This article introduces three methods of measuring noise figure: gain method, Y-factor method and noise figure tester method . A comparison of these three methods is given in tabular form.
In wireless communication systems, the noise figure (NF) or the corresponding noise factor (F) defines the noise performance and contribution to receiver sensitivity. This application note elaborates on this important parameter and its different measurement methods.
Noise Figure and Noise Figure
Noise figure (NF) is also sometimes referred to as noise factor (F). The simple relationship between the two is:
NF = 10 * log10 (F)
Noise figure (noise figure) contains important information about the noise performance of an RF system, and is defined by the standard as:
Many commonly used noise figure (noise factor) formulas can be derived from this definition.
The following table shows typical RF system noise figure:
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The method of measuring noise figure varies from application to application. As can be seen from the above table, some applications have high gain and low noise figure (low noise amplifier (LNA) in high gain mode), and some applications have low gain and high noise figure (mixer and LNA in low gain mode). ), some have very high gain and wide range noise figure (receiver system). The measurement method must therefore be chosen carefully. This article will discuss the noise figure tester method, the gain method, and the Y-factor method.
Using a Noise Figure Tester
The noise figure test/analyzer is given in Figure 1.
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figure 1
A noise figure tester, such as Agilent's 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). Measure the output of the device under test using a noise figure analyzer. Since the analyzer knows the input noise and signal-to-noise ratio of the noise source, the noise figure of the DUT can be calculated internally and displayed on the screen. For some applications (mixers and receivers), a local oscillator (LO) signal may be required, as shown in Figure 1. Of course, certain parameters such as frequency range, application (amplifier/mixer), etc. must be set in the noise figure tester before measurement.
Using a noise figure meter is the most straightforward way to measure noise figure. Also the most accurate in most cases. Engineers can measure noise figure over a specific frequency range, and the analyzer can simultaneously display gain and noise figure to aid in the measurement. Analyzers are frequency limited. For example, the Agilent N8973A can operate from 10MHz to 3GHz. When measuring very high noise figure, such as noise figure exceeding 10dB, the measurement result is very inaccurate. This method requires very expensive equipment.
gain method
As mentioned earlier, in addition to directly using the noise figure tester, other methods can be used to measure the noise figure. These methods require more measurements and calculations, but under certain conditions, these methods are more convenient and accurate. One of the commonly used methods is called the "gain method", which is based on the definition of noise factor given earlier:
In this definition, noise arises from two factors. One is the interference arriving at the input of the RF system, which is different from the desired wanted signal. The second is due to random perturbations of the RF system carrier (LNA, mixer and receiver, etc.). The second case is the result of Brownian motion, applied to thermal equilibrium in any electronic device, the available noise power of the device is:
PNA = kTΔF,
Here 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 spectral density PNAD = -174dBm/Hz.
We thus have the following formula:
NF = PNOUT - (-174dBm/Hz + 20 * log10(BW) + gain)
In the formula, PNOUT is the total output noise power measured, and -174dBm/Hz is the power spectral density of ambient noise at 290°K. BW is the frequency bandwidth of interest. Gain is the gain of the system. NF is the noise figure of the DUT. Each variable in the formula is logarithmic. To simplify the formula, we can directly measure the output noise power spectral density (dBm/Hz), then the formula becomes:
NF = PNOUTD + 174dBm/Hz - Gain
In order to measure noise figure using the gain method, the gain of the DUT needs to be predetermined. The input to the DUT needs to be terminated with a characteristic impedance (50Ω for RF applications and 75Ω for video/cable applications). The output noise power spectral density can be measured using a spectrum analyzer.
See Figure 2 for the device measured by the gain method.
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figure 1
As an example, we measure the noise figure of the MAX2700. The measured gain is 80dB at the specified LNA gain setting and VAGC. Next, install the instrument as shown above, and terminate the RF input with a 50Ω load. Read out the output noise power spectral density on the spectrum analyzer as -90dBm/Hz. For stable and accurate noise density readings, choose the optimal RBW (resolution bandwidth) and VBW (video bandwidth) as RBW/VBW = 0.3. The calculated NF is:
-90dBm/Hz + 174dBm/Hz - 80dB = 4.0dB
The gain method can be used in any frequency range as long as the spectrum analyzer allows it. The biggest limitation comes from the noise floor of the spectrum analyzer. It can be seen in the formula that when the noise figure is low (less than 10dB), (POUTD - gain) is close to -170dBm/Hz, and usually the gain of the LNA is about 20dB. So we need to measure the noise power spectral density of -150dBm/Hz, which is lower than the noise floor of most spectrum analyzers. In our case, the system gain is so high that most spectrum analyzers can accurately measure noise figure. Similarly, if the noise figure of the DUT is very high (eg, higher than 30dB), this method is also very accurate.
Y-factor method
The Y-factor method is another commonly used method for measuring noise figure. In order to use the Y-factor method, an ENR (redundancy to noise ratio) source is required. This is the same source of noise mentioned earlier in the Noise Figure Meter section. The device diagram is shown in Figure 3:
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image 3.
ENR heads typically require a high voltage DC power supply. Such as HP346A/B noise source needs 28VDC. These ENR heads can work in a very wide frequency band (such as 10MHz to 18GHz for HP346A/B), and have standard noise figure parameters at specific frequencies. The following table gives specific values. The noise figure at frequencies between the markers can be obtained by extrapolation.
Table 1. ENR of noise head
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Turning the noise source on or off (by switching the DC voltage on and off), engineers can use a spectrum analyzer to measure the change in the power spectral density of the output noise. The formula for calculating noise figure is:
In this formula, ENR is the value given in the table above. Usually the NF value of the ENR header will be listed. Y is the difference in output noise power spectral density when the noise source is on and off.
This formula can be obtained from:
The ENR noise head provides two noise sources for noise temperature:
T = TH for hot temperature (when DC voltage is applied) and T = 290°K for cold temperature.
The ENR noise head is defined as:
Redundant noise is obtained by biasing the noisy diodes. Now consider the amplifier (DUT) power output ratio at cold temperature T = 290°K vs. hot temperature T = TH: Y = G(Th + Tn)/G(290 + Tn)
= (Thursday/290 + Sun/290)/(1 + Sun/290
This is the Y-factor method, whose name comes from the formula above.
According to the noise figure definition, F = Tn/290+1, F is the noise factor (NF = 10 * log(F)), therefore, Y = ENR/F+1. In this formula, all variables are linearly related, and the noise coefficient formula above can be obtained from this formula.
We again use the MAX2700 as an example to demonstrate how to measure noise figure using the Y-factor method. The device diagram is shown in Figure 3. Connect HP346A ENR to RF input. Connect 28V DC voltage to the noise source. We can monitor the output noise power spectral density on a spectrum analyzer. Turn on/off the DC power supply, the noise spectral density changes from -90dBm/Hz to -87dBm/Hz. So Y = 3dB. For stable and accurate noise power spectral density readings, RBW/VBW was set to 0.3. Obtained from Table 2, ENR = 5.28dB at 2GHz, so we can calculate the value of NF to be 5.3dB.
Summarize
In this article, three methods for measuring the noise figure of RF devices are discussed. Each method has its advantages and disadvantages and is suitable for specific applications. The table below summarizes the advantages and disadvantages of the three methods. In theory, the measurement results of the same RF device should be the same, but due to the limitations of RF equipment (availability, accuracy, frequency range, noise floor, etc.), the best method must be selected to obtain correct results.
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