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Design of Satellite Modem

This tutorial on VSAT covers following.

This page describes design of satellite modem which describes satellite modem block diagram and design tips with L band 950-1450MHz modem example. It mention various discrete components used in the design which include Micro-Controller module, Direct Digital Synthesizer(DDS), Encoder,decoder,Modulator-Demodulator,variable attenuator,MMIC amplifier,Oscillator,synthesizer and more. This modem is used in satellite earth station which is interfaced with L band Up converter and L band downconverter.

Let us understand design with example of L band satellite modem and demonstrate step by step approach to design this satellite modem. The frequency band for this modem is 950-1450 MHz. The satellite modem is used in earth earth station or VSAT station which provides voice/data connectivity to remote areas via satellite network. satellite modem connects with Multiplexer/demultiplexer on one side and with RF Transceiver on the other side.

Multiplexer multiplexes many small bandwidth voice channels and data channels and combines the same to produce high bandwidth baseband channel which is given as input to the satellite modulator part, which modulates the same on some carrier with modulation type choosen. Similarly Demultiplexer demultiplexes high bandwidth baseband channel from the satellite demodulator part to small low bandwidth channels which is connected to telephone/computers.

Transceiver used here will convert L band to C band/Ku band and vice versa in transmit/receive path. Following diagram depicts basic block schematic of the satellite modem for our design.

L band satellite modem design

As shown in the figure modem consists of two cards one M&C Card and another modulator/demodulator card. M&C Card houses Micro-Controller (DS80c320), EPROM, NVRAM and other discrete components. Modulator/demodulator is a multi-layer mixed signal board which houses Direct Digital Synthesizer (AD9851), STEL2030C (Encoder-Decoder), RF2480/RF2422 Modulator, AD558 (D to A Converter), SPDT switch(AS164-80 from Alpha industries) , 20dB Coupler (using Micro-strip), FS-PLL-S-A02 Synthesizer, ALPHA variable attenuator (in 1 dB and 0.5dB steps) , reference Oscillator card, MMIC amplifier(MSA1104) and other SMD components. M&C Card monitor and control various parameters of satellite modem's Modulator and demodulator portion. It has RS485 on board for remote operation. Software is embedded in EPROM on M&C Card.

L band filter has been designed using L and C components which is inserted after modem IC. This filter will take care of removing carriers other than desired ones. Attenuators will take care of changing power level both in modulator and demodulator part. Crystal oscillator will take care of providing reference clock as well as reference to synthesizer. Synthesizer will select frequency in L band with step size as designed. At various levels AND gate is used for providing ON/OFF functionality.

Following parameters are designed to monitor and control on this satellite modem.

Modulator Part:
1. Transmit Frequency
2. Power Level
3. Modulator ON/OFF
4. Modulation type selection BPSK/QPSK
5. Data rate
6. Differential Encoder ON/OFF
7. Scrambler ON/OFF
8. Modulator data (inverted/Normal)
9. Modulator clock source and phase
10.Modulator alarms
11.Modulator pure carrier mode

Demodulator Part:
1. Receive Frequency
2. Demodulator input power level
3. Demodulation type
4. Data rate
5. Data type inverted/Normal
6. Differential Decoder ON/OFF
7. Descrambler ON/OFF
8. Demod clock source and phase
9. Demod Eb/N0
10.Demod BER
11.Demod Alarms

As mentioned above M & C card based on Micro-controller circuitry will control and monitor above parameters. The other part consists of discrete board(using digital ICs) and RF board(based on microstrip design concept). We recommend buying RF modules rather than designing rf circuit of your own to cut down design time as well as avoid unexpected design problems between digital and RF parts.

Modulation types

BPSK-This page describes BPSK modulation technique with equation and constellation diagram.
QPSK -This page describes QPSK modulation technique with equation and constellation diagram.
QAM-This page describes QAM modulation technique with equation and constellation diagram.
MSK-GMSK MSK modulation,GMSK modulation and GMSK demodulation.
8PSK 8-PSK modulation or multilevel PSK or phase shift keying modulation technique.
BPSK vs QPSK -Difference Between BPSK and QPSK modulation techniques.
QPSK vs OQPSK vs pi/4QPSK-Difference between QPSK,OQPSK and pi/4QPSK modulation techniques

RELATED LINKS

stripline basics and types
Microstrip line variants
Slotline basics and types
Finline basics
CPW(Coplanar Waveguide) basics
RF converter design
C band RF Transceiver basics
RF filter design
RF vector Signal generation/analysis
Microstrip line basics

               

RF and Wireless Terminologies


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