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No. 2D730-148E*O
(3)
Description of operation
(a)
SCW summation
The SCW reception signals from the transducer are amplified to 20 dB (fixed gain of
the preamplifier in the TR board) and are input to the 2nd preamplifier in the CB.
The CB has a 64-ch reception signal line. When the sampling marker is moved in
the SCW, the delay tap is selected accordingly from among the 16 taps on the delay
line in the eight 8 x 16 multiplexers including 8 chs. Then algebratic summation
is
performed and reception beam forming is implemented.
The delay time within one cycle of the carrier waveform
is sufficient because the
pulsed wave is periodic and a minimum quantification
step of 20 ns is implemented
with a maximum delay of 500 ns.
The reception signals after algebratic summation
differ depending
on the type of
transducer.
Therefore the signal levels are matched using the variable attenuator
RF
gain and an appropriate
dynamic range is allocated.
The reception signals of the pencil (CW) and summation
output of the SCW are
switched by the multiplexer.
Control of the 8 x 16 multiplexer, selection of the RF Gain, and output switching
control between the pencil and SCW are performed
by the address decoder (EPLD)
based on the code of the TC bus from the RC.
(b)
Quadrature
demodulation
section
1) Selectable BPF
The address decoder selects an appropriate
band-pass filter (BPF) depending
on
the carrier frequency.
2) Quadrature
demodulator
A clock with a go-degree
phase difference in carrier frequency is input to the two
diode mixers and quadrature
demodulation
is performed for reception signals
passed through the selectable BPF.
The clock is generated
by the TXPG based on the system clock (four-phase,
40
MHz) from the RC and is supplied to the mixer by shifting the level using the
driver.
3)
Baseband filter
The DC (carrier elements) and added elements are eliminated by the BPF behind
the mixer. The HPF behind the BPF is a fourth-order
active filter and allows
clutter elements to be removed.
The required Doppler elements can be extracted
The LPF is an antialiasing filter for the ADC and the cutoff is fixed to 156 kHz.
This filter is a fourth-order
active filter.
4-18
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