Specifications

49
Now that we can view our signal on the display screen, let’s look at amplitude
accuracy, or perhaps better, amplitude uncertainty. Most spectrum analyzers
are specified in terms of both absolute and relative accuracy. However,
relative performance affects both, so let’s look at those factors affecting
relative measurement uncertainty first.
Before we discuss these uncertainties, let’s look again at the block diagram
of an analog swept-tuned spectrum analyzer, shown in Figure 4-1, and see
which components contribute to the uncertainties. Later in this chapter, we
will see how a digital IF and various correction and calibration techniques
can substantially reduce measurement uncertainty.
Components which contribute to uncertainty are:
• Input connector (mismatch)
• RF Input attenuator
• Mixer and input filter (flatness)
• IF gain/attenuation (reference level )
• RBW filters
• Display scale fidelity
• Calibrator (not shown)
An important factor in measurement uncertainty that is often overlooked
is impedance mismatch. Analyzers do not have perfect input impedances,
and signal sources do not have ideal output impedances. When a mismatch
exists, the incident and reflected signal vectors may add constructively or
destructively. Thus the signal received by the analyzer can be larger or
smaller than the original signal. In most cases, uncertainty due to mismatch
is relatively small. However, it should be noted that as spectrum analyzer
amplitude accuracy has improved dramatically in recent years, mismatch
uncertainty now constitutes a more significant part of the total measurement
uncertainty. In any case, improving the match of either the source or
analyzer reduces uncertainty
1
.
Chapter 4
Amplitude and
Frequency Accuracy
Input
signal
RF input
attenuator
Mixer
IF gain
Display
IF filter
Log
amp
Envelope
detector
Video
filter
Pre-selector, or
low-pass filter
Sweep
generator
Local
oscillator
Reference
oscillator
Figure 4-1. Spectrum analyzer block diagram
1. For more information, see the Agilent PSA
Performance Spectrum Analyzer Series Amplitude
Accuracy Product Note, literature number
5980-3080EN.