FM Systems GB-1 User manual

GB-1
GROUND-LOOP BLOCKER
INSTRUCTION BOOK
IB6329-01

TABLE OF CONTENTS
DESCRIPTION 2
MOUNTING INSTRUCTIONS 2
HOW TO CABLE THE GB-1 2
POWER SUPPLY INSTALLATION 4
OPERATION 4
CARE AND MAINTENANCE 4
APPLICATIONS (WHERE TO USE THE SYSTEM) 4
CCTV INSTALLATION TIPS 4
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DESCRIPTION
This product blocks and removes the most common problems in
CCTV video installations. The GB-1 also restores the picture
quality before it goes to the monitor or recorder. The unit is
placed at the monitor site or anywhere a video problem exists.
Ground-Loop Bars in CCTV pictures caused by AC power ground-loops
are cancelled completely.
The GB-1 will eliminate ground-loop problems induced by
lightning protection equipment. It also corrects for video level
loss and loss of picture definition caused by long cable runs and
twisted pair transmission systems up to 3000 feet. The unit
prevents Ghosting in the picture by absorbing and eliminating the
reflected video ghost signal. It will also restore the correct
impedance to down-stream video signals to prevent secondary
ghosting in the picture.
Longer cable runs can be accommodated by placing additional
GB-1 units at intermediate locations to be used as line extenders.
In this way, a mile or more of cable loss can be corrected and 60
cycle bars found particularly on long cable can be eliminated.
The GB-1 is a single channel unit with independent video
level and definition controls as well as ground-loop blocking. A
fully isolated secondary video output is supplied for systems that
require additional monitor locations, so the GB-1 will also
operate as a Distribution Amplifier.
MOUNTING INSTRUCTIONS
The rugged one piece mounting structure allows you to mount
the unit firmly in place with two screws. Select a place to mount
the unit away from harsh or wet environments, indoors is
recommended. The GB-60 should be located near the monitor
equipment or the place you wish to bring the video signal to.
Select a position that gives you the best access to cable the
system and reduces the labor in installation.
HOW TO CABLE THE GB-60
Attach the cable coming from the camera to the “Video Input”
BNC. Then attach a cable from the BNC connector marked “Video
Output 1” to the monitor equipment. BE SURE TO TERMINATE THE END
OF THE VIDEO CABLE WITH A 75 OHM TERMINATION OR PROPERLY TERMINATE
INTO OTHER EQUIPMENT.
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POWER SUPPLY INSTALLATION
The GB-1 is powered by a 24 VAC wall mount power transformer.
Connect the 24 VAC to the screw terminal connector marked “24 VAC.
The Green LED will flash continuously when the power is applied.
OPERATION
When video is applied to the input connectors of the GB-1 an
internal active video clamp blocks the 60 Hz ground-loop current
from interfering with the CCTV video picture. Next the video goes
into an adjustable gain stage that lets the installer adjust for
cable loss on long cable runs. A second control allows the
installer to adjust for high frequency losses that cause reduction
in picture quality. The down stream video impedance is then
matched to the 75 Ohm cable to prevent picture Ghosting
downstream.
CARE AND MAINTENANCE
There is no routine maintenance or calibration required with
this equipment. There are no field adjustable controls inside the
box. Opening the box will void your warranty. All adjustments are
made through the front panel.
APPLICATIONS (WHERE TO USE THE SYSTEM)
This system can be used anywhere that a video signal in coax
cable exists. It can be used to improve Twisted pair transmission
system response when used with a UTP balun. Video pictures on long
cable runs will benefit from the installation of the GB-1. In a
long cable run the GB-1 can be placed at the mid point between the
camera and the monitor to improve the transmission level of the
signal.
CCTV INSTALLATION TIPS
This section will discuss video ground loop problems in CCTV
installations. The cause and how it effects picture quality with
solutions to eliminate the problem.
When Video Ground Loop problems or 60 cycle Bars occur, they are
easy to see on a video monitor. They look like a horizontal band
or bar across the video monitor, that slowly moves up the video
screen. These bars can be barely noticeable, or can be so bad that
the video monitor loses lock and breaks up the picture. If the
video camera is Line-Locked to the 60 cycle main power, the bars
may stand still in the picture, but they still obscure picture
definition and create customer complaints.
The source of the 60 cycle bar originates from the power
industries use of local grounds to balance their power grid.
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Everywhere 60 cycle power is used, a local ground is attached to
the power grid to return all unbalanced current flow to ground. As
an example, you will notice that every main power breaker box will
have a ground wire or conduit going to a ground rod or similar
device connected to an earth ground. Every correctly installed
power outlet will have a connection to this ground.
Not all grounds are created equal. In fact the earth ground in one
building is most likely to have a different voltage potential
relative to any other building, even grounds inside the same
building will have different voltages between them, based on the
uneven current flow of the power load.
Here is how the 60 cycle bar gets into your video picture. If you
connect a coaxial cable to a monitor or other equipment that plugs
into the 60 cycle main power and the other end of the coaxial
cable becomes grounded locally for any reason a Ground Loop is
created. Any difference in the 60 cycle voltage between these two
ground points will create a current flow in the shield of the coax
that induces the 60 cycle AC voltage into your video signal. It is
easy to measure these differential voltages, simply disconnect the
video cables at the monitor point and using your voltmeter on AC
volts, measure between any two shields of the incoming video
cables, you will be amazed at the difference.
The solution is to never connect both ends of a video cable to
local grounds. Any cable can be grounded at one end without
inducing the ground loop current. When you run coax cable from one
building to another, it is acceptable to install through
connection points, but do not allow the shields to come into
contact with one another or the local ground.
A coaxial connector laying in a cable tray or conduit box can
accidentally contact ground, don't let this happen. Use tape on
the connector to prevent accidental grounding. Also try not to
attach the camera to any structure that is likely to be grounded.
Remember that the camera is already grounded at the opposite end
of the coaxial cable by the monitor equipment.
At the monitor station you may have many pieces of equipment
connected together, like a (Quad, Tape Recorder, Monitor) all of
which plug into the main 60 cycle power. This will not present a
problem if you plug all of the equipment into the same power line
at the monitor point. Making sure that all the equipment share the
same ground point at the monitor station. Also try to keep the
video cables between equipment, (the service loops) as short as
possible.
If you already have an installation that has 60 cycle bars, there
are some steps you can take to solve the problem. If coaxial cable
shields are connected together anywhere in the system, separate
them if possible.
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Similarly remove all but one ground connection on each coaxial
cable if possible, the ground is usually at the monitor end of the
coaxial cable because the monitor equipment plugs into the 60
cycle main power supply which is grounded.
Sometimes a ground loop problem can be reduced by reversing the AC
plug on the power transformer used to power the camera, or reverse
the 24 VAC power connection to the camera. This technique will not
work on DC powered cameras.
If the problem still persists, a GB-1 can be installed at the end
of the coaxial cable to eliminate the 60 cycle bars. This is much
easier and cost effective when you consider the labor cost to find
and solve the ground-loop problem.
With an understanding of Ground Loop problems and the use of the
GB-1, you should be able to keep the 60 cycle bars out of your
CCTV installations.
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