دارة المحدد (limiter) .. كتاب التلفزيون الملون والعادي

​18-5 LIMITER circuit:

The function of this circuit is to remove the variables in the amplitude of the medium frequency signal of the FM audio, and thus the signal gets rid of the largest amount of interference pulses with it, which may arrive with the audio signal or caused by the device’s amplification circuits that may enlarge some frequencies more than others for reasons related to the response curve. Because it is practically impossible to obtain a perfect curve with a flat top and right sides, as in Figure (A10-18). Limiting the amplitude of the FM signal does not have any harmful effect on the IF band or the audio information it carries.

The limiter can be binary, so the limitation is on one side, and it may be composed of two oppositely shaped diodes - it works to determine the signal from the top and bottom, and one or more link capacitors reserve the continuous component. The limiter may be valve or transistor. In modern devices, an integrated circuit IC performs various tasks and functions. One of these functions is to identify the signal with an accuracy of up to (100) microvolts, such as the circuit (MC1590) Figure (11-19), while in the diodes method, the determination is about (1) volt only. The process of determining the amplitude can be done in a separate stage or by using an FM modulation detector, and this detector does not respond to the capacitive changes of the FM signal.

transistor selector:

The scheme can be determined in one stage, but the best identification requires two stages - the figure - says the first detection stage is binary (D) according to this binary has a short time constant that enables it to identify and cut the jamming pulses. The second stage is the Q2 transistor, which has a longer time constant, and it detects and cuts changes with a longer period of time.

The working principle of the circuit.

The transistor: the selected amplifier receives the 5MHz FM input signal from the IF amplifier.

The 5.5 MHz FM signal appears in a collector circuit and the diode ​ connects to both ends of the primary coil of the transformer (55) MHz T1 - so  determines the negative parts of the signal and any AM capacitive modulation in it.

The polarity of the signal in the transformer T fluctuates and is applied to the base of the selector transistor Q. This transistor works in the saturation point because of the positive bias of the base provided by the voltage part (R-R2), and whatever the positive part of the signal on collector 2 is negative in the base of Q2.

This negative signal interrupts Q and thus defines the second half of the input signal. Figure (13-18) shows a simplified circuit for a specific working principle with two diodes to determine the signal from both sides.​