requency. At this point the heterodyne frequency reaches zero. This point is known as zero beat. As the receiver continues to be tuned in the same direction the audio note starts to rise in frequency as the local oscillator starts to move away on the other side of the signal. This means that there are two points where a particular audio frequency is obtained. This audio image cannot be removed without the addition of more electronics in the mixer. As the Receivers 127 requirements for this circuitry are normally quite exacting most direct conversion sets live with the problem. The direct conversion receiver is finding widespread use in cellular telecommunications applications where its simplicity helps to keep the costs low. Using a sophisticated mixing technique it is possible to extract the transmitted data from the received signal and apply it to the digital circuitry in the phone for processing. The superhet To overcome the limitations of receivers such as the TRF or direct conversion set another type of receiver topology evolved and gained popularity in the 1920s and 1930s. Called the superhet, which is short for supersonic heterodyne, this type of set is widely used today. It operates by changing the frequency of the incoming signal to a fixed frequency intermediate stage where it can be more easily filtered and amplified before being demodulated to give an audio signal that is amplified in the normal way. Using the mixing process it is possible to change the frequency of a signal up or down as shown in Figure 5.9. To illustrate this, take the example of a signal at 2.0 MHz. This mixes with the local oscillator at 1.5 MHz to give a signal at the intermediate frequency of 0.5 MHz or 500 kHz. This signal represents the difference mix product. The sum appears at 3.5 MHz and is easily filtered out as it is very well removed from the frequency of the filters. Unfortunately there is another mix product which produces a signal that can also pass through the intermediate frequency stages. The first Resulting signal