If Amplifier Circuit; Audio Amplifier Circuit; Squelch Circuit; Microphone Amplifier Circuit - Kenwood NX-340 Service Manual

2.1.2.3

IF Amplifier Circuit

The first IF signal is passed through a four-pole monolithic crystal filter (XF400) to remove the adjacent channel signal. The filtered
first IF signal is amplified by the first IF amplifier (Q403) and is then applied to the IF system IC (IC400).
The IF system IC provides a second mixer, AGC+BPF, PLL FM detector, noise squelch and RSSI circuit.
The second mixer mixes the first IF signal (49.95MHz) with the signal of the second local oscillator output (Q400) and produces the
second IF signal of 450kHz.
The second IF signal is passed through the internal bandpass filter of the IF system IC to remove the adjacent channel signal. The
filtered second IF signal is amplified by the limiting amplifier and demodulated by the internal discriminator of the IF system IC.
The demodulated signal is routed to the audio circuit.
2.1.2.4

Audio Amplifier Circuit

The demodulated signal from IC400 is sent to an AF amplifier through IC705, and is routed to an audio power amplifier (INT:IC709,
EXT:IC707) where it is amplified and output to the speaker.
2.1.2.5

Squelch Circuit

Part of the AF signal from the IC400 enters the FM IC (IC400) again, and the noise component is amplified and rectified by a filter and
an amplifier to produce a DC voltage corresponding to the noise level.
The DC signal from the FM IC goes to the analog port of the MCU (IC710). IC710 determines whether or not to output sounds from
the speaker by checking if the input voltage is higher or lower than the preset value.
To output sounds from the speaker, IC710 sends a high signal to the EXTSPSW line and turns IC707 on through Q709, Q710, Q713
and Q716. This explanation is for the external speaker amplifier. For the internal loudspeaker amplifier, IC710 sends a high signal to
the INTSPSW line and turns IC709 on through Q711 and Q714.
2.1.3 Transmitter System
2.1.3.1

Microphone Amplifier Circuit

The signal from the microphone is limited by the AGC circuit, which is composed of D700, D701, Q705, Q706 and the mute switch
(Q704). IC705 is composed of a high-pass filter, low-pass filter and pre-emphasis/IDC circuit.
The signal from the microphone and the low speed data from the MCU (IC710) enter the baseband IC (IC705) and pass through each
path and are mixed inside the IC.
The output signal from the audio processor MOD2 goes to the VCO modulation input. The other output signal from MOD1 goes to the
TCXO modulation input.
2.1.3.2 Drive and Final Amplifier Circuit
The signal from the T/R switch (D15 is on) is amplified by the pre-drive amplifier (Q201) to 20mW.
The output of the pre-drive amplifier is amplified by the drive amplifier (Q206) and the RF final amplifier (Q208) to 5.0W (1W when the
power is low).
The drive amplifier and the RF final amplifier consist of two MOS FET stages.
The output of the RF final amplifier is then passed through the antenna switch (D203 and D204) and harmonic filter (LPF) and is
applied to the antenna terminal.
IC400 IC705
Baseband
FM IC
IC
Fig.3 Audio amplifier circuit
IC705
MIC
Baseband
D700,701 MIC
Q705,706 AGC
LSD
SW
MICM
OUT
Q704
Fig.4 Microphone amplifier circuit
IC707
AF PA SPO EXT
(EXT) SP
IC709
AF PA INT
(INT) SP
MOD2
VCO
X1
TCXO
MOD1
(No.RA003<Rev.004>)1-5