Why do my optical encoder signals look this way?

I received an email from a customer who was seeing “not symmetrical” data channels on some optical encoders we shipped to them

Hi Jim,

The data channels are not symmetrical and there appears to be something superimposed on the index. Attached are pictures. Can I return these for your evaluation?

Regards,

Al

Below are the screen shots that he sent:

 

My response was as follows:

Hi Al,

What you are seeing is normal and expected for an encoder with a ribbon cable with unlodaded channel termination. What you are seeing is crosstalk between the channels, and possibly some external signal noise else being picked up by the cable.

When the channels are loaded the irregularities in the signals go away (as shown in the second photo).

Jim

Channels with 220Ohm differential loading

Channels without loading

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Quantum Devices, Inc. (QDI) Barneveld, WI, was established as a Wisconsin corporation in October 1989, as an outgrowth of activities in the general area of optoelectronics. The main goal of QDI is to provide customers with a complete source for all stages of product design and development, starting from concept to a marketable commodity. These products include application specific Silicon Photodiodes, Optical Encoders and Light Emitting Diodes (LED) for commercial, industrial and medical applications. QDI's photodiodes and LED's are used in instrumentation for photosynthesis and photobiological research, biomedical and medical instrumentation for measuring blood sugar levels (glucometer), oxyhemoglobin and pulse rate (pulse oxymeter). One of the most dramatic products developed at QDI was the application of LED lighting systems for use as the light source for Photodynamic Therapy (PDT). Sales of QDI products include both domestic and international markets. Quality and reliability are very important concepts in maintaining our company wide commitment to overall product performance. Quality simply means continuous process improvement. We are committed to continually increasing our product excellence through increased quality and reliability. Quantum Devices, Inc. is ISO 9001 certified and the ISO Standard will provide the guidance for the vital function of maintaining our commitment to constantly improve our product quality. Quantum Devices is now proud to introduce it's new line of rotary encoders. Incorporating QDI's patented sensor technology, our QPhase™ Family of Encoders feature high resolution, increased frequency response and superior reliability over temperature (0 to 120 degrees C). QDI incremental and absolute encoders set a new standard and are backed by a 2 year factory warranty. We invite you to request an evaluation unit, additional information or price quotations. Mission Statement Quantum Devices is dedicated to the principle that light provides the power for all life on earth. We believe the quality, delivery and control of light is essential to the wellness of man and his advancement into the future.

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