Dual-comb spectroscopy has become a topic of growing interest in recent years due to the advantages it offers in terms of frequency resolution, accuracy, acquisition speed, and signal-to-noise ratio, with respect to other existing spectroscopic techniques. In addition, its characteristic of mapping the optical frequencies into radio-frequency ranges opens up the possibility of using non-demanding digitizers.
In this paper, we show that a low-cost software deﬁned radio platform can be used as a receiver to obtain such signals accurately using a dual-comb spectrometer based on gain-switched semiconductor lasers.
We compare its performance with that of a real-time digital oscilloscope, ﬁnding similar results for both digitizers. We measure an absorption line of a H13C14N cell and obtain that for an integration time of 1 s, the deviation obtained between the experimental data and the Voigt proﬁle ﬁtted to these data is around 0.97% using the low-cost digitizer while it is around 0.84% when using the high-end digitizer.
The use of both technologies, semiconductor lasers and low-cost software deﬁned radio platforms, can pave the way towards the development of cost-efﬁcient dual-comb spectrometers.
Sivan and his collaborators developed inexpensive 434 MHz band tracking tags for bats that emit radio pings every few seconds. These pings do not contain any location data, however the location is accurately tracked by several USRP SDRs with high accuracy GPSDO oscillators set up around the target tracking area. A radio direction finding technique known as "time difference of arrival" or TDoA is used to pinpoint the location of each tag. Sivan writes:
A wildlife tracking system called ATLAS, developed by Sivan Toledo from Tel-Aviv University in collaboration with Ran Nathan from the Hebrew university, enabled a science breakthrough reported in an article in Science that was published yesterday.
The system uses miniature tracking tags that transmit radio pings in the 434 MHz bands and SDR receivers (Ettus USRP N200 or B200). Software processes the samples from receivers to detect the pings and to estimate their time of arrival. The overall system is a "reverse-GPS" system, in the sense that the principles and math are similar to GPS, but the role of transmitters and receivers is reversed. A youtube video explains how the system works. SDR-RTL dongles can certainly detect the pings, but their oscillators are not stable enough to accurately localize the tags.
The system has been used to track 172 wild bats (in batches, some consisting of 60 simultaneously-tagged bats). The results showed that bats can make novel shortcuts, which indicates that they navigate using a cognitive map, like humans. The system, and other ATLAS systems in the Netherlands, England, Germany, and Israel are also tracking many different animals, mostly small birds and bats.
The video below shows the bats being tracked on a map accelerated to 100x.
The Science article itself is mostly about the discoveries on bat behaviour that were made by the system. However the YouTube video embedded below explains a bit more about how the technical radio side works.
A Technical Overview of the ATLAS Wildlife Tracking System
Over on YouTube the Ham Radio 2.0 has uploaded an interview with Scotty (WA2DFI) from TAPR who talks about a new software defined radio called the TangerineSDR that is expected to be released mid-2020.
The TangerineSDR will focus on scientific use cases such as the HamSci Personal Space Weather Station, ground satellite stations, academic research and RF sniffing. The goal is to have a modular SDR that can be produced cheaply for educational institutions, whilst having the capability to be upgraded to a high performance version for the space weather station.
The TangerineSDR is a Modular Software Defined Radio Project with the following objectives:
Development of SDR radios that allow experimentation in a variety of radio modes.
Provide support to unaffiliated other groups that need these radios to support their mission.
To provide hardware modularity so that the user can have a functioning radio with different subsets of the possible components.
To allow varying performance so that beginners can have a functioning radio with a minimum of parts, yet allow an expert user more functionality as needed.
To allow users to experiment with differing configurations of data collection, networking, transport and visualization.
In the Video Scotty shows off a mock-up of the TangerineSDR. The video description by Ham Radio 2.0 reads:
Presented at the TAPR Digitial Communications Conference of 2019, Scotty, WA2DFI, shows us a mock-up of a newly designed radio for Space Weather, and many other things, dubbed the Tangerine SDR. This modular radio is planning to be in production by mid-2020, with a working prototype to show at the 2020 Orlando Hamcation. Take a look at this short video and let me know what you think.
Tangerine Scientific SDR Space Weather Radio - FirstLook
In the experiment a laser is fiber optically coupled to an eletro-optic phase modulator, which modulates a 400 MHz FM signal onto the light. The light is then passed into a Carbon monoxide absorption cell with a photodiode used to take the spectroscopic measurements. The signal from the photodiode is passed into a LNA and then into the Airspy where the signal can then be processed on the PC.
The paper is very technical, but describes the setup, and how they characterized and calibrated the Airspy for their measurements. They conclude with the following:
A successful demonstration of a commercially available software defined radio as a lock-in amplifier was performed. For this purpose, the tuner front end and back end were characterized. The sensitivity and non-linearity of the receiver circuit was measured and analyzed. Acquisition of a CO spectral line was demonstrated using FM-spectroscopy with a repetition rate of 1 kHz. This proves the usability of an off-the-shelf SDR as a cheap but powerful lock-in amplifier by adding PLL driven frequency generators. The drawback of the arbitrary initial phase of the used phase locked loops can be either solved by software or hardware measures.