t is also not particularly good when selective fading occurs on short wave signals. Under these conditions certain of the frequencies of a signal are not received for short periods of time, and this can lead to very high levels of distortion. SSB and Morse demodulation To receive Morse and single sideband it is necessary to have additional circuitry. In order to hear the Morse signal it is necessary to beat the incoming signal with an internally generated one SO that the typical Morse signal of a tone being turned on and off can be heard. If a normal AM detector was used, then a series of clicks and thumps would be heard as the carrier was turned on and off. The internal oscillator is generally called a beat frequency oscillator or BFO. The incoming signal and the beat signal are passed into a mixer where they are mixed together. The resulting audio tone is equal in frequency to the difference between the signal and the BFO. As the mixing process is a multiplication process, and the output is the product of the two input signals, this type of detector is often called a product detector. The same type of detector is also used for receiving single sideband. Here the beat frequency oscillator is used to reintroduce the carrier as shown in Figure 5.35. As a result of this application the oscillator may also be known as the carrier insertion oscillator or CIO. Frequency where carrier for the SSB signal would be Receiver passband Amplitude Wanted signal Frequency positioned Figure 5.35 Action of a beat frequency oscillator Receivers 153 The frequency of the BFO needs to be set to give the optimum performance. The signal being received should be tuned for the best reception, by placing it in the middle of the receiver pass-band. The setting of the oscillator is then adjusted to suit. This may often mean that it is just outside the pass-band of the set. However, this does not matter as it comes after the selectivity. Synchronous AM demodulation Improvements in amplitude modulation reception can be achieved by using a sys