AMPLITUDE Modulation Transmitter

Radio transmitter may be classified into following categories:

1. I.      According to the type of modulation used
2. II.      According to the service involved
3. III.      According to the frequency range involved.

Classification of according to the type of modulation used:

i) Amplitude Modulation Transmitters: In A.M transmitter, the modulating signal amplitude modulates the carrier. Such transmitters are used for radio broadcast on long, medium and short eaves, radio telephony on short waves, radio telegraphy on short waves, television picture broadcast on every short waves or ultra short waves.

ii) Frequency Modulation transmitters: in an FM transmitter, the signal voltage frequency modulates the carrier. Such transmitters are used for broadcast in V.H.F and U.H.F range, television sound broadcast in V.H.F and U.H.F ranges, radio telephony communication in V.H.F and U.H.F range over short distances

iii) Pulse Modulation Transmitter: In a pulse modulation transmitter, the signal voltage alters some characteristic of the pulses. Theses characteristics of the pulse altered on modulation may be pulse width, pulse position, pulse amplitude, pulse frequency or pulse code. Accordingly the various methods of the pulse modulation are:

• Pulse width modulation
• Pulse position modulation
• Pulse amplitude modulation
• Pulse frequency modulation
• Pulse code modulation.

Classification of according to the type of service involved:

i) Radio Broadcast Transmitters: Theses transmitters are designed for transmitting speeches, talks, music, dramas etc for the information and reception of people. These broadcast transmitters may be either amplitude modulated or frequency modulated. The A.M transmitters operate on long waves, medium waves and short waves and radiate carrier power as low as about 1 kW and high as 100 kW. The F.M broadcast transmitters operate on very short waves or on ultra waves and radiate carrier power of the order of 100 kW or so.

ii) Radio Telephony Transmitters: These transmitters are designed for transmitting telephone signals over long distance by radio means. A radio telephone transmitter uses certain special as volume compressors, privacy devices, peak limiters etc. Radio telephone transmitters may be either of amplitude modulation type or frequency modulation type.

iii) Radio Telegraph Transmitters: A radio telegraph transmitter transmits telegraph signals from one radio station to another radio station. It may use either amplitude modulation or frequency modulation. When point-to-point radio communication is involved, the transmitting antennas are highly directive so that the electromagnetic energy is beamed into a narrow beam directed towards the receiving antenna at the receiving radio station.

IV)Television transmitters: Television broadcast requires two transmitters one for transmission of picture and other for transmission of sound. Both operate in very high frequency or ultra high frequency range. The picture transmitter is amplitude modulated by the picture signal occupying a band about 75 MHz. The total bandwidth occupied by one television channel is about 7 MHz.

v) Radar Transmitters: Radar (Radio Detection & Ranging) may be of two types:

A) Pulse radar and

B) C.W (Continuous Wave) radar.

Pulse radar transmitter uses pulse modulation of carrier. It uses high output power typically 100 kW peak and operates at microwave frequencies typically 3000 MHz or 10,000 MHz. The C.W radar transmitter may use frequency modulation of the carrier voltage or may utilize Doppler Effect.

Vi) Navigational Transmitter: A number of navigational aids using special types of radio transmitters and receivers are used these days for sea and air navigation. Also radio aids are used for blind landing of aircrafts. Typically radio aids of landing are:

A) I.L.S (Instrument Landing System) and

B) G.C.A (Ground Controlled Approach).

In addition to these, several other radio means are provided at air portfor surveillance. Accordingly a large number of radio transmitter of varied types, frequency and power are required depending upon the operation desired.

Classification of according to the type of Carrier frequency:

• Long Wave Transmitters:

These transmitters operate on long waves i.e. on frequencies below 300 kHz. Such long wave transmitters are used for broadcast in temperate countries, where atmospheric disturbance on long waves are not severe.

• Medium Wave Transmitters:

These transmitters operate on long waves i.e. on frequencies below 500-1650 kHz and usually used for broadcast.

• Short Wave Transmitters:

These transmitters operate on long waves i.e. on frequencies below 3-30 MHz and usually used for broadcast

• V.H.F and U.H.F Transmitters:

These transmitters operate on V.H.F and U.H.F range and are used for F.M broadcast, television broadcast, F.M radio telephony etc.

• Microwave Transmitters:

These transmitters operate on 100 MHz and are used in radar, television relay, microwave link between two countries etc.