LeCroy 9300C Series, Operator'S Manual

Page: 229 / 275

C –16
Appendix C
Where M
min
is the minimum magnitude, fixed at about 0.001 of
the full scale at any gain setting, below which the angle is not
well defined.
The dBm Power Spectrum:
dBm PS
M
M
M
M
n
ref
n
ref
= ×





 = ×






10 20
10
2
2
10
log log
where M
ref
= 0.316 V (that is, 0 dBm is defined as a sine wave of
0.316 V peak or 0.224 V RMS, giving 1.0 mW into 50
Ω
).
The dBm Power Spectrum is the same as dBm Magnitude, as
suggested in the above formula.
dBm Power Density:
( )
dBm PD dBm PS ENBW f
= − × ×
10
10
log
∆
where ENBW is the equivalent noise bandwidth of the filter
corresponding to the selected window, and
∆
f is the current
frequency resolution (bin width).
7. The FFT Power Average takes the complex frequency-
domain data R'
n
and I'
n
for each spectrum generated in Step
5, and computes the square of the magnitude:
M
n
2
= R'
n
2
+ I'
n
2
,
then sums M
n
2
and counts the accumulated spectra. The
total is normalized by the number of spectra and converted
to the selected result type using the same formulae as are
used for the Fourier Transform.