L Band satellite Modem design | Satcom Modem Block Diagram

This page describes L Band Satellite modem design with satcom modem block diagram. Both modulation and demodulation working of satellite modem at L Band frequency are described.

Let us understand step wise guide to design L band satellite modem. As this modem is used for satellite communication, it is also known as satcom 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. The RF transceiver should support L band Up converter and L band down converter in order to interface seamlessly with L band satellite modem.

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.

VSAT system

Transceiver used here will convert L band to C band/Ku band and vice versa in transmit/receive path. Refer VSAT overview for more information on how satellite modem interface in the VSAT system. Following satcom modem block diagram depicts modulator and demodulator parts of a satellite modem. In the VSAT system in which L-band modem is employed, RF Transceiver operates at L band. In this RF transceiver up converter converts L band to C band frequency and down converter converts C band to L band frequency. Here L band refers to frequencies in the range 950 to 1450 MHz where as C band refers to 5925 to 6425 MHz in the uplink and 3700 to 4200 MHz in the downlink direction.

L Band Satcom Modem Block Diagram

L band satellite modem Block Diagram
Figure-1: L Band Satellite modem Block diagram (Modulator part)

As shown in the modem block diagram, it 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 satellite Modem design

This satcom modem has been designed using discrete components as mentined in the modulator part of the satcom block diagram above. 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). There are satellite modem ICs available from various manufacturers which can house functionalities of multiple modulator and demodulator features. This will reduce the size as well as cost requirements. Now-a-days entire Satellite modem physical layer can be developed on DSP (Digital Signal Processor) which can further minimizes time and cost in satellite modem design and development.

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


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