Back in January of this year we posted about PhD student Lucas Riobó's work that about about using an RTL-SDR to create a low cost optical "high-speed real-time heterodyne interferometer". In that work he used an RTL-SDR as a data acquisition tool for an optoelectronic front end sensor (opto = visual light). This allowed him to translate optical data into an RF signal, which could be received by the RTL-SDR, and then easily processed in a PC.
In this work, a general architecture for the implementation of software-defined optoelectronic systems (SDOs) is described. This concept harnesses the flexibility of software-defined hardware (SDH) to implement optoelectronic systems which can be configured to adapt to multiple high speed optical engineering applications. As an application example, a software-defined optical interferometer (SDOI) using the LimeSDR platform is built. The system is tested by performing high speed optical detection of laser-induced photoacoustic signals in a concentrated dye solution. Using software modifications only, conventional single carrier and also multicarrier heterodyne techniques with space and frequency diversity are performed.
A main difference with the other article described in this post, is that we could also use the transmission path of the LimeSDR to perform many modulation waveforms of the electromagnetic fields which will interfere, to provide a noticeable performance improvement in single-shot interferometric measurements.
Thanks to PhD student Lucas Riobó of the University of Buenos Aires, Argentina for submitting his very interesting work on creating a "High-speed real-time heterodyne interferometer" with a low cost RTL-SDR dongle. This is a new application for the RTL-SDR that we have not yet seen.
Interferometers are tools that combine two separate electromagnetic waves (e.g. radio or light) and analyze the interference pattern created by their combination. One usage for example is creating a radio telescope interferometer using multiple small radio dishes. The result is that you can get the same resolution as a much larger dish without the cost of needing to build a huge dish. This has been done before with RTL-SDR's and Pulsar detection.
The paper and concept is fairly complex for someone without a background in optical science, but basically it seems that Lucas has created an optical interferometer that interfaces with an RTL-SDR dongle via a wideband optoelectronic front-end. This allows the optical data to be translated into an RF signal which can then easily be analysed with the low cost RTL-SDR. A system like this reduces costs and allows for much easier data acquisition and processing on the PC. He writes:
As you may know, optical Interferometry is a family of techniques in which the superposition of electromagnetic waves (in the optical range of the spectrum), cause the phenomenon of interference in order to extract information. In this work, we implement an optical heterodyne interferometer. This interferometer, the waves (laser beams) that superpose have a frequency shift f0 between them. When the beams interfere, the intensity from the combination of the beams (interferogram) is a sinusoid signal at a frequency f0 (i.e. a carrier signal). In this work, one of the beams reflects over a sample that has a mechanical deformation. Therefore, this information is encoded in the phase of the carrier signal.
We applied the RTL-SDR dongle to demodulate the carrier signal to extract the phase information. Instead of using an antenna, we put a photodiode with a transimpedance amplifier (TIA). Thus, since the signal obtained from the photodiode and the TIA is proportional to the interferogram, the phase/frequency recovery techniques are the same as those used in telecommunications systems (i.e. we can use many demodulation algorithms developed by the community).
Amateur radio astronomer Peter W East has recently uploaded a new document to his website. The document details how he built a quad RTL-SDR based receiver for his radio astronomy experiments in interferometry and wide-band pulsar detection (pdf – NOTE: Link Removed. Please see his website for a direct link to the pdf “Quad RTL Receiver for Pulsar Detection”. High traffic from this post and elsewhere has made the document go offline several times). Interferometry is a technique which uses multiple smaller radio dishes spaced some distance apart to essentially get the same resolution a much larger dish. Pulsars are rapidly rotating neutron stars which emit radio waves, and the strongest ones can be observed by amateur radio telescopes and a receiver like the RTL-SDR.
The Quad receiver has four RTL-SDR’s all driven by a single TCXO, mounted inside an aluminum case with fans for air cooling. He also uses a 74HC04 hex inverter to act as a buffer for the 0.5 PPM TCXO that he uses. This ensures that the TCXO signal is strong enough to drive all four RTL-SDRs.
Whilst all the clocks are all synced to a single master clock, synchronisation between the RTL-SDR’s is still difficult to achieve because of jitter introduced by the operating system. To solve this he introduces a noise source and a switch. By switching the noise source on and off, correlation of the signal data can be achieved in post processing.
In the document Peter shows in detail how the system is constructed, and how it all works, as well as showing some interferometry results. The system uses custom software that he developed and this is all explained in the document as well.