Manual Lowrance X-15 (page 8 of 14) (English)

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higher

settings

of the knob. When

cruising,

other times when

just simple

bottom-contour

infor-

mation is

desired,

the

Sensitivity

setting

can be low. In

deep

water

or over

soft, muddy bottoms,

(which produce

weak

echoes),

the

setting

will have to be

higher.

When

high Sensitivity settings

are

used,

a second

bottom echo

will

appear

This is

normal and is

caused

the

returning signal

re-

flecting

off the

surface of the wa-

ter,

making

second

trip

to the

bottom and back.

When detailed

information about

brush

piles,

individual fish

or the

thermocline is

desired,

the Sen-

sitivity

knob should be set at

about

point.

Refer to the illustration

below to see what effect the

Sen-

sitivity setting

has on the recorded

information.

Recording

individual fish with an

"arched"

signature

can

usually

accomplished

trolling speed

with

the

Sensitivity

knob at the ¾

point,

higher

Refer to the sec-

tion

"Arched

Signatures,"

page

10 for

more information about this

important

function

your

recorder.

Suppression

Control

The

Suppression

Control is

used to reduce interference from

noise.

Noise,

in electronic

terms,

any

undesired

signal.

It can be

caused

electrical

source,

(such

as the

engine's ignition

sys-

tem),

air bubbles in dis-

turbed water which is

called cavi-

tation noise. In both

cases,

the

noise could

produce

unwanted

marks

on the

paper (See Figure

45.) Fortunately,

noise

pulses

are

relatively

short in time com-

used on all the Lowrance's vari-

able

suppression

flashers and

graphs. Basically,

it works on the

principle

that most noise

pulses

are of

relatively

short duration. If

the receiver circuit can be ad-

justed

that it will

accept only

long pulses,

then the short

pulses

will be

cancelled

out,

and

only

the

desired

information

(fish,

bottom,

structure,

etc.)

will be

displayed.

course,

the

transmitter's

pulse

width would have

to be increased

at the

same time so that the return

echoes would be

accepted by

the

receiver

This is

exactly

what

the Low-

rance

suppression system

does.

The transmitter's

pulse

width is in-

creased

the front

panel sup-

pression

control,

and the

receiver

"tracks" the amount of increased

pulse

width,

cancelling

out

any

narrow noise

pulses,

and

display-

ing only

the return echoes from

fish or the

bottom,

etc.

(Note:

Re-

ceiver

sensitivity

is not diminished

at all

this

process.)

The

only disadvantage

to this

system

resolution,

or the

ability

separate

targets,

is diminished

when the

pulse

width is increased.

A 200

(micro-second)

transmit-

ter

pulse

width used on the X-15

when

power

is first

turned

on,

will

allow the unit

display

two fish or

targets

that

are

only

6 inches

apart.

other

words,

if two fish

that are six inches

apart

are dis-

played

on the

graph paper,

they

will show

as two

separate

arches when the transmitter is

operating

with a 200

cs pulse

width.

Now,

if we increase the

transmitter

pulse

width to 400

gs,

(by rotating

the

Suppressor

control

clockwise)

those same two fish

arches will

blend

togetherand

show

as one fish br

possibly

even a "blob" on the

paper

With

a 400

transmit

pulse

width,

those same two fish will

have to be at least 12 inches

apart

before

they

will

show

as two

separate

arches on the

graph.

This is

why

it is

important

to leave

the

Suppressor

control turned

down to minimum

when

looking

for fish.

However,

it is

helpful

sometimes

to have

longer pulse

widths when

noise is

not a concern. When

using

sonar in

deeper water,

easier for the unit to

detect a

longer pulse

coming

back

from the

bottom or

from the fish.

This s

called

"Probability

of Echo

Detec-

tion."

The

probability

of the unit

being

able to detect an

echo re-

turning

from

deep

water is di-

minished

because the further

the

sound

pulse

has to

travel,

the

weaker

it becomes.

broadening

the

pulse width,

essence,

larger signal

is transmitted and it

is easier for

the receiver to

detect it.

For this

reason,

the micro-

computer

the X-15 automati-

cally

increases the initial

pulse

width of

the unit as

deeper

lower

limits are

set

the

operator.

(Note:

Initial

pulse

width is the

pulse

width

of the unit when the

Suppressor

control is turned to

minimum.)

INITIAL

TRANSMIT

PULSE

WIDTH

(Micro-seconds)

10 110

130

160

40-200

200

From

210 feet to 600

feet,

the initial

pulse

width increases

the same rate as

the

depth.

For

Figure

LLI

Igure

SENSITIVITY: TOO LOW

Figure

LOWER

LIMIT

(Feet)

SENSITIVITY:

PROPER SETTING

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Lowrance X-15 Manual

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