Agilent Spectrum Analysis Basics - Agilent Technologies

Image response: A displayed signal that is actually twice the IF away from
the frequency indicated by the spectrum analyzer. For each harmonic of the
LO, there is an image pair, one below and one above the LO frequency by the
IF. Images usually appear only on non-preselected spectrum analyzers.
Incidental FM: Unwanted frequency modulation on the output of a device
(signal source, amplifier) caused by (incidental to) some other form of
modulation, e.g. amplitude modulation.
Input attenuator: A step attenuator between the input connector and first
mixer of a spectrum analyzer. Also called the RF attenuator. The input
attenuator is used to adjust level of the signal incident upon the first mixer.
The attenuator is used to prevent gain compression due to high-level and/or
broadband signals and to set dynamic range by controlling the degree of
internally generated distortion. In some analyzers, the vertical position of
displayed signals is changed when the input attenuator setting is changed,
so the reference level is also changed accordingly. In modern Agilent
analyzers, the IF gain is changed to compensate for input attenuator
changes, so signals remain stationary on the display, and the reference
level is not changed.
Input impedance: The terminating impedance that the analyzer presents to
the signal source. The nominal impedance for RF and microwave analyzers
is usually 50 ohms. For some systems, e.g. cable TV, 75 ohms is standard.
The degree of mismatch between the nominal and actual input impedance
is given in terms of VSWR (voltage standing wave ratio).
Intermodulation distortion: Unwanted frequency components resulting
from the interaction of two or more spectral components passing through
a device with non-linear behavior (e.g. mixer, amplifier). The unwanted
components are related to the fundamental components by sums and
differences of the fundamentals and various harmonics, e.g. f 1 ±f 2 , 2f 1 ±f 2 ,
2f 2 ±f 1 , 3f 1 ±2f 2 , and so forth.
Linear display: The display mode in which vertical deflection on the display
is directly proportional to the voltage of the input signal. The bottom line
of the graticule represents 0 V, and the top line, the reference level, some
non-zero value that depends upon the particular spectrum analyzer. On
most modern analyzers, we select the reference level, and the scale factor
becomes the reference level value divided by the number of graticule
divisions. Although the display is linear, modern analyzers allow reference
level and marker values to be indicated in dBm, dBmV, dBuV, and in
some cases, watts as well as volts.
LO emission or feedout: The emergence of the LO signal from the
input of a spectrum analyzer. The level can be greater than 0 dBm on
non-preselected spectrum analyzers but is usually less than –70 dBm on
preselected analyzers.
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