Receiver Circuit Configuration; Receiver Front-End - Kenwood TK-80 Service Manual

TK-80
■ Frequency data selection
The SPSL input of the DDS IC selects whether to use the
data in the internal frequency shift register or the data from
the parallel input as frequency modulation data.
■ Frequency modulation
The MDEN input of the DDS IC enables or disables fre-
quency modulation. When frequency modulation is enabled,
frequency data is added, and the result is input to the phase
data operation section.
■ Phase data operation
The target frequency phase data is output by accumulat-
ing 28 bit frequency data in the 28-bit phase accumulator.
F = Fs/2 28 x Dsum
OUT
Fs: DDS IC input frequency/2
Dsum: Frequency data + Frequency modulation data
If 2 25 is set for Dsum when 1/8 Fs is output, the phase data
must be increased by π/8.
So far, 28 bit absolute value operation has been used, but
a 28 bit signed operation can also be used, assuming that the
MSB is a sign. If complement data of 8000000 to FFFFFFFF
(hex) is set, the phase moves in the negative direction for
positive data.
■ SIN-ROM
Phase data from the phase data operation section is con-
verted to sine wave data of 0000 to FFFF (hex) in 16 bit offset
binary format (Fig. 3).
ANT
12
CIRCUIT DESCRIPTION
FINAL UNIT (X45-3520-21) TX-RX UNIT (X57-4660-21)
RX
~30MHz
TX
Fig. 4 Receiver front-end
25
π/8 = 2

3. Receiver Circuit Configuration

The configuration of the receiver circuit is double conver-
sion with a first IF of 73.045MHz and a second IF of 10.695
MHz in SSB, AM, CW mode.
The incoming signal from the antenna passes through the
antenna switch relay of the final unit, then through the 30
MHz low-pass filter, and goes to the TX-RX unit. The signal
passes through a 30MHz low-pass filter in the TX-RX unit,
and goes through the 8-segmented band-pass filters. If AIP
is off, the signal passing through band-pass filter is amplified
by the RF amplifier, Q8, Q9, and is input to the first mixer, Q4
to Q7. If AIP is on, the signal bypasses Q8 and Q9 and goes
directly to the first mixer. It is mixed with the LO1 signal by
the first mixer to produce a first IF signal of 73.045MHz.
~30MHz
1.605MHz~ ~1.605MHz
D10
D9
AIP ON
D23 D24
1.605~2.5MHz
D11 D12
2.5~4.5MHz AIP OFF
D25 D26
D14
D13
4.5~8.0MHz Q8,9
D15 D16
8.0~10.5MHz
D17 D18
10.5~14.5MHz
D19 D20
14.5~21.5MHz
D21 D22 D50
21.5~30MHz
28
2π = 2
FFFFH
0000H
25
7π/8 = –2
Fig. 3
1st MIX
~30MHz Q4~7
IF
LO1
73.145~
103.045MHz