Satellite transponder with equalising level limiting adaptor
Created 01 August 2005
This STELLA transponder was first presented and demonstrated at the 20th AMSAT-UK Colloquium on 30 July 2005 at the University of Surrey, Guildford UK.
Link to the zipped paper (13.5Mb)
Traditionally multiple access linear transponders within the amateur satellite service have used a single automatic gain control (AGC) across their entire passband. Single AGCs can often limit transponder access due to interference from other services or high power uplinks of some users. Using DSP technology, it is now possible to provide automatic multiple adjustable notch filtering, facilitating equal access opportunities for all transponder users.
The paper describes the implementation of the IF stages of a linear transponder. For practical demonstration purposes, it uses the RF downconverter and upconverter transponder hardware designed and built by David Bowman G0MRF, described here.
The technologies used are based heavily on the SDR-1000 software defined radio designed by Gerald Youngblood http://www.flex-radio.com.
Set up in the traditional manner on the infamous G6LVB kitchen counter. This demonstration version uses a PC and USB soundcard as an alternative to the embedded hardware to demonstrate the CODEC and DSP functions with visible spectrum.
Description and layout of the hardware. The DSP IF is connected to the conventional transponder IF only for its filtered input stages: the conventional AGC itself is not used when the DSP IF is used.
Block diagram of the STELLA embedded hardware. The Audio CODEC used (Cirrus Logic CS4272) will support a passband of up to 180kHz. The low power DSP used (a 150MFLOP TMS320VC33) will perform a 2048 bin FFT in 500us and drawing only 200mW of power.
Close up of the Quadrature Oscillator, Quadrature Sampling Detector and Exciter
Overview of the STELLA software functionality
Demonstration of how the STELLA software automatically notches strong signals
Mail Howard, G6LVB