HDM8513AP datasheet(22/67 Pages) HYNIX

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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 HDM8513A 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 17 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.

Absolute

Value

255

256

Absolute

Value

255

256

In Phase

Component

Average

Magnitude

Instantaneous

Deviation

Average

Deviation

In Phase

Component

FIGURE 16: NOISE MEASUREMENT CIRCUIT

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