Datawell 45100, Руководство пользователя

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A.3.2 Noise
Natural noise is always present and has many appearances. Since most of the natural noise
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is
generated in our atmosphere due to a phenomenon called “weather”, the noise depends on the
season, the part of the day and the propagation characteristics (see below).
An increase of natural noise will degrade the signal to noise ratio and therefore will decrease the
link range.
Technical details:
Noise can be almost constant or it can have a bursty nature -usually caused by lightning- and it
goes up and down in power dependent on the part of the day. Summers are usually much
noisier
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than winters in the Northern hemisphere. Natural noise is almost independent of the
receiving direction when using a directional antenna.
Noise can also be caused by extraterrestrial objects such as the Sun and Jupiter which are very
noisy around 30 MHz. This type of noise sources is called galactic noise.
Fortunately the amount of natural noise is limited to a maximum of approximately 30 dB above
the thermal noise limit and thus will never be able to “overdrive” the receiver at its input, this in
contrast with interference which is capable of overloading the receivers input.
A.3.3 Interference
Interference originates from man made equipment and is therefore often called “man made
noise”. Interference signals can be of an intentional source, such as another transmitter or from
an unintentional source, for instance the spark ignition from a car.
Over the last 20 years interference has increased significantly and over the next 20 years there is
no reason to assume that this increase will be halted, therefore in the case of erroneous data link
it is sensible to investigate if interference is the main cause of the errors.
Like natural noise, the amount of interference limits the maximum distance between the buoy
and its receiver. Maximizing the distance to interfering sources and/or using a directional
antenna will minimize the impact of interferers on the buoy signal.
Technical details:
An important distinction can be made between in-band interferers and out-of-band interferers.
An in-band interference falls in the signal bandwidth of the receiver (plus and minus 100 Hz
around the receive frequency for the RX-D2) and it directly harms the translation process
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from
an analog buoy signal into a digital data stream. In band interferers can be identified with the
RX-D2 by listening to the speaker or by connecting the line out signal from the backside of the
RX-D2 (see
chapter 2 ) to the soundcard input of a PC and use an audio analysis program to
observe the signal quality.
An interferer does not fall within the receivers signal bandwidth of 200 Hz but still falls within
the receivers bandwidth. This will harm the quality of the received buoy signal if it is strong
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At 30 MHz.
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A 10 to 25 dB variation is expected between summer and winter in The Netherlands.
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This translation process involves demodulation, bit synchronization, frame synchronization and error
correction.