المميز (Discriminator) .. كتاب التلفزيون الملون والعادي

18-6 Discriminator:

The discriminant is used to detect the frequency modulated FM wave, and its simplified circuit (Fig. 18-14) consists of three resonant circuits, often in series or in parallel. In these circuits, the highest voltage is generated at the resonant frequency, so the first circuit, L, tunes at the middle frequency of sound, 5.5 MHz, while circuit L and circuit L, one of them tunes at a frequency higher by (100 kilohertz) and the second at a frequency lower by (100 kilohertz), i.e. the resonant frequency of the circuit.
(= 5.4 MHz and a circuit of 56 MHz), thus obtaining two curves for the response as in Figure (15-18). The output voltage of the circuit. As shown in Figure (16-18), due to the connection of load resistors and diodes in the circuit, and the voltages formed on the two resistors are almost in opposite mode. At point X, the resultant of the two efforts is zero. As for the other frequencies, the resultant of the two voltages forms the response curve of the total circuit of the discriminator, which resembles the letter S at all frequencies of the discriminator. The straight part of the curve between the two DC points is often extended to more than the required higher and lower frequency, to ensure its straightness in the specified field.

This feature is no longer used in modern devices. However, we have explained it because the principle of its work is similar to the work of the discriminator that was used after it, which is the Fooster-Seeley discriminant (Fig. 17-18). In which it was replaced by a secondary circuit with one tuning capacitor, two diodes, and two load resistors (1 - R2). It has a curve in the result: similar to the first discriminator. With the existence of the discriminant, there must be a circuit with a limit and amplitude, because the discriminator is sensitive to changes in the signal amplitude. It can be said that the Distinguished does not have the excellent quality and performance for detecting FM waves, so other better detectors are used and are not affected by amplitude changes and do not require a specific circuit.

7-18 Audio Detector:

The detector extracts the frequency-modulated audio signal from the medium-frequency wave of the IF sound carrying it - and returns it to the audio frequency that does not exceed
(15 kHz) The detector has several types and is distinguished from the distinguished one in that it is not affected by the capacitive changes of the AM signal, which, as we mentioned, interferes with the frequency-modulated signal, whether through interference and interference pulses, or because some frequency bands are enlarged more than others in the field of sound-carrying frequency or medium frequency IF. these reagents.

Ratio Detector:

Figure (18-18) This detector extracts frequencies that carry low-frequency audio information without being affected by the capacitive changes of the signal that enters it, which is modulated at FM frequency. And the circuit is in the figure
(18 - 19) It is used in practical and economical ways - and it is developed from the practical circuit shown that operates on the phase-shifting feature. In order to understand the principle of work, we will follow the points of passage of the current, and letters have been placed on the points of the circuit to follow up on that. second .

The second route is from the D1 runway to its board, to the following points (C-B), then to the ground, then ascends through to point D, then to points (A-E), and finally back to the runway again.

Now suppose that the diode D2 has a current that has one path, from the cathode to the anode to (A-E-F) through D1 to (C-B) and then back to the cathode again. From these two opposite currents arises the difference that represents the voltage of the detector's audio output