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High-Accuracy Noise Figure
Measurements Using the
PNA-X Series Network Analyzer
Application Note
Table of Contents
Overview of Noise Figure ................................................................................... 3
What is noise figure? ......................................................................................... 3
Importance of noise figure accuracy ........................................................... 5
Noise Figure Measurement Techniques ...................................................... 6
Y-factor method .................................................................................................... 7
Cold source method ............................................................................................ 8
Accuracy Limitations ............................................................................................ 9
Assumptions of the Y-factor method ............................................................ 9
Noise figure measurement uncertainty contributions .......................... 10
PNA-X's Unique Approach ................................................................................ 19
Option choices .................................................................................................... 19
Correcting for noise-parameter effects ...................................................... 21
Measurement comparisons of PNA-X and Y-factor method ............... 23
Scalar noise calibration .................................................................................. 30
Sweep considerations ...................................................................................... 31
Using the standard receivers for measuring noise figure ................... 32
Noise-power parameters ............................................................................... 38
Measuring frequency converters ................................................................. 38
Measuring differential devices...................................................................... 40
Measuring noise parameters ....................................................................... 42
Calibration Overview ............................................................................................ 43
Vector noise calibration ................................................................................... 43
Standard receiver noise calibration ............................................................. 45
Scalar noise calibration ................................................................................... 46
Calibration for frequency converters........................................................... 46
On-wafer calibration ........................................................................................ 47
Moving the noise calibration plane ............................................................ 49
Practical Measurement Considerations .................................................... 50
Ambient temperature setting ........................................................................ 50
Noise averaging.................................................................................................. 50
Optimizing S-parameter power level ........................................................... 54
Optimizing power sensor level during calibration .................................. 55
Compression and damage levels ................................................................ 56
Interference ........................................................................................................ 58
Additional Resources .......................................................................................... 59
Application notes ............................................................................................... 59
Magazine articles............................................................................................... 59
Papers .................................................................................................................... 59
Web ....................................................................................................................... 59
2
Overview of Noise Figure
What is noise figure?
Noise figure is a figure-of-merit that describes the amount of excess noise
present in a system. Minimizing noise figure reduces system impairments
that result from noise. In our personal lives, noise degrades the image quality
of TV pictures, and adversely impacts the voice quality of cell phone calls. In
military applications like radar, receiver noise limits the effective range of the
system. With digital communications, noise increases the bit-error rate. System
designers always try to optimize the overall signal-to-noise ratio (SNR) of the
system. This can be done by increasing the signal, or by reducing noise. In a
transmit/receive system like a radar system, one possibility is to increase the
radar's transmitted power by using bigger, more powerful amplifiers, and/or by
using larger antennas. Decreasing the path loss between the transmitter and
receiver also helps increase SNR, but path loss is often defined by the operating
environment and cannot be controlled by the system designer. SNR can also be
increased by decreasing receiver-contributed noise, which is usually determined
by the quality of the low-noise amplifier (LNA) at the front end of the receiver. In
general, it is easier and less expensive to decrease receiver noise (and achieve
a better noise figure) than to increase transmitter power.
The definition of noise figure is simple and intuitive. The noise factor (F) of a
network is defined as the input SNR divided by the output SNR:
F = (Si/Ni)/(So/No), where
Si = input signal power
So = output signal power
Ni = input noise power
No = output noise power
Noise figure (NF) is simply the noise factor expressed in decibels: NF = 10*log (F)
This definition is true for any electrical network, including those that shift the
frequency of the input signal to a different output frequency, such as an up or
down converter.
To better understand the concept of noise figure, consider an amplifier where
the output signal is equal to the input signal multiplied by the gain of the
amplifier. If the amplifier is perfect, the output noise is also equal to the input
noise multiplied by the amplifier's gain, resulting in the same SNR at both the
input and output of the amplifier. For any real-world amplifier however, the
output noise is larger than the input noise multiplied by the gain, so the SNR at
the output is smaller than that at the input, resulting in F being greater than one,
or NF being greater than 0 dB.
