further increase (tight coupling) results in the formation of the double-humped characteristic shown in curve (III), where the output at resonance has decreased.
Two tuned circuits are often mounted in a screening can, the coils generally being wound the necessary distance apart on the same former to give the required coupling. The coupling is then said to be 'fixed'.
