[Direct digital SDR receiver for satellites]
Direct digital SDR receiver principle
As I’ve introduced it in an earlier post, the DRU-244A phase coherent SDR receiver digitizer card has no significant input pre-selection. Thus, it can implement direct digital SDR reception. The bandwidth of the input network makes it possible to digitize the signals in the upper Nyquist bands (which is referred to as under sampling). The platform may be used as a Direct Digital Radio (DDR) receiver for the VHF and UHF bands. We just need to add some input gain to compensate for the slope in input sensitivity at higher frequencies. The SRM-3000 Software-Defined Radio (SDR) application is prepared for this type of operation, and can tune to the equivalent frequency in the baseband.
Preparation of the VHF/UHF direct digital SDR receiver
I’ve already made some sensitivity tests in the 70 cm HAM radio band. See earlier post: Direct-digital-uhf-sdr-radio-receiver-dru-244. I’ve waited for an opportunity to test it on a real target, which ended up being the MASAT-1 – the first Hungarian cube sat. Yesterday, I had a chance to visit the ground control station of the university, where the folks have a tracking antenna with 20 dB gain. I’ve connect my direct digital SDR receiver to the split antenna signal.
To prepare I’ve tested two input pre-selector filters around the 437.345 MHz downlink frequency. One was a Mini-Circuits HPF-LPF cascade, the other was a ceramic filter for the 433 MHz ISM band for radio remote controllers. Both showed 1.5 dB insertion loss, which seems be acceptable; there is no significant input noise figure reduction, and hence significant loss of sensitivity.
I’ve also prepared a Mini-Circuits connectorized block LNA with 20 dB gain and <1 dB noise figure. This seemed to sufficiently improve sensitivity, and thus provided reception capability for direct digital SDR receiver.
Visiting the satellite control ground station
I’ve checked the satellite tracking information on-line, and showed up at the station at the right time to set up the rig. The station operator told me that they had a high-selectivity coax resonator filter installed before their 20 dB low noise preamp, so my pre-selector filter proved unnecessary. We had set up a computer display with incoming packets from some other stations, which helped us checking the reception in the area. We had nothing else to do, so we just waited for the satellite signal to appear on the display. I’ve utilized the +/- 12.5 kHz bandwidth to cover the doppler shift.
Receiving the satellite with the SDR receiver
At the predicted time, we’ve observed the first signals at the high side of the display. The observed doppler shift was more than +10 kHz.
Later, as the satellite got closer to us, the doppler shift got smaller. I’ve slowed down the waterfall display; this way we could see the doppler shift during the whole transmission.
On the last picture, we can see the uplink command packet right below the zero frequency. Our receiver seems to have been tuned a couple kHz below the exact frequency. However, the DRU-244A SDR receiver platform has an external 10 MHz reference input, so next time a GPS clock reference can be employed to keep the frequencies more accurate.Share Quadrus SDR