It is important to note that when measuring and comparing noise figures, the
test system is assumed to provide perfect 50-ohm terminations at the input and
output of the device-under-test (DUT). In real-world scenarios however, this is
never the case. Later, we will discuss the accuracy implications if our test system
is not exactly 50 ohms, and we will show how calibration and measurement
methods can overcome the errors produced from an imperfect 50-ohm source
match.
3
Another way to express the amount of noise added by an amplifier or system
is in terms of effective input temperature (Te). To understand this parameter,
recall that the amount of noise available from a passive termination can be
expressed as kTB, where k is Boltzmann's constant, T is the temperature of the
termination in Kelvin, and B is the system bandwidth. For a given bandwidth, the
amount of noise is proportional to temperature. Therefore, the amount of noise
produced by a device can be expressed as an equivalent noise temperature,
normalized to a 1 Hz bandwidth. For example, the amount of electrical noise
coming out of a commercial noise source with a 15 dB excess-noise ratio (ENR)
is equivalent to a termination at 8880 K. The noise factor of any real device
can be expressed as an effective input noise temperature. While Te is not the
actual physical temperature of the amplifier or converter, it is the equivalent
temperature (in degrees Kelvin) of an input termination connected to a perfect
(noise-free) device that would produce the same amount of additional noise at
the output. Te is related to the noise factor as:
Te = 290*(F - 1)
A plot of Te versus noise figure is shown in Figure 1. While the majority of LNAs
are described using noise figure, Te is often used for LNAs that have noise
figures that are less than 1 dB. Te is also useful for mathematical calculations
involving noise powers.
Figure 1. Effective noise temperature versus noise figure
4
Importance of noise figure accuracy
One of the goals of this application note is to give the reader a better
understanding of accuracy issues related to noise figure measurements.
Measurement accuracy is important in both R&D and manufacturing environments.
In R&D, better noise figure accuracy means that there will be a better correlation
between simulations and measurements, helping designers refine circuit
models faster. But higher accuracy also means that a system designer can
better optimize transmit/receive systems like those used in radar applications.
When assigning performance values to all of the individual components of the
system, the system designer must add a guard band based on measurement
accuracy, since a component designer will measure their device to verify its
performance. For noise figure, improved measurement accuracy and smaller
guard bands mean the LNA can have better specifications, which in turn means
that lower-power transmit amplifiers can be used for the same overall system
SNR. This translates to smaller, lighter, and cheaper transmitters, all of which is
very important for airborne and spaceborne applications.
In manufacturing, improved measurement accuracy also allows use of smaller
guard bands, which provides better correlation among multiple test stations.
This means fewer products must be reworked, resulting in higher yields and
improved throughput, and lower test costs. Smaller guard bands also allow
better device specifications, yielding more competitive products that command
higher prices or attain higher market share.
5
Noise Figure Measurement Techniques
There are two main techniques for making noise figure measurements.
The most commonly used method is called the Y-factor or hot/cold-source
technique, and it is used with Agilent's noise figure analyzers, and spectrum
analyzer-based solutions.
The Y-factor method uses a calibrated noise source consisting of a specially
designed noise diode that can be turned on or off, followed by an attenuator
to provide a good output match (Figure 2). When the diode is off (i.e., no bias
current is present), the noise source presents a room-temperature termination
to the DUT. When the diode is reversed biased, it undergoes avalanche break-
down, which creates considerable electrical noise over and above that provided
by a room-temperature termination. This amount of extra noise is characterized
as an "excess noise ratio" or ENR, and for a given noise source, ENR varies
versus frequency. Typical noise sources have nominal ENR values that range
from 5 dB to 15 dB, depending on the value of the internal attenuator. Using the
noise source, two noise-power measurements are made at the output of the
DUT, and the ratio of the two measurements, which is called the Y-factor, is used
to calculate noise figure. The Y-factor method also yields the scalar gain of the DUT.
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