Part II: Settings and Measurements
6-2 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, the 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
DS2831 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.
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 DS2831 is equipped with triggering technology that accurately locks
onto a specific VITS line and provides precise measurement results.
CSO/CTB Measurement Gated CSO/CTB Measurement – Frequency Domain Waveform
NOTE
: Using such line-removing/inserter equipment for CTB
measurements will cause a slightly audible "ticking" noise
in your television set 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 CSO/CTB Measurement – Time Domain Waveform
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