BENISON V5
- 5 -
CIRCUIT DESCRIPTION
1) Receiver System
The receiver system is a double superheterodyne system with a 21.7MHz first IF and a 450kHz second IF.
1. Front End
The received signal at any frequency in the 245.0000- to 245.9875-MHz range is passed through the
low-pass filter (L29, L28, L14, L13,C224, C220, C223,C219 and C222) and tuning circuit (L12 and D27),
and amplified by the RF amplifier (Q29). The signal from Q29 is then passed through the tuning circuit (L9
and L11 varicaps D25 and D26) and converted into 21.7MHz by the mixer (Q28).The tuning circuit, which
consists of L12, L9, varicaps D27 and D26, L11, varicaps D25 , is controlled by the tracking voltage form
the CPU so that it is optimized for the reception frequency. The local signal from the VCO is passed through
the buffer (Q24), and supplied to the source of the mixer (Q28). The radio uses the lower side of the
superheterodyne system.
2. IF Circuit
The mixer mixes the received signal with the local signal to obtain the sum of and difference between them.
The crystal filter (FL3, FL2) selects 21.7MHz frequency from the results and eliminates the signals of the
unwanted frequencies. The first IF amplifier (Q17) then amplifies the signal of the selected frequency.
3. Demodulator Circuit
After the signal is amplified by the first IF amplifier (Q17), it is input to pin 16 of the demodulator IC (IC14).
The second local signal of 21.25MHz (shared with PLL IC reference oscillation), which is oscillated by the
internal oscillation circuit in IC13 and crystal (X4), is input through pin 2 of IC14. Then, these two signals
are mixed by the internal mixer in IC14 and the result is converted into the second IF signal with a
frequency of 450kHz. The second IF signal is output from pin 3 of IC14 to the ceramic filter (FL1), where the
unwanted frequency band of that signal is eliminated, and the resulting signal is sent back to the IC14
through pins 5.The second IF signal input via pin 5 is demodulated by the internal limiter amplifier and
quadrature detection circuit in IC14, and output as an audio signal through pin 9.
4. Audio Circuit
AF signals from the FM IF IC (IC14, pin 9) are applied to the mute switch (IC4, pin 1) via the AF filter circuit
(IC14b, pins 6,7). The output signals from pin 11 are applied to the AF power amplifier (IC9, pin 4) after
being passed through the [VOL] control (W1).The applied AF signals are amplified at the AF power
amplifier circuit (IC9, pin 4) to obtain the specified audio level. The amplified AF signals, output from pin 10,
are applied to the internal speaker (SP1) as the “SP”signal via the [SP] jack when no plug is connected
to the jack.
5. Squelch Circuit
A squelch circuit cuts out AF signals when no RF signals are received. By detecting noise components in
the AF signals, the squelch switches the AF mute switch. A portion of the AF signals from the FM IF IC
(IC14, pin 9) are applied to the active filter section (IC14, pin 8) where noise components are amplified and
detected with an internal noise detector. The active filter section amplifies noise components. The filtered
signals are rectified at the noise detector section and converted into “NOIS”(pulse type) signals at the
noise comparator section. The “NOIS”signal is applied to the CPU(IC6, pin 14). The CPU detects the
receiving signal strength from the number of the pulses, and outputs an “AFB”signal from pin 33. This
signal controls the mute switch (IC4) to cut the AF signal line.
2) Transmitter System
1. MICROPHONE AMPLIFIER CIRCUIT
The microphone amplifier circuit amplifies audio signals with +6 dB/octave pre-emphasis characteristics
from the microphone to a level needed for the modulation circuit. The AF signals from the microphone are
applied to the microphone amplifier circuit (IC1C, pin 10). The amplified AF signals are passed through the
low-pass filter circuit (IC1D, pins 13, 14) via the mute switch (IC4, pins 4, 3). The filtered AF signals are
