Specifications

HOW TO MEASURE CCTV CAMERA OUTPUT IMPEDANCE.
The vast majority of coaxial cables are either 50 Ohm or 75 Ohm
Characteristic Impedance. This Characteristic Impedance is determined by the ratio
of the diameters of the shield and core wire and the Dielectric Constant of the
insulating material between the core wire and the shield. The CCTV Industry chose
to use the 75 Ohm standard for coaxial cable.
Since a coaxial cable must be terminated at both ends of the cable with
resistances equal to the cable Characteristic Impedance to prevent reflections from
impairing the picture,
cameras must have an internal source impedance of 75
Ohms, and Monitors must be provided with a 75 Ohm termination.
THE PROBLEM.
In the past, CCTV cameras could be counted on to have proper 75 Ohm
source impedances, but recently there have been a rash of cameras showing up on
the market that do not have a 75 Ohm source impedance. In fact they exhibit almost
zero output impedance even though the specification sheets that come with them
specify 75 ohms! Clearly the cameras do not meet their own specifications and thus
could be returned for not meeting their own published specifications.
PROBLEMS CAUSED BY INCORRECT TERMINATIONS.
The worst of termination problems occur when the camera has a zero source
impedance, or the 75 Ohm impedance is left off at the Monitor location. By far the
worst effects occur when both zero Ohm source impedance exists
and the 75 Ohm
termination is removed at the Monitor.
Ghosts and Ringing become more evident as the length of the coaxial cable
becomes greater. The ringing effect (where a white-to-black or black-to-white
transition in the picture is repeated many times in close succession) occurs with
short cable runs of 50 to 200 feet. Obvious Ghosts appear when the cable is 500-
1000 feet long. The first of many successive Ghosts will be about 1/2 inch to the
right of the original object on a TV monitor screen with about 1000 feet of cable.
Another problem caused by zero source impedance cameras is that signals
placed on the coaxial cable to control Pan and Tilt of the camera will be shorted out
by the zero Ohm output of the camera and may cause remote Pan and Tilt systems
to be intermittent or fail.
The same intermittent or failed condition may be induced into other equipment
that is using the same coaxial cable to transmit special signals.
CCTV TRAINING MANUAL PAGE 10 OF 28