| Double frequency changing |
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In a simple frequency changing receiver, the local oscillator (Fol) activates the mixer followed by the middle frequency amplifiers (Fi). Therefore, it is possible to receive two frequencies : (Fol+Fi) and (Fol-Fi). The choice between these two frequencies (desired frequency and "image" frequency) are realized by bandpass filters. More the middle frequency is low, more it is difficult to realize the separation of the two received frequencies, but the amplification and the demodulation are easier and more efficient with narrow band. More the middle frequency is high, more the separation of the received frequencies is efficient, but the amplification and the demodulation in narrow band are more complex. The double frequency changing allows to keep the benefits of the two previous cases without having the drawbacks, and by this fact, is used commonly in high-performance receivers. |
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