Acoustic-to-electric transduction by gas discharges

 This will be a stream-of-consciousness post on my favorite technology right now: the use of gas discharges to transduce acoustic waves into electronic signals. This type of transduction takes advantage of the fact that the electrical resistivity between two electrodes is directly proportional to the gas pressure. The pressure perturbation of an acoustic wave will therefore cause a minute fluctuation of the discharge current if the voltage between the electrodes is held fix. The transduced signal can be extracted simply by high-pass filtering the discharge current. Amplification of the signal is non-trivial. Several efforts have been made to use this transduction mechanism to make a viable "plasma microphone", including patents by Tri-Ergon's Vogt, Engl and Massolle (filed in 1921) and by Westinghouse's Thomas (filed in 1922). The Thomas device used a conventional atmospheric discharge and was used for experimental AM broadcasts but was never commercialized. The Vogt device ("Das Kathodophon") used a thermionic cathode and did see some commercial use in Germany from the late 1920s to late 1930s. There has also been academic research, for example by Fransson et al. in the early 1970s, and more recently by Bequin. The major obstacle to technological viability seems to be that atmospheric discharges are electrically noisy and produce noxious gases (ozone and nitrous oxides).

Instead of revisiting the hard problem of developing a plasma microphone to transduce sound propagating through air, back in 2016 I decided to try to tackle the simpler problem of using a gas-discharge tube to transduce acoustic waves propagating in solids and fluids. Instead of the multiple kV needed for atmospheric discharges, gas-discharge tubes can be operated with voltages as low as 80 V. Their discharges are also much less noisy and obviously noxious gases are not produced by noble gases (neon and argon) confined inside a sealed glass tube! This technology is now becoming mature enough to use and there are patents pending (I'm the inventor, the US patent is assigned to Princeton University). This post is a quick update on the technological development level.

I have been focusing on technical applications, especially the sensing of ultrasound in the frequency range of 60 kHz to 1 MHz. Spontaneous acoustic emissions at these frequencies can be produced when stress energy is released during early stages of metal fatigue. Detecting these acoustic emissions can therefore be used to predict catastrophic structural failure of metal structures (steel bridges, drilling platforms, etc.). Piezoelectric acoustic sensors are used for this application, but they have very limited service life under harsh operating conditions (including high ambient heat and neutron radiation). For the frequency range of interest here, I have found that commodity neon glow lamps make excellent transducers! Here are photos of an NE-2 type neon glow lamp powered off (to the left) and powered on (to the right) with a characteristic glow surrounding the cathode:

Here's is a comparison of the performance of a similar gas-discharge tube (same 1/4" diameter as the NE-2, but two thirds the length) with a commercial piezoelectric sensor:

As can be seen, the plasma sensor has a much flatter frequency response (that is, its sensitivity depends much less on frequency) resulting in better signal quality. At room temperature the plasma sensor is six times less sensitive at 100 kHz (the primary resonance of the piezo sensor), but six times more sensitive at 600 kHz. In the frequency range of interest, the plasma sensor sensitivity is within a 6dB range and most of that variation is probably due to the ultrasound transmitter used for the testing. Interestingly enough, for this test the plasma sensor performed better at 150 degrees Celsius than at room temperature with a factor of 2.5 increase of sensitivity. Here's is the same plot on a log scale:

This is pretty decent performance for a $0.12 gas-discharge tube versus a ~$100 commercial piezo sensor!

There are high-temperature piezo sensors available for temperatures up to 200 degrees Celsius (but using piezoelectric materials with reduced sensitivity). Here are the results of a high-temperature test with an NE-2 as the transducer:

As can be seen, after some transients as the tube heated up, the plasma sensor functioned normally (its sensitivity remained unchanged) for 100 hours at an ambient temperature of 400 degrees Celsius. In this case the sensitivity did thus not increase at higher temperature, as it did in the previous test. The transducer functioned normally after cooling off to room temperature, but a day later it was unable to form a discharge at the normal operating voltage. Our hypothesis is that air slowly leaked into the tube during cooling off. We are unaware of other ultrasonic acoustic sensors that function at these temperatures.

The plasma sensor should be able to withstand significant neutron irradiation, but we have yet to test this hypothesis. We expect the service life to be somewhat reduced as sputtering of cathode material is accelerated by neutron embrittlement.

In summary, the plasma ultrasonic acoustic sensor is now a viable alternative to conventional, piezoelectric sensors, especially in harsh operating conditions, or if low signal distortion is  a priority.

