JW Fishers SSS-100K PC Operation And Maintenance Manual page 20

TIME VARIABLE GAIN (TVG)
The sonar Processor contains a TVG circuit on the PC board that receives the echo signals from the fish preamplifier
boards. The TVG circuit amplifies and makes time variable gain adjustments to the signal to make up for signal losses
which occur when the echoes are traveling through the water. Each channel has its own TVG circuit. Each TVG circuit
has its own set of operator controls which are located of the Sonar Processor's top panel.
Left Input
From Fish
Right Input
From Fish
The signal return (shown below) to the Sonar Processor is the signal that we would expect to see at the output of the
fish preamps. The amplitude of the signal that reaches the Computer determines the color of the image. If the TVG circuit
did not modify the signal shown below (Fig 2), then the display would start out good, but the bottom return
would quickly turn to a color indicating no return (Fig 3). If the return from target #2 was real strong, it might
Fish "pop out" of the light background and print a light image as shown below.
Acoustic Wave
Target #1
10 m
A
m
p
#1
l
i
t
u
d
e
Transmit pulse
Target #1
TVG Board
Sonar Processor PC Board
Left Channel TVG Circuit
Right Channel TVG Circuit
Fig 1
100 m Range Switch
Fig 2
Signal to Sonar Processor
Shadow
Fig 3
Print Head - Right Channel Display
100 m
8"
(if no TVG)
20
Left Output
to Interface
to CPU Board
Board
Right Output
to Interface
to CPU Board
Board
Target #2
#2
Shadow
Next transmit pulse
Target #2