كاشف fm نوع (pll) حلقة الطور المقفل .. كتاب التلفزيون الملون والعادي

10-18 FM Detector: Locked Loop Detector (PLL)

It is a single integrated circuit that also uses synthetic frequencies for other purposes. We are now concerned with the function of the circuit as an FM frequency modulation detector.

circuit work:

The PLL detector circuit consists of four sections, as shown in the block diagram (Fig. 18-23).
These sections are:

1 - Phase detector

2 - Low pass filtering

3- Amplifier

4- A voltage-controlled oscillator (VCO) and it is noted that there are no tuned circuits in it - and the sensitivity of these circuits is about (1) to 2 millivolts - with upper limits around (15-20) millivolts. Above this level, the suppression of AM capacitive changes can lose its function, and this detector receives two signals at the input, which are:

1- The medium frequency signal of sound (55) megahertz.

2 - A signal from a voltage controlled oscillator (VCO) which generates the same mid frequency as the IF sound.

The detector works on the stability of the (VCO) frequency always with the IF frequency of the audio - if the average frequency of the audio changes, the (VCO) frequency also changes - and since there is always a change in the IF audio appropriately to the original audio signal, the resulting DC error voltage increases in the amplifier D.C. This voltage is an exact copy of the transmitted audio signal. The PLL detector thus acts as an uncomposed FM detector, and the error voltage (audio) polarity determines the result of the instantaneous difference between the IF frequency and the (VCO) frequency that deviates above and below the central IF frequency of the audio. The error voltage is zero when VCO IF frequency = 5.5 MHz.

Connecting the circuit with external components:

The IF signal enters audio via capacitor C to pin 12 of the integrated circuit
(Fig. B-23-18) The capacitor C4 is connected to terminal 2 and 3 and is used to set the VCO frequency to approximately 5.5 MHz - the input signal must be at least (1) to 2 millivolts (R,C and R circuit) , C) is connected to terminals (14 - 15) and forms a low-pass filter - and the sound output is taken from terminal (9) - the Co capacitor and the internal resistance of (IC) are connected to terminal (10) and form a high-frequency reduction circuit for deemphasis.

11-18 Digital Frequency Modulation Detection Circuit:

It is characterized by the absence of the need for files and does not require any settings, including:

A - Logic circuit detector (multi-vibration - monostable) - as in Figure (18-24).

b. FM peak difference detector, as shown in Figure (18-25).

The circuit consists of eight transistors and a circuit tuned at (5.5 MHz). It is part of an integrated circuit IC that performs the functions of all audio stages in the TV set.

The complete circuit of the sound stage of a transistor device:

Figure (18-26) shows all the circuits in the transistor sound stage in a colored device.


​​​​​​