We take a look at a recurring issue within optical fiber installations that’s been causing many units to appear dead-on-arrival.

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In a previous column, we noted that as newer optical fiber products are released, manufacturers are gradually understanding that emulating short copper cable with long-distance active optical fiber is a bit more complex than it appears. Our latest tests on these new designs have confirmed this. While they are not perfect, these products represent a step toward creating a copper-like expe

rience with optical fiber. But we continue to receive complaints about dead-on-arrival (DOA) units, intermittent performance, and long-term failures with optical fiber installations. One recurring issue has been input dynamic range impacting performance.

First, let’s clarify that the issue is not exclusive to optical fiber. It can arise with any active coupling device that communicates with any interface, in this case, HDMI. Let’s try to clear this up by gaining a better understanding of how input dynamic range can affect system designs and product selection for active optical devices.

How Input Dynamic Range Can Affect System Designs

An input stage is generally similar in structure across devices but can have different limits. Audio has certain limits, as do analog television and digital video. A ‘safe zone’ represents the input limits an electronic device can manage. If the input signal is too high, it will saturate; if it is too low, the signal can get buried in the noise floor. Ideally, the signal should be in the middle.

Now take that same device, but with defined voltage levels for which it is designed. Levels from 0.25 to 0.75 volts can easily operate within this device’s safe zone. However, once the levels move outside of this window, problems can occur.

By examining each source, DPL Labs discovered that not all are the same. Some have slightly low output, while others have slightly high output. In many cases, using Active Optical Cables (AOCs) some sources will function while others may not. Using these same sources on different input stages can yield different results.

It is very rare for a source to output too much unless it is being amplified, pushing its output beyond the Vmax safe zone limits. This sometimes happens when AOCs are introduced into a system and have a strong output, causing the signal destination’s input stage to saturate into its Vmax limit.

If the source device has too low of an output, the opposite can happen, where the input device cannot even detect the incoming signal. In this case, some form of amplification is required. Remember, these values can change from one source to another.

We found the same issues with input stages to AOCs. The transmission signal gets converted to light energy, but the same rules apply, with some sources working and others not. It does not mean the cable is defective, because the cable device will function once the signal enters the AOC’s safe zone.

Fixing Issues with Input Dynamic Range in AOCs

The key is to determine each AOC’s input dynamic range. Once this is known, the integrator only has to look at the dynamic range numbers and match the source device to the cable. Easy, right? Unfortunately, not. Life would be much easier if these numbers were available, but we shouldn’t hold our collective breath.

For the past year, DPL has added yet another series of tests and procedures to all active cable devices. Instead of testing each device by the input standard only, DPL has 12 additional tests for each device at every data rate and at both standard input levels and low input levels close to the standard’s lower limit. This way, we can see each device’s input dynamic range. If it fails to achieve a range for safe operation, it goes back to the member’s engineering team until they get it right. All AOCs listed on the DPL website have passed this crucial dynamic range test.

ABOUT THE AUTHOR

Jeff Boccaccio is president of DPL Labs. Jeff can be reached at info@dpllabs.com