SDA-5500/SDA-5510 Sweep Configuration: Sweep History and Theory
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should be within N/10+X dB of flat at its output. Stealth sweep can
automatically calculate whether an amplifier passes this test. X is set in
the SWEEP configuration menu as the Sweep Limit Variable. Within the
sweep display, entering the Limit Check mode allows the amplifier
number (N) to be set. A limits check box appears with a check mark for a
good amp, and an X for a bad one.
Sweep References
The accuracy of a cable system sweep can be improved using references.
Since the goal of the cable network is usually to have unity gain throughout,
a sweep reference can be used to remove both measurement error and
response problems in parts of the network that are not currently being
adjusted.
To understand how this works, consider Figure 14-22. The goal of the
network is to provide the same signal as A at B, C, and D. However, during
adjustment of the system amplifiers (downstream from the optical node), the
frequency response of headend and optical node cannot be adjusted. Use of
references can eliminate their contribution to frequency response problems.
When you have no control over the parts of the network "upstream" from B,
think of B as having a perfect, or flat, response. The goal is to make C and D
match B as closely as possible.
Fig. 14-22 A Small Cable Cascade
To take a reference, perform a system sweep at point B and store this in the
sweep receiver’s memory. After sweeping at points C and D, compare their
response to the memorized trace from B (the reference sweep), rather than
using an unreferenced trace from the headend. Using an unreferenced
trace, you would be trying to cancel out problems in the headend, fiber, or
node with the downstream amplifiers, instead of just trying to minimize the
impairments from system amplifiers themselves.
When they're used correctly, references can also reduce measurement
error. Because the relative measurement accuracy of sweep equipment is
much better than its absolute accuracy, a more accurate sweep is possible
using references. Taking the reference trace with the same transmitter and
Headend
Optical Node
A
Fiber
B C D
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