Digital Signal Processing Pipeline

Summary

Here is a diagram summarizing the DSP setup for the PAPER Polarimetry project:

NRAO Computers

As illustrated above, the Soyuz computer is connected to the Ettus Software Defined Radio (SDR); data from the Ettus is then sent to the computer Maia via a TCP block in GNU Radio (the server flowgraph which is on Soyuz will be attached here upon its completion). The two data streams from the two channels of the Ettus are interleaved in GNU Radio on Soyuz and then de-interleaved when they are received by the client TCP block in a complementary flowgraph on Maia, which handles further processing and storing the data.

Maia will be connected to Ethernet in the 45-foot trailer, so remote users can access it through SSH / VNC to retrieve the collected data.

Ettus SDR

As noted above, the Ettus is connected to Soyuz. Data from two of its ports are streamed in to GNU Radio. The Ettus has 4 SMA ports, but can only accept two separate data streams; this is because each pair of ports corresponds to the in-phase and quadrature pair from a single signal. This must be taken into account when considering DSP steps in GNU Radio and post-processing. (Port RF1 corresponds to the in-phase component of channel 1, RF2 corresponds to the quadrature component of channel 1, and RF3 and RF4 correspond to the I and Q components of channel 2, respectively).

GNU Radio

The GNU Radio DSP pipeline consists of two flowgraphs -- a TCP server flowgraph (on Soyuz), which simply interleaves the two data streams from the two separate channels and then sends it to the second flowgraph via a TCP Sink block. The second flowgraph (on Maia) receives data through a TCP client Source block; the data streams are then de-interleaved and further processing (correlation, etc.) steps will be performed to them.

Note: an external clock source (10 MHz) must be used in order to ensure that the phase of the two incoming data channels from the Ettus matches up. A uniform phase offset must also be applied, but the value of the offset does not vary with frequency.

Raspberry Pi

Maia will run the Raspberry Pi client program to receive data from the temperature sensor attached to the RPi. More info about the RPi system can be found here: RPiSensorSetup

-- EllieWhite - 2019-06-26