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The 18.6GHz model adopts tracking preselector, protecting all its
passband from image interference frequencies, thus it can offer at least 50dB
image suppression.
You can pre-determine whether the signal is true or fake by adjusting the
center frequency and observe the direction and speed of spectral line
movement. For example, if the central frequency is increased, the true signal
will move left, while the image signal will move right. The moving speed of the
true signal is equal to the speed the center frequency shifts, while the fake
response caused by the harmonic wave mixing would have a faster moving
speed.
The second image interference is inherent in the zero-IF receiver, and
suppression is usually up to 60dB, occasionally drops to 40dB. The second
image signal is often very close to the true signal, the typical distance is 6 ~
31MHz, but with a narrow SPAN, the distance will become closer, can be
about 1MHz minimum.
In the range of 750MHz~6GHz, usually only the image interference of
the zero-IF receiver occurs, and its characteristics resemble the second image
interference's mentioned.
Second image suppression might become obvious after a long period of
consistent operation. User could consider pausing or rebooting the machine
to re-calibrate the IQ, so as to mitigate the suppression. In addition,
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