3: Audio/Filter Board
3-2
PRC1099A-MS
3.1.1
Transmit Path
The transmit signal path begins at a microphone or headset connected to one
of the front panel audio connectors. It is routed through the front panel Audio
connector (50 OHM TX AUDIO) and then applied to VOGAD device U8 on
the Audio/Filter board.
VOGAD
The transmit audio signal is applied to U8C, a single, integrated circuit
VOGAD (voice operated gain adjusting device). The VOGAD is referred to as
a compandor because it expands or compresses the audio to maintain a
consistent output audio signal level applied to the balanced modulator
regardless of the input signal level.
Op amp U8A is the input signal amplifier for the compandor that provides a
set point for compandor U8C. The gain of U8A is set using resistor divider
R133 and R134. R134 and C153 form a low-pass filter for the U8A input to
filter low frequency noise. Resistor R132 sets the reference voltage at
approximately half of Vcc to prevent large audio signals from getting clipped
or distorted.
Compandor U8C expands low level audio signals and compresses high level
signals to maintain a consistent audio signal level regardless of the audio
input. Capacitor C148 sets the attack and release time of the compandor which
determines how fast the compandor reacts to changes in the audio signal.
Op amp U8D is the output signal amplifier that brings the audio signal to the
level required for the balanced modulator. The voltage divider network of
resistors R126 and R127 set the gain of output signal amplifier U8D.
Accessory
Connector Audio
Input
Attenuator and RF filter C31, L13, C32 and R46 attenuate and filter the high
level, 0 dBm audio input (600 OHM TX AUDIO) from the Accessory
connector before it is applied to pin 1 on balanced modulator U6.
Balanced
Modulator
Balanced modulator U6 mixes the audio signal with the beat frequency
oscillator (BFO) frequency of 1650 kHz (USB) or 1647 kHz (LSB) and then
suppresses the carrier frequency. The amplified and leveled audio output
signal from U8D is applied to pin 1 of U6. The BFO from the Synthesizer
board is applied to pin 10 of the balanced modulator. The output from U6 is a
low-distortion, 1650 or 1647 kHz, double-sideband signal with a suppressed
carrier.
Potentiometer R50 applies an adjustable offset current to pin 4 that balances
the modulator precisely to provide maximum carrier suppression.
In CW mode, the CW tone oscillator is applied to U6 through R72.
The supply voltages for the U6 and the VOGAD are supplied from the T8
line. In data mode (
OPT
), these components are supplied from the +8V line
and are not switched off in receive mode. This eliminates disturbances when
using high speed switching in the ARQ mode.
