Comtech EF Data OM20 - Manual (Page 40)

OM20 Universal Outdoor Modem OM20 Theory of Operation
MN-OM20– Revision 5 4–2
4.1.1 OM20 L-Band Printed Circuit Card
The L-Band Printed Circuit Card consists of an analog modulation function, an analog complex
downconversion, and two wide-band digital synthesizers. The block diagram of the L-Band
Assembly is shown in Figure 4-2.
In the modulator, analog in-phase (I) and quadrature (Q) signals are generated on the Digital
Baseband Printed Circuit Card, routed to the L-Band/IF Printed Circuit Card, and modulated at
the desired frequency. The L-Band modulated signal is then passed through a microprocessor
controlled variable attenuator providing gain control of the output signal.
In the complex downconverter, the signal for demodulation is amplified and sent through a
variable wideband attenuator for AGC. The gain-controlled signal is then passed through a
complex downconverter to a low IF.
Quadrature
Demodulator
IF
B
oar
d
C
onn
ect
or
(
40
-P
in
H
eader
)
Demodulator I
Demodulator I Inv.
Demodulator Q
Demodulator Q Inv.
AGC
L-Band
Synthesizer
(950-1750)
PDA
Analog Q Inv.
Analog I Inv.
L-Band
Synthesizer
(950-1750)
Quadrature
Modulator
Analog Q
Analog I
Q AA
Filter
I AA
Filter
Switch
RxLB
TxLB
L-Band
C
oupl
er
DCSA
L-Band
Figure 4-2. L-Band Card Block Diagram