Simrad ES80 Reference Manual page 472

Simrad ES80 Reference Manual
The principle is shown in the illustration. Curve (A) has
set to Fast, and the level is increased from 0
Ramping
V to maximum level using from minimum two (2) up
to maximum ten (10) cycles. At the end of the pulse,
maximum ten down to minimum two cycles are used
to reduce the output level.
The number of cycles used depend on the q-factor
(bandwidth relative to centre frequency) for the
connected transducer. The number of ramping cycles
will be upward limited to the number of cycles in half
a pulse length.
Curve (B) has
Ramping
level using the first half the pulse duration. The second half of the pulse is then used to
reduce the output level.
Advantages
• Fast ramping normally means that only the first few cycles of the transmit signal are
used to ramp up the signal amplitude and the last few cycles are used to ramp down
the amplitude.
For short pulses, the ramping might converge to Slow ramping.
The advantage with Fast ramping is that the transmitted pulse contains more energy
relative to the pulse length. This increases the signal/noise ratio. When you apply
"chirp", the transmission frequency changes from a low frequency at the beginning of
the pulse to a high frequency at the end of the pulse. The combination of chirp and
Fast ramping will significantly increase the signal/noise ratio.
• Slow ramping means that the first half part of the transmit signal is used to ramp up
the amplitude and the last half part is used to ramp down the amplitude.
The advantage with Slow ramping is that the pulse have a smooth start and stop. This
significantly reduces the transients in to the matched filter and therefore you get
reduced side lobes in time (range) at a cost of slightly reduced range resolution at the
output of the matched filter for FM pulses.
The side lobes in time (range) will thus be lower with Slow ramping than with Fast
ramping. Smaller side lobes in time (range) will reduce "crosstalk", as a strong echo
from one range will not contaminate weak echoes from other ranges. Increased echo
discrimination is particularly useful with echoes of different strength.
A raised cosine window is used as ramping function.
Ramping when using FM transmission
• Fast ramping will result in maximal received signal power for a chosen a longer
effective pulse duration and transmit power, and a wider effective frequency
bandwidth than Slow ramping.
Fast ramping is normally used to increase frequency bandwidth, range resolution,
and signal-to-noise ratio.
470
set to Slow. The output level is increased from 0 V to maximum
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