Agilent Spectrum Analysis Basics - Agilent Technologies

Following the input filter are the mixer and the local oscillator, both of
which add to the frequency response uncertainty. Figure 4-2 illustrates what
the frequency response might look like in one frequency band. Frequency
response is usually specified as ± x dB relative to the midpoint between the
extremes. The frequency response of a spectrum analyzer represents the
overall system performance resulting from the flatness characteristics and
interactions of individual components in the signal path up to and including
the first mixer. Microwave spectrum analyzers use more than one frequency
band to go above 3 GHz. This is done by using a higher harmonic of the local
oscillator, which will be discussed in detail in Chapter 7. When making
relative measurements between signals in different frequency bands, you
must add the frequency response of each band to determine the overall
frequency response uncertainty. In addition, some spectrum analyzers
have a band switching uncertainty which must be added to the overall
measurement uncertainty.
+0.5 dB
Frequency response
Signals in the same harmonic band
0
- 0.5 dB
BAND 1
Specification: ±0.5 dB
Figure 4-2. Relative frequency response in a single band
After the input signal is converted to an IF, it passes through the IF gain
amplifier and IF attenuator which are adjusted to compensate for changes in
the RF attenuator setting and mixer conversion loss. Input signal amplitudes
are thus referenced to the top line of the graticule on the display, known as the
reference level. The IF amplifier and attenuator only work at one frequency
and, therefore, do not contribute to frequency response. However, there is
always some amplitude uncertainty introduced by how accurately they can be
set to a desired value. This uncertainty is known as reference level accuracy.
Another parameter that we might change during the course of a measurement
is resolution bandwidth. Different filters have different insertion losses.
Generally, we see the greatest difference when switching between LC filters
(typically used for the wider resolution bandwidths) and crystal filters
(used for narrow bandwidths). This results in resolution bandwidth
switching uncertainty.
The most common way to display signals on a spectrum analyzer is to use a
logarithmic amplitude scale, such as 10 dB per div or 1 dB per div. Therefore,
the IF signal usually passes through a log amplifier. The gain characteristic
of the log amplifier approximates a logarithmic curve. So any deviation from
a perfect logarithmic response adds to the amplitude uncertainty. Similarly,
when the spectrum analyzer is in linear mode, the linear amplifiers do not
have a perfect linear response. This type of uncertainty is called display
scale fidelity.
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