Posted on Feb 6, 2014

Mixers (sometimes known as frequency converters), modulators, balanced modulators and other circuit blocks are considered below. Each works on the same basic principles. A mixer circuit normally has two inputs - from two separate signal sources. In the diagram below, the sources are two oscillators. Each oscillator is a generator producing a sinewave output, one at frequency

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f1 and the other at frequency f2. We will use numerical examples later. The mixer multiplies the signals together. You don`t need to know the details. Just remember that the output comprises a complex mixture of separate sinewaves at many different frequencies. The major output frequencies are shown on the diagram. The main point to note is that the output comprises the two separate input frequencies f 1 and f 2 and their sum, (f 1 + f2), and their difference, (f 1 - f2). In practice, there are other component signals too - but we can ignore those. A filter - which can be any one of various sorts - selects the required output from the mixer. In this diagram, a simple parallel tuned circuit is shown. The output will normally be tuned to the SUM, (f 1 + f2), or tuned to the DIFFERENCE, (f 1 - f2), signal as required. Consider Oscillator 1 to generate a 9 MHz signal and Oscillator 2 to generate a 5 MHz signal. The output from a mixer will contain these two signals, plus their sum, 14 MHz, and the difference, 4 MHz. The mixer output tuned circuit could be tuned to 14 MHz if that output was required, or tuned to 4 MHz, should that output be required. The output from a mixer contains many more combinations of frequencies - generated from the harmonics of the input signals mixing with the component signals. For purposes of this amateur radio examination these can be ignored. Almost any electronic device, diode, transistor, valve,...

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