An RF-Quiet Treadmill

Introduction

In 2007 I purchased a Horizon Fitness CT61 treadmill from Amazon. Johnson Fitness manufactures and sells treadmills under the Horizon Fitness, Vision Fitness, and Matrix Fitness brands.

My spouse regularly uses the treadmill in a second floor bedroom. My 6-band coaxial-cable trap dipole HF antenna as well as a 5/8 wavelength 2 meter vertical antenna are located in our attic approximately 5 meters above the treadmill. I often operate on the 80-10 meter HF and 2 meter VHF bands at the same time the treadmill is in use. My favorite mode is JT65-HF which features a waterfall spectrum display. I have never experienced or observed on the waterfall any radio frequency interference (RFI) from this treadmill.

Discussion of radio frequency interference from treadmills is a recurring topic on the RFI mailing list. Because I have never experienced any RFI from our treadmill, I decided to create this web page to show the design considerations that I believe are responsible for the lack of radio frequency interference from this fitness device.

Treadmill Tear-Down

motor Disclaimer: I have no knowledge if other Johnson Fitness treadmills share a similar design and RFI performance to this model.

The treadmill belt is powered by a large pulse-width modulated (PWM) DC motor rated 15 Amps at 90 Volts DC.

RFI Suppression

toroids The IEC power receptacle and power switch do not appear to contain a line filter.

The manufacturer placed separate toroids on the treadmill belt DC motor power and ground conductors. I measured the size of each toroid as 31 mm OD, 18.3 mm ID, and 15.5 mm H (similar to the dimensions of an FT-114-A toroid.)

The two DC motor power conductors (red and black) each make 4 turns through one of the toroids.

The single DC motor ground conductor (green/yellow) makes 4 turns through a separate toroid.

I do not know what toroid material was used, and I did not measure its permeability. I also did not measure the drive waveform with an oscilloscope.

Grounding

grounding The treadmill belt DC motor ground conductor (green/yellow), the AC line ground conductor (green/yellow), and the electronics ground conductor (green/yellow) all terminate at a single point to a stud bonded to the treadmill chassis.

Shielding

The treadmill belt DC motor and drive electronics are mounted above a steel chassis but are otherwise unshielded. They are covered only by a non-conductive plastic shroud, so there appears to have been no attempt or need to shield radiated emissions from within the treadmill.

Summary

I hope this brief glimpse inside a treadmill that has proved to be RF-quiet at my QTH might prove useful to others who need to remediate interference from a not-so-RF-quiet treadmill.

Author: John DeGood, nu3e AT arrl DOT net
Last updated: Sun 2 Oct 2016