Question { BARC, 6934 }

how the capacitor and battery are related in symbol,explain?

Answer

Chinky Gupta's answer is only partially correct.

The symbol for a cell (not a battery) is two parallel lines of unequal length, the shorter one of which is also thicker/heavier than the other. The longer, and thinner, line is the positive pole, the shorter and thicker one is the negative.

A battery may be represented by drawing the correct number of cells in series, or, for brevity, drawing two cells separated by a dotted line.

Question { 10083 }

What is Modulation and Demodulation?

Answer

If we radiate a simple r.f. carrier it conveys no information other than the fact that its transmitter is working. Not much use is it?

The name "carrier" suggests that it is capable of carrying information.

The information is applied to the carrier so that the carrier's characteristics are changed. It is these changes which represent the information being carried.

Changes to the carrier's characteristics are called modulation.

The two most common ways that the information can change its carrier's characteristic are by altering the amplitude of the carrier (it should come as no surprise to learn that this is called amplitude modulation, abbreviated to a.m.), and by altering the frequency of the carrier (which, of course, is f.m.)

The process of recovering the information imposed upon a carrier (the modulation) is called demodulation or detection.

Exactly the same processes are encountered in audio.

If an audio note has its amplitude varied (modulated) to and fro (up and down) it is called tremolo - in r.f. we call it a.m. (it's the same thing).

If an audio note has its frequency varied back and forth, it is called vibrato - in r.f. we call it f.m. (it's the same thing).

Question { 10870 }

Why TV transmission is carried out in the UHF and VHF bands?

Answer

A television transmission requires a wide bandwidth - very much wider than that of an audio transmission.

The bands reserved for audio broadcastin, both m.f. and h.f., are much narrower than the bands allocated for television; too narrow to accommodate even a single station to operate satisfactorily.

The v.h.f. and u.h.f. bands allocated to television are wide enough to accommodate several television transmissions.

Question { IIITM, 2775 }

Is there any tradeoff between voice quality and bandwidth, how?

Answer

Yes there is.

The narrower the bandwidth, the less information which can be carried.

The opposite being true - the wider the bandwidth, the more information can be carried.

Take, as an example, an a.m. broadcast station. Frequency separation between channels is, in the U.S.A., 10kHz, and in the U.K. 9kHz.

The maximum bandwidth transmitted must not exceed 10kHz or 9kHz, depending on the country.

For a 10kHz bandwidth, that of each sideband must not exceed 5kHz, or 4.5kHz in the case of 9kHz channel spacing. In practice the actual bandwidth is a little narrower to prevent one station interfering with another.

This means that the maximum audio frequency which an a.m. broadcast station can transmit is either 4.5kHz or 4kHz depending on the channel separation.

Now consider television transmissions. There is obviously a lot more information to carry and the bandwidth must therefore be much wider.

Question { TCS, 14344 }

Which modulation is good for Radio Broadcasting.
AM(Amplitude Modulation) or FM(Frequency Modulation)?

Answer

Either of them may be considered "good" dependent on the service provided.

In the medium frequency broadcast band (535kHz to 1700kHz) a.m. is chosen because of its relatively narrow bandwidth - about 8kHz, so that many stations can operate in that band.

In the v.h.f. broadcast band (87.5MHz to 108MHz) f.m. is used so that the wider bandwidth necessary for reasonably good transmission of music, plus a 38kHz sub-carrier for stereo transmission, may be used.

In the aeronautical v.h.f. band, a.m. is used. This is to avoid what is called the "capture effect" of a stronger f.m. transmission overriding a weaker one, which could mean that a weak signal from an aircraft in distress might not be heard.

So, both a.m. and f.m. are "good" for broadcasting.

Question { Spiro Solutions, 2353 }

in what condition transistor act as switch and amplifier?

Answer

To be used as a switch, a transistor will be either not conducting and therefore be an open switch, or fully conducting (in saturation) and therefore be a closed switch.

To be an amplifier a transistor has to have a standing bias such that it is operating in its linear mode.