Agilent Spectrum Analysis Basics - Agilent Technologies

Relative uncertainty
When we make relative measurements on an incoming signal, we use either some
part of the same signal or a different signal as a reference. For example, when we
make second harmonic distortion measurements, we use the fundamental of the
signal as our reference. Absolute values do not come into play; we are interested
only in how the second harmonic differs in amplitude from the fundamental.
In a worst-case relative measurement scenario, the fundamental of the
signal may occur at a point where the frequency response is highest, while
the harmonic we wish to measure occurs at the point where the frequency
response is the lowest. The opposite scenario is equally likely. Therefore,
if our relative frequency response specification is ±0.5 dB as shown in
Figure 4-2, then the total uncertainty would be twice that value, or ±1.0 dB.
Perhaps the two signals under test might be in different frequency bands of the
spectrum analyzer. In that case, a rigorous analysis of the overall uncertainty
must include the sum of the flatness uncertainties of the two frequency bands.
Other uncertainties might be irrelevant in a relative measurement, like
the RBW switching uncertainty or reference level accuracy, which apply
to both signals at the same time.
Absolute amplitude accuracy
Nearly all spectrum analyzers have a built-in calibration source which
provides a known reference signal of specified amplitude and frequency.
We then rely on the relative accuracy of the analyzer to translate the absolute
calibration of the reference to other frequencies and amplitudes. Spectrum
analyzers often have an absolute frequency response specification, where
the zero point on the flatness curve is referenced to this calibration signal.
Many Agilent spectrum analyzers use a 50 MHz reference signal. At this
frequency, the specified absolute amplitude accuracy is extremely good:
±0.34 dB for the ESA-E Series and ±0.24 dB for the PSA Series analyzers.
It is best to consider all known uncertainties and then determine which
ones can be ignored when doing a certain type of measurement. The range
of values shown in Table 4-1 represents the specifications of a variety of
different spectrum analyzers.
Some of the specifications, such as frequency response, are frequency-range
dependent. A 3 GHz RF analyzer might have a frequency response of ±0.38 dB,
while a microwave spectrum analyzer tuning in the 26 GHz range could have
a frequency response of ±2.5 dB or higher. On the other hand, other sources
of uncertainty, such as changing resolution bandwidths, apply equally to
all frequencies.
Table 4-1. Representative values of amplitude uncertainty for common spectrum analyzers
Amplitude uncertainties (±dB)
Relative
RF attenuator switching uncertainty 0.18 to 0.7
Frequency response 0.38 to 2.5
Reference level accuracy (IF attenuator/gain change) 0.0 to 0.7
Resolution bandwidth switching uncertainty 0.03 to 1.0
Display scale fidelity 0.07 to 1.15
Absolute
Calibrator accuracy 0.24 to 0.34
52