Vaisala RVP900, Руководство пользователя

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R
V
P
8
Me
as
ur
ed P
owe
r (2dB
pe
r Maj
or
D
iv
)
Power at Antenna Feed (2dB per Major Division)
1.2dB Detection
Threshold (LOG)
10L
og(I
o
)
–
5
d
B
S
N
R
M
in
D
et
ec
ta
b
le
10Log(N)
Figure 47 Model Intensity Curve - Power at Antenna Feed (2dB per Major Division)
The procedure is to connect a calibrated signal generator to the radar receiver and inject
known power levels to generate a calibration plot of measured power versus the inserted
power at the antenna feed, similar to that in shown in the figure.
The calibration reflectivity
dBZ
o
is computed from the radar constant and the value of
I
o
,
which is the intercept of the straight line fit (green) with the noise level.
To understand why this geometric construction yields the value of
I
o
, Let G
dB
represent the
overall gain of the RF and IF components leading up to RVP900. The green line can be
interpreted as the response of an ideal noise-free amplifier having gain GdB , while the red
curve is the response of the real-world amplifier(s) whose equivalent front-end noise is
I
o
:
(Red) 10log
10
(P
OUT
) = GdB + 10log
10
( P
IN
+ I
0
)
(Green) 10log
10
(P
OUT
) = GdB + 10log
10
( P
IN
)
The measured receiver noise is the horizontal asymptote of the red curve, that is, the value
of the red curve when the input power
P
IN
is 0:
10log
10
(N) = GdB + 10log
10
(I
0
)
Intersecting this measured noise level with the green straight line gives:
Chapter 7 – Processing Algorithms
215