Talk About the Returning and Overshooting of the Signal

[fangshou]

Take TF card CLK as an example.

The waveform detected by the TF card CLK receiver (shrapnel) is as follow.

FIG.1 TF card CLK receiver waveform (shrapnel)

TF card CLK path string is 0Ω and 50Ω, the magnitude of measure at shrapnel keeps constant, and the TF card CLK input impedance is high resistance. Between the test point and the TF card internal DIE, there is a piece of shrapnel and TF card gold finger, which is more perceptual. The test waveform has a back collapse, with overshooting, which can be concluded as high load + inductance + capacitance. Use ADS for simulation, simulation and results are shown below.

 
FIG.2 TF card CLK simulation diagram (0Ω)

 
FIG.3 TF card CLK simulation results (0Ω)

Through the figure above you can see that the source end of the string was 0Ω, internal DIE had overshooting but no returning, while the shrapnel had returning and overshooting, the source end had steps, returning and overshooting. The step on the source end was caused by the mismatch between high impedance and load. And the waveform of the source end is shown below.

 
FIG.4 TF card CLK source waveform

The waveform detected by the source also has a similar step, which is basically consistent with the simulation results.

Within the TF card, the DIE has overshoot only. It can get a string of a small resistance at the source end, increase the source impedance, and reduce the voltage current injected to the microstrip line, which is to reduce the driving force of the source. In this way, the charging speed of the capacitance can be slowed down, to avoid the capacitance getting overshooting.

After debugging, the source end turns 22Ω series resistance. The effect is good, you can see at the following figure.

FIG.5 TF card CLK receiver waveform (shrapnel)

FIG.6 TF card CLK simulation results (22Ω)

From the measured waveform at the above shrapnel and the emulation waveform of the red (TF card internal DIE), we can know that the overshooting had already disappeared mostly.