3. Effect of Bias-T circuit on SI
Necessary characteristics of Bias-T inductors – wide frequency range
In an ideal inductor the impedance rises proportionally to the frequency, but this is not the case in actual inductors. The impedance curve tends to have the shape of an arch.
To find out at which frequencies a Bias-T inductor for PoC applications needs to have high-impedance characteristics, we measured the SerDes signal components on a frequency axis.
The results show that the SerDes signal frequencies are distributed over a wide range, and that a Bias-T inductor therefore needs to have high impedance over a wide frequency range.
PoC system (SerDes) signal frequency component measurement method
Characteristics of Bias-T circuit used for verification
A single ordinary inductor cannot cover a wide frequency range, so it is necessary to use multiple inductors with different self-resonance frequencies in combination. In contrast, LQW32FT series inductors developed for Bias-T applications are able to cover a wide frequency range with a single device, thereby reducing the number of inductor components required.
We checked the difference in signal integrity (SI) of SerDes signals when multiple inductors are used in combination and when an LQW32FT series inductor developed for Bias-T applications is used.
When multiple inductors are used in combination, the impedance curve is unstable and the signal waveforms are distorted. In contrast, when using an LQW32FT series inductor, the signal waveforms are not distorted and transmission proceeds normally.
Signal line transmission characteristics (S21) and reflection characteristics (S11)
Reduction of waveform quality due to degradation of transmission line transmission characteristics.
The S21 transmission characteristics of the Bias-T signal transmission reveal that better characteristics are achieved by using the LQW32F series.
Better S11 reflection characteristics can also be achieved by using the LQW32FT series.
4. Effect of cables on SI
To check the effects automotive coaxial cable characteristics have on waveforms, we passed signals from a signal generator through automotive coaxial cable and monitored the waveforms with an oscilloscope. We also evaluated cable transmission loss characteristics using S21.
Cable transmission loss characteristics (S21)
When we looked at the effects of cable length, we found that making the cable longer resulted in a noticeable decrease in the quality of high-frequency waveforms, demonstrating that the effects of cables on SI cannot be ignored. When evaluating Bias-T inductors, it is necessary to check the S parameters using an evaluation system that includes the cables.
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