Tektronix 2715, User Manual

Page: 146 / 360

CATV Measurements
5–36
2715 User Manual
When the CARRIER-TO-NOISE measurement is initiated on an analog channel,
the 2715 performs the following:
NOTE . When you are measuring C/N on an in-service basis at a test frequency
within the video band, the useful sensitivity may be limited by noise on the video
signal being carried.
1. If in-service basis is selected, finds a group of quiet lines (if none are
specified in the channel table).
2. Tunes to the visual carrier frequency and sets the following: RBW to
300 kHz with no video filter, span to 200 kHz/div, sweep speed to 50 ms/div
(200 ms/div if the modulation polarity is positive as in the French L system).
The 2715 makes a single sweep, and then does a peak search with the marker
and records the marker value as the carrier level.
3. Performs one of the following (unless in auto in-service basis):
a. If normal basis (in-service not selected), returns to frequency and span
settings that display the whole channel across the screen. Selects 30 kHz
RBW and turns the video filter on. Initially places the marker on the
minimum point of the waveform, and if necessary, changes the reference
level setting so that the low point is at least 10 dB above the bottom of
the screen.
b. If interactive in-service basis is selected, returns to frequency and span
settings that display the whole channel across the screen. Selects
100 kHz RBW and turns the video filter off. Enables gating and peak
acquisition mode. Initially places the marker at 2 MHz above the visual
carrier, and if necessary, changes the reference level setting so that the
low point of the trace is at least 10 dB above the bottom of the screen.
4. If in interactive mode, prompts the user to adjust the marker to the desired
position for the noise measurement. If in auto mode with pause-for-carrier-
off, prompts the user to turn off the carrier.
5. Measures the noise at the location shown in Table 5–8 using the applicable
built-in noise measurement routine, which applies the noise measurement
corrections for filter BW, detector errors, and instrument noise floor
proximity. If measuring on the in-service basis, the routine includes
switching to zero span and averaging for a substantial number of TV vertical
sweep periods.
CARRIER-TO-NOISE