Chapter 5. Theory Of Operation; Modulator; General Description; Specifications - Comtech EF Data SDM-309B Installation And Operation Manual

5.1 Modulator

5.1.1 General description

The modem Modulator card is a 10.25" x 14" card (Figure 5-1) that fits in the top left
slot of the modem chassis. Its function is to create a QPSK modulated carrier within the
50 - 90 MHz or 100 - 180 MHz range from the digital data stream that is provided by the
Interface card. Several subSections make up the card. These include:
1) The Digital Interface
2) The Scrambler/Differential Encoder
3) The Convolutional Encoder
4) The I/Q Nyquist Filters
5) The Modulator
6) The Output Amplifier
7) The RF Synthesizer
8) The Clock Synthesizer
A block diagram of the modulator is shown in Figure 5-2. A detailed description of the
subSections appears in Section 5.1.3.

5.1.2 Specifications

Modulation type
Frequency Range
Frequency Select Method
Frequency Step Size
Frequency Stability
Channel Spacing
Phase Error
Filtering Type
Spectral Occupancy
MN/U-SDM309B Rev. #
THEORY OF OPERATION
Chapter 5.
QPSK (BPSK optional)
50 to 90 MHz or 100 to 180 MHz
Synthesized
2.5 KHz
10 ppm
.7 Times the Data Rate
Divided by the Encoding Rate.
2.5 ° max.
Nyquist, Pre-equalized
See Figure 5-3
dB at ± .75 Symbol Rate
5-5–1