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Radio 101: Understanding the Radio Telegraph Transmitter
Radio Telegraph Transmitter is essentially similar to radio telephone transmitter except for the following differences firstly in the place of a conventionally telephone modulator, a telegraph modulator is used, secondly modulation is done at a lower carrier power level and thirdly class c amplifiers are used throughout, even after modulation since bandwidth involved is small. Further for radio telegraphy work, it is not essential to have the transmitter highly free from noise and hum modulation. In practice even self rectifying circuits with large hum are often used. There are two basic methods of telegraph modulation. Accordingly radio telegraph transmitters may be classified into two categories firstly those using amplitude modulation of the carrier and secondly those using frequency modulation of the carrier. Amplitude modulation radio telegraph radio telegraph transmitters are of two types. The first type is those using amplitude modulation of the carrier. The second type is those using frequency modulation of the carrier.
Amplitude modulation radio telegraph may be of two types firstly interrupted continuous wave type and secondly modulated continuous wave type. In interrupted continuous wave type of modulation, the carrier voltage is interrupted or keyed in accordance with some standard telegraph code say the Morse code. Thus during the mark interval, the carrier is allowed to reach the transmitter output while during the space interval, the carrier is disallowed or interrupted. This is the conventional type of telegraph modulation. In the modulated continuous wave type of modulation any audio tone interrupted according to the telegraph signal, amplitude modulates the carrier. Carrier voltage is produced in crystal controlled master oscillator. A buffer amplifier isolates the master oscillator form the load. Harmonic generators following the buffer amplifier raise the carrier frequency to the desired value and also raise the carrier power some what. In the modulator, a carrier at a low power is keyed in accordance with the modulating DC telegraph signal. The modulated carrier is raised in power in a chain of tuned class C amplifiers. The output is then fed to the transmitting antenna.
The frequency modulation radio telegraph transmitters may be of two types firstly Frequency Shift Keying Transmitter. In a Frequency Shift Keying Transmitter, the carrier frequency gets shifted to a slightly different value during making interval. Thus the carrier voltage is continuous but has different frequencies during marking and spacing intervals. Second is frequency modulated continuous wave type, in this type an audio tone interrupted according to the telegraph signal frequency modulates the carrier. Out of these various methods, the on-off keying and frequency shift keying are most commonly used. Direct keying in the oscillator is rarely done since this introduces incidental phase and frequency modulations. Keying is usually done is one the subsequent amplifiers at a low carrier power level.