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6.4 Where does the image interference appear?
In the range of 0~750MHz(0-500MHz for left port) and above 6GHz, there
may be two kinds of image interference, namely the image interference of the
first mixer and the second image interference. For 10.8GHz model, the
frequency of the 1st IF is about 2.65 GHz, and the image interference point is
about 5.3 GHz away from the observation frequency point. The severity of the
image interference depends on the performance of the preselector.
Using a high LO in 0~750MHz, an image signal of 5.3GHz~5.8GHz may
appear, which is usually more than 60dB lower than the real signal. Because
the performance of low-frequency antennas at high frequencies is often not
ideal and can bring in more image suppression, this part of image interference
has minor influence.
Above 6GHz, 10.8GHz model and 18.6GHz model adopt different
strategies for LO as well as for preselection.
Low LO is used by 10.8GHz model above 6GHz, and the signals within 0-
5.5GHz might become an image interference. However this frequency band
comes with strict preselector offering at least 50dB image suppression. Thus
the major concern for 10.8GHz is spurious response introduced by the second,
third LO harmonic wave.
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