Keysight Technologies CXA N9000B, Manual

Page: 75 / 88

N9069C Noise Figure Measurement Guide 75
Measurement Related Tasks and Concepts
Setting the Signal Analyzer to Make a Measurement
Selecting the Input Attenuation Range
The noise figure measurement application has a default input attenuation
calibration range of 0 dB to 8 dB, and a step size of 4 dB.
The disadvantage of wide ranges of attenuator calibration is the number of
calibration sweeps, and the time the calibration routine takes. The advantage
of wider ranges is freedom from needing to repeat the calibration when using a
higher output power DUT.
In the Noise Figure Measurement Application, the attenuators do not
autorange. There is therefore a risk of overdriving the analyzer. If the input
signal power level is greater than –26 dBm in low band (0 to 3.6 GHz), or
greater than –31 dBm in high band (above 3.6 GHz), the preamp can go into
compression and the accuracy of your results will be adversely affected. In
most cases, 0 dB attenuation is adequate.
A guide to the input power that can be handled by the X-Series analyzers at
each frequency range is shown in Table 3-4 on page 75 . To check for
overdriving of the analyzer, that is, compression occurring at the preamp stage,
set the attenuation ( AMPTD , Attenuation tab) to 0 dB and note the noise figure
of your DUT. Now increase the attenuation by one step (4 dB) by pressing the
up-arrow key. If your noise figure changes too much, such as 0.3 dB,
attenuation is required for accurate measurements. The error will decline to
40% as large with each additional 4 dB step of attenuation and then the
remaining error with higher attenuation is about two-thirds as large as the
change in NF observed. When you choose increased attenuation to avoid
compression errors, you also get increased uncertainty due to the imperfectly
calibrated noise of the analyzer relative to the noise of the DUT. Usually, any
noise level that is large enough to cause compression is so large that it makes
the analyzer noise an insignificant contributor of uncertainty.
Table 3-4 Power Detection and Ranging on X-Series Signal Analyzers
Frequency Attenuation
Setting
Maximum
Input Power
for High
Accuracy
Approximate DUT Characteristics
a
10 MHz to
3.6 GHz
0 dB –26 dBm
Over the full bandwidth
b
, a DUT with NF = 5 dB and Gain <
44 dB, or NF = 15 dB and Gain < 37 dB
4 dB –22 dBm Gains 4 dB higher than the 0 dB attenuation case
8 dB –18 dBm Gains 8 dB higher than the 0 dB attenuation case
12 dB –14 dBm Gains 12 dB higher than the 0 dB attenuation case