HP 8591C, User Manual, Page: 298 / 373

This test is used to compare the frequency response at a reference point in
the system to any other points in the system. It can also be used to see how
the frequency response changes with time or temperature.
Start frequency, stop frequency, resolution bandwidth, video bandwidth,
and reference level should be the same for the reference trace as the
measurement trace. If two points are compared that have greater than 8
gain difference, the reference level can be adjusted to position the trace on
the display.
The system frequency response test measures cable TV distribution system
flatness without the need for a sweep generator or other signal source. This
measurement technique does not require shutting down the system, does
not interfere with any signals on the system, and works on systems with or
without horizontal-sync suppression scrambling. If channels are scrambled,
any time-varying scrambling should be disabled.
Note that this test is only available in Channel mode.
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