Short note on VSB correction

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written 6.7 years ago by

teamques10 ★ 68k

Another important function assigned to IF section is to equalize the amplitudes of side-band components, because of vestigial sideband transmission.
The need for vestigial side-band correction the picture carrier frequency gain must be 6 db 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 side-band 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. 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 diagram.
As an example, suppose that the receiver is switched to channel 3 on Band I. The local oscillator frequency for channel 3 is (55.25 + 38.9) 91.15 MHz. 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 40.4 MHz (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 31.9 MHz (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 40 dB.

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