Vaisala RVP900 - Figure 47 Model Intensity Curve - Power at Antenna Feed (2 Db Per Major Division)

RVP8 Measured Power (2dB per Major Div)
Power at Antenna Feed (2dB per Major Division)
1.2dB Detection
Threshold (LOG)
10Log(I
o
)
–5dB SNR
Min Detectable
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