1 introduction
The noise coefficient is a major measurement parameter in the development of microwave products and the production process, which is one of the key parameters of the characterization receiver and its components in the case of thermal noise, and the measurement test of the noise coefficient is more An important part of the noise metering test.
Define the noise factor (f) in the engineering to have a determined input and output, and the input end source impedance is in 290. When k (room temperature), the network input signal noise power ratio is the ratio of the noise power ratio of the network output terminal signal.
Where Si is the input signal power, Ni is input noise power, SO is output signal power, NO is output noise power, and G is two port network gain, NA is noise power of two port network itself.
It explicitly indicates that the network generates noise is caused by the deterioration of the network input signal-to-noise ratio, that is, the transmission network has a size of the total noise power contribution to the output end of the output. The noise factor (NF) is the log of the noise factor, defined as follows:
Repeatable, high noise coefficient measurement method is very important, discuss three typical methods: noise testing instrument, gain method, and y coefficient method, and verify the accuracy of the Y coefficient method through experiments.
2 noise test instrument method
Using a noise tester is a direct method of measuring a noise coefficient, in most cases it is also accurate. Engineers can measure noise coefficients within a particular frequency range. The noise tester can simultaneously display the gain and noise factor to help measure. The test schematic of the noise tester is shown in Figure 1.
The noise test instrument test noise coefficient is a y coefficient method. First, the noise tester itself is a receiver, which can be used to test the noise power of the loser; the second noise tester needs to control the power-on and unpowered state of a noise source, and test the measurement (DUT).
Noise Coefficient Tester, such as an AV3984 noise factor analyzer, generating a 28VDC pulse signal drive noise source (AV16604), which generates noise drive DUT. DUT's noise coefficients can be displayed internally and displayed on the screen by the analyzer known noise and signal-to-noise ratio.
Using the noise coefficient tester is a direct method for measuring the noise coefficient. The measurement personnel can measure the noise coefficient within a particular frequency range, and usually the noise analyzer is more accurate in ultra-low noise measurements. When the measurement is high, the measurement results are inaccurate.
3 gain method
The current noise coefficient measurement mainly uses a dedicated noise coefficient tester, but does not have such a dedicated device or the required test frequency range is not in its range, the spectrum analyzer can be measured, that is, the gain method, the method for frequency The measured can be measured within the frequency range of the spectrum versators used.
A measurement formula can be obtained based on the definition of the noise coefficient:
In the formula (3), Pout is the measured noise power spectral density, and -174 dBm / Hz is a power spectral density of environmental noise when 290 ° K (room temperature), and BW is the frequency bandwidth of interest, and Gain is the gain of the system. Each variable in equation (3) is logged, in order to simplify the formula, we can directly measure the output noise power spectral density (DBM / Hz), and the equation (3) becomes:
Taking the receiver as an example, the test principle is shown in Figure 2.
The gain of the receiver (the level range of the receiver can receive) with the signal source and the spectrum can be used, and if the interested is the noise coefficient when the receiver small signal is close to the receiver, it can be selected to be close to the sensitivity level, such as less than - 100 dBm signal strength); to obtain a stable and accurate noise density reading, the selected resolved bandwidth (RBW) and the video bandwidth (VBW), making the base noise on the spectrometer look clean. The smaller the video bandwidth, the smaller the base noise displayed on the spectrometer, the more accurate the POUT reading.
This method can be applied to any frequency range as long as the spectrum analyzer is allowed. Usually the noise analyzer is more accurate in ultra-low noise measurements. This approach is also very accurate for applications where the system gain is very high and the noise coefficient is very accurate. The limit is from the noise substrate from the spectrometer.
4 y coefficient method
The Y coefficient method is a typical method for measuring the noise coefficient. The Y coefficient refers to the ratio of the two corresponding noise power obtained at the output of the DUT when the input of the DUT is in 2 different credits. The noise source is an indispensable device for the Y coefficient method. The noise source is a noise generator capable of generating two different noise power. It is generally necessary to drive the voltage with a DC pulse power supply. When the DC drive voltage is powered, it is equivalent to the noise source. For the hot state, the output large noise power is output; the DC drive power is turned off, called a cold state, called a cold state, and outputs the noise power at room temperature. The difference between the heat temperature and the cold temperature of the noise source is referred to as the ultra-noise ratio (ENR) of the noise source.
The test principle is shown in Figure 1, and the working equation is exported below in order to further illustrate its working principle.
When the noise source is transferred to the noise temperature T0, the output power Pn of the DUT is:
In the formula (5), K is the Bordsman constant, 1.38 * 10-23J / k, B is bandwidth (Hz), g is gain, TE is the measured network equivalent input noise temperature (k). Similarly, when the noise source is transferred to the noise temperature TN, the output noise power Pn of the DUT is:
The noise figure meter is provided gain is C, in both cases, the noise figure meter power detected CP0 and CPn are namely:
Separate TE and expressed as F, you can get:
In the formula, the ENR is the ultra-noise ratio (dB), Y = CPN / CP0, and the power ratio, and F is the noise coefficient (DB).
Figure 3 shows the composition schematic of the Y-system number value generator.
First, the noise source is operated in the noise temperature T0 (cold state), and the RF attenuator is placed in 0 (DB), and the standard attenuator is placed in A0 (DB), and the indicator is indicated in a location; then, the noise source works in Noise Temperature TN (hotstream), adjust the standard attenuator to return the indicator of the indicator to the original position. At this time, the attenuation reading is an AN, then a = an - a0 (db), the expression of the Y coefficient can be obtained:
The Y coefficient method is suitable for the measurement of a wide range of noise coefficients, simple and accuracy, only need to know the noise source, and measure the noise source cold, the output noise power in the heat of the heat, no need to measure DUT gain Therefore, this method is used when verifying the noise factor accuracy.
5 experimental verification
The Y-factor generation formula (9) given in equity (10), in turn, as shown in Table 1.
We use the AV3984 microwave noise factor analyzer for experiments, and its operating frequency is 10 MHz ~ 26.5 GHz, and the experiment block diagram is shown in Figure 4.
According to the connection experimental instrument shown in Fig. 4, the noise coefficient performance index of the AV3984 is ± 0.25 dB, and the test data is shown in Table 2.
The standard attenuator is used in the experiment, and the PHOT value of the noise coefficient analyzer is recorded by recording the PHO of the noise coefficient analyzer, and the noise coefficient measurement is obtained:
The results show that the deviation of the data is in 0.25 dB. The Y coefficient method can measure a large noise coefficient range, which is suitable for any frequency band, not limited by gain, and more accurate.
Smart noise source used in the experiments, excess noise ratio table may be automatically uploaded to the noise figure analyzer can also upload on command when needed from the noise source. For noise sources that need to manually enter a super-noise ratio table, it is necessary to confirm that the super-noise ratio of the use of the super-noise ratio used before the measurement is matched.
Noise detection circuit
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