Agilent Technologies 22A Specifiche Scaricare il pdf (Pagina 50)

The general expression used to calculate the maximum mismatch error

in dB is:

Error (dB) = –20 log[1 ± |(

analyzer

)(

source

)|]

where

is the reflection coefficient

Spectrum analyzer data sheets typically specify the input voltage standing

wave ratio (VSWR). Knowing the VSWR, we can calculate

with the following

equation:

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 ±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 quite 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.

(VSWR–1)

(VSWR+1)

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Agilent Technologies 22A Specifiche Pagina 50