Alc Circuit - SEA 222 Operator's Manual

5.4.14
5.4.15
Configuring the power amplifier in this fashion has the advantage
of providing for better system redundancy, better heat distribution for
the output devices and additionally provides a much wider range of
useable output power devices.
OUTPUT LOW PASS FILTER(s)
Four
1 ow
pass fi
1
ters are provided to cover the frequency range from
2 - 23 MHz. The highest frequency filter, which covers the 16 - 23 MHz
spectrum is in the circuit at all times. When lower frequencies are in
use, a lower frequency filter is placed in .series with the 16 - 23 MHz
filter. This provides much greater total VHF stopband rejection than
would the use of independent filters. Note that the high frequency
filter is a
5
pole ellipti.cal function design, while the lower fre-
quency filters are 7 pole elliptical function types. This is possible
because of the natural drop in spurious outputs from the power ampli-
fier at higher frequencies. Filter selection is through small power
relays which are operated by the control computer through serial relay
driver, Ul.
ALC CIRCUIT
The ALC circuit provides a DC level which is proportional to the
RF output at the antenna connector on the rear panel. This DC level is
used to control the gain of the two I.F. Amplifiers on the main board
in order to prevent transmitter overload and resultant nonlinearity.
An additional function of the ALC circuit is to provide a visual
"modulation" indication of the transceiver front panel. This helps the
operator determine that he is properly modulating the transceiver.
The voltage across RS is derived from the actual RF output voltage
through a voltage divider and diode detector CRl and CR2. The output
from the detector is applied to RS/C3 which has a time constant of
approximately 250 mSec. This fast attack/medium fast release voltage
waveform is buffered by emitter follower Ql, routed to the main board
through Jl and the interboard cable, and then applied through steering
diodes to the ALC control buss and the LED driver circuitry.
Thus, it can be seen that the main board gain control potentiometer,
RlOl, sets the average gain
11
floor
11
for the transmitter and the ALC
potentiometer adjusts the attack threshold for the ALC feedback
voltage. Under normal drive conditions, the instantaneous ALC feedback
voltage will not exceed the idling DC level-on the ALC buss. Under
high drive conditions, when the output attempts to exceed 150 watts,
the ALC feedback will override the
nc
level and reduce transmitter gain
to prevent distortion.
35
I