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6-4 Types of Frequency Traps in Medium Frequency Amplifiers:

Frequency traps absorb or suppress unwanted frequencies, such as the sound carrier frequencies of the lower adjacent channel or the image carrier frequencies of the upper adjacent channel. That is, if the voter is tuned to the third channel, the trap suppresses the sound carrier for the second channel, and another trap suppresses the image carrier for the fourth channel. This avoids and protects the received channel image signal from interference and distortion of the signals that may be entered.

In fact, there are five types of traps used in the amplifier: IF image, which is as shown in Figure (6-9) and (6-10).

1- The serial trap.
2- The branching trap
3- The suction trap (aspirator).
4- Trap T.
5- The damper trap.

1- The serial trap:

It is a circuit of a coil and a capacitor in parallel tuned at a certain resonant frequency - placed between two stages of the intermediate frequency - the circuit is tuned sharply at the desired frequency, so that it rejects this frequency and the frequency close to it, thus preventing its signal from reaching another stage and allowing the rest of the frequencies By passing by.​​​​​​

3- The suction trap (aspirator):

It is a commonly used circuit called a rejection circuit), consisting of a coil and a fixed capacitor. The coil L1 is connected with the coil L2 inductively, and L1 represents the load of the medium frequency amplifier IF. When the resonant frequency of the circuit of this trap passes, a large current strength is generated due to the inductive connection between the trap and the L1 coil, and thus the voltage of this signal decreases to a very small value - and it does not pass to the next stage - and this trap is distinguished from others in that it absorbs or drops the total power of the tuned-frequency signal on him. The frequency of the absorption trap can be changed by changing the position of the ferrite core inside the coil, as shown in the figure (6-11).

4- Trap Model T:

It is the most complex, but the most effective. In Figure (6-13), we find that the trap circuit consists of elements (1-21) and it is tuned to the frequency of the rejected signal - and the resistance R has a calculated value, and its work is to dampen the signal that the circuit is tuned to.
The ratio of the rejected signal to the allowed signal is (70 or 60) meaning that the permissible signal strength is 50 to 60 times greater than the rejected signal strength due to the existence of this trap.

5 - the damping trap:

It works to reduce the amplification gain of tuned frequencies - it is usually used in the transistor emitter circuit or cathode circuit in valves and is called (cathode trap). The traps often used in damping are absorption traps and serial traps, Figure (6-9).