Tektronix 2715, Руководство пользователя

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CATV Measurements
2715 User Manual
5–37
Table 5–8: Locations of Noise Measurement
Mode Measurement Location
In-service interactive At marker
In-service auto At +2.7 MHz above carrier
Interactive At marker
Auto At lowest channel edge
Pause At +2.7 MHz above carrier
Digital Average whole channel
6. Subtracts the noise level from the previously measured carrier level (both
expressed in log (dB) units) and displays the result as C/N in dB.
7. If in-service basis is selected, verifies that the quiet lines measured are still
quiet. Turns preamp on or off as needed. Stores measured waveform in
Display Register A.
NOTE . The Carrier-to-Noise test result may be marked with an asterisk (
*
). This
indicates that the measured noise level is within 2 dB of the 2715’s calculated
noise level. Under these conditions, the inaccuracy is always on the conservative
side; the actual Carrier-To-Noise ratio will be better than the measured value.
The gated Carrier-to-Noise test result may also have 1000, 2000, or 3000 added
to it. This indicates that events occurred during the measurement which may
cause an inaccurate result. If so, the inaccuracy is always on the conservative
side; the actual Carrier-To-Noise ratio will be better than the measured value.
If 1000 is added to the result, the instrument detected that one or more of the
quiet lines found at the start of the test were not quiet at the end of the test.
If 2000 is added to the result, the instrument increased the signal level at the 1st
mixer input above optimum to ensure adequate signal level for accurately-timed
gating control. For Carrier-to-Noise measurements, this should occur only on
channels that are substantially weaker than the general system level.
If 3000 is added to the result, both of the above events occurred.
When performing in-service tests, the instrument checks for quiet lines at lines
10 through 20 and 272 through 282 on NTSC systems or at lines 6 through 20
and 318 through 332 on PAL systems. If the only quiet lines available are beyond
this range, then you must specify the quiet line in the channel table, using the
Cable TV RF Measurements Software for Windows. In this case, the in-service
test is run on the specified line, even though it is outside the normal range, but
there is no verification that the line is still quiet after the test.