LeCroy WavePro 7 Zi series Operator's Manual page 484

WavePro 7Zi
A BlackmanHarris window is applied to the input data to minimize leakage effects. The net
result is that nbpw will provide excellent results even if frequency changes occur due to spindle
speed variations. If the actual frequency differs from the specified frequency, and the bin width
is 0.105% (minimum resolution bandwidth), the resulting power will be reduced from the actual
as in this table:
Frequency Difference dB Reduction
0.03% 0.3 dB
0.06% 1.1 dB
0.1% 3 dB
If the bin width is greater than 0.105%, the frequency difference for which a specified dB
reduction will occur will scale proportionally to the bin width/0.105.
nbpw results are presented in dBm.
Parameter
Selecting this parameter accesses a frequency setting dialog.
Settings
owrt Overwrite
The ratio of residual to original power of a low-frequency disk waveform overwritten by a higher
Definition
frequency waveform.
owrt measures the residual power of a low-frequency LF waveform after it has been overwritten
by a high-frequency HF waveform. The LF waveform should be stored to memory (M1-M4). The
HF waveform can then be input to the DDA, and overwrite calculated where:
where V
frequency after the HF waveform write, and V
waveform at the LF base frequency. The calculation is performed by the DDA making a narrow-
band power measurement (see nbpw parameter description) at LF, for both the HF and LF
waveforms, and subtracting the second result from the first. A menu enables the choice of
which waveform, HF or LF, is assigned to which DDA channel or trace (1, 2, 3, 4, M1 to M4, or
F1 to F8). The menu button is used to set the input for HF or LF, while the input for the selected
Description
waveform is set with the associated knob. The owrt results are presented in dB. All averaging,
including statistics and trend average, is performed on linear units. Average results are
converted to dB.
Note: In typical use, it is preferable to use nbpw to measure the LF waveform, and then the
residual LF in the HF separately, instead of the owrt parameter. Overwrite is the difference
between the nbpw readings in dB. There are two reasons why this is preferable: 1) nbpw, with
statistics on, provides average power readings. With owrt the low frequency signal is typically
a stored single-shot acquisition due to the difficulty finding a suitable trigger for time domain
averaging of a head signal. 2) owrt computes both nbpw results each time. If the LF is stored,
this is not necessary. So nbpw will take twice as many acquisitions as owrt and achieve a
more stable average result in the same amount of time.
Selecting this parameter accesses a frequency setting dialog. This frequency is used to
calculate nbpw for both the HF and LF waveforms. If a large number of digits is used, selection
Parameter
of the exact frequency may be difficult. In this case, a number with fewer digits and less
Settings
precision should be chosen for the approximate frequency, then the precision should be
increased as desired and the exact value chosen.
pw50 Pulse Width 50
483
owrt = 20 log (V /V ),
r o
is the residual V of the sine wave component of the HF waveform at the LF base
r rms
is the V of the sine wave component of the LF
o rms
WP700Zi-OM-E-RevA