o frequency mixing is a non-linear process that involves the level of one signal affecting the level of the other at the output. This process involves the two signal levels multiplying together at any given instant in time and the output is a complex waveform consisting of the product of the two input signals as shown in Figure 5.5. From this it can be imagined that signals on different frequencies are produced, and this is in fact the case. Figure 5.5 Mixing two signals together. The two top signals are the input signals, and the bottom one is the resulting output 124 Newnes Guide to Radio and Communications Technology Amplitude Originating signals Resultant signal f1 (1.0 MHz) and Resultant signal f1 f2 (0.25 MHz) f2 (0.75 MHz) f1 + f2 (1.75 MHz) Frequency Figure 5.6 Frequencies produced by mixing two signals together It is found that new signals occur at frequencies equal to the sum and difference of the two original signals as shown in Figure 5.6. In other words if the two input frequencies are f1 and f2, then the two resultant signals will appear at f1 + f2 and f1 - f2. To give an example, if the two original signals are at frequencies of 1 MHz and 0.75 MHz as in Figure 5.5, then the two resultant signals will appear at 1.75 MHz and 0.25 MHz. If the two frequencies are very near together, for example 1 MHz and 1.001 MHz, then one of the resultant frequencies will appear in the audio part of the spectrum and can be amplified by an audio amplifier and passed into a loudspeaker SO that it can be heard. The circuit used to multiply two signals together is called a mixer or a multiplier as a result of its multiplying action. Either term is equally acceptable, although the term mixer is more commonly used in circuit applications. To make up a typical direct conversion receiver a number of circuit blocks are required. These are shown in Figure 5.7. In this the signals first pass into a radio frequency amplifier and tuning stage. This fulfils a AF amplifier Mixer amplifier with audio filter Local oscillator Figure 5