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Agilent
Fundamentals of RF and Microwave
Noise Figure Measurements
Application Note 57-1
Table of Contents
1. What is Noise Figure? ..............................................................................3
Introduction .......................................................................................................3
The importance of noise in communication systems ...............................4
Sources of noise...............................................................................................5
The concept of noise figure ...........................................................................6
Noise figure and noise temperature.............................................................7
2. Noise Characteristics of Two-Port Networks .........................8
The noise figure of multi-stage systems .....................................................8
Gain and mismatch ..........................................................................................9
Noise parameters .............................................................................................9
The effect of bandwidth..................................................................................10
3. The Measurement of Noise Figure ..................................................11
Noise power linearity ......................................................................................11
Noise sources ...................................................................................................11
The Y-factor method ........................................................................................12
The signal generator twice-power method .................................................13
The direct noise measurement method .......................................................13
Corrected noise figure and gain ....................................................................14
Jitter ..................................................................................................................14
Frequency converters ......................................................................................15
Loss ............................................................................................................15
Lo noise .....................................................................................................15
Lo leakage.................................................................................................15
Unwanted responses ..............................................................................15
Noise figure measuring instruments ............................................................16
Noise figure analyzers ............................................................................16
Spectrum analyzers.................................................................................16
Network analyzers ..................................................................................17
Noise parameter test sets .....................................................................17
Power meters and true-rms voltmeters ..............................................17
4. Glossary...............................................................................................................18
5. References .........................................................................................................28
6. Additional Agilent Resources, Literature and Tools............30
2
Chapter 1.
What is Noise Figure?
Introduction
Modern receiving systems must often process very weak This application note is part of a series about noise measure-
signals, but the noise added by the system components tends ment. Much of what is discussed is either material that is
to obscure those very weak signals. Sensitivity, bit error ratio common to most noise figure measurements or background
(BER) and noise figure are system parameters that character- material. It should prove useful as a primer on noise figure
ize the ability to process low-level signals. Of these param- measurements. The need for highly repeatable, accurate and
eters, noise figure is unique in that it is suitable not only for meaningful measurements of noise without the complexity of
characterizing the entire system but also the system compo- manual measurements and calculations has lead to the devel-
nents such as the pre-amplifier, mixer, and IF amplifier that opment of noise figure measurement instruments with simple
make up the system. By controlling the noise figure and gain user interfaces. Using these instruments does not require
of system components, the designer directly controls the noise an extensive background in noise theory. A little noise back-
figure of the overall system. Once the noise figure is known, ground may prove helpful, however, in building confidence and
system sensitivity can be easily estimated from system band- understanding a more complete picture of noise in RF and mi-
width. Noise figure is often the key parameter that differenti- crowave systems. Other literature to consider for additional in-
ates one system from another, one amplifier from an other, and formation on noise figure measurements is indicated through-
one transistor from another. Such widespread application of out this note. Numbers appearing throughout this document in
noise figure specifications implies that highly repeatable and square brackets [ ] correspond to the same numerical listing in
accurate measurements between suppliers and their custom- the References section. Related Agilent Technologies literature
ers are very important. and web resources appear later in this application note.
The reason for measuring noise properties of networks is to
minimize the problem of noise generated in receiving systems.
One approach to overcome noise is to make the weak signal
stronger. This can be accomplished by raising the signal power
transmitted in the direction of the receiver, or by increasing the
amount of power the receiving antenna intercepts, for exam-
ple, by increasing the aperture of the receiving antenna. Rais-
ing antenna gain, which usually means a larger antenna, and
raising the transmitter power, are eventually limited by govern-
ment regulations, engineering considerations, or economics.
The other approach is to minimize the noise generated within
receiver components. Noise measurements are key to assuring
that the added noise is minimal. Once noise joins the signals,
receiver components can no longer distinguish noise in the
signal frequency band from legitimate signal fluctuations. The
signal and noise get processed together. Subsequent raising
of the signal level with gain, for example, will raise the noise NFA simplifies noise figure measurements
level an equal amount.
3
The importance of noise in The output signal-to-noise ratio depends on two things--the
input signal-to-noise ratio and the noise figure. In terrestrial
Communication systems systems the input signal-to-noise ratio is a function of the
transmitted power, transmitter antenna gain, atmospheric
The signal-to-noise (S/N) ratio at the output of receiving sys- transmission coefficient, atmospheric temperature, receiver
tems is a very important criterion in communication systems. antenna gain, and receiver noise figure. Lowering the receiver
Identifying or listening to radio signals in the presence of noise noise figure has the same effect on the output signal-to-noise
is a commonly experienced difficulty. The ability to interpret ratio as improving any one of the other quantities.
the audio information, however, is difficult to quantify because
it depends on such human factors as language familiarity, In satellite systems, noise figure may be particularly important.
fatigue, training, experience and the nature of the message. Consider the example of lowering the noise figure of a direct
Noise figure and sensitivity are measurable and objective broadcast satellite (DBS) receiver. One option for improving
figures of merit. Noise figure and sensitivity are closely related receiver noise figure is to increase the transmitter power,
(see Sensitivity in the glossary). For digital communication sys- however, this option can be very costly to implement. A better
tems, a quantitative reliability measure is often stated in terms alternative is to substantially improve the performance of the
of bit error ratio (BER) or the probability P(e) that any received receiver low noise amplifier (LNA). It is easier to improve LNA
bit is in error. BER is related to noise figure in a non-linear performance than to increase transmitter power.
way. As the S/N ratio decreases gradually, for example, the
BER increases suddenly near the noise level where l's and 0's
become confused. Noise figure shows the health of the system
but BER shows whether the system is dead or alive.
Figure 1-1, which shows the probability of error vs. carrier-to-
noise ratio for several types of digital modulation, indicates
that BER changes by several orders of magnitude for only a
few dB change in signal-to-noise ratio.
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