Coupled circuits

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In a receiver having two tuned circuits, both circuits must be tuned to the same frequency.
De 2 coils of the tuned circuits must have a certain distance in between, via the magnetic field around the coils, the circuits are coupled to each other.
The distance between the coils determines the degree of coupling, the smaller the distance, the higher the coupling.

The -3dB bandwidth of a coupled circuit is higher than with a single tuned circuit.

The suppression of frequencies at some distance from the tuning frequentie is in a coupled circuit better than with a single circuit, this is good for suppressing strong local station.

In the pictures below you see the response of the coupled circuit at different degrees of coupling.
For comparison the response of a single tuned circuit is shown as a dotted line.

The pictures are taken from a radiohandbook, and are about a coupled filter for 465 kHz, but the theory is usefull for all coupled circuits, so also for crystal receivers with two tuned circuits.

The coupling between the circuits is too low, because the distance between the coils is too high.

There is no maximum powertransfer from one coil to the other.

The bandwidth is higher compared to a single circuit.

Critical coupling
There is maximal powertransfer.

The bandwidth is higher than in a single circuit, and also also higher than a undercoupled circuit.

The response curve is flat in the top over a smal part.

The coupling is too high because the distance between the coils is too small.

The bandwidth is too high.

The curve shows two peaks, the higher the coupling , the higher the distance between the peaks, and the deeper the dip between the peaks.


The critically coupled circuit is the best compromis between sensitivity and bandwidth.

Ther are several methods for adjusting coupling.
The methods shown below are mostly used in crystal receivers.

The coils are side by side

Coupling is adjusted by varying the distance between the coils.


Coils behind each other.

Coupling is adjusted by varying the distance between the coils.



One coil is turnable.

Coupling can be adjusted by varying the angle between the coils.

With both coils in the same direction, the coupling is maximal.
Turning the angle towards 90 degrees, the coupling will decrease.

With a angle of 90 degree, there is no coupling at all.


Capacitive coupling

Coupling is adjusted by a variable capacitor with a very low value (e.g. 1 pF).

There must be no coupling via the magnetic field, so the coils must be placed at a right angle.

Capacitive coupling is also useable when magnetic coupling is difficult, for instance when using ringcore or potcore coils.

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