VSAT Modem basics

This page covers VSAT modem basics, VSAT modem testing and VSAT modem vendors.

The modem used in VSAT application is referred as VSAT modem. It is also known as satellite modem. There are different bands of VSAT modem available based on IF frequencies of use in its construction. Typically VSAT modems are available in 70+/-18 MHz or 140+/-18 MHz or L band (950 to 1450 MHz) range.

Following diagram depicts modules used in the design of L Band Satellite Modem:
• Baseband part which consists of FEC modules (Convolutional encoder & decoder), differential encoder & decoder, scrambler & descrambler etc.
• Modulator & demodulator which takes care of modulation and demodulation respectively.
• Modulator module takes information as input and produces modulated carrier as output at IF frequency.
• Demodulator module takes modulated carrier as input and produces information (voice/data) as output.

VSAT modem L Band

Synthesize is required in VSAT modem to select different RF carriers as required. Variable RF attenuators are used to provide different attenuation or gain values for testing as well as maintenance of VSAT network. Switches are used for various baseband loop back as well as IF loop back testing purpose in case of faulty conditions of VSAT modem. Usually VSAT modem is interfaced with Multiplexer/Demultiplexer using RS422/V.35 etc. interfaces.

VSAT modem testing

VSAT modem Loop Back Tests

Following tests are conducted during maintenance and installation phase of VSAT modem:
• Baseband Loopback: It tests baseband part of the VSAT modem.
• IF loopback: It tests baseband as well as modulator-demodulator IC of this modem.
• RF loopback: It tests modem including RF part of the VSAT terminal.
• Bit Error Rate testing for short duration and long duration.
• Voice call flow and data flow through VSAT modem
• Functional tests under various environment tests (temperature, vibration etc.)

VSAT modem vendors or manufacturers

Following table mentions VSAT modem vendors or manufacturers:

VSAT Modem Vendor or manufacturer
Comtech EF Data
Radyne Comstream, Inc.


Refer following sub topics related to VSAT technology:
Article on VSAT   VSAT NMS protocol  RF Terminology   Satellite Terminology  Access types  SCPC  MCPC  RF Transceiver  C band RFT  Microstrip  RF upconverter design   Power Amplifier  what is modulation  satellite Modem  Link Budget