HP 70000 series, User Manual

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A
Theory of Operation
Spectrum Analyzer
The HP 70000 Series modular spectrum analyzer systems are superheterodyne receivers , which
are electronically tuned by applying an adjustable saw-tooth voltage ramp to a voltage-tuned
local oscillator (LO). The input signal is mixed with a signal from the LO , the output of the
mixer is amplied, ltered, detected, and then converted to digital values which are stored in
the spectrum analyzer .
Stored measurement results can be retrieved several ways . If the spectrum analyzer is
connected to a display (for example , HP 70004A color display , HP 70205A graphics display or
HP 70206A system graphics display), measurement results can be displayed. Measurement
results can also be monitored with a computer . Figure A -1 illustrates the basic spectrum
analyzer functions .
Figure A -1. Simplied Spectrum Analyzer Block Diagram
F or an input signal, F
S
, the LO frequency is tuned so that
F
LO
0 F
S
= F
IF
where
F
LO
is the local oscillator frequency
F
S
is the tuning frequency (displayed response)
F
IF
is the mixing product
When this equation is satised, the input signal can be detected.
When the full voltage range of the sweep ramp is applied to the local oscillator , it sweeps
through its frequency range , yielding a wide range of frequencies at the mixer output.
However , since the intermediate-frequency (IF) amplier passes only frequencies within its
passband, measured spectrum for a continuous-wave signal is simply a plot of the passband of
the resolution bandwidth lters in the IF amplier .
Theory of Operation A-1