HDM8515 Datasheet, PDF (23/75 Pages) Hynix Semiconductor – DVB/DSS Compliant Receiver

3.3 Noise Measurement Circuit
When the DBS system is being installed in any place, the most difficult part of the installation is
accurate pointing of the antenna toward the satellite. Inaccurate pointing results in loss of margin
and greater potential for outages in adverse weather conditions. Existing systems use information
from the demodulator forward error correction circuits to provide a measure of antenna pointing.
Unfortunately, this method is useful over a range of only several dB above system threshold.
The HDM8515 employs a unique circuit for accurate measure of signal strength over a 20 dB range
of signal to noise ratio. This method, illustrated in the block diagram, makes use of the fact that
the demodulator provides 8 bits of resolution for each of the quadrature output components. This
high resolution provides a means of measuring the noise component with great accuracy.
The eight bit in-phase demodulator filter output is detected by an absolute value circuit, then
passed through an IIR to provide a measure of average signal amplitude. Each sample is then
subtracted from this average amplitude to provide an instantaneous noise sample. The absolute
value of these noise samples are then averaged by a second IIR to provide a measure of the noise
which is roughly proportional to the noise power and inversely proportional to signal to noise ratio.
Finally, the Figure 16 illustrates the results of simulations under different noise conditions. This
figure illustrates that for signal-to-noise ratio as high as 19 dB, the noise measurement circuit
provides a meaningful measure of signal power with worst case resolution of 1 dB.
In-Phase
Component
8
Absolute
Value
255
256
R
8
8
Absolute
Value
Average Instantaneous
Magnitude Deviation
255
16
256
Average
Deviation
R
8
16
FIGURE 15: NOISE MEASUREMENT CIRCUIT
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