Kenwood TKR-740 Service Manual page 33

4.2 Narrow IF circuit
The narrow IF circuit consists of tw o-pole M CF XF2, four-
pole M CF XF4, IF amplifier Q25, IF amplifier Q32, FM detector
IC IC7,crystal oscillator X1 and ceramic filters CF1, CF3. The
unw anted components of the signal are removed by tw o-pole
M CF XF2 and four-pole M CF XF4, and the resulting signal is
amplified by IF amplifiers Q32. The FM IC IC7 produces the
second IF signal (450 kHz), ceramic filters CF1 and CF3 remove
unw anted components and an IF amplifier amplifies the signal,
and the quadrature detector circuit FM -detects the signal to
produce a baseband signal and outputting it to pin 15.
The baseband signal passes through analog sw itch IC18,
inversion amplifier IC15 (B/2), AF amplifier IC11 (A/2 and B/2),
and goes to the Y0 input of multiplexer IC9 and the V2 input of
electronic volume control IC9. The level of the signal that
CIRCUIT DESCRIPTION
enters V2 of the electronic volume control IC is adjusted, the
signal passes through the hysteresis circuit AF sw itch Q34,
goes to IC7 noise filter input (pin 17), and high-frequency
components are selected by an HPF consisting of an external
RC netw ork. The signal is noise-detected and compared, and
the noise squelch signal (N-DET) is fed to DC sw itch Q36.
The signal strength analog voltage (RSSI) from the tw o second
IF amplifiers in IC7 is compared w ith the reference voltage
set by electronic volume control V4 by the internal RSSI
comparator, and the RSSI squelch signal (C-DET) is output from
pin 20 of IC7. C-DET enters DC sw itch Q37 and is ANDed
w ith the N-DET by DC sw itch Q38. A squelch signal (SC) is
output from connector CN6.
Fig.15 Narrow IF circuit
TKR-740
33