TK-90
21
Receive Circuit
The receive circuit is a double conversion structure with
first IF: 73.095MHz and second IF: 10.695MHz.
■
From Antenna to Preamplifier
The received signal from the antenna terminal is passed
into the Final unit (X45-378), the surge trap (D3), the transmit/
receive switching relay (K1), and the image signal eliminating
filter, and is then output to the TX-RX unit (X57-721).
The signal from the Final unit CN1 is input into TX-RX unit
CN1. It is then passed through the attenuator circuit (K1: de-
fault setting 20dB, the image signal eliminating filter, the
serge absorption limiter, and is then input into the bandpass
filter circuit. The bandpass filter circuit is divided into 8 fre-
quency ranges. A breakdown of the frequencies is shown in
table 1. This bandpass filter passes not only the received
signal but also the transmit signal at the time of transmission.
After the bandpass filter circuit, there is a preamp (Q103)
with approximately a 10dB gain. This preamp can be turned
ON/OFF using the FPU. It is then passed through a local os-
cillator signal eliminating filter, and input into the first mixer.
■
From First Mixer, RF Output
(Receive Second IF: 10.695MHz)
The signal input into the first mixer is converted to the first
IF frequency (73.095MHz). The first mixer uses a high-perfor-
mance circuit with quad-connects JFETs (Q105, 106, 108,
109). Its dynamic range characteristics is therefore equal a to
high-class model. The next step, MCF, uses 2, 2-pole MCFs.
By passing not only the received signal but also the transmit-
ted signal through this narrow-band MCF, nearby spurious
signals are reduced. The signal input/output terminals TP1
(CN4) and TP2 (CN5) for MFC adjustment are located on the
print base.
The signals which passed through MCF are amplified with
the receiving first IF amplifier (Q183) which is gain controlled
(AGC), and are then converted to receiving second IF (10.695
MHz) at the receiving second mixer. This mixer employs pas-
sive type which uses a diode and is a dual-directional circuit
which converts not only received signal frequencies but also
transmitted signal frequencies.
CIRCUIT DESCRIPTION /
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ANT
RX
TX
ATT
–20dB
RF BPF
30kHz~
30MHz
Q103
PRE AMP
Through
(PRE AMP OFF)
Q105,106,108,109
RX 1st Mixer
MCF
XF101
73.095MHz
Q183
1st IF AMP
D183
RX 2nd Mixer
10.695MHz
RX 2nd IF
Q184
LO2 AMP
LO2
62.4MHz
AGC
Q107
LO1 AMP
LO1
73.125~103.095MHz
Band
Filter range
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1
30kHz~1.605MHz
2
1.605MHz~2.5MHz
3
2.5~4.1MHz
4
4.1~7.5MHz
5
7.5~10.5MHz
6
10.5~14.5MHz
7
14.5~21.5MHz
8
21.5~30.0MHz
Table 1
BPF range
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Fig. 3
From antenna to TX-RX unit /
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