Learning Event 5:
IDENTIFY VIDEO TESTING EQUIPMENT AND ITS OPERATION
This section will discuss the grating generator, dot bar generator and video
sweep marker generator. We will go through their purpose and operation, as well as
interpret their output patterns.
The grating generator provides a convenient means of checking and adjusting
the linearity of television deflection circuits.
It generates a timing signal
synchronized by standard synchronizing generator or the deflection circuits of the
receiver under test, and injects this signal into the video circuit being tested.
The pattern produced has the appearance of a grating as shown in Figure 3-21.
Grating test pattern
a. The block diagram, Figure 3-22, shows the typical
circuitry necessary to produce a satisfactory grating pattern.
(1) The desired pattern is produced by inserting the horizontal and
vertical pulses from either a standard television synchronizing generator or the
deflection circuits of a television receiver, as previously stated.
(2) The vertical pulses are then multiplied 15 times, while the horizontal
pulses are multiplied 20 times.
They are vectorially added in the adder circuit
and the output is applied to a clipper.
(3) The output pattern of the grating generator is determined by the bias
point of the clipper circuit.
When the bias is adjusted so that either the
horizontal or vertical signal extends above the clipping level, the resulting
output is a grating pattern.