Chapter 3 _______________________________________________________ Hardware Installation
VAISALA_______________________________________________________________________ 77
Both of these quantities depend only on the ratio of the two powers;
therefore, the two equations define a parametric relationship in the
dimensionless variable R = ( N
LNA
/ N
IFDR
) . on page 76 was created by
sweeping the value of R from 1/9 to 9. The solid red curve shows the locus
of ( ΔDynamicRange, ΔSensitivity ) points, and the dashed green curve
shows R (expressed in dB) as a function of ΔDynamicRange. For example,
when the LNA noise power is equal to the IFD noise power, R is 1.0 (0 dB)
and there is a 3 dB reduction in both sensitivity and dynamic range.
The recommended operating region is the portion of the curve that limits
the loss of sensitivity to between 1.4 dB and 0.65 dB. The attendant loss of
dynamic range falls between 5.5 dB and 8.5 dB, respectively. Each axis of
the plot has an important physical interpretation within the radar system:
- The horizontal axis is equivalent to the increase in the RVP900 report
of filtered power when the IF-Input coax cable is connected, versus
disconnected. This is an easy quantity to measure, and thus provides
a simple way to check the overall gain of the LNA/Mixer/IF
components.
- The vertical axis is equivalent to a worsening of the LNA/Mixer noise
figure. This can also be interpreted as the amount of transmit power
that is, in some sense, "wasted" when observing very weak echoes. If
you have installed an expensive LNA with a very low noise figure,
then you need to pick an operating point that makes the most of
preserving that investment.
on page 76 can be used to calculate the net gain that is required by the
front-end components, and to predict the final system performance:
1. Choose an operating point that balances the need for sensitivity versus
dynamic range. For this example, we allow a 1 dB loss of sensitivity
from the theoretical limit of the LNA/Mixer, and assume a bandwidth
of 0.5 MHz.
2. For a 1 dB loss of sensitivity, the ΔDynamicRange is first determined
from the solid red curve as 7 dB. The required noise ratio R is then
read vertically on the dashed green curve as 6.1 dB.
3. The RF/IF gain must bring the front-end thermal noise at
-112 dBm/MHz up to a level that is 6.1 dB higher than the IFD noise
density of -87 dBm/MHz. The gain is: (-87 dBm/MHz
+6 dB) - (-112 dBm/MHz) = 31 dB. This gain does not depend on
bandwidth, and therefore is correct for all pulse width/bandwidth
combinations.
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