The meaning and measurement method of noise
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1. Definition of noise
The noise factor is a measure of the messy signal added to the signal by the device under test (DUT) when the RF signal passes through it, mainly due to the irregular thermal motion of the electrons in the device. In order to measure the degree of this deterioration, the concepts of noise factor F (Noise Factor) and noise figure NF (Noise Figure) are introduced. That is to say, the noise factor quantifies the degree to which the DUT reduces the signal-to-noise ratio of the signal;
Figure 1.1 Changes in signal-to-noise ratio before and after the useful signal passes through the DUT
Noise Factor: F = SNRIN/SNROUT
Noise Figure: FdB = 10lg(SNRIN/SNROUT)
In the RF transceiver link, the receiver link usually pays special attention to the noise factor, because it determines the sensitivity of the receiver. The lower the noise factor, the higher the receiving sensitivity. From the application perspective, the receiver noise factor is crucial to radar and military and civilian communication systems. High NF is the main factor limiting the maximum detection distance of the radar.
2. Noise testing principle
2.1 Two noise measurement methods
2.1.1 Y-factor method
The Y-factor method for measuring noise coefficient requires equipment including a spectrum analyzer and a standard ENR noise source. By turning the noise source power supply (28V DC) on and off to make it cold and hot, the total power in the two states is measured, and the noise of the DUT itself is calculated.
Figure 2.1 Schematic diagram of noise measurement using the Y-factor method
Figure 2.2 Y-factor method for calculating noise measurement
Figure 2.3 Connection diagram (a) Calibration steps (b) Measurement steps
Three main steps to taking a measurement
1). Calibrate the noise figure of the test equipment;
2). Cascade the DUT and the equipment and measure;
3). Calculate the noise figure;
Some points to note when selecting noise sources and spectrum analyzers:
1). The calibration process requires that the difference between turning the noise source on and off is at least 3dB, so we must choose a noise source whose ENR is at least 3dB greater than the noise figure of the spectrum analyzer, that is:
ENR > NFSA+ 3dB
2). The measurement requires that the difference between turning the noise source on and off is at least 5dB, so we must choose a noise source whose ENR is at least 5dB greater than the noise figure of the DUT, that is:
ENR > NFDUT+ 5dB
3). To solve the difference between the measurement and calibration steps, the difference between the calibration and measurement steps is required to be less than 1dB. This can be achieved by selecting the spectrum analyzer and preamplifier so that the NF+ gain of the DUT is at least 1dB higher than the NF of the spectrum analyzer.
NF DUT + Gain DUT > NF SA + 1dB
If the above three points are achieved, the NF data measured by the Y-factor method will be credible;
2.1.2 Cold source method (gain method)
The absolute power of the DUT's output noise is the result of the DUT's inherent noise being amplified by its gain. If the amplifier's gain can be accurately measured, the amplified input noise can be deducted from the measurement result, thereby calculating the noise factor. Therefore, measuring the DUT's gain is crucial, and the cold source method is also called the gain method.
The cold source method uses a signal analyzer to measure the gain and output noise absolute power of the DUT, as shown in Figure 2.4.
Figure 2.4 Using the cold source method requires terminating the input of the DUT
The cold source method is usually most effective for high-gain LNAs because the signal analyzer can effectively measure signals that are significantly higher than the source noise floor.
Like the Y-factor method, the cold source method requires calibration of the noise figure and gain of the receiver that characterizes the internal noise of the instrument. This calibration requires a noise source, or a power meter can be used to make a swept frequency measurement to determine the effective noise bandwidth of the receiver.
Figure 2.5 Calculation method of noise coefficient using cold source method
Figure 2.5 is a graph showing the relationship between output noise power and input noise power. The gain of the DUT can be measured alone to obtain the slope of this straight line. Then, only one power measurement is required to determine the intersection of this straight line and the Y-axis, thereby determining the position of the straight line in the graph, so that the noise coefficient of the DUT can be derived.
Figure 2.6 System block diagram of NF measurement using vector network cold source method
The entire calibration consists of three steps
1). Connect the noise source to port 2 of the vector network.
Measuring noise power in hot and cold states
Measuring the matching of noise sources in hot and cold states
2). Connect the through piece (between port 1 and port 2)
Measure the gain difference between 0, 15, and 30 dB settings
Measure the load match of a noise receiver
Measure the Γs of an electronic calibration module used as an impedance tuner
3). Connect the electronic calibration module (between port 1 and port 2)
Measuring the error terms of conventional S parameters
Measurement of the receiver noise power at different Γs using an electronic calibration kit (without using a tuner)
3. Summary
|
Application Scenario |
advantage |
shortcoming |
| Y- Factor Method |
Broadband NF measurement |
NF can be measured at any frequency regardless of gain; |
When measuring large noise, the error is large; |
| Cold Source Method |
High-gain or high-noise DUT measurements |
Simple operation and instrument settings;
suitable for DUT testing in any frequency band;
very accurate in measuring high-noise DUT; |
Limited by the limitations of the spectrum analyzer, it cannot measure low-gain or low-noise DUTs;
it is susceptible to the uncertainty of the voltage standing wave ratio (VSWR); |
The above is what I want to share with you. I hope it will be helpful to you~~
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