التردد المتوسط في جهاز الاستقبال .. كتاب التلفزيون الملون والعادي

Chapter six :

IF image mid-frequency amplifier stage

6-1 Intermediate frequency in the receiver:

After entering the RF radio frequency signal - which carries the image and sound information - to the voter who amplifies this signal, and outputs it the local oscillator signal that generates a frequency higher than the frequency of the receiving channel by a fixed amount called Intermediare Frequency (IF) Figure ( 6 - 1) We can get four different frequencies as a result of the process of interfacial conflict (super heterodyne). These frequencies are:

1- The frequency of the receiving channel.

2- The frequency of the local oscillator.

3 - (oscillator frequency + frequency of the receiving channel)

4 - (Frequency of the oscillator - the frequency of the receiving channel)

The difference in frequency in the oscillator and the frequency of the receiving channel is what the output of the mixer tunes to. The average frequency is called the IF of the image. Where it is according to the European system
as follows :
The average frequency of the visual signal = (389) megahertz.
The average frequency of the audio signal = (3304) megahertz. They are separated by 5.5 MHz

The method of super heterodyne conflict, which is used in all receiving systems and devices, has eliminated the method of direct reception of the station's frequency, and in it there is no need for a local oscillator or mixer, and it does not contain a medium frequency. Treating the frequency of each station and enlarging it is not profitable, enlarging, and good clarity. The most important features of the super heterodyne method are:

1- The receiver operates on the same fixed average frequency with all channels and stations, and it is sufficient to receive a station by composing or tuning the radio frequency and changing
The frequency of the local oscillator, and this is what makes the average amplification circuits less expensive and better with less distortion or noise, because the response curve is in the best design.

2 - The average frequency is lower than the frequencies of the TV channels - its circuits help in obtaining an enlargement factor and better selectivity at a lower cost. For example, channel number /5/ has a frequency from 174 to 181 MHz. The frequency of the local oscillator when received is +1 = 214.1 MHz or 181 + 33.15 = 21415 MHz.


That is, we combine the smallest frequency of the channel with the highest frequency of the intermediate frequency band


Or we collect the highest frequency of the lowest frequency of the medium frequency band with It is noted that the frequency of the sound carrier wave is higher than the frequency of the image carrier wave by 5.5 MHz in transmission, and the opposite in reception, i.e. the frequency of the sound carrier is lower by (55) MHz.


3- The intermediate frequency enters into two or three amplification stages and is called the intermediate frequency stage of the image. The basic function of the IF stage is:




1- Giving the RF signal the largest gain percentage in the device stages.

2 - Purifying the signal from unwanted interference and jamming

3- Preventing the audio signal from interfering with the visual signal.

4- Separating the middle frequency of the sound from the middle frequency of the image signal.

e- Canceling or dampening the interference of nearby stations.

6- It gives the IF signal sufficient amplitude (amplitude) to operate the visualization detector. Capacitive modulation amplifier (AM), visual amplifier, and frequency modulation (FM) sound detector.

The circuits of the medium frequency amplifier in monochrome and color television are similar. However, in color, they have more accurate and complex specifications, as they have to pass a wider band without fading or distorting.
- 2 IF amplifiers in color TV:

It usually consists of three amplification stages. The signal input must match its impedance at the input of the first stage of the IF amplifier and the output of the voter - and is determined by the length of the coaxial cable connecting them. The sound trap reduces the amplitude of the sound carrier signal of the same channel and prevents the entry of the sound carrier signal frequency of the adjacent channel.

The audio carrier signal of the same channel, after weakening it and reducing its capacity as much as possible, passes to the visual detector, and a tuned circuit in the third stage of the IF amplifiers extracts the audio carrier.

There is also another sound trap between the third stage of the IF amplification and the visual signal detector, and its function is to prevent sound interference and access to the image detector. As for the AGC signal, it is applied to the first and second stage of the medium frequency amplifier, as in black and white television. Three signals enter the IF stages:

1 - The output signal of the channel voter and its frequency is fixed. It is the difference between the frequency of the local oscillator and the frequency of the receiving station, ie = frequency of the oscillator - frequency of the station, which is the intermediate frequency (IF). It is a fixed frequency = 36.5 MHz.

2 - AGC automatic gain governor signal. It generates a signal that adjusts the intensity of the received signal so that it remains within a certain level. The AGC signal arrives in a feedback manner to the bases of the transistors or the network of valves, and works to increase or decrease the amplification of the circuit. Its signal can be negative or wave - depending on the type of circuit.

​​​​​3 - The DC voltage feed signal that feeds the stages of the circuit - The supply voltage varies according to the type and design of the device, and it does not exceed (25 volts) in transistor devices and integrated circuits