Installation Instructions

Hardware Installation
RVP8 User’s Manual
September 2005
2–12
noise power of the IFD over some bandwidth. Similarly, let N
LNA
represent the LNA/Mixer
thermal noise power over that same bandwidth, and after amplification by all RF and IF stages.
Note that N
IFD
is primarily due to the quantization noise that is introduced by the A/D converter,
whereas N
LNA
has its origins in the fundamental thermal noise of the receiving system. The
reduction of receiver sensitivity is the amount by which the LNA thermal noise is increased over
the original level established by the front-end components:
DSensitivity + 10 log
10
( N
LNA
) N
IFD
) * 10 log
10
( N
LNA
) + 10 log
10
ǒ
1 )
N
IFD
N
LNA
Ǔ
Likewise, the reduction of RVP8 dynamic range is the amount by which the IFD quantization
noise is increased over its stand-alone value:
DDynamicRange + 10 log
10
( N
LNA
) N
IFD
) * 10 log
10
( N
IFD
) + 10 log
10
ǒ
1 )
N
LNA
N
IFD
Ǔ
Note that both of these quantities depend only on the ratio of the two powers; hence, the two
equations define a parametric relationship in the dimensionless variable R + ( N
LNA
ń N
IFD
).
Figure 2–2 was created by sweeping the value of R from 1/9 to 9. The solid red curve shows the
locus of ( DDynamicRange, DSensitivity ) points, and the dashed green curve shows R itself
(expressed in dB) as a function of DDynamicRange. For example, when the LNA noise power
is equal to the IFD noise power, R is 1.0 (0dB) and there will be a 3dB 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 1.4dB and 0.65dB. The attendant loss of dynamic range will fall between 5.5dB and
8.5dB respectively. Each axis of the plot has an important physical interpretation within the
radar system.
S The horizontal axis is equivalent to the increase in the RVP8’s 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.
S 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 will want to pick an operating point that makes the most of
preserving that investment.
Figure 2–2 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 your need for sensitivity versus dynamic
range. For this example, we will allow a 1dB loss of sensitivity from the
theoretical limit of the LNA/Mixer, and will assume a bandwidth of 0.5MHz.
2. For a 1dB loss of sensitivity, the DDynamicRange is first determined from the
solid red curve as 7dB. The required noise ratio R is then read vertically on the
dashed green curve as 6.1dB.