استخدام دارة تصفية (sawf) في ضبط منحني الاستجابة وعرض الحزمة لمرحلة (if) .

6-5 Using the SAWF filter circuit to adjust the response curve and the bandwidth of the IF stage:

The Surface Acoustic Wave Filter (SAW) Fig. (6) - (14) (6) - (15) is a surface acoustic wave (SAW) element used in the mid-frequency amplifier section of television - an alternative For the circuits composed of a coil and an LC capacitor, as well as used as a frequency trap. Zenith devices used it instead of a three-phase integrated circuit, so it passes the required bandwidth and ensures the accuracy of the response curve.
It is silicon oxide with physical stress such as flexion). Conversely, this particular material is subjected to an electrical voltage that makes it fall under physical stress, and this phenomenon is used in crystal oscillators, filters, etc., and the best materials used in SAW filters are a thin layer of zinc oxide, lithium tantalate, bismos germanium oxide, and quartz.

SAW is a non-magnetic electric waves that travel across the surface of the bioelectric material, and its frequency is very high to several GHz, but its speed is slow, not exceeding (2010) the speed of magnetic waves. Therefore, SAW circuits are used in addition to the field of filtering and filtration in line delay circuits in color, and in pulses Electronic processors and the various elements of microwaves (microwaves) and their circuits.

The SAW circuit suppresses unwanted frequencies such as the sound carrier for the lower channel and the image carrier frequency for the higher channel, and suppresses all frequencies that exceed the bandwidth of the correct intermediate frequency. Thus, the response curve and field width for the middle frequency stage are in its best specifications. Figure (6- (16) shows the (S.WIF) circuit in a color television set.

6-6 Method of extracting the audio signal: The image and sound signals pass together in the high and medium frequency amplifier stages, but they do not amplify in the same proportion - rather the wave traps suppress the audio signal in the first medium frequency amplification stage in order to prevent its interference with the image (Fig. 6-17) This method is called the common voice method and it is common now. The other way is the separate sound method. It was used in the past and is no longer used today. We will explain both methods: 1- The separate sound method: the sound signal is separated after the first stage of amplifying the middle frequency of the IF image, by means of a well-tuned resonant (suction) circuit at the frequency of the sound signal. And for this
The method has some advantages in terms of the independence of the sound in most of its stages, performing accurate toning of the sound, and reducing the occurrence of a common failure of the image and sound together - but some obstacles emerged, including the difficulty of maintaining the frequency of the local oscillator with a narrow band, especially at high frequencies, and the occurrence of microphone interference in the oscillator, which may makes him change its frequency 2- The common sound method: in which the two signals enter together until the image detector. Then the image signal is output to its designated stages, as does the sound carrier wave of the image colliding, resulting in a difference in frequency of 5.5 MHz. This frequency carries all the information of the FM-modulated sound. It goes to the stage of amplifying the middle frequency of the sound, then to the sound detector, then to the loudspeaker, and finally to the speaker. The common audio method has the advantage of achieving an increase in image contrast and clarity when the signal is weak. By re-tuning the local oscillator by fine tuning to a lower frequency by (1) MHz, the carrier frequency signal will decrease.
The average for the image is from 38.9 MHz to (379) MHz), so it changes the response curve and takes advantage of the upper part of the inclined side of the curve - and this reset causes a decrease in the interstitial frequency of the sound from (33.4 to MHz), even if that happens in the audio method. The separate sound is to make the sound lost because the width of the sound pass band does not exceed (300 kHz), while the sound is not affected in the common sound method.

Also, any shift in the frequency of the local oscillator does not affect the sound emitted by the device, but rather changes the average frequency of both the sound and the image.