Mismatch & Measurement Error Reducing Technique

The general expression used to calculate the maximum mismatch error
in dB is:

As an example, consider a spectrum analyzer with an input VSWR of 1.2 and
a device under test (DUT) with a VSWR of 1.4 at its output port. The resulting
mismatch error would be Â± 0.13 dB.

Since the analyzer's worst-case match occurs when its input attenuator is
set to 0 dB, we should avoid the 0 dB setting if we can. Alternatively, we can
attach a well-matched pad (attenuator) to the analyzer input and greatly
reduce mismatch as a factor. Adding attenuation is a technique that works
well to reduce measurement uncertainty when the signal we wish to measure
is well above the noise. However, in cases where the signal-to-noise ratio is
small (typically 7 dB), adding attenuation will increase measurement error
because the noise power adds to the signal power, resulting in an erroneously
high reading.

Let's turn our attention to the input attenuator. Some relative measurements
are made with different attenuator settings. In these cases, we must consider
the input attenuation switching uncertainty. Because an RF input
attenuator must operate over the entire frequency range of the analyzer,
its step accuracy varies with frequency. The attenuator also contributes
to the overall frequency response. At 1 GHz, we expect the attenuator
performance to be quit good; at 26 GHz, not as good.

The next component in the signal path is the input filter. Spectrum analyzers
use a fixed low-pass filter in the low band and a tunable band pass filter
called a preselector (we will discuss the preselector in more detail in
Chapter 7) in the higher frequency bands. The low-pass filter has a better
frequency response than the preselector and adds a small amount of
uncertainty to the frequency response error. A preselector, usually a
YIG-tuned filter, has a larger frequency response variation, ranging from
1.5 dB to 3 dB at millimeter-wave frequencies.