• N
IFDR
represent the stand-alone (terminated input) noise power of the IFD over some
bandwidth.
N
LNA
has its origins in the fundamental thermal noise of the receiving system.
• N
LNA
represent the LNA/Mixer thermal noise power over that same bandwidth, and
after amplification by all RF and IF stages.
N
IFDR
is primarily due to the quantization noise that is introduced by the A/D converter.
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:
= 10log
10
+
10log
10
= 10log
10
1 +
The reduction of RVP900 dynamic range is the amount by which the IFDR quantization
noise is increased over its stand-alone value:
= 10log
10
+
10log
10
= 10log
10
1 +
Both quantities depend only on the ratio of the two powers. The two equations define a
parametric relationship in the dimensionless variable R = ( N
LNA
/ N
IFDR
) . The
previous
figure 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 (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 1.4 dB ... 0.65 dB. The attendant loss of dynamic range falls between 5.5
dB ... 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 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.
When designing your RF and IF components, remember that the final amplifier driving the
RVP901 IFDR must be capable of driving up to, perhaps, +14 dBm, so that signals above
saturation can be correctly measured.
For example, you can use the calculation shown in the previous
figure 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.
RVP900 User Guide M211322EN-J
70
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