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PALO ALTO, Calif., Dec. 1, 2003
It is no surprise that noise is the enemy
of RF and microwave designers everywhere. Noise limits the ability
of a communications receiver to detect weak signals, and impedes
designers from achieving the best receiver performance. Noise
in transmitted signals degrades performance, not just of the
transmitted signal, but in the surrounding spectrum as well.
Because noise is ubiquitous, the RF and microwave industry long
ago created a measurement parameter called noise figure to quantify
how much noise a component or system adds to a signal that passes
through it.
Although noise figure is but one of many parameters
used to describe noise and receiver sensitivity in RF and microwave
systems, it has become the most useful and widely used. Measurement
of noise figure has always required high levels of accuracy
and repeatability between successive measurements and between
different instruments. Accuracy and repeatability ensure agreement
between the specified performance measurements made by component
and subsystem manufacturers and the measurements made by their
customers.
Noise Figure Basics
Noise figure as a measurement parameter came
into use in the early 1940s when engineer Harold Friis defined
it as the signal?to?noise ratio (SNR) at the input of an RF
or microwave device divided by the SNR at its output, expressed
in decibels (dB). As its name implies, SNR is the ratio of signal
level to noise level in a given transmission environment. The
greater the SNR, the more the signal exceeds the noise, thus
making the signal more detectable. In contrast, the lower the
noise figure value the better, since noise created by a microwave
component, subsystem or system would ideally add no noise to
the signal as it passes through. This is never the case, since
all electrical devices add some noise, but devices that add
the least noise are best and have the lowest noise figures.
Just How Important Is Noise Figure?
The importance of noise figure to overall
system performance and cost is hard to overestimate. For example,
halving the noise figure of a direct broadcast satellite (DBS)
receiver from 2 dB to 1 dB has nearly the same effect on performance
as increasing the power of the satellite transponder by as much
as 25 percent. Obviously, manufacturers will find it far more
expensive to increase the power of a space?borne transmitter
than to improve the performance of a receiver's low?noise amplifier
(LNA) in a terrestrial terminal.
In a satellite receiver production line, noise
figure can often be reduced by 1 dB simply by adjusting impedance
levels or selecting appropriate transistors. This 1?dB noise
figure reduction has about the same effect as increasing the
antenna area by 25 percent. Increasing antenna size raises the
cost, size and weight of its steering mechanism and support
structure, and may make the antenna too large for applications,
such as DBS, where aesthetics are a prime consideration.
In a wireless communications system, a base
station with a low noise figure allows the mobile devices it
communicates with to reduce their transmit power, which has
a positive effect on both the battery life and on the size and
corresponding weight of the batteries they use.
Noise is also important in transmitter design.
For example, excessive noise in a wireless base station's linear
power amplifier can degrade the reception quality in adjacent
channels, rendering it non-compliant with regulations governing
interference.
Making Noise Figure Measurements
There are several techniques and types of
instruments that can be used to measure noise figure, ranging
from dedicated noise figure analyzers to spectrum analyzers,
network analyzers and true rms power meters. As might be expected,
dedicated noise figure analyzers provide the least measurement
uncertainty, followed by spectrum analyzers (if equipped with
a preamplifier).
The Agilent ESA-E Series mid-performance spectrum
analyzers all have optional integrated preamplifiers (option
1DS), which allows them to deliver fully specified noise figure
measurements from 10 MHz to 1.5 or 3 GHz depending on analyzer
frequency range. The Agilent ESA-E Series spectrum analyzers
complement the PSA Series high-performance spectrum analyzers
and the Agilent NFA Series noise figure analyzers. It is the
most effective solution if a mid-performance spectrum analysis
tool is already required in an application. In the past, measuring
noise figure with a spectrum analyzer required multiple steps
and several mathematical calculations that made the process
tedious and error prone. The ESA-E Series' new noise figure
measurement personality now automates almost the entire process,
including calculations. The result is a solution that is very
accurate and easy to use. The new personality is an integral
part of the spectrum analyzer's rich environment of general-purpose
capabilities, which include one-button power measurements, phase
noise and modulation analysis with links to 89601A VSA software.
For advanced spectrum analysis and excellent
instrument uncertainty, the user has a choice of the PSA Series
spectrum analyzer. The PSA has all the functionality expected
of a high-performance spectrum analyzer and a noise figure measurement
personality with an identical user interface to the ESA-E Series.
Consequently, customers can seamlessly move from one instrument
to the next without having to worry about re-acquainting themselves
with the nuances of each instrument.
It might seem that users of the ESA-E series
and the PSA Series spectrum analyzers would no longer need a
dedicated noise figure analyzer. However, all three instruments
are uniquely suited to specific environments.
Spectrum analyzers are one of the designer's
most common, versatile and useful measurement tools, and are
found on nearly every test bench. Their multifunctional nature
gives them the ability, for example, to first locate spurious
signals and then measure the noise figure of a device at frequencies
where these signals will not interfere with noise measurements.
Thus, the ESA-E Series with the noise figure personality is
an ideal solution for designers who need diverse capabilities
at an economical price. It is the industry's most flexible spectrum
analyzer with a unique card cage structure ideally suited for
customizability. The PSA Series offers the most advanced spectrum
analysis with a leading-edge combination of flexibility, speed,
accuracy and dynamic range.
In contrast, noise figure analyzers are very
specific application instruments designed only to measure noise
figure, gain and related metrics. They can do this faster, easier
and with greater accuracy over a much broader frequency range
than spectrum analyzers or other instruments. Hence, they are
the top choice for the best possible measurement uncertainty,
especially for frequencies above 3 GHz. For fully specified
performance up to 26.5 GHz, the fastest most accurate instruments
are the Agilent NFA Series noise figure analyzers.
# # #
Contact:
Janet Smith, Agilent
+1 970 679 5397
janet_smith@agilent.com
Heather Van Schoiack
Weber Shandwick, for Agilent
+1 425 452 5457
hvanschoiack@webershandwick.com
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