Vaisala RVP900 - Choosing Intermediate Frequency

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:
 = 10log
10


1
+


2
 1


1
where the two noise factors are the linear representations of the noise figures that were
expressed in decibels:
 = 10log
10

If 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 RVP901 IFDR is equivalent to a noise factor
of 501, therefore we have:
3 = 10log
10
1.58 +
501  1


1
from which we solve:


1
= 1204 30.8
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.
4.2.13
Choosing Intermediate Frequency
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
There are many reasons for staying clear of the Nyquist frequency multiples. Most of these
considerations apply to all types of digital processors, and are not
specific to RVP900.
As an example of what can go wrong at the Nyquist frequencies, suppose that an
intermediate frequency of 35 MHz was used. This is only 1 MHz away from the
(approximately) 36 MHz sampling rate. The external anti- alias filter must now be designed
more carefully, since a spurious input signal at 37 MHz is aliased into the valid 35 MHz band.
If the valid signal bandwidth were 2 MHz, then the anti-alias
filter has the dicult task of
passing 34 MHz to 36 MHz free of distortion, while rejecting everything above 36 MHz. The
filter's transition zone has to be very sharp, and this is dicult to achieve.
RVP900 User Guide M211322EN-J
72