HP 70000 series - A Theory of Operation; Spectrum Analyzer Operation

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 amplied, 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.
Simplied
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 satised, 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) amplier 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 amplier.
Theory of Operation A-1