SECTION 4CIRCUIT DESCRIPTION
4-1 RECEIVER CIRCUITS
4-1-1 RF CONVERTOR CIRCUIT (CONV UNIT)
The RF convertor circuit converts 1025-1999.99999 MHz
RF signals to 200-989.99999 MHz RF signals.
(1) 30.00000-1024.99999 MHz signals
RF signals from the antenna connector (J1) pass through
the switching relays (RL1, RL2) to bypass the RF convertor
circuit. The bypassed RF signals either bypass or are
passed through the RF attenuator circuit (R17-R19.
R25-R27) then applied to the RF-B unit via J3.
(2) 1025.00000-1999.99999 MHz signals
RF signals from the antenna connector (J1) are applied to
the RF amplifier circuit (ICI, IC3) via the switching relay
(RL1) and tunable high-pass filter (D3-D5, D20, D21.
L2-L4). The amplified signals are mixed with convertor LO
signals at the RF convertor circuit (1C2) to produce
200.00000-989.99999 MHz RF signals. The converted RF
signals are applied to the RF-B unit via J3 after bypassing
or passing through the attenuator circuit.
The convertor PLL circuit (IC6; VCO, IC7: PLL 1C) gener-
ates a1000 or 1010 MHz LO signal and applies them to the
convertor circuit directly or divides them by 2at 108.
4-1-3 VHF/UHF RF CIRCUIT (RF-B UNIT)
The RF circuit amplifies the received signals within the
range of frequency coverage and fitters out-of-band signals.
The RF circuit consists of 4bandpass After circuits with an
RF amplifier for each.
The received signals from the CONV unit are passed
through to the tunable bandpass filter via the switching relay
(RL1), then ampliAed at the RF ampllAer circuit. The
amplified RF signals are again passed through another
bandpass Alter to suppress out-of-band signals. The Altered
signals are ampliAed at the other RF amplifier circuit (IC3),
then applied to the 1st mixer circuit after passing through
the low-pass or bandpass with tuned notch circuit.
The tunable bandpass Alters employ varactor diodes to tune
the center frequency of the RF passband for wide band-
width receiving and good image response rejection. These
diodes are controlled by the CPU (MAIN unit. IC33) via the
voltage amplifier circuit (IC2).
AGa-As FET is used for the RF ampliAers (Q1-Q4) to
provide high sensitivity within wide-band coverage, and also
to provide 10 dB amplifying gain.
•Convertor LO signals
Receive frequency Convertor LO frequency
1025-1199 99999 MKz 500 MHz
1200-1989 99999 MHZ 1000 MHz
1990-1999 99999 MHz 1010MHz
4-1-2 RF ATTENUATOR CIRCUIT
(CONV UNIT)
The attenuator circuit attenuates the signal strength up to
30 dB to protect the RF ampliAer from distortion when
excessively strong signals are received.
The RF attenuator circuit consists of 2separate attenuator
circuits connected in series. The 1st stage of the RF
attenuator circuit (R17-R19) provides 20 dB attenuation;
the 2nd stage (R25-R27) provides 10 dB attenuation via a
“n"type attenuator.
•Tunable bandpass filter and RF amplifier
Receive frequency
(MHz)
BPF select
signal Varactor diodes RF amp.
30.0- 89.99999 BPF1 D3. D4 Q1
90.0- 242.09999 BPF2 D9-D12 Q2
242.1- 499.99999 BPF3 D15, Die, D33, D34 Q3
500.0-1024.99999 BPF4 D19-D23 Q4
4-1-4 TUNED NOTCH CIRCUIT (RF-B UNIT)
The tuned notch circuit activates while RF signals higher
than 500 MHz are received. The circuit prevents the 1st LO
signal from entering the antenna connector and also re-
duces IF disturbance.
The tuned notch circuit {D29-D32, L51-L54, L74) is de-
signed between the high-pass (L82, C166-C168) and low-
pass Alter (L55, L56, C96-C98, Cl 00). The high-pass filter
reduces IF disturbance (266.7 MHz), and the tuned notch
and low-pass Alter circuits prevent the 1st LO signal from
entering the antenna connector.
•VHF/UHF RF and mixer circuits
30.0-1 999.9999a MHz CONV unit
30.0-989.99999 MHz
—
10 de
ATT
RF-B unit
^~^BPF2^
-^BPF3^
¥
BPF4/
Convertor LO signal
IC3
r^h
Bu^ -6<^
1st LO signal
2nd LO signal •
MIX unit
778.7 MHz
iC2
266.7 MHz
-<8>10.7 MHz
to
MAIN
unit
4-1
