Sabtu, 23 April 2011

modulation should be aimed in voice transmission

modulation should be aimed in voice transmission

The power of a modulated wave is found from the formula:

Pmod=(1+m2/2) x Pcarr

Where Pmod=Power of the modulated wave,

M=degree of modulation,

Pcarr=power in the carrier frequency.

The power in an amplitude modulated wave is divided between the carrier and the two side bands. The carrier power is constant, and so, the side band power is the difference between the carrier power and the total power in the modulated wave. The above formula is to find the power of the modulated wave when carrier is modulated by single sinusoidal tone.

If the carrier power=50 watts

%ge of modulation=100 or 1

degree of modulation=1

Then the power of the modulated wave,

Pmod=(1+m2/2) x 50=3/2 x 50 = 75 watts

Since the carrier power = 50 watts; the two side bands have 25 watts in them, i.e.

25/75 x 100% = 33.3% of the total power with 100% modulation.

In case of 50% modulation with same carrier power we have,

Pmod=(1+0.52/2) x 50 = 2.25/2 x 50 = 56.25 watts

Now the side bands have only 6.25 watts (since 56.25-50 = 6.25)

Since all the intelligence being transmitted is contained in the side bands, the desirability of a high percentage of modulation is crystal clear. A comparatively low powered, but well modulated transmitter often produces a stronger signal at a given point than does a much higher powered, but poorly modulated, transmitted the same distance from the receiver.

RF Oscillator

This is the stage where the carrier frequency intended to be used is generated by means of Crystal Oscillator Circuitry or capacitance-inductance based Variable Frequency Oscillator (VFO). The RF oscillator is designed to have frequency stability and power delivered from it is of little importance, hence can be operated with low voltage power supply with little dissipation of heat.

Buffer Amplifier

The low power RF carrier output from the RF oscillator is amplified in this portion and it also keeps the RF oscillator and power amplifier circuits separate electrically imparting frequency as desired by the amateur can be done in this stage, when the carrier frequency multiplication technique is applied here. In it the Morse key for keying out carrier continuous wave can be accomodated.


Audio information is impressed upon the carrier frequency at this stage

Balanced Modulator

In this type of modulator, while the audio information (voice) is impressed upon the carrier frequency, at the same time its output gives a signal without carrier frequency but yet with the two side band frequencies carrying the voice/audio information.

Side-band filter

It discards out any one of the side band.

RF linear Amplifier

RF power amplification is done here and this stage is coupled to the antenna system through antenna impedance matching circuitry. Care is taken at this stage so that no harmonic frequency is generated which will cause interference in adjacent band (splatter) on other bands.

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