The reflection, that is, the echo on the transmission line, is caused by the discontinuity of the impedance. A mismatch between the source and load impedances causes reflections on the line, and the load reflects a portion of the voltage back to the source. If the load impedance is less than the source impedance, the reflected voltage is negative; otherwise, the reflected voltage is positive. Ideally, the output impedance, transmission line impedance, and load impedance are equal. At this point, the impedance of the transmission line is continuous and no reflection occurs. The amplitude of the reflected voltage signal is determined by the source reflection coefficient rS and the load reflection coefficient rL.
The key to solve the transmission line reflection is impedance control. Impedance matching can suppress transmission line reflection. There are four matching termination methods: parallel termination, Thevenin equivalent parallel termination, AC termination and series termination. Here, the Thevenin equivalent parallel termination method is used to control the input impedance of the detector circuit, and then the circuit topology is extracted to simulate the transmission characteristics of the circuit before and after the termination.
Before the termination, the waveform has distortion on the rising edge, which may cause misoperation. The matching termination effectively eliminates the distortion of the signal, and the monotonicity is very good, and the original signal is pulled up on the rising edge, and the level switching is advanced in advance, the steady state time of the signal is increased, and the rising edge of the signal is relatively stable. Although there is an overshoot in the high-level maintenance phase, it has no effect on the signal confirmation, and the signal quality is ideal. In addition, the length of the signal transmission line also has a certain influence on the reflection. The simulation found that when the transmission line is long, the predicted reflection phenomenon occurs; when the transmission line is short, the simulation waveform and the analysis result agree very well. Therefore, the wiring length is different and the processing method should be different. In general, trace lengths are less than 2 inches and are handled by LC circuits with lumped parameters; greater than 8 inches are treated with transmission line circuits with distributed parameters.
As the operating frequency of the system increases, the routing delay can no longer be ignored when the rising or falling edge of the signal is steep. It plays a vital role in the establishment and maintenance of the signal, and may even affect the timing of the system, causing misoperations, so it must be considered. The MCM high-speed circuit design requires that the phase deviation of the memory chip should not be too large, so the wiring delay from the driver to the receiver should be approximately equal. The length of the signal line has a great influence on the transmission quality, which may cause the signal to be distorted during transmission. The signal transmission quality deteriorates as the line length increases. For long signal lines, the source or terminal matching method should be used to improve the transmission quality. The signal integrity simulation tool can be used to easily simulate the delay from the driver to each chip, and then adjust the layout and wiring according to the simulation results to meet the predetermined requirements.
Each signal of the detector should be kept at the same transmission delay as much as possible. This requires that the wiring be kept as long as possible. For weak differences, the wiring can be extended or shortened according to the simulation results. After the wiring is completed, the transmission delay of the input signal is simulated by Spectra Quest software. The specific parameters are shown in Table 2. It can be seen that the relative delay does not exceed 0.2 ns, and the simulation results are ideal.
In addition to analyzing the reflection and delay of the signal in the time domain, EMI (electromagnetic interference) is also an important aspect of high-speed circuit design.
Electromagnetic interference, including excessive electromagnetic radiation and sensitivity to electromagnetic radiation, can cause electromagnetic interference effects if the operating frequency is too high, the signal changes too fast, or the layout and wiring are unreasonable. The EMI simulation is performed on the change of the wiring strategy and the increase of the detector circuit before and after the terminal matching. The noise generated by the signal continues from 0 to 2 GHz, the range is very wide, and the radiation intensity of each frequency is not the same. The radiation intensity of some frequencies exceeds the limit, that is, the electromagnetic interference of the signal at this frequency is beyond the reach of the system. To the extent that measures should be taken to reduce their radiation levels. Perform impedance control as described above and minimize the wiring length. It can be seen that the frequency wave exceeding the limit has fallen below the horizontal line, and the radiation intensity at each frequency point has decreased, and the entire radiation intensity has been reduced. This shows that for the transmission signal, changing the wiring length and adding an appropriate matching termination network not only improve the signal transmission characteristics, but also reduce the electromagnetic radiation intensity and improve the signal quality.
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