Transmitter Block Diagrams 

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How to draw them!                                                QUESTIONS

This is is a "block diagram" of a simple transmitter. Before the actual stages are discussed, consider the diagram itself.  It is drawn to show the "signal flow" entirely from left to right, shown by the arrows. 

The CW Transmitter 

The simplest of all transmitters is one for sending Morse code - a CW (Continuous Wave) transmitter as shown in the diagram above.   SIGNALS  

An oscillator generates the signal and it is then amplified to raise the power output to the desired level. A Morse key is used to chop the transmission up into the "dots" and "dashes" of Morse code.   OSCILLATORS    MORSE CODE  

The oscillator runs continuously. The Driver / Buffer are isolation stages, to isolate the oscillator from the sudden load-changes due to the keying of the amplifier. This minimises frequency "chirp" on the transmitted signal. 

The oscillator is usually supplied with DC from a voltage-regulated source to minimise chirp (slight changes in the output frequency) due to variations in the supply voltage. 

Several driver and buffer stages may be used. The keying may be in the final amplifier alone - usually in the cathode or emitter lead - or may also be applied to the driver stage too.   

A "keying relay" may be used to isolate the Morse key from the transmitter circuits, to keep high voltages away from the operator's Morse key. In the interests of operator safety, the moving bar of the Morse key is ALWAYS kept at earth potential. 

The A M Transmitter                                                                             To top of page

This is a diagram of a typical Amplitude-Modulated transmitter. The principles of each block and the principles of Amplitude Modulation are treated in   OSCILLATORS   and in   SIGNALS . 

The block diagram is derived from the CW transmitter. 

The modulated stage is usually the final amplifier in the transmitter. This is known as "high-level" modulation. If a following amplifier is used to raise the output power level, it must be a linear amplifier. 

The SSB Transmitter

The generation of a SSB signal is treated in   SIGNALS  and in MODULATORS . A transmitter takes the generated signal and first translates it with a mixer / VFO combination to the required output frequency then amplifies it to the required power output level using a linear amplifier. A linear amplifier is needed to preserve the signal waveform in all ways except to increase the output amplitude. 

The F M transmitter                                                                                    To top of page

Again, the principles of each block have been treated elsewhere.   SIGNALS 

The modulator can be one of several types. The simplest to understand is probably to consider the voltage-controlled oscillator.   OSCILLATORS   

Applying an audio signal to the varicap diodes in the circuit example given in the Oscillator discussion will change the frequency of the oscillator in accord with the modulation. This increases the frequency swing with increased audio loudness, and the rate of swing with increasing audio frequency  - hence providing Frequency Modulation. 

In VHF hand-held transceivers, the oscillator will be generated by a phase-locked-loop to get "channel switching" facilities. The frequency modulation may then be generated by applying the audio signal to the PLL. 

The Frequency Multiplier stage is an RF amplifier with a tuned output - the output tuned to a harmonic of the input signal. 

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