Mixers and oscillators

  
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The multiplication operation that is required to generate a signal of a frequency equal to the difference of the input frequencies, can be implemented in several ways. Actually a multiplied signal generally also appears when two signals are added and subsequently put through a non-linear device, like a pn-junction in a diode or transistor. Example
Mixers and oscillators - schematic

s of this type of mixer are a transistor biased into non-linear operation, a simple two-diode mixer and more advanced four-diode balanced mixers. When using a square wave oscillator, the multiplication operation simply comes down to reversing the polarity of the bat signal at the frequency of the local oscillator. This can be implemented by using an analog switch to switch between the bat signal and an inverted version of the bat signal. In the picture on the right a basic implementation of this idea is shown. The leftmost opamp is configured as an inverting amplifier to give the signal some extra boost. The rightmost opamp is an inverting amplifier with gain -1. A 4053 is used to switch the output to either the output of opamp 1A or opamp 1B. The positive inputs of the opamps are biased at half the supply voltage. These can be built with a single opamp or a NE555 (pdf) for example. In these type of oscillators, the frequency is inversely proportional to the RC product. The nonlinear (inversely proportional) relationship between the RC product and the frequency is not very convenient. This can be alleviated somewhat by using a logarithmic potmeter, but in that case the clockwise position of the potmeter corrresponds to the lowest frequency. These use a resonant LC-circuit to generate a relatively pure frequency (little harmonics). Disadvantages are the nonlinear relationship between LC and frequency and the lack of variable...



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