Types Of Measurement

While we have defined spectrum analysis and vector signal analysis as
distinct types, digital technology and digital signal processing are blurring
that distinction. The critical factor is where the signal is digitized. Early
on, when digitizers were limited to a f w tens of kilohertz, only the video
( baseband) signal of a spectrum analyzer was digitized. Since the video signal
carried no phase information, only magnitude data could be displayed.
But ven this limited use of digital technology yielded significant advances:
flick r-free displays of slow sweeps, display mark rs, different types of
averaging, and data output to computers and print rs.

Because the signals that people must analyze are becoming more complex, the
latest generations of spectrum analyzers include many of the vector signal
analysis capabilities previously found only in Fourier and vector signal
analyzers. Analyzers may digitize the signal near the instrument s input,
after some amplification, or after one or more downconvert r stages. In any
of these cases, relative phase as well as magnitude is preserved. In addition to
the benefits noted abov , true vector measurements can be made. Capabilities
are then determined by the digital signal processing capability inherent in th
analyzer s firmware or available as add-on software running either internally
( measurement personalities) or externally ( vector signal analysis software)
on a computer connected to the analyzer. An example of this capability is
shown in Figure 1-7. Note that the symbol points of a QPSK ( quadrature
phase shift k ying) signal are displayed as clusters, rather than single points,
indicating errors in the modulation of the signal under test.

Figure 1-7. Modulation analysis of a QPSK signal measured with a
spectrum analyzer

We hope that this application note gives you the insight into your particular
spectrum analyzer and enables you to utilize this versatile instrument to
its maximum potential.