“Africa and remote sensing are not for sissies”

South African Synthetic Aperture Radar II

 

The credit for the design and construction of the FDU is given to Darren Coetzer, who completed a Masters of Science in Electrical Engineering from the University of Cape Town in 2004.

FDU description

The Frequency Distribution Unit (FDU) provides the local oscillators and clocking frequencies required by the radar system.

SASAR II has two intermediate frequency (IF) stages of 158 MHz, and 1.3 GHz respectively. It also has a final RF stage of 9.3 GHz. The multiple stages of the radar serves two purpose:

· to decrease the presence of image frequencies

· To provide multiple frequency modes of operation

The FDU provides the following frequencies

· 158 MHz: 1st IF LO

· 1132 MHz: 2nd IF LO

· 4000 MHz: frequency is doubled and amplified to produce RF LO of 8 GHz

· 150 MHz: Clocking frequency for Digital Pulse Generator (DPG)

· 210 MHz: Clocking frequency for Sampling Unit (SU)

All frequencies, except the 4 GHz, is obtained from a programmable synergy frequency synthesizer. The frequencies are amplified and filtered as required to provide the correct power level to drive the mixers.

A mixer is a 3 port passive device that accepts 2 frequency inputs at port 1 and 2, and at the output port 3 there will be the sum and differences of the two frequencies. We can select the frequencies we desire and discard the rest. The mixer essentially multiplies the two frequencies, translating them to an higher, or lower frequency.

Remember that the receiver stage of the radar system is essentially the reverse of the transmitter stage, so the same LO’s are used to preserve the coherence of the system.

Currently the production of the system frequencies is established through a windows based program, CodeLoader2. This is however inconvenient as it increases the number of redundant systems available. The Radar Controller Unit (RCU) has been set up to program the digital synthesizers.

The multiple stages of the SASAR II allow the radar to operation at 3 different frequencies:

· X band: 9.3 GHz (3cm)

· C band: 5.3 GHz (6cm)

· L band: 1.3 GHz (15cm)

An example of some of the mixers used in the project, courtesy of Minicircuits and Miteq