Kongsberg Simrad ST90, Reference Manual

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LFM (Linear Frequency Modulation)
In LFM transmissions the frequency changes during the duration of the pulse. The
change is linear. Use the
Frequency
function to define the centre frequency used by the
transmitted pulse. Use the
Bandwidth
function to define the frequency range during
the transmission.
Each frequency modulated transmission ("ping") from the ST90 system consists of a
frequency sweep around the centre frequency. The difference between the lowest and
the highest frequency in each transmission is referred to as
bandwidth . The selected
bandwidth will be distributed equally on both sides of the centre frequency.
Example
If the chosen operating frequency is 17 kHz, and you set the bandwidth
to 2 kHz, the frequency range in each transmission starts on 15 kHz and
ends on 19 kHz.
HFM (Hyperbolic Frequency Modulation)
In HFM transmissions the frequency changes during the duration of the pulse. The
change is hyperbolic. At the beginning of the pulse the frequency increases slowly,
while gradually increasing faster and faster towards the end of the pulse. Use the
Frequency
function to define the centre frequency used by the transmitted pulse. Use
the
Bandwidth
function to define the frequency range during the transmission.
Each frequency modulated transmission ("ping") from the ST90 system consists of a
frequency sweep around the centre frequency. The difference between the lowest and
the highest frequency in each transmission is referred to as
bandwidth . The selected
bandwidth will be distributed equally on both sides of the centre frequency.
Note
HFM (Hyperbolic Frequency Modulation) is only available if your transceiver is fitted
with LPT32 transceiver boards. To use the pulse type, you must make changes to the
setup files. To find out which transceiver boards your transceiver is fitted with, see the
Transceiver
page in the
Diagnostics
dialog box.
Pulse Duration
The pulse duration can be selected according to the current range and what kind of
targets you are looking at. The longer range you wish to use, the longer pulse duration
should be selected. However, when using CW transmissions a long pulse duration will
reduce the resolution. It will also cause the ST90 system to transmit less frequent.
Further, increased pulse duration will cause reduced source level when using
consecutive transmissions. For example, if the ST90 system is set up to use one
horizontal ping and two vertical pings these are transmitted in series. The first ping
Simrad ST90 Reference Manual