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Radio Detectors

March 28, 2014

German physicist, Heinrich Hertz (1857-1894), demonstrated the transmission and reception of radio waves in 1887. In his experiments, he used a spark gap as a generator of radio waves, and he detected the electromagnetic radiation when it induced sparks in another gap connected to an antenna. When Hertz turned the spark voltage supply on and off, it was a primitive form of modulation called on-off keying.

In the twentieth century, when the generation and reception of radio waves became more manageable, other modulation types were invented. With today's digital communication technology, there are too many modulation schemes to list in an article like this. Possibly the most important type to most people is quadrature amplitude modulation (QAM), used in wireless local area networks.

The US Federal Communications Commission once listed the basic modulation types shown in the table, where "A" signifies amplitude modulation, and "F" signifies frequency modulation. This was the list I needed to memorize when I took the exam for my commercial radiotelephone operator license in the 1960s.
TypeDescription
A0Unmodulated
A1On-Off Telegraphy
A2Amplitude Modulated Telegraphy
A3Telephony ("AM Radio")
A4Facsimile
A5Television
F0Unmodulated
F1Frequency Shift Telegraphy
F2Frequency Modulated Telegraphy
F3Telephony ("FM Radio")
F4Facsimile
F5Television
Before Edwin Armstrong invented frequency modulation in 1933, all modulated radio was amplitude modulated. Simple devices, such as the Branly coherer, were used as early detectors of radiotelegraphy (A1 in the above table), but they weren't capable of detecting speech signals (radiotelephony). For that, you needed a rectifier, and the first such rectifiers were the simple "cat's whisker" semiconductor diodes shown in the figure.

After a while, vacuum tube rectifiers became the radio detectors of choice, since they didn't need as much attention as a cat's whisker diode. Today, there's a wide variety of semiconductor diode types available, priced as low as a few cents per piece.

crystal detector and circuit

A crystal radio circuit (left) and a "cat's whisker" detector diode (right). Early hobbyist crystal radios were more likely to have a variable inductor than a variable capacitor. The inductor was usually copper wire wrapped on a round oatmeal container with a slider to connect to different turns. The diode detector D1 was generally a cat's whisker diode formed from a galena (lead sulfide) crystal making contact to a metal wire. This type of diode is known as a Schottky diode. (Left and right images via Wikimedia Commons.)


The diode detector is sufficient for most applications, but other radio detection tasks, such as those in radio astronomy and medical imaging (MRI), require better sensitivity. Scientists at the Niels Bohr Institute of the University of Copenhagen (Copenhagen, Denmark) have teamed with colleagues at the Technical University of Denmark (Kongens Lyngby, Denmark) and the Joint Quantum Institute, a joint operation of the National Institute of Standards and Technology and the University of Maryland (College Park, Maryland) to create an optomechanical radio detector.[2-5]

The detector is based on the fact that nanomechanical oscillators will couple strongly to microwave and optical fields.[2] The device is essentially an upconverter that translates radio frequencies to light frequencies, so a megahertz radio signal is converted to hundreds of terahertz.[5] This allows signal detection using quantum optical techniques used in quantum-limited signal detection.[2]

The usual way to reduce noise in a detector is to eliminate thermal noise by cooling to liquid helium temperatures. Such temperatures, although very low, still allow some thermal noise. Says Eugene Polzik, head of the Quantum Optics Research Center at the Niels Bohr Institute,
"We have developed a detector that does not need to be cooled down, but which can operate at room temperature and yet hardly has any thermal noise. The only noise that fundamentally remains is so-called quantum noise, which is the minimal fluctuations of the laser light itself."[4]
The device has an antenna that couples radio signals to an aluminum-coated 50 nanometer thickness membrane formed from silicon nitride. The membrane is one plate of a capacitor. Although the device is isolated in a vacuum chamber (see figure), it's not cooled. The thermal noise is reduced by the thinness of the membrane. A bias voltage of less than ten volts is enough to induce strong coupling between the radio-frequency voltage and the membrane's displacement.[2]

Nanomechanical resonator for radio detection in a vacuum chamber

Nanomechanical resonator for radio detection being placed in a vacuum chamber. Vacuum operation isolates the membrane, thereby allowing high-Q resonance.

(Photograph: Ola Jakup Joensen, Niels Bohr Institute.)


Optical coupling is made by reflecting a laser from the membrane surface.[2] The radio and light waves interact non-linearly with the mechanical resonance of the membrane.[2,4] The radio signals can then be detected as an optical phase shift with quantum-limited sensitivity.[2] The noise equivalent temperature is just 40 mK, and the detector sensitivity limit is 5 pV per square-root hertz.[2]

As Polzik summarizes,
"This membrane is an extremely good oscillator and that is why it is so ultrasensitive. At room temperature, it works as effectively as if it was cooled down to minus 271°C, and we are working to get it even closer to minus 273 degrees C, which is the absolute minimum. In addition, it is a huge advantage to use optical detection, as instead of using ordinary copper wires to transmit the signal, you can use fiber optic cables, where there is no energy loss."[4]

Artist's impression of the radio-light mixing at the membrane detector.

Artist's impression of the radio-light mixing at the membrane detector.

(Artist's impression by Mette Høst.)


References:

  1. Edwin H. Armstrong, "Radio signaling system," US Patent No. 1,941,066, December 26, 1933.
  2. T. Bagci, A. Simonsen, S. Schmid, L. G. Villanueva, E. Zeuthen, J. Appel, J. M. Taylor, A. Sørensen, K. Usami, A. Schliesser and E. S. Polzik, "Optical detection of radio waves through a nanomechanical transducer," Nature, vol. 507, no. 7490 (March 6, 2014), pp. 81-85.
  3. Mika A. Sillanpää and Pertti J. Hakonen, "Optomechanics: Hardware for a quantum network," Nature, vol. 507, no. 7490 (March 6, 2014), pp. 45-46.
  4. Ultra sensitive detection of radio waves with lasers, Niels Bohr Institute Press Release, March 5, 2014.
  5. Up-Converted Radio - The way to treat radio waves in a noisy environment is to turn them into visible light, Joint Quatum Institute Press Release, March 6, 2014

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