3: Audio/Filter Board
PRC1099A-MS
3-7
The oscillator is keyed by pulling the limiter's inverting input down near
ground via Q19. The CW key is attached to the base of Q19 through the tone
key input at J3 pin 6.
Q19 conducts when the key is open. When the key is closed, correct bias is
restored and the oscillator starts smoothly from a zero amplitude to the level
where the 20% clipping occurs in the limiter. When the key is opened, the
oscillator stops within a few cycles as Q19 saturates. The keyed envelopes are
trapezoidal. When the transmitter gain is enough to reach ALC limits, the
transmitter reaches full output before the oscillator does. The attack and decay
times of the transmitter are less than those of the oscillator.
CW Hold Timer
and Gate
PNP transistor Q17 with associated components R57, R58, R59, D12, and
C91 form a hold timer that holds the transmitter in the transmit mode for a
short time after the key is released. Diode gates D13, D14, and D15 isolate the
following different functions; ATU tune, tone key, CW key.
3.1.5
Control Circuitry
T/R Switching
The transmit/receive (T/R) switching is controlled by the PTT line. Closing
the PTT line activates double-pole, high-reliability, fast-acting relay K13. One
pole of K13 switches the antenna from the receive signal to the transmit. The
other pole switches the reg8V line to transmit (T8) and receive (R8)
control lines. These two control lines provide for transmit/receive functions
throughout the receiver.
Contrast Control
Differential amplifier U3D and the associated circuitry make up a temperature
sensing contrast control for the Display board. Variable resistor R107 sets the
reference voltage at the non-inverting input of U3D. The voltage at the
inverting input is supplied through Q25 that is essentially connected as a
diode. As the temperature and the contrast of the display change, the
characteristics of Q25 change. Once inverted through U3D and applied to the
display, this contrast voltage provides a more constant display contrast.
Function Decoder
The serial data from the processor is clocked through shift register U7 into
U9, where it is decoded. This data controls several functions on the
Audio/Filter board listed below.
Output
State
Function
Q1 (pin 4)
Normal: low
Active: high
Sets the transceiver to 5W RF output by
turning on low power switch Q22 (refer to
“Low Power Switch” on page 3-6). Initiated by
front panel Power switch setting.
Q2 (pin 5)
Normal: high
Active: low
Activates squelch hold circuit U2C and U2D
(refer to “Squelch Circuit” on page 3-4).
Q3 (pin 6)
Normal: high
Active: low
Mutes transmit audio by grounding audio from
VOGAD through D8 to balanced modulator U6
