VOXSAT
By Gustavo Carpignano
MISSION REVIEW
The VOXSAT satellite is aimed at educational and research problems. Signals from the satellite are to be received as short audio messages on Earth by VHF-FM raceivers. The satellite will also include an FM crossband Repeater, with an uplink in the 70 cm amateur radio band and a downlink in 2 meter amateur radio band.
1. Voxsat upper view without cover
The satellite is to be put in orbit inside
MODULE M, a test Russian satellite. Its active existence depends of the power supply resources available. The satellite is being developed by AMSAT ARGENTINA with the help of AMSAT RUSSIA.
2. Module M with Voxsat
VOXSAT consists of a common modular structure that has the purpose of containing the different functional block. The structure is made of thin aluminum that makes it strong and light and provides effective shielding of the RF signals. All the frames are similar.having a rectangular form of 23 cm x 18 cm x 2.5 cm with a bottom and upper cover and a hole for a DB25 connector. The different Aluminum parts were painted to prevent corrosion during the manipulation and transport. Each VOXSAT module has a unique external communication port via a DB25 connector.
All modules are connected via a common bus. This simplifies the removal of any module for test or change.The structure has three modules :
The command section consists of one little controller unit. An PIC microcontroller receives and interprets the ground commands. Since the spacecraft control and environment is supplied by the principal mission, the need for telemetry is very small so not telemetry will be transmitted about the operational parameters.
For Record and Play audio, we are using a new generation of chips designed by ISD Inc. Each chip has the capability of loading audio signals for one minute inside a EEPROM of 500 kb and retaining the information for ten years.The command and analog block section was made on three small boards and stacked in one of the standard frames.
POWER SUPPLY
All the elements within the payload such as the on board computer and other systems require regulated voltages for their proper operation. To meet these needs, the satellite contains a switching regulator that converts the voltage supplied by the main mission.The switching regulator converts the +28 V of the Russian satellite into the payload needs of +10 V and +5 V. The +10 V is supplied to the rf parts.
The cunent required is nearly 1 Ampere. The + 5 V regulator uses a 7805. The + 5 V supplies a well regulated voltage to all the digital systems. Since all digital components are CMOS. very little current is required from this regulator. The switching regulator was made on a small printed circuit board and assembled in one of the standard frames of the satellite.
The VOXSAT has two high efficiency transmitters for the 2 meter band. The power output is near 4 W. The final transmitter effinency achieved was better than 50 % while maintaining good harmonic suppression and overall frequency stability.
The transmitters consist of a pair of Exciters and RF Amplifiers.The exciters provide about 5 mW of rf drive at the output frequency to the rf amp. An MRF652 VHF power transistor is used for the amplifier. It was selected for its high operating efficiency.
One transminer is used as a beacon for analog broadcast downlink. The other is used as the primary repeater downlink.
Each exciter and rf amp was made on a double side printed circuit board. Each pair of exciters and rf amps were assembled on a pair of standard frames.The beacon transmitter frequency will be on 145.995 MHz and the downlink repeater and audio broadcast frequency is 145.810 MHz.
RECEIVERS
The FM receivers for 432 MHz are used for control and repeater uplink. The heart of these receivers is the MC3359 chip. Each receiver has a GaAsFET pre-amplifìer and helical biters. Each receiver is made on double sided printed circuit board and staked on a standard frame. The uplink repeater frequency will be 435.990 MHz.
ACKNOWLEDGMENT
We are thankful to all AMSAT members around the world that in one way or another have helped. We especially thanks Leonid Labutin, UA3CR, (SK in 1998), Eugeni Labutin. RA3APR , and Sergej Samburov RV3DR , for their great patience with our group.