Chapter 3 _______________________________________________________ Hardware Installation
VAISALA_______________________________________________________________________ 79
noise level is around -87 dBm. Since an ideal unity gain amplifier has
produced a noise power of -114 dBm in an equivalent bandwidth, the
noise figure of the IFDR is 27 dB.
When two amplifiers are cascaded, so that the output of the first drives the
input of the second, the overall gain is the product of the two linear gains
G
1
lin
and G
2
lin
, and the overall noise figure is computed from the two noise
factors F
1
lin
and F
2
lin
as:
where the two noise factors are simply the linear representations of the
noise figures that were expressed in deciBels:
Suppose that our first amplifier is an LNA/Preamp with a 2 dB noise figure
(noise factor 1.58), and we want to know what gain it must have such that,
when cascaded into the IFDR, the overall noise figure is 3 dB. The 27 dB
noise figure of the IFDR is equivalent to a noise factor of 501, therefore we
have:
from which we solve G
1
lin
= 1204 (30.8 dB). This agrees with the 31 dB
of gain that was computed in the example of the previous section for the
same RF/IF components and desired overall performance.
3.2.13 Choice of Intermediate Frequency
The RVP900 does not assume any particular relationship between the A/D
sample clock and the receiver's intermediate frequency. You may operate
at any IF that is at least 2 MHz away from any multiple of half sampling
rate. At 72 Mhz sample, the multiple are nominally 18 MHz, 36 MHz,
54 MHz, 72 MHz, and 90 MHz). The valid frequency bands are thus:
6-16 MHz, 20-34 MHz, 38-52 MHz, 56-70 MHz, 74-88 MHz
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