Tektronix 2715, Руководство пользователя

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CATV Measurements
2715 User Manual
5–43
FREQUENCY RESPONSE
The FREQUENCY RESPONSE test displays the difference between spectra
taken at different times or places in the system (mode 1), or a raw spectrum of
the entire channel table (mode 2). The modes are selectable from the FREQ
RESP SETUP menu.
A mode 1 frequency response test has two parts. The first part is acquisition of
the reference and the second is comparing the current spectrum with the stored
reference spectrum. The reference may be acquired either by measurement or by
recalling a previously stored reference from NVRAM.
A mode 2 frequency response test sets the instrument up into a mode that sweeps
between specified frequency limits in a 5 MHz resolution bandwidth.
When the FREQUENCY RESPONSE test is initiated, the start and stop
frequency limits default to the minimum and maximum frequencies in the
selected channel table.
If a mode 1 test has been selected, a reference must be acquired. A reference is
acquired by using the ACQUIRE A REFERENCE selection in the FREQ RESP
SETUP menu. The default start and stop frequency limits may be changed prior
to acquiring the reference, if desired. (Whenever the start or stop frequencies are
changed, the current reference is marked as invalid.) The instrument will sweep
between the start and stop limits specified in this menu, apply a digital filter to
the resultant sweep, and retain the resultant waveform and instrument setup. The
digital filter is used to smooth the reference to reduce sensitivity to slight
frequency errors and differences in program material between the reference
sweep and the subsequent frequency response peaks.
During reference acquisition, the reference name is assigned by linking the site
name, current date, the start frequency, and the stop frequency. The name may be
changed with the EDIT CURRENT REFERENCE NAME selection. This
selection can only be used if there is a currently valid reference.
The reference is acquired with the 5 MHz RBW filter. The goal is to capture the
video carrier peaks throughout the frequency range.
If a valid reference exists, the mode 1 test (WITH REF) may be run. The first
step is to recall the reference settings into the 2715 hardware. Then after each
sweep, the raw sweep data is filtered using the identical digital filter used to filter
the reference data. The reference waveform is then subtracted from the current
waveform and the difference written into the C register for display on the screen.
This process continues until the user terminates the test by pressing [W] or [Z].
At termination, the last difference waveform is left saved in the C display.
Running The Mode 1 Test