Icom IC-505 Operation Manual page 16

SECTION МІ CIRCUIT OPERATION |
The ІС-505 transceiver, designed to be used for 50 MHz SSB, CW, and optional FM
applications, employs a circuit with a 13.99 MHz first IF of single conversion superheterodyne
(and double conversion superheterodyne with 455 kHz second IF for FM). It basically consists
of five units: MAIN (A), MAIN (B), PLL, LOGIC, and front-panel EF units. These units are
discussed in detail in the following sections.
1. MAIN (A) Unit | (
Consists of the antenna switching circuit and receiving circuitries, such as RF amplifier, IF
amplifier, and product detector circuits, as well as transmitting circuits such as balanced .
modulator, IF amplifier, and power amplifier circuits.
An incoming signal from the antenna is fed to the antenna switching circuit through the
low-pass filter, then to the RF ampiiner which is a dual-gate MOS РЕТ (3SK74) featuring а
low noise figure.
The amplified signal from the RF amplifier is then sant via the band-pass filter to the mixer
circuit, with a local oscillator frequency signal from the PLL circuit, and they are mixed to
get a 13.99 MHz first IF signal. Note that the tracking control signal from the PLL unit is -
always applied to tune the band-pass filter for operating frequency. | |
The 13.99 MHz first IF signal is then sent to the small-size, high-performance crystal filter
to provide the required pass-band response for SSB operation. It is then sent to and
amplified at dual-gate MOS FET's (38K74's) and ІС ВА401. Here, the signal is amplified to
the appropriate level before being sent to the product detector IC (uPC1037H) at the next
stage. At the product detector, the input signal and the signal from BFO circuit are mixed
and converted into an AF signal, which is sent to the AF amplifier circuit of the MAIN (B)
unit.
A part of the signal of IF amplifier is passed through the AGC detector and is further
applied as an AGC voltage to both the RF and IF amplifier circuits. Similarly, a partial
signal from the mixer circuit is passed through noise amplifier and noise detector circuits,
and is used to control the noise gate (Noise Blanker) circuit. This prevents the noise signal
from going to the next stage (especially when the noise blanker is on).
In the transmit mode, a voice signal being amplified by the mic amplifier circuit of the
MAIN (B) unit is applied to the balanced-modulator IC (uPC1037H), which is also used as
the product detector in the receive mode. This IC mixes BFO signals and AF signals to
generate DSB signals. The DSB signal is further buffered, amplified, and is passed through
the crystal filter (which is also used in the receive mode) where it is converted into a 13.99
MHz SSB signal.
S METER
DC
MAIN (8)
ANT — UNIT
TOW POWER
IW POWER DRIVE
AMP AMP.
to
from PLL`