TK Chin talks about the requirements for differential pairs in his blog post "Differential Pairs: What You Really Need to Know." In real-world applications, we use a copper trace in a printed circuit board (PCB) or a copper wire in a cable assembly to implement a differential pair. Longer PCB traces or cables exhibit higher transmission losses, which can degrade the signal quality. In this article, I will explain how insertion loss can affect the signal quality of a differential pair and explain how the equalizer can eliminate this effect.
What is insertion loss?
The transmission loss consists of two parts: skin loss at low frequencies and dielectric loss at high frequencies. The skin effect loss depends on the cross-sectional area of ​​the interconnect; for example, the width of the PCB trace and the metal thickness, or the wire diameter of the cable. When the frequency is below several hundred megahertz, the skin effect loss is the main transmission loss and is proportional to the square root of the frequency. When the frequency is high, the dielectric loss becomes the main transmission loss. The amount of dielectric loss depends on the material properties of the dielectric and is proportional to the frequency.
Insertion loss is a common term used to describe the transmission loss of an interconnect. It is the ratio of the voltage at the load in both cases with and without the interconnect. The network analyzer can measure insertion loss by amplitude and phase. Figure 1 shows the typical insertion loss of two PCB traces on an FR4 board: one trace is 5 inches (blue) and the other trace is 10 inches (red), but both have equal trace widths (5 Mil). As you can see from Figure 1, the insertion loss characteristics are the same as those exhibited by the low-pass filter, and the amount of signal attenuation increases as the frequency increases. The loss increases linearly with the length of the PCB trace.
Figure 1: Insertion loss of FR4 PCB traces
Why insertion loss degrades the signal
The data transfer serial bit stream contains logic 1 and 0 of different durations. In Figure 2, you can see that the transmitter waveform consists of data bits of longer duration (lower frequency pulses) and shorter duration (higher frequency pulses). Their amplitudes are approximately equal and the flip paths are almost the same, resulting in a clean, fully open data eye.
When the signal is transmitted through the PCB traces, the low-pass filter effect slows down the pulse turn-over time, and short-duration pulses do not have enough time to reach its full amplitude. In addition, the attenuation of high-frequency pulses is also greater than the attenuation of low-frequency pulses: when reaching the destination, their amplitudes are very different. Because longer duration pulses and shorter duration pulses have different amplitudes, the flip path changes and produces time domain jitter. This type of jitter depends on the data pattern and is often referred to as intersymbol interference (ISI). Figure 2 shows the receiver waveform and the corresponding eye diagram. The jitter caused by the insertion loss of the differential pair is very significant.
Figure 2: Signal degradation due to insertion loss
How TI Equalizers Can Solve This Signal Deterioration Problem
The fundamental problem of signal degradation described above is caused by unequal amplitude pulses, which are generated by low-pass filters. The solution to this problem is to cancel the signal attenuation with the goal of achieving equal pulse amplitudes. The equalizer is a specially designed high-pass filter whose transfer function is equal to the reciprocal of the interconnect's low-pass filter transfer function. There are many common equalizer implementations. You can use a high-gain continuous-time linear equalizer (CTLE) that offers more gain at high frequencies and less gain at low frequencies. Alternatively, you can use a high-pass filter that produces attenuation at low frequencies. This filter is commonly used as a transmitter equalizer in many de-emphasis driver designs. In addition, there are many digital implementations, such as finite impulse response filters (FIRs) or decision feedback equalizers (DFEs) used in retimers.
Figure 3 shows the TI DS125BR800A with CTLE that eliminates the ISI jitter caused by the interconnect. By selecting an appropriate amount of equalization that matches the insertion loss characteristics of the interconnect, the Repeater clears the ISI jitter and provides a clean data eye diagram at the destination of the received message.
Figure 3: CTLE Repeater repeater eliminates ISI
Texas Instruments' portfolio of signal conditioning devices is rich and varied, allowing you to compensate for the effects of insertion loss on differential signals and meet the needs of many common communication protocols.
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