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Deerfield Laboratory, Inc.: Ground Loop Analysis and Solutions

Introduction
The Problem: Ground Loop Noise
The Problem: Symptoms
Solution 1: Video Isolation Choke
Solution 2: Video Isolation Transformer
Application Notes

Introduction
Video transmission systems can easily be protected against the intrusion of noise and hum. Simply connect a video isolation transformer or choke anywhere in the susceptible line. Both of these components yield high quality pictures. They are not filters, so do not distort the video signal. They merely prevent noise from entering the signal ports.

The Problem: Ground Loop Noise
Hum and crosstalk are likely to occur when the elements of an electronic system are connected by transmission lines. Ground return paths generally exist which are external to the ground conductor of the transmission line. (The braided shield is the ground conductor on a coax line). Noise energy is introduced into such a loop by way of an equivalent generator in the ground return path, or by magnetic induction into the ground loop. If ground potential differences in the AC power feeds are the cause, then hum results. If the noise source derives from another line, then crosstalk results. Long distances between ground points, poor grounding practices, and inherently compromised grounding environments increase the probability that ground loops will cause interference.

Symptoms:
Ground loop noise is most often in the form of 60 Hz hum. Horizontal lines slowly moving up the monitor are the characteristic interference pattern of hum. Ground loops also make a transmission line susceptible to crosstalk and stray pickup from high frequency sources. Once a video signal has been infected by hum or other noise, it is impossible to filter out because the noise frequency falls within the video band.

Solution 1: Video Isolation Choke
The video isolation choke presents a simple transmission line to the signal, so that its signal amplitude capability is essentially unlimited, and its bandwidth extends from DC to that frequency where line losses become excessive. Isolation increases with increasing noise frequency. The choke provides protection against moderate levels of 60 Hz hum, and protection against very high levels of RF noise.

Solution 2: Video Isolation Transformer
The video isolation transformer is an extremely broad bandpass 1:1 isolation transformer. Its hum isolation is greater than the choke. It can sustain very high noise voltages without degradation. Isolation decreases with increasing noise frequency. A choke can be added inline to increase protection from high frequency noise. Signal amplitude at low frequencies is limited by core saturation.

Application Notes
The choke is primarily used in Broadcast TV because it passes the DC component of the signal. It is used in studio, and in remote ENG. The transformer is primarily used in CCTV: security, manufacturing, avionics, display, etc. Both transformer and choke can pass camera control signals. Models are available for tight space requirements, and for many different special circumstances.


Deerfield Laboratory, Inc. (ph)650-632-4090

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