Tektronix 2715, Руководство пользователя

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CATV Measurements
5–18
2715 User Manual
In-service measurement capability includes measurement of C/N, CSO, and ICR
(on negative modulation polarity signals only). Gating hardware in the 2715
Spectrum Analyzer allows these measurements to be made on normal modulated
TV signals without disruption of programming. This is done by interrupting the
signal processing path of the spectrum analyzer during times when active video
and/or undesired test signals would interfere with the measurement.
The advantage of using gating techniques is that you can perform more
measurements without interrupting service to your subscribers. Also, carrier-to-
noise may be measured within the video band of the channel, which allows you
to make meaningful measurements through a set-top converter without using
extra equipment or complicated procedures.
NOTE . In-service measurements are not available when measuring digital
channels.
Note the following limitations that apply when using gated measurements:
H
Gated measurements are not generally available on scrambled channels,
since the gate timing hardware depends on normal horizontal and vertical
sync pulse structure for its operation.
H
Measurement speed is reduced in some cases, since it takes longer to obtain
trace filling and/or averaging when the gate is open so little of the time
(typically less than 1%).
H
Selection of beat frequencies is more difficult in interactive CSO mode
because the gating is not compatible with ordinary video filtering. Visually,
the dominant feature of the resulting screen display is the collection of noise
peaks (as opposed to the average noise level viewed with appropriate video
filtering). A technique for minimizing the impact of this effect is suggested
in the section on CSO measurement details; however, there will always be
some loss in the ability to see weak CSO components. This does NOT
impact the accuracy or dynamic range of the earnest CSO measurement
taken once you have defined the frequencies to be checked.
H
Since the channel is active during in-service measurements, it is possible for
the measurement accuracy to be affected adversely by residual or inaccurate
video content in the line(s) being measured. For example, if a noisy video
signal is applied to the modulator at the head end, the resulting noise
sideband power will be indistinguishable from noise introduced by the
distribution system. Noisy video can limit the sensitivity for both C/N and
CSO measurements taken in-service. When measuring ICR, noise is not a
concern, but the 2715 assumes that the test signal in the specified line is
spectrally flat at the head end. If this is not true, the 2715’s flatness will be
misstated. A technique for assessing flatness and compensating for a nonflat
video source is suggested in the section on ICR measurement details.
In-Service Measurement
Capability