In the past, USA-Satcom has used full communication receivers (e.g. Icom IC-R8500) for Inmarsat operation, but for long term monitoring this was wasteful. This project is an L-Band downconverter that takes the entire Inmarsat passband 1525-1555 MHz and down converts this to a lower frequency (IF) that an can be used by an SDR
The downconverter is made of the following components (mostly available from mini-circuits):
- L-Band Bandpass Filter
- 2-port splitter (this allows the use of a communication receiver if needed)
- Chain of 3 L-Band amplifiers, in this case a total of about 90dB of gain
- Diode Mixer
- GPS referenced Local Oscillator feeding the Mixer
- IF amplifier and low-pass filter
- 4-port splitter (this allows the use of multiple SDRs)
The concept of a downconverter is fairly simply, but in practice it takes some empirical engineering for the design to be optimal. You can see various attenuators in different places, this is to optimized the RF and LO levels going into the mixer. The final design gives active passband signals in the range of -10 to 5dBm, which is the sweet spot for the particular SDR that is being used.
Here is a picture showing the layout and wiring.
Final assembled L-Band downconverter mounted in operating console.