Agilent Spectrum Analysis Basics - Agilent Technologies

Agilent data sheets describe the ability to resolve signals by listing the 3 dB
bandwidths of the available IF filters. This number tells us how close together
equal-amplitude sinusoids can be and still be resolved. In this case, there
will be about a 3 dB dip between the two peaks traced out by these signals.
See Figure 2-7. The signals can be closer together before their traces merge
completely, but the 3 dB bandwidth is a good rule of thumb for resolution of
equal-amplitude signals 3 .
Figure 2-7. Two equal-amplitude sinusoids separated by the 3 dB BW
of the selected IF filter can be resolved
More often than not we are dealing with sinusoids that are not equal in
amplitude. The smaller sinusoid can actually be lost under the skirt of the
response traced out by the larger. This effect is illustrated in Figure 2-8. The
top trace looks like a single signal, but in fact represents two signals: one at
300 MHz (0 dBm) and another at 300.005 MHz (–30 dBm). The lower trace
shows the display after the 300 MHz signal is removed.
3. If you experiment with resolution on a spectrum
analyzer using the normal (rosenfell) detector mode
(See “Detector types” later in this chapter) use
enough video filtering to create a smooth trace.
Otherwise, there will be a smearing as the two
signals interact. While the smeared trace certainly
indicates the presence of more than one signal, it is
difficult to determine the amplitudes of the individual
signals. Spectrum analyzers with positive peak as
their default detector mode may not show the
smearing effect. You can observe the smearing by
selecting the sample detector mode.
Figure 2-8. A low-level signal can be lost under skirt of the response
to a larger signal
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