SG-2000 MANUAL
133
The SGC Building, 13737 S.E. 26th St. Bellevue, WA. 98005 USA
©1995, SGC, Inc.
TEL: (206) 746-6310 FAX: (206) 746-6384
21.16
Output Low Pass Filters
(Schematic J30100500 2/2)
One of six low pass filters located on the LPA assembly is selected by the
MicroProcessor program. These filters eliminate harmonic distortion products
from the linear power amplifier output signal.
The following table lists the upper cut-off frequencies of the six bandpass filters
selected in the LPA.
2.9MHz
4.6MHz
9.0MHz
15.6MHz
20.7MHz
30.0MHz
21.17
Automatic Level Control
(Schematic J30100500) 2/2
Following the low pass filter, the forward power is sampled by a peak detector
consisting mainly of the transformer T14 and the diode D18. A DC signal
proportional to the forward power appears at the node of R71/C103; the reverse
power appears at R69/C99. The forward DC representative is then amplified by
IC2A and given a release time constant defined by C68 and R79. This is then fed
to a summing device used to control the level going to the AGC. The AGC in
turn controls the audio level going to the modulator (
IC202)
to maintain output
power level of 150 watts.
21.18
Modulation Characteristics, Audio Frequency Response
In order to test the modulation characteristics, a single tone, variable frequency
signal was applied to the microphone input of the transceiver. The ALC circuitry
was disabled. The RF output of the transmitter as a function of the audio
frequency was measured and plotted. Results are shown graphically below.
Audio frequency was varied over the range of 100 Hz to 5000 Hz.