Skewed Encoder Waveform

I received this e-mail from a potential customer who is trying to determine why his Encoder waveform doesn’t look right.  His name has been changed to protect his identity.

Hi Jim

I have just come across your Web page on RPM calculation using an optical encoder and oscilloscope. I was keen to test out this method of RPM calculation so rigged up my little encoder and oscilloscope without hesitation. I don’t seem to be getting a nice wave wave form across my display, its rather skewed. Could you just point out where I’m going wrong?

Really enjoyed reading your articles. Look forward to hearing back from you soon.


Eddie’s photos are below:

Hi Eddie,

I would love to say the problem is that you aren’t using a Quantum Encoder….

But instead it looks like you are just missing a ground reference for the scope.   There is usually a little black alligator clip hanging off the side of the scope probe. That clip needs to be attached to the signal common on the encoder (black or negative on the power supply)

The red arrow below indicates where the ground clip should connect to the scope probe.

The reason your waveform looks  skewed is because the absence of a ground reference causes the scope to pick up ambient 60 Hz noise (it is everywhere, outlets, lights etc.) and couple it with your encoder signal.

Connecting the scope ground to the incremental encoder signal common will clear that right up.

Below is a picture of a scope probe with the ground clip.

Take care,


Jim Miller is a Design/Application engineer working for Quantum Devices Inc.

He can be reached at (608) 924-3000, or via e-mail at


Air gap in high resolution optical encoders

As the resolution of optical encoders increases, the distance from sensor to disk decreases. In the incremental encoder industry, this distance is called the “Air gap”.  In the side view photo of an optical encoder above, the two red lines indicate the air gap between the sensor and disk  in a QD145 incremental rotary encoder.

In the photo below I have added a human hair to show perspective.

For more information on optical encoders, contact Quantum Devices  at (608) 924-3000.

Incremental Encoder Engineer interview

I was featured in an interview with EEweb.


Jim Miller – Application and Design Engineer at Quantum Devices

How did you get into electronics/ engineering and when did you start?

I started when I was pretty young, like nine or ten, taking apart radios and using an old wood-burning tool to desolder components from the circuit boards I scavanged. I would pick up anything that was broken or being thrown out and tear it apart. I had no idea what I was doing but eventually stumbled onto some of the “Engineers Notebooks” that Forrest M. Mims III wrote for Radio Shack, those gave me the knowledge I was missing. Before long I was able to blow fuses out in the house on a regular basis. I have come a long way since then – we now have circuit breakers.

Can you tell us about your work experience/ history before becoming an Applications Design Engineer at Quantum Devices?

I have mainly worked with Industrial controls for Food & Beverage, and Pharmaceutical companies, which always includes quite a bit of PLC Programming. That is why you will see a bit of Ladder Logic in with some of the Quantum Devices Blog posts that I do. While I predominantly work with discrete electronics today, the industrial controls experience dovetails nicely with our Optical Encoder lines. Encoders are used on the back of motor and often in industrial applications, so I am able to better understand the way an end user might be trying to implement a design, and to some extend the way they might think.

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