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Agilent PN 4395/96-1
How to Measure Noise Accurately Using the
Agilent Combination Analyzers
Product Note
Agilent Technologies 4395A/4396B Features of the Combination
Network/Spectrum/Impedance Analyzers
Analyzer Each of the combination analyzers
(4395A and 4396B) offers high per-
formance, is economically designed,
Introduction and contains vector network, spec-
One of the major concerns in C/N trum, and impedance measurement
(carrier/noise) ratio measurement is functions available in one instrument.
understanding the factors that influ- This design strategy allows measure-
ence noise measurement accuracy. In ment of gain, phase, group delay, noise,
noise measurements, different spec- spurious, C/N ratio and more--all of
trum analyzers provide different which are indispensable for evaluat-
measurement results caused by a dif- ing the performance of electronic In addition, they use digital filters
ference of the signal processing algo- components and circuitry in the with a steep shape factor to provide
rithms or different RBWs (resolution important 500 and 1800 MHz fre- substantially improved performance
bandwidths). quency range. For spectrum analysis, in analyzing closely-spaced signals.
in particular, these combination ana- The analyzers are designed with
In this note, we will compare the com- lyzers cover an extremely wide fre- utmost care to minimize internal
bination analyzers (Agilent 4395A and quency range (4395A: 10 Hz to generation of noise, thus allowing
4396B) and conventional spectrum 500 MHz, 4396B: 2 Hz to 1.8 GHz) signals of extremely low levels to be
analyzers (Agilent 4195A, 3588A, and feature the stepped FFT (fast measured without sacrificing its
3589A, 3585A/B) with respect to Fourier) technique (4395A: all resolu- measurement speed. Furthermore,
noise measurement accuracy, and tion bandwidth, 4396B: resolution the time gated spectrum analysis
explain why the combination analyz- bandwidth of 1 Hz to 3 kHz), providing function (Option 1D6) is optionally
ers ensure higher accuracy in noise a sweep time 20 to 100 times shorter available for repetitive burst signal
measurement. than the conventional analyzers. analysis.
Differences between Analyzer 2. Difference in RBW and shape factor The RBW filter (IF filter provided at
Models Noise is defined as spectral energy the last filtering stage in an analyzer)
Measurement results vary from one that is present over an entire fre- can be broadly divided into two types:
analyzer model to another most fre- quency band. Consequently, meas- analog and digital filters. An analog
quently due to the following: ured noise level varies depending on RBW filter may cause up to 20% inac-
the RBW (resolution bandwidth) of curacy in its bandwidth. This means
1. Difference in signal detection the filtering and the shape factor that the indicated noise level can be
method between spectrum analyzers used by each analyzer. inaccurate by as much as 1.5 dB. A
digital filter, however, causes no more
2. Difference in RBW and its shape than 1% inaccuracy in its bandwidth,
factor which results in an inaccuracy of 1 dB
or less.
Each difference is discussed in detail
through comparison between the Conventional Detection Method
IF Stage
combination analyzers and other Mixer
spectrum analyzers: Input
Y
1. Difference in signal detection method RBW Filter Logarithmic Detector X Display
Amp.
between spectrum analyzers
Spectrum analyzers use one of the
following detection methods:
Local Oscillator Sweep Control
A. Conventional detection method
using a logarithmic amplifier and an True RMS Value Based Detection Method
IF Stage
envelope detector Mixer
Input Digital Detection
A