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simple modulated signals. With GNURadio connected, KC908 is even capable
of outputing relatively complicated digital modulating signals.
1.1 Theory
KC908 contains a zero IF receiver, covering a range of 0.75-6ghz.(The left
port's range is 0.5-6Ghz). By mixing, the frequency outside this mentioned
range is moved to the range of 0.75-6ghz. The zero IF receiver samples the
baseband directly to get the digital IQ signal. The bandwidth of digital IQ
signal is 40MHz, spectrum result can be obtained by FFT. Because baseband is
generated by analog mixer, local oscillation leakage can not meet the high
requirement of spectrum analysis. In order to solve this problem, the
instrument discards a sideband and poor performance part of the FFT result
and only takes 15MHz width to display, which is the origin of effective real-
time bandwidth (15MHz). If the sweep span is wider than the effective real-
time bandwidth, the LO scans the whole sweep width with 15MHz intervals
and splices the results.
Since the spectrum is generated by splicing FFT results, the display effect,
as well as certain concepts, would definitely differ greatly from conventional
sweeping spectrometers. RBW, resolution bandwidth, for example, no longer
means the bandwidth of resolution filter. Instead, it means the equivalent
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