Transmitter Block Diagrams

Question File Number 18



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Transmitter Block Diagrams

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.

CW TX

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.

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The A M Transmitter

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.

AM TX

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.

SSBTX2

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The F M Transmitter

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.

FMTX

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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Further details about NZART can be obtained from the web at  NZART. There are several categories of NZART membership which include Transmitting and Non-Transmitting. Anyone interested in radio can join. E-mail enquiries to  nzart@nzart.org.nz will bring details about NZART Membership. On-line details about  Joining NZART - Membership has advantages.
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Compiled Sun Nov 28 2010 at 8:41:44pm


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