| 摘要 |
There is provided an image rejection mixer (100) for downconversion of an incoming signal to a lower intermediate frequency signal, using the absolute difference of frequencies between the incoming signal and the local oscillat or signal. The image rejection mixer is set up to accept and downconvert incomi ng signals at two discrete frequencies fh and fl, where the local oscillator frequency is positioned mid-way between those frequencies fh and fl. There a re two signal paths (S1, S2) in the image rejection mixer, either one or the other of which is chosen by positioning a controllable single pole, double throw (130) switch into one of its two switch positions. The output of the switch is led to a mixing element (116), from which an intermediate frequenc y signal is derived. The first of the two signal paths (S1) from the input to the image rejection mixer to the mixing element has a bandpass characteristi c centred at one or the other of the two input frequencies of interest; the other signal path (S2) has a bandstop characteristic centred at the same frequency. By choosing one or the other of the signal paths, an incoming signal at a designated one or the other of the incoming signal frequencies m ay be downconverted, and the image signal and any noise at that image signal frequency will be rejected.
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