Digital storage
All spectrum analyzers require some form of display storage to retain, on a CRT
screen, the relatively slow-moving results of a swept spectrum measurement. Early
spectrum analyzers used CRTs with long-persistence phosphors (or storage meshes
behind the CRT face) to maintain a visible trace throughout an entire frequency
sweep. Modern spectrum analyzers use digital technology to convert the analog
output from an analyzer’s video detector to binary numbers in an internal memory.
These values are then displayed on the analyzer’s CRT screen.
Although digital storage requires a display with a finite number of frequency points,
there are tremendous advantages to digitizing measurement results. Many
functions, such as trace math, were unobtainable with older spectrum analyzers.
Digitizing measurement results also makes it easy to save and recall traces and to
transfer measurement data to other instruments (for example, over the GPIB).
Zero response and DC measurements
What is zero response?
When viewing frequency spans that start at 0 Hz (or very close to 0 Hz), a spectral
line is usually visible at the extreme left of a spectrum analyzer’s display. This is
called zero response or LO feedthrough. In an FFT spectrum analyzer, zero
response is caused by residual dc that originates in the analyzer’s own input
amplifiers. Zero response gives the illusion of a dc offset, even if the input signal
has no dc component—and this occurs even if the analyzer has an ac-coupled input.
In the 89400 series vector signal analyzers, some degree of zero response is always
present in the 0 Hz bin (sometimes called the dc bin). The residual dc that causes
this offset may also leak into the first several bins as well. If you don’t want to see
any zero response on the analyzer’s display, simply start the frequency span several
bins above 0 Hz.
Can spectrum analyzers measure DC?
Most spectrum analyzers are not intended to measure dc. However, analyzers such
as the 89400 series analyzers can measure very low frequencies. This analyzer can,
in fact, measure dc, but not without including a dc offset of its own that can
contribute to (or obscure) a dc offset in the input signal. As we mentioned, this
internal offset is caused by residual dc that originates in the analyzer’s input
amplifiers. Thus, measurement performance at dc is not specified.
Fundamental Measurement Interactions
19 - 9
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