Mumbai university > Electronics and telecommunication Engineering > Sem 6 > Television Engineering

Marks: 05

Years: May 2016

• An important function assigned to IF (Intermediate frequency) section is to equalize the amplitudes of sideband components because of vestigial sideband transmission.

• For vestigial sideband correction, the picture carrier frequency gain must be 6dB down on the IF frequency response curve.

• It is also necessary to shape the IF response curve around the picture IF frequency in such a way that all lower video frequencies which got a boost on account of partial lower sideband transmission (besides the full upper side band) are duly attenuated and get restored to their actual level.

• This is achieved by suitable tuning and shaping the response of the IF stages.

• This is fully illustrated in the below figure which shows ideal overall response of the IF section.

• In the IF amplifier circuitry, provision must be made for rejection of signals from adjacent channels.

• For this purpose, special tuned circuits called trap circuits are connected in the signal path in such a way that the offending frequencies are removed.

• These trap circuits are disposed at convenient places in the IF amplifiers.

• Their position will vary from receiver to receiver but generally they are placed in the input circuit of the first IF amplifier.

• The way in which these unwanted adjacent channel IF signals appear is illustrated in the above figure.

• As an example, suppose that the receiver is switched to channel 3 on Band 1. The local oscillator frequency for channel 3 is (55.25 + 38.9) 91.15MHz.

• This would beat with the channel picture and sound carrier frequencies to give the desired picture and sound IF frequencies.

• Besides these, the sound carrier of channel 2 which is close to the beginning of channel 3, will beat with the local oscillator to give unwanted difference frequency of 40.4MHz (94.15 – 53.75) which would lie close to the upper skirt of desired IF response.

• Similarly, the picture carrier of upper adjacent channel 4, will also beat with the local oscillator to produce another unwanted difference frequency signal of 31.9MHz (94.15 – 62.25).

• This is close to the lower skirt of IF response. The trap circuits are designed to attenuate these two adjacent channel interfering frequencies by about 40dB as shown in the above figure.

• It is understood that such interference would occur only if transmitters operating at channels 2 and 4 are located close to the transmitter operating at channel 3.