e signals consist of an oscillation that is keyed on and off, the circuitry can be kept very simple. Indeed it is possible to make a Morse transmitter with only two or three transistors and a handful of other components. However, more sophisticated designs can naturally be made if required. Figure 6.10 shows the block diagram of the very basic Morse transmitter. All that is required is an oscillator to generate the signal, a power amplifier to increase its level, a method of switching the signal on and off, and finally the matching and filtering circuitry at the output. Switching Morse circuitry Amplifier and power Matching Oscillator amplifier and filtering Figure 6.10 Block diagram of a simple Morse transmitter In the very simplest circuits the oscillator may be crystal controlled to ensure sufficient stability, although this does limit the flexibility of the set. Often a VFO is employed to enable the set to operate over a band of frequencies. This could be an LC tuned oscillator or a more complicated unit based around a synthesizer. However, for a transmitter of this simplicity it is unlikely that a sophisticated synthesized oscillator would be used. Transmitters 195 The power amplifier is used to increase the level of the signal. In this type of transmitter where no analogue modulation is applied to the carrier, the PA will be operating in class C to give the maximum efficiency. The final stage is the filtering and matching circuitry. For a low power transmitter it is often possible for the output device to give a good match to 50 ohms with little or no impedance transformation. As a result only filtering may be required. For a transmitter of this nature this will consist of a low-pass filter. This will remove the spurious signals that will be harmonics of the required signal. Typically this may be a five pole filter with a cut-off just above the operational frequency. Amplitude modulation transmitter A huge number of signals in the radio spectrum are modulated to carry audio. The simplest way of achieving th