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3.1. Automatic Gain Control
is decreased by a small amount. On the other hand, if it is lower then the gain factor is
increased. Note the word slightly in the last sentence. This is because, in order to avoid
the gain factor to be oscillating over and over again, we define a range of values for which
no change of the gain factor is performed. Typically, this range is around
[0.9 ∗ P owerRef erence, 1.1 ∗ P owerRef erence]
.
With all this at hand, we can write the pseudo-code for the AGC:
while true do
AvgP wr = 0;
gain = 1;
pwrRef = −12dB;
tav = 1/213 ;
for i = 0 to W indowSize do
current IQ − Sample = current IQ − Sample ∗ gain;
AvgP wr = tav ∗ IQpwr + (1 − tav) ∗ AvgP wr;
if (AvgP wr < 0.9 ∗ P wrRef ) then
gain = 1.1 ∗ gain;
end if
if (AvgP wr > 1.1 ∗ P wrRef ) then
gain = 0.9 ∗ gain;
end if
end for
end while
As an example of the workings of the last algorithm, in picture 3.2 we show an example
of a real DAB signal received at NXP Offices in Leuven. It turned out that the power of
the incoming signal was not very good to be subsequently processed. Nevertheless, as can
be seen the AGC performs well its task and amplifies the signal to a amplitude such that
it can be processed with all the available resolution in the CoolFlux architecture (24 bits).
For a better understanding of the AGC block we show in picture 3.3 the average signal
power of the last signal (in blue) in superposition with the gain factor (in red). Presumably
the power of the input signal is increased (maybe due to multipath propagation) at around
point 12 in the x-axis. Consequently, the gain factor is immediately decreased to maintain
the power of input signal at safe levels. As an observation, an excess in power does not
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