Furuno CN-24, Service Manual

Page: 12 / 90

2-3
Fig. 2.2 TX waveform Fig.2.3 TX trigger: TSMT
Receiver Circuit
The received signal from the transducer is first amplified by the RF amplifier and then
mixed with the local oscillator output to get a 455 kHz IF signal. The IF signal (Fig.2.4 CH1)
is amplified and fed to the next stage's full wave detector. The output of the detector (Fig.
2.4 CH2) is A/D converted to eight bit echo data at specific sampling intervals.
The AMP PWR CONT circuit su 5 V (AMP5V), which is controlled by APWR signal
from the CPU, to the TVG amplifier only during the reception period to conserve battery
consumption.
Fig.2.4 Waveforms of received echo signal Fig.2.5 Waveform of carrier signal
TVG Circuit
Triggered by the TVG (Time Varied Gain) signal from the CPU on the CONT A board, the
TVG circuit generates an exponentially rising TVG voltage (TVG curve). This voltage
controls the gain of the amplifier in such a way that the gain is minimum at the time of
transmission and gradually increases with time. The TVG curve is factory set to a 30 log
curve suitable for fish detection.
Fig.2.6 shows the waveforms of the TVG voltage.
Measuring point: P4 #1 - P4 #4
Frequency:75 kHz
100V/DIV
0.2ms/DIV
Measuring point: TP6 on TRS.A board
Frequency:75 kHz
2V/DIV
0.5ms/DIV
Measuring point: CH1 -- TP3
CH2 ---TP2(ESIG)
Measuring point: TP7 (carrier)
CH1
CH2
CH1
200mV/Div
1ms/Div
CH2
2V/Div
1ms/Div
CH1
200mV/Div
1us/Div