Specifications

The “square-wave” type of UPS will put out the same 60Hertz frequency but
its output waveform is that of a square-wave. A 60Hertz square-wave signal
contains the “fundamental” or lowest frequency of 60Hertz, however it also has
every “harmonic” or multiple of that frequency all the way up to and beyond the
video frequency range. Strong high level signals in the Kilo-Hertz region (1000Hz)
and even in the Mega-Hertz region (1000,000Hertz) will interfere directly with
video. These interfering signals are multiples of 60 Hertz and are locked to the line
rate which makes the interference stand still on the screen if your cameras are line
locked. If not line locked the interference will slowly move across the screen.
HOW IT GETS IN.
One way that the interfering signal can get into the CCTV system is from a
power supply. If a square-wave type UPS is used to power the camera, the high
frequency interfering signals can enter through the AC or DC power supply and
become part of the video by cross-talking directly to the video amplifiers in the
camera. The high frequency interference will get through the cameras power
regulators because the camera was designed to filter out 60Hz AC, not hundreds
of Kilohertz of AC. DC powered camera’s can have the same problem for the
same reason, however the interfering signal will always be moving on the screen
because DC cameras cannot be line locked.
GROUND LOOPS AND THE COMMON POWER SUPPLY.
Another way interfering signals can enter the video is through an induced
ground loop. The UPS is grounded and if the camera is also grounded some of
the UPS energy can flow down the coax cable to the cameras ground. This
sharing of grounds will induce the interfering signal onto the video. A ground at the
camera can be a tricky thing. If you are using a DC powered camera, the coax
cable shield and the negative power supply wire are the same at the camera. This
means that if the negative power supply is grounded anywhere, then your camera
coax shield is also grounded at that point and it can induce a ground loop.
If you are using a common power supply to power multiple cameras and
even one camera is grounded then the ground connection will back up to all of the
cameras. Common power supplies for DC cameras are especially susceptible to
this problem. The common power supply ground loop may not affect all the
cameras depending on the distance of coax and the grounding conditions at each
camera.
CCTV TRAINING MANUAL PAGE 20 OF 28