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TK-2212/2217

5) Audio Amplifier Circuit

The demodulated signal from IC201 goes to AF amplifier
through IC301.
The signal then goes through an AF volume control , and
is routed to an audio power amplifier (IC302) where it is
amplified and output to the speaker.

Fig. 3 Wide/Narrow switching circuit

CIRCUIT DESCRIPTION

1. Frequency Configuration

The receiver utilizes double conversion. The first IF is 38.85

MHz and the second IF is 450 kHz. The first local oscillator
signal is supplied from the PLL circuit.

The PLL circuit in the transmitter generates the necessary

frequencies. Fig. 1 shows the frequencies.

Fig. 1 Frequency configuration

1) Front End (RF AMP)

The signal coming from the antenna passes through the
transmit/receive switching diode circuit, (D103,D104,D105
and D106) passes through a BPF (L214 and L213), and is
amplified by the RF amplifier (Q205).
The resulting signal passes through a BPF (L209 and L211)
and goes to the mixer. These BPFs are adjusted by variable
capacitors (D203,D204,D205 and D206). The input voltage
to the variable capacitor is regulated by voltage output from
the microprocessor (IC405).

2) First Mixer

The signal from the front end is mixed with the first local
oscillator signal generated in the PLL circuit by Q1 to
produce a first IF frequency of 38.85 MHz.
The resulting signal passes through the XF201 MCF to cut
the adjacent spurious and provide the opitimun
characteristics, such as adjacent frequency selectivity.

Fig. 2 Receiver section

2. Receiver

The frequency configuration of the receiver is shown in Fig. 2.

3) IF Amplifier Circuit

The first IF signal is passed through a four-pole monolithic
crystal filter (XF201) to remove the adjacent channel signal.
The filtered first IF signal is amplified by the first IF amplifier
(Q203) and then applied to the lF system IC (IC201). The
IF system IC provides a second mixer, second local
oscillator, limiting amplifier, quadrature detector and RSSI
(Received Signal Strength Indicator). The second mixer
mixes the first IF signal with the 38.4MHz of the second
local oscillator output (TCXO X1) and produces the second
IF signal of 450kHz.
The second IF signal is passed through the ceramic filter
(CF201) to remove the adjacent channel signal. The filtered
second IF signal is amplified by the limiting amplifier and
demodulated by the quadrature detector with the ceramic
discriminator (CD201). The demodulated signal is routed
to the audio circuit.

4) Wide/Narrow Switching Circuit

Narrow and Wide settings can be made for each channel
by switching the demodulation level.
The WIDE (low level) and NARROW (high level) data is
output from IC405, pin 45.
When a WIDE (low level) data is received, Q202 turn on.
When a NARROW (high level) data is received, Q202 turn off.
Q202 turns on/off with the Wide/Narrow data and the
IC201 detector output level is switched to maintain a
constant output level during wide or narrow signals.

BPF

IC301

IC302

AF PA

IC201

IF,MIX,DET

ANT

AQUA-L

CF201

TCXO

TUNE

X3 multiply