Rules for sub-GHz RF transmitters in the US

After a couple of years of absence from this sort of work, I need microcontrollers to communicate by RF for a couple of projects. I had a box full of cheap 315 and 433 MHz RF modules, as well as some 915 MHz LoRa ones, but decided to spend some time searching for alternatives in the 216 to 960 MHz frequency range. I jotted down some notes and subsequent thoughts that I’ll share in the form of this blog post.

Use of RF transmitters in the US is regulated by the Code of Federal Regulations Volume 47 (Telecommunication), Part 15 (Radio Frequency Devices), Subparts A (General) and C (Intentional Radiators). Transmitters “shall be certificated”, unless they are “home-built devices” as defined by paragraph 15.23. When I buy an RF module I assume, probably naively, that the seller provides me with a device from a manufacturer that has received the necessary certification for it. Do note that if you upgrade the antenna, or make any other modifications, you are responsible for any resulting violations of FCC rules.

For home-built devices, not built from a kit, you don’t need certification as long as you built five or less and they are for personal use only. You still need to make a reasonable effort to ensure they follow FCC rules. But it’d be possible to build some Proof-of-Concept devices that operate away from the usual frequencies, and still be in compliance.

I made a plot of the maximum field strength that is allowed for continuous (per paragraph 15.209) and periodic (15.231e) transmissions. Except for in the 915 MHz ISM band, continuous transmissions are severely limited to a field strength of not more than 200 μV/m, measured 3 m from the antenna. For a typical whip antenna (a monopole without a ground plane) that translates into a radiated power of less than -50 dBm, or 10 nW! That seems impractically low. In the 470-806 MHz UHF band, continuous transmission is not allowed. For intermittent transmissions, FCC will increase your power budget (but limits the bandwidth to 0.25% of the center frequency). Specifically 15.231e allows periodic transmissions of no more than one second duration, separated by a silent period of 10 seconds or 30 times the transmission duration, whichever is longer. The field strength limit is 1.5 mV/m at 216 MHz and ramps up to 5 mV/m at 470 MHz. For a 315 MHz transmitter, periodic operation is then allowed at -30 dBm and at 433 MHz at -25 dBm. When manually operated as a remote control, and for other even more specialized applications, higher transmitter power is allowed. Paragraph 15.205 defines four restricted bands where no unlicensed transmissions are allowed. In the ISM band from 902-928 MHz both continuous and periodic transmissions can have a field strength of 50 mV/m (corresponding to an output power of about 0.5 mW).

I also plotted the frequency ranges populated by TV broadcasts and 5G, as well as dotted lines to mark the 315, 433, 868 and 915 MHz frequencies for which RF modules are widely available. So why the ubiquity of these frequencies? Well, 915 MHz is easy, since it’s an ISM band and allows much higher transmission power. Similarly, 433 MHz is an ISM band in Europe and Africa but there is nothing particular about it in the US. I’m guessing the day job of 315 MHz transmitters is as remotes for garage-door openers. From my perspective, the 868 MHz ones are the least common ones (I didn’t find a single one in my box of RF modules). I think it’s used for LoRa in Europe so it probably has some special status there. In the US it’s inside the 5G n26 band downlink!

Garage-door openers commonly used frequencies anywhere in the 300 to 400 MHz range until the mid 1990s. I have a couple of retired ones from “Whistler Auto-Mation Products” with adjustable frequencies. 390 MHz then seems to have become the canonical frequency. In 2004 DOD deployed new Land Mobile Radios that operate in the range 380 to 399.9 MHz and garage doors close to military bases promptly stopped working! After this debacle, 315 MHz seems to be the frequency of choice. This is an interesting lesson on the use of unlicensed (“Part 15”) devices, they should be able to handle interference from authorized spectrum users.

No dramatic changes in the last four to five years, as expected. The higher UHF TV channels have all been taken over by 5G. UHF TV used to go up to channel 83 (890 MHz), but now stops at channel 36 (608 MHz). The 868 MHz RF modules seem like a bad idea in the US, since they can interfere with cell phone signals. “Channel 13.5”, the frequency range of 216 to 470 MHz (so nicknamed because it’s between the VHF 13 and UHF 14 TV channels) seems to be the sweet spot for lightweight radio communication over tens of meters. I’ll probably end up comparing 315 and 433 MHz and use the one with least local noise and interference.