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How to analyze the impedance and loss of the PCB connector
The impedance and loss of the PCB is critical to the transmission of high speed signals from the PCB connector. Because of the analysis of such a complex transmission channel, the impact of the signal on the signal can be explored by transmitting the channel impulse response. The impulse response of the circuit can also be obtained by transmitting one narrow pulse. The ideal narrow pulse should be a narrow pulse of infinitely narrow width and very high amplitude. When this narrow pulse is transmitted along the transmission line, the pulse will be broadened, and the shape after widening is related to the response of the line. Mathematically, the impulse response of the channel and the input signal can be convolved to obtain the waveform of the signal after transmission through the channel. The impulse response can be obtained from the step response of the channel, since the differential of the step response is the impulse response, so the two are equivalent.
In a specific case, an ideal narrow pulse or an infinitely steep step signal does not exist, which is difficult to generate and has poor precision control. Therefore, in actual tests, a sine wave is used for testing to obtain a frequency domain response, and The time domain response is obtained by the corresponding physical layer test system software. Compared to other signals, sine waves are easier to form, and their frequency and amplitude accuracy are easier to control. Vector network analyzer VNA (vector network analyzer) can accurately measure the reflection and transmission characteristics of transmission channels to different frequencies by sine wave sweeping in the frequency range up to several tens of GHz. The dynamic range is more than 100dB, so modern is in progress. In the analysis of high-speed transmission channels, a vector network analyzer is first used for measurement.
The reflection and transmission characteristics of the sine wave of different frequencies of the system under test can be expressed by S-parameter. The S-parameter describes the characteristics of the transmission and reflection of the sine wave of the DUT for different frequencies. If the reflection and transmission characteristics of the transmission channel for sine waves of different frequencies can be obtained, it is theoretically possible to predict the influence of the real digital signal after passing through this transmission channel. Since the real digital signal can be seen in the frequency domain, it can be considered by many Composed of sine waves of different frequencies.
For a single-ended transmission line, it includes four S parameters: S11, S22, S21, S12. S11 and S22 respectively reflect the reflection characteristics of sine waves of different frequencies for 1 port and 2 ports, S21 reflects the transmission characteristics of sine waves of different frequencies from 1 port to 2 ports, and S12 reflects the port characteristics from 2 ports to 1 port. The transmission characteristics of sine waves of different frequencies. For differential transmission lines, because there are a total of four ports, the S parameters are more complicated, for a total of 16. Typically, a 4-port or more port vector network analyzer is used to measure the differential transmission line to obtain its S-parameters.
If the 16 S-parameters of the measured differential line are obtained, many important characteristics of the pair of differential lines have been obtained. For example, the SDD21 parameter reflects the insertion loss characteristic of the differential line, and the SDD11 parameter reflects its return loss characteristic.
More information can be obtained by performing inverse FFT transformation on these S parameters. For example, the SDD11 parameter transform obtains a time domain reflection waveform (TDR: Time Domain Reflection), and the time domain reflection waveform can reflect the impedance variation condition on the measured transmission line. The SDD21 result of the transmission line can be inverse FFT transformed to obtain its impulse response, and then the waveform or eye diagram of the digital signal with different data rates after the pair of differential lines is predicted. This is a very useful piece of information for digital design engineers.
It can be seen that the vector network analyzer (VNA) is used to measure the transmission channel of the digital signal. One aspect emulates the analysis method of the radio frequency microwave, and can obtain the characteristics of the transmission channel with very precise frequency in the frequency range of several tens of GHz; On the one hand, by performing some simple time-domain transformation on the measurement results, it is possible to analyze the impedance variation on the channel and the influence on the real signal transmission, and thus assist the digital engineer to distinguish the backplane, cable, and connection in the early stage. The advantages and disadvantages of the device, PCB, etc., without having to wait until the final signal goes wrong, and then rush to deal with it.
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