Turning the Pluto SDR into a Network Analyzer

Usually when we see a project using a software-defined radio (SDR), the SDR’s inputs and outputs are connected to antennae, but [FromConceptToCircuit]’s project connected an ADALM-Pluto SDR to an RF bridge and a few passive components to make a surprisingly effective network analyzer (part two of the video).

The network analyzer measures two properties of the circuit to which it is connected: return loss (S11) and insertion gain or loss (S21). To measure S21, the SDR feeds a series of tones to the device under test, and reads the device’s output from one of the SDR’s inputs. By comparing the amplitude of the input to the device’s output, a Python program can calculate S21 over the range of tested frequencies. To find S11, [FromConceptToCircuit] put an RF bridge in line with the device being tested and connected the bridge’s output to the SDR’s second input. This allowed the program to calculate the device’s impedance, and from that S11.
The RF bridge and other components introduce some inaccuracies to the measurements, so before making any other measurements, the system is calibrated with both a through connection and an open circuit in place of the tested device. The RF bridge’s directivity was the biggest limiting factor; transfer back from the bridge’s output line caused the reflection under load to exceed the reflection of an open circuit in some frequency ranges, at which point the analyzer couldn’t accurately operate.

[FromConceptToCircuit] was eventually able to make measurements throughout most of the 0.1-3 GHz range with a dynamic range of at least 10 dB, and expects a more directive RF bridge to give even better results. If you’d like to repeat the experiment, he’s made his Python program available on GitHub.

We’ve previously seen [FromConceptToCircuit] use the Pluto SDR to make a spectrum analyzer. We’ve also featured a guide to the Pluto, covered a project that improved its frequency stability, and seen it used to transmit video.