Interphase Twinscope Operation Manual page 69

"False" Bottom
Echoes Caused by
Sidelobe Returns.
Transmit Pulse Sidelobe Echo
Receiver Input Signals
Receiver Gain (TVG)
Sidelobe Echo
Receiver Output Signals
How Time Varible Gain Helps Reduce Sidelobe
Effects
Desired Echo
Receiver Gain Level
Desired Echo
typical display of the sidelobe echoes appears as an arc
at the same distance as the bottom depth , and in the
worst case, from the bottom below to the surface ahead.
After using the Twinscope in different situations, with
different gain settings, you should become proficient in
identifying the bottom echoes caused by the
transducer's sidelobes. To minimize the sidelobe
effect, the gain should be reduced.
However, in some situations, you may want to ignore
the sidelobe effect and increase the gain to achieve a
better display of the bottom far-forward of the vessel.
TVG (Time Variable Gain)
As the acoustic signal travels through the water it is
attenuated in strength and also loses strength because
the signal is being spread over a larger and larger area.
For this reason, distant targets appear weaker than
close targets - even if both targets are of identical size.
The Twinscope's receiver circuit includes a Time
Variable Gain (TVG) feature where the receiver's gain
is controlled by the microprocessor and increases in
value as the depth or forward range increases. This
feature tends to compensate for the weakening of the
signal at large distances as the receiver's gain will be
increased as the signal gets smaller. It is also helpful
in minimizing sidelobe problems as shown in the
diagram at left.
In the diagram, a boat is shown with the main
transducer beam aimed far forward. However, the
sidelobe beam is also striking the shallow bottom
directly beneath the boat. Both the desired echo (the
one from the far forward beam) and the sidelobe echo
will reach the receiver as shown in the graph just
beneath the boat. Note that the transmit signal, the
sidelobe echo and the desired echo have been
arbitrarily shown as the same size.
The diagram just below shows how the TVG on the
receiver changes with distance (or time). The farther
the target from the boat, the larger the receiver gain.
Finally, the bottom diagram shows the output of the
receiver after amplification with the TVG feature. Note
that the TVG has reduced the sidelobe echo compared
to the desired echo.
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