4.3.2 Transmitter Circuit
A signal from the microphone is fed to a pre-emphasis operations amplifier IC3, and
modulates VCO (Q4) through active LPF IC2.
The VCO output signal from Q4 is sent to the RF power amplifiers IC1, Q1 and Q2 through
buffer amplifier Q3. The RF signal from IC1 is fed to the antenna through a low pass filter.
The DC voltage correlative to the RF output is detected by D2 and Q9, amplified by Q24
and fed to IC1. The output power voltage from IC1 controls the RF power to keep RF
output at a constant level.
4.3.3 Receiver Circuit
The signal from the antenna passes through the single tuned band pass filter, and is
amplified by RF amplifier Q17, and is fed into a triple tuned band pass filter. The
signal is then mixed by Q19 (first mixer) and produces the first IF signal of 21.6 MHz.
This signal is sent to a crystal filter (21.6 MHz) and first IF amplifier Q20, mixed by
IC4, the second mixer, and becomes an audio signal after detection.
The output of the first IF amplifier Q20 is fed into IC4. IC4 contains the second mixer,
second local oscillator, 455 KHz amplifier, quadrature detector and DC switching
A 455 KHz ceramic filter is installed between pins 3 and 5 of IC4 to examine the
selectivity of this unit.
The detector output is separated into audio and noise components by an RC filter.
The noise component is fed back to the noise amplifier section of IC4. Its output is
rectified by a diode in IC4 and then fed to the switching amplifier in IC4.
The signal from IC4 is amplified by IC5 to drive the speaker while the receiver is in
the squelched condition. Muting control of IC5 is carried out by the CPU IC201.
Weather Alert Tone Detecting Circuit
If a weather alert tone is included in the AF signal from IC4 while receiving the
weather service broadcast, IC6 detects it and notifies an alert condition to CPU IC201.