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Keywords: rf,rfic,wireless,noisefigure measurement,gain,meter,y factor, rf ics, rfics
TUTORIAL 2875
Three Methods of Noise Figure Measurement
Nov 21, 2003
Abstract: Three different methods to measure noise figure are presented: Gain method, Y-factor method,
and the Noise Figure Meter method. The three approaches are compared in a table.
Introduction
In wireless communication systems, the "Noise Figure (NF)," or the
related "Noise Factor (F)," defines the noise performance and
contributes to the receiver sensitivity. This application note
describes this important parameter and details ways to measure it.
Click here for an overview of the wireless
components used in a typical radio
transceiver.
Noise Figure and Noise Factor
Noise Figure (NF) is sometimes referred to as Noise Factor (F).
The relationship is simply:
NF = 10 * log10 (F)
Definition
Noise Figure (Noise Factor) contains the important information about the noise performance of a RF
system. The basic definition is:
From this definition, many other popular equations of the Noise Figure (Noise Factor) can be derived.
Below is a table of typical RF system Noise Figures:
Category
MAXIM Products Noise Figure* Applications Operating Frequency System Gain
LNA
MAX2640
0.9dB
Cellular, ISM 400MHz ~ 1500MHz
15.1dB
LNA
MAX2645
HG: 2.3dB
WLL
3.4GHz ~ 3.8GHz
HG: 14.4dB
LG: 15.5dB
WLL
3.4GHz ~ 3.8GHz
LG: -9.7dB
Mixer
MAX2684
13.6dB
LMDS, WLL
3.4GHz ~ 3.8GHz
1dB
Mixer
MAX9982
12dB
Cellular, GSM 825MHz ~ 915MHz
2.0dB
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Receiver System MAX2700
3.5dB ~ 19dB PCS, WLL
1.8GHz ~ 2.5GHz
< 80dB
* HG = High Gain Mode, LG = Low Gain Mode
Measurement methods vary for different applications. As shown in the table above, some applications
have high gain and low noise figure (Low Noise Amplifiers under HG mode), some have low gain and
high noise figure (mixers and LNAs under LG mode), some have very high gain and wide range of noise
figure (receiver systems). Measurement methods have to be chosen carefully. In this article, a Noise
Figure Meter as well as two other popular methods - "gain method" and "Y factor method" - will be
discussed.
Using a Noise Figure Meter
Noise Figure Meter/Analyzer is employed as shown in Figure 1.
Figure 1.
The noise figure meter, such as Agilent N8973A Noise Figure Analyzer, generates a 28VDC pulse signal
to drive a noise source (HP346A/B), which generates noise to drive the device under test (DUT). The
output of the DUT is then measured by the noise figure analyzer. Since the input noise and Signal-toNoise ratio of the noise source is known to the analyzer, the noise figure of the DUT can be calculated
internally and displayed. For certain applications (mixers and receivers), a LO signal might be needed, as
shown in Figure 1. Also, certain parameters need to be set up in the Noise Figure Meter before the
measurement, such as frequency range, application (Amplifier/Mixer), etc.
Using a noise figure meter is the most straightforward way to measure noise figure. In most cases it is
also the most accurate. An engineer can measure the noise figure over a certain frequency range, and
the analyzer can display the system gain together with the noise figure to help the measurement. A
noise figure meter also has limitations. The analyzers have certain frequency limits. For example, the
Agilent N8973A works from 10MHz to 3GHz. Also, when measuring high noise figures, e.g., noise figure
exceeding 10dB, the result can be very inaccurate. This method requires very expensive equipment.
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Gain Method
As mentioned above, there are other methods to measure noise figure besides directly using a noise
figure meter. These methods involve more measurements as well as calculations, but under certain
conditions, they turn out to be more convenient and more accurate. One popular method is called "Gain
Method", which is based on the noise factor definition given earlier:
In this definition, "Noise" is due to two effects. One is the interference that comes to the input of a RF
system in the form of signals that differ from the desired one. The second is due to the random
fluctuation of carriers in the RF system (LNA, mixer, receiver, etc). The second effect is a result of
Brownian motion, It applies in thermal equilibrium to any electronic device, and the available noise power
from the device is:
PNA = kTΔF,
Where k = Boltzmann's Constant (1.38 * 10 -23 Joules/ΔK),
T = Temperature in Kelvin,
ΔF = Noise Bandwidth (Hz).
At room temperature (290ΔK), the noise power density PNAD = -174dBm/Hz.
Thus we have the following equation:
NF = PNOUT - (-174dBm/Hz + 10 * log 10 (BW) + Gain)
In the equation, PNOUT is the measured total output noise power. -174dBm/Hz is the noise density of
290°K ambient noise. BW is the bandwidth of the frequency range of interest. Gain is the system gain.
NF is the noise figure of the DUT. Everything in the equation is in log scale. To make the formula
simpler, we can directly measure the output noise power density (in dBm/Hz), and the equation
becomes:
NF = PNOUTD + 174dBm/Hz - Gain
To use the "Gain Method" to measure the noise figure, the gain of the DUT needs to be pre-determined.
