Eye diagrams don’t lie: ReDrivers, before and after

USB, HDMI, or analog signals degrade to mush over long signal traces or cables; check out these “before and after” videos for proof.

Note: this particular blog posting is sponsored by Pericom Semiconductor.

I’ve been writing about Pericom Semiconductor’s signal integrity products for the last few months, and it’s been shocking to see digital signals acting like analog audio waveforms and bouncing around like techno music on a DJ’s VU meter.

Noise, attenuation, cross-talk and other high frequency nasties take clean rise/fall edges at the signal source and turn them all sloppy and jittery after only a few interconnects…or long cable runs.

Here’s what a good high frequency SERDES signal should look like (right).

Before and after

What better way to showcase the benefits of signal integrity ReDrivers and adaptive equalization (AEQ) than with some scope pix?

Seeing is believing: check out the following eye diagrams.

#1 USB 3.0 – 5 Gbps never looked so good

Using a standard PC motherboard and a 36-inch trace test board, the degraded (closed “eye”) USB 3.0 signal is shown below. Note that a USB 3.0 receiver at the destination end would not recover data and the system just wouldn’t work.  The cause: PCB vias, traces, connectors, and cabling ruin the 5 Gbps digital signal.

Figure 1 USB closed eye

This USB 3.0 system wouldn’t work due to noise, jitter and out-of-spec signals.

By adding a tiny (2mm x 2mm) USB 3.0 ReDriver amplifier, the figure below shows a restored signal at the receiver and 5 Gbps data flow.  Note the open “eye”, the margin above/below the signal continuum, and the tight timing on the overlapped traces. This is a very clean signal and the system works within specs. (The complete PC motherboard set-up is in this USB 3.0 video. )

Figure 2 USB after ReDriver#2 HDMI seeing is believing

Our “always on” society bombards us with LCD screens everywhere we go. HDMI is the preferred video source in home theatre, conference center, digital signage, and other remote LCD setups. Second and third screens connected to modern laptops also rely on HDMI. But at 2.5 Gbps, HDMI is a serial standard that’s highly susceptible to signal integrity problems.

Using a PC’s display port as the video source, an extreme case was rigged in a lab using a long 30 m HDMI cable. The SI results are predictably bad, as shown below.

Figure 3 HDMI closed eyeAdding a signal recovery and conditioning HDMI ReDriver, the unrecognizable signals from above were astoundingly improved to an open “eye” with ample margin, as shown below. HDMI ReDrivers not only return the signal to acceptable levels, they come in flavors that add desirable video features such as equalization, splitters, level shifts, color correction, and more. (Short video with demo set-up is here.)

Figure 4 HDMI open eye#3 CCTV analog video

While not a digital signal, I threw this one in because it’s a real world example of poor analog signal integrity that you can see on a screen.

Scenario: Most closed-circuit TV surveillance systems are still analog because they’re cheap, and systems are characterized by long analog cable runs from source to viewing destination. PCB connectors, traces and video CODEC ICs further degrade signals.

The lab set-up: 500m COAX cable; signal generator; AEQ “on” and “off”. In the “Before” picture lousy, lossy signals and attenuation create a distorted and “wavey” test pattern. Unlike degraded digital (serial) USB 3.0 and HDMI signals that won’t recover at the receiver, SI-affected analog signals still carry information but the output is unacceptable in surveillance applications.

Figure 5 distorted analogAfter a video decoder is added at the receiving end and adaptive equalization algorithms are enabled (AEQ = “on”), the result is shown below. The image is recovered and enhanced with no loss of detail. This is after a whopping 500 m (~ 1/3 mile) of cable. (Video is here.)

Figure 6 AEQ afterConclusions

Signals degrade, and the faster things clock or the longer are the traces/cables, the worse things get. In gigahertz signals, ReDrivers from companies like Pericom Semiconductor clean up signals and make usable PCIe, USB 3.0, HDMI and other standards. And even in pure analog signals, video decoders with DSP-based AEQ can dramatically clean up signals.