Ctb / Cso - Deviser S7200 User Manual

Part II: Settings and Measurements
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CTB / CSO

Typically, CTB/CSO measurements require that system engineers first measure the video carrier level (where
the sync tip of the modulation is the absolute peak level); then remove the video modulation by shutting
down the modulator - or simply remove the modulation from the carrier – to allow accurate noise
measurement. Unfortunately, removing the modulation interrupts service.
A better way to measure CTB/CSO accurately is to perform an in-service "gated" measurement, with the
help of gating technology and quiet line deletion/insertion in the vertical blanking interval of video signals
that are off the viewable screen (usually below line 12). If you choose a higher VITS line, you may encounter
video modulation, and thus visible interference on the television picture.
Occasionally, VITS lines are already quiet enough when video signals are delivered by the Broadcaster. It is
highly recommended to use your own VITS inserter to remove any measurement uncertainty. CTB distortion
falls exactly at the video carrier frequency. But CSO distortion typically falls ±0.75 MHz and ±1.25 MHz from
the video carrier. In a PAL-D system, CSO production falls ±0.25MHz and ±1.25MHz from the carrier.
When we measure CSO and CTB, the test results are in terms of C/CSO and C/CTB. In the gated mode, the
S7200 will first measure absolute carrier level, then trigger the appropriate VITS line # (preset for each
channel – can be set in the channel plan editor) to measure noise.
The CTB interference signal, however, falls exactly under the video carrier frequency; and since the video
carrier amplitude is likely to be higher than the interference, a gated CTB measurement must use a line-
removing/inserter equipment to remove one line, or at least one field of one line, of the video carrier signal.
At the screen above, press Continue (F3) to proceed to testing. "Please turn off the carrier!" will appear.
NOTE: Using such line-removing/inserter equipment for CTB measurements will cause an audible "ticking"
noise in your television speaker output. This test requires that the test signal be inserted only for the duration
of the actual test, and not left "on" all the time – so as to avoid customer concerns about audio problems.
Gated CTB/CSO Measurement – Frequency‐Domain Waveform (left) and Time‐Domain Waveform (right)
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Many other types of test signals may be inserted for gated mode
testing, including in-channel frequency response, differential gain,
and differential phase. A quiet line is absolutely required for
measuring CCN, CSO, CTB, or cross-modulation. The S7200 is
equipped with triggering technology that accurately locks onto a
specific VITS line and provides precise measurement results.
Use the arrow keys to select and modify the fields on the left. First
choose a channel to measure.