Then the input of the DUT is terminated with the characteristic impedance (50Ω for most RF
applications, 75Ω for video/cable applications). Then the output noise power density is measured with a
spectrum analyzer.
The setup for Gain Method is shown in Figure 2.
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Figure 2.
As an example, we measure the noise figure of the MAX2700. At a specified LNA gain setting and
VAGC, the gain is measured to be 80dB. Then, set up the device as show above, and terminate the RF
input with a 50Ω termination. We read the output noise density to be -90dBm/Hz. To get a stable and
accurate reading of the noise density, the optimum ratio of RBW (resolution bandwidth) and VBW (video
bandwidth) is RBW/VBW = 0.3. Thus we can calculate the NF to be:
-90dBm/Hz + 174dBm/Hz - 80dB = 4.0dB.
The "Gain Method" can cover any frequency range, as long as the spectrum analyzer permits. The
biggest limitation comes from the noise floor of the spectrum analyzer. As shown in the equations, when
Noise Figure is low (sub 10dB), (POUTD - Gain) is close to -170dBm/Hz. Normal LNA gain is about
20dB. In that case, we need to measure a noise power density of -150dBm/Hz, which is lower than the
noise floor of most spectrum analyzers. In our example, the system gain is very high, thus most
spectrum analyzers can accurately measure the noise figure. Similarly, if the Noise Figure of the DUT is
very high (e.g., over 30dB), this method can also be very accurate.
Y Factor Method
Y Factor method is another popular way to measure Noise Figure. To use the Y factor method, an ENR
(Excess Noise Ratio) source is needed. It is the same thing as the noise source we mentioned earlier in
the "Noise Figure Meter" section. The setup is shown in the Figure 3:
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Figure 3.
The ENR head usually requires a high DC voltage supply. For example, HP346A/B noise sources need
28VDC. Those ENR heads works are a very wide band (e.g.10MHz to 18GHz for the HP346A/B) and
they have a standard noise figure parameter of their own at specified frequencies. An example table is
given below. The noise figures at frequencies between those markers are extrapolated.
Table 1. Example of ENR of Noise Heads
HP346A HP346B
Frequency (Hz) NF (dB) NF (dB)
1G
5.39
15.05
2G
5.28
15.01
3G
5.11
14.86
4G
5.07
14.82
5G
5.07
14.81
Turning the noise source on and off (by turning on and off the DC voltage), an engineer measures the
change in the output noise power density with a spectrum analyzer. The formula to calculate noise figure
is:
In which ENR is the number given in the table above. It is normally listed on the ENR heads. Y is the
difference between the output noise power density when the noise source is on and off.
The equation comes from the following:
An ENR noise head provides a noise source at two "noise temperatures": a hot T = TH (when a DC
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voltage is applied) and a cold T = 290°K. The definition of ENR of the noise head is:
The excess noise is achieved by biasing a noisy diode. Now consider the ratio of power out from the
amplifier (DUT) from applying the cold T = 290°K, followed by applying the hot T = TH as inputs:
Y = G(Th + Tn)/G(290 + Tn) = (Th/290 + Tn/290)/(1 + Tn/290).
This is the Y factor, from which this method gets its name.
In terms of Noise figure, F = Tn/290+1, F is the noise factor (NF = 10 * log(F))Thus, Y = ENR/F+1. In
this equation, everything is in linear regime, from this we can get the equation above.
Again, let's use MAX2700 as an example of how to measure noise figure with the Y-factor method. The
set up is show above in Figure 3. Connect a HP346A ENR noise head to the RF input. Connect a 28V
DC supply voltage to the noise head. We can monitor the output noise density on a spectrum analyzer.
By Turning off then turning on the DC power supply, the noise density increased from -90dBm/Hz to 87dBm/Hz. So Y = 3dB. Again to get a stable and accurate reading of the noise density, RBW/VBW is
set to 0.3. From Table 1, at 2GHz, we get ENR = 5.28dB. Thus we can calculate the NF to be 5.3dB.
Summary
In this article, three methods to measure the noise figure of RF devices are discussed. They each have
advantages and disadvantages and each is suitable for certain applications. Below is a summary table of
the pros and cons. Theoretically, the measurement results of the same RF device should be identical,
but due to limitations of RF equipment (availability, accuracy, frequency range, noise floor, etc), we have
to carefully choose the best method to get the correct results.
Noise
Figure
Meter
Suitable
Advantage
Applications
Disadvantage
Super low
NF
Expensive equipment, frequency range
limited
Convenient, very accurate when
measuring super low (0-2dB) NF.
Very high
Easy setup, very accurate at
Gain
Gain or very measuring very high NF, suitable for
Method
high NF
any frequency range
Y
Wide range
Factor
of NF
Method
Limited by Spectrum Analyzer noise
floor. Can't deal with systems with low
gain and low NF.
Can measure wide range of NF at any When measuring Very high NF, error
frequency regardless of gain
could be large.
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More Information
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Application Note 2875: http://www.maximintegrated.com/an2875
TUTORIAL 2875, AN2875, AN 2875, APP2875, Appnote2875, Appnote 2875
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