The first post describes the challenge of designing a low-cost aperture array. In the next post, an low-noise active antenna receiver was described. The next step is to down converter, digitize and process the data.
Down conversion and sampling
Below the ground plane of the receiver, the output of each receiver is connected to a rtl-sdr dongle. These dongles cost about $16 when bought in large quantities, have a bandwidth of 24 – 1766 MHz, a stable clocks (<1 PPM), can power the receiver through its input, and a IQ sample rate of 2.4 MHz.
The rtl-sdr dongles are connected with USB cables to an Odroid C2. The Odroid has 4 cores which is able to process the 4 channels in real time. It also has 2GHz of RAM and Gigabit Ethernet which is handy for data buffering and transport.
Power-over-ethernet is used to power the tile. A passive splitter and a cheap 12-24V to 5V power supply (LDNIO DL-C28) is used to power the Odroid. The Odroid then powers the rtl-sdr dongles through USB, and the rtl-sdr dongles power the receiver through it’s input.
Cost of receiver
The first prototype being developed will consist of 64 elements, grouped into 8 tiles, each having 8 receiver elements. The approximate costs is given in the table below:
|Approximate cost per element|
|Receiver (PCB & metal box)||$18|