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T H E O R Y
O F
O P E R A T I O N
5 Theory of Operation
Learn about the internal blocks that make up the Signal Hound
T
he Signal Hound is built around a narrow-band IF-to-bits receiver with a
maximum bandwidth of 250 KHz. It receives up to 2 Megabytes of I/Q data
each second, which it processes into a trace. To bring you a low cost,
compact spectrum analyzer, we used recently developed high-level RF
integrated circuits. The block diagram, below, illustrates the major elements of the
design. Image rejection is accomplished by mixing high side and low side (sometimes
with separate IF frequencies), then masking together the results.
5.1.1
The Four Sweep Modes
Zero Span: the Signal Hound operates by continuously mixing a fixed LO
⇒
frequency with the RF input and displaying the instantaneous amplitude or
frequency of that signal as a function of time.
Below 6.5 KHz RBW or VBW: the Signal Hound processes the spectrum in 200
⇒
KHz sections (or smaller), by mixing two fixed LO frequencies (typically one
above the RF frequency and one below). These are masked together to produce
the displayed trace. LO and IF frequencies are selected automatically to best
cancel image and spurious responses.
Figure 5: Simplified Block Diagram
27
Chapter
5
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