SECTION 3CIRCUIT DESCRIPTION
3-1 VHF RECEIVER CIRCUITS
3-1-1 ANTENNA SWITCHING CIRCUIT
(HPF UNIT)
The received signals that pass through the LPF UNIT are
separated as VHF and UHF signals on the HPF UNIT and are
applied to each RF amplifier circuit.
The HPF UNIT contains ahigh-pass filter, low-pass filter and
an antenna switching circuit. The VHF signals pass through
the low-pass filter (L3-L5, C6~C10) and the antenna
switching circuit (C12~C14, L6, L7, D2). The VHF signals
and are then applied to the V*RF UNIT. The high-pass filter
(C1~C5, LI, L2) is designed for UHF receiving and does not
allow VHF signal passage.
3-1-2 RF AND 1ST MIXER CIRCUITS
(V.RF UNIT)
The RF and 1st mixer circuits are located on the V»RF UNIT
where the signals are amplified at Q1 and Q2 and are then
converted to a1st IF signal at Q3. The frequency of the 1st
IF signal is 30.875 MHz.
Q3 converts the RF signal using the V*PLL output signal
coming from the "LLO” line.
The VHF RF circuit adopts atuned bandpass filter circuit that
tunes the filters to the center frequency of the receiving
signal using varactor diodes (D2, D4, D5). APLL lock
voltage is used for the filter tuned signal.
3-1-3 IF CIRCUIT (DET UNIT)
The 1st IF signal passed through the crystal filter (F11 on the
MAIN UNIT) enters the DET UNIT. The DET UNIT converts
the 1st IF signal to a2nd IF signal and then detects AF
signals. The unit also has asquelch circuit that is explained
in Section 3-1 -5.
The 1St IF signal amplified at Q3 is applied to the FM IF 1C
(IC1 ,pin20). IC1 contains mixer, oscillator, limiter amplifier,
quadrature detector and meter detector circuits.
IC1 converts the entered signal from pin 20 to a455 kHz
2nd IF signal using the oscillated signal and outputs the 2nd
IF from pin 4to pass through the crystal filter (FI1 ). The 2nd
IF signal re-enters IC1 (pin8) where it is converted to an AF
signal at aquadrature detector (IC1 ,XI ). The AF signals are
output from IC1 (pinll) and are de-emphasized with -6
dB/oct. at the integrating circuit (R8, C28, C29) and are then
applied to the AF UNIT.
3-1-4 AF CIRCUIT (AF UNIT)
The AF UNIT contains the AF drive amplifier (Q1 ,Q2), power
amplifier (IC1), noise squelch switch (Q4~Q6) and tone
squelch switch (Q3).
The AF signals from the DET UNIT are amplified at the AF
drive amplifier and are passed through the [VOL] control on
the VR UNIT and are then amplified at the power amplifier
(IC1) to obtain the speaker driving level.
3-1-5 SQUELCH CIRCUIT
(DET AND AF UNITS)
In an FM receiver, noise signals are output from the speaker
when no signal is received and are suppressed when a
signal is received. The noise squelch circuit cuts off the
noise signals using this phenomenon.
Noise components in AF signals (20 kHz and above) are
amplified at the active filter (IC1, RIO, R11, Cl 4, C15 on the
DET UNIT) and are rectified at D1 to convert to DC voltages.
These voltages switch Q2. Thus, while receiving no signal,
the "LOW” signal is output from the DET UNIT as the
“BUSY” signal.
If the "BUSY” signal goes down, the “AFS” signal goes
down. The “AFS” signal controls the power source
(Q4-Q6) of the AF power amplifier (IC1). Thus, when the
"BUSY” signal is at “LOW,” the AF power amplifier does not
operate.
When an optional tone squelch unit is installed, 05 on the
MAIN UNIT controls the “AFS” signal using the “TSQL”
signal.
Fll
IF CIRCUIT
3-1
