Specifications

The key idea here is that such intermittent or failed conditions are caused by a
CCTV camera with a zero output impedance, not a failure in the Pan and Tilt or
other equipment.
THE SOLUTION.
Buy only CCTV cameras that exhibit 75 Ohm output impedance! But how can
you be sure a particular camera really has 75 Ohm output impedance when the
spec sheets for that camera proclaim 75 Ohms? The answer of course is to test the
output impedance before installing them in the field. Unfortunately this measurement
cannot be made directly with your trusty Volt-Ohmmeter. The output impedance of a
CCTV camera is not a static Resistance reading, but a dynamic AC Reactance
measurement.
CORRECT MEASURING OF CAMERA OUTPUT IMPEDANCE.
Connect the camera to be tested through a short 75 Ohm coaxial cable to an
Oscilloscope, Waveform Monitor or CM-1 Camera Master. Arrange to be able to
place a precision Termination (75 Ohms +/- 1%) at the Oscilloscope or meter.
Measure the amplitude of the sync pulse with the termination in place. The sync
pulse should read about 40 I.R.E (0.286 Volts Peak-to-Peak). Now remove only the
75 Ohm Termination. The sync level will Double (80 I.R.E 0.571 Volts Peak-to-Peak)
if the camera has the correct source impedance. A typical defective camera well
read almost the same amplitude with or without the termination on it. An actual
output impedance measurement may be made by using the formula in the following
TABLE 1.
SYNC PULSE PASS / FAIL TEST TABLE 1.
UNTERMINATED TERMINATED SOURCE IMPEDANCE
Vp-p IRE Vp-p IRE
0.571 80 0.286 40
PASS 75 Ohms (=/- 5%)
0.286 40 0.286 40
FAIL NEAR ZERO Ohms
Even though exact readings will vary between various CCTV cameras, this
test will suffice as a PASS/FAIL test. If a more exact measurement is desired, the
following formula may be used.
CALCULATION OF PRECISE SOURCE IMPEDANCE.
U - T U = Un-terminated Reading of Sync IRE or Voltage.
S= ----- X 75 T = Terminated Reading of Sync IRE or Voltage.
T S = Internal Source Termination in Ohms.
CCTV TRAINING MANUAL PAGE 11 OF 28