Comtech EF Data SDM-309B - Manual (Page 125)

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