VLF Radio Engineering
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This implies that electrically small, low-loss transmitters are also low bandwidth and therefore, by the Shannon—Hartley theorem, have limited data bitrates Direct antenna modulation DAM decouples bandwidth from Q t 21 , 22 , 23 , 24 , 25 , 26 , In one embodiment, the resonant frequency is actively shifted coincident with changes in the input drive frequency. DAM enables operation at a frequency outside the fractional bandwidth of the passive antenna.
In a frequency shift keying FSK modulation scheme, the carrier and hop frequencies each correspond to a different resonant frequency which changes at the FSK rate. Because an active transmitter is not a linear time-invariant system, the Bode-Fano limit does not constrain the bandwidth. As well as high bandwidth and efficiency, the radiated signal magnitude should be maximized.
The effective d , allowable stress, and Q m should be maximized.
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The lumped Q m of the resonator system includes mounting losses, external dampening, and internal losses in the piezoelectric material itself In this mode, thermoelastic dissipation and Akhiezer damping are low. The LN is supported only at two points near the longitudinal center and an input signal applied across the metalized end of the crystal and an adjacent coaxial toroid couples power into the resonator. The radially coupled fields excite the length-extensional mode. At resonance, the input impedance is primarily resistive and both the velocity at the end of the crystal and the directly proportional dipole moment are maximal.
Illustration of how a piezoelectric resonator can be used as a transmitter.
This manuscript highlights a conceptual demonstration of an active piezoelectric transmitter. Both experiments and simulations illustrate how the efficiency of the transmitter system can be increased while not constraining the bandwidth. In addition, a significant magnetic field is measured in the near field and it drops off consistent with an electric dipole. To demonstrate this concept, experiments are performed with a 9. The velocity measurement is non-intrusive to the resonator operation and can be used to calculate the dipole moment. Multiphysics simulations show a linear correlation of velocity with dipole moment near resonance.
We use this attribute to more easily characterize transmitter behavior in a controlled laboratory setting. Assuming this Q t , a multi-physics simulation 29 calculates the dipole moment, surface and near fields, and induced stress. For a peak dipole moment of The multi-physics simulation is compared to measured data see Fig. The simulation calculates both the input impedance magnitude and the peak output velocity which closely correspond to the measured values.
It is anticipated that this concept scales at least an order of magnitude below and above in carrier frequency. Measured ring-down waveforms with the modulation electrical relay closed. With the electrical relay open, the measured Q is about k. The experimental data is shifted down in frequency by The piezoelectric resonator is a harmonic oscillator with parameters such as stiffness, mass, and external capacitance determining the resonant frequency This dependence of resonant frequency on stray capacitance enables the DAM technique. A time-varying capacitance can modulate the resonant frequency outside the passive system bandwidth.
A discrete capacitor couples the modulation plate to an electrical relay which shorts and opens this capacitance to ground coincident with the change in the input signal frequency. The two input frequencies match the resonant frequency either with the relay open or closed.
The modulation mechanism must not spoil the Q. Efficient modulation promptly converts the energy resonating at one frequency to the second frequency. Velocity or electric dipole magnitude during modulation should be approximately the same as when the transmitter resonates at only one frequency.
If one tone is substantially higher or lower magnitude than the other during modulation, then the tuning of the input drive frequency to the two resonant frequencies is not matched. Also, the average input power to the crystal ideally is constant regardless of the modulation rate.
The input signal frequency is swept with the electrical relay first in the closed position, then in the open position see Fig. Differing relay losses result in a different Q k versus k for the two states. To achieve approximately constant amplitude during modulation, the higher-Q signal is driven slightly off resonance. Without DAM, the crystal slowly charges and discharges depending upon the drive frequency, while the amplitude with DAM is relatively constant. The passive fractional bandwidth of a system with a Q t of , is 2. Measured peak crystal velocity at two values of external capacitance the modulation relay open or closed.
Note that one of the resonant frequencies is intentionally detuned to minimize the effect of different Q on the amplitude of the output signal. Measured crystal velocity for three different frequency shift keying FSK rates using direct antenna modulation. The two frequencies are clearly distinguishable for all rates and the velocity magnitude decreases only slightly from about 0. Only at the lowest FSK rates are the differences between the relay open versus closed states distinguishable due to the different Q for each state. The upper limit for FSK rate for the present system is likely determined by the relay switching characteristics.
Ideally, the relay switching time should be much less than the FSK period. The slight increase in input impedance and decrease in peak velocity magnitude versus FSK rate is attributed to the increasing influence of switching losses. Effect of frequency shift keying FSK rate on input impedance and power. Measured time varying impedance a , b and input power c , d for without direct antenna modulation DAM a , c versus with DAM b , d.
Various FSK rates are shown. As the FSK rate increases, the input power goes to zero for the without DAM case as the crystal does not build up to full resonance at either frequency. The impedance slightly increases for the DAM case versus increasing FSK rate, and the input average power slightly decreases. The robustness of the modulation technique is apparent considering the long relay switching time.
One cycle of Therefore, the relay switching time is not well synchronized to the drive frequency and one or more RF cycles pass as the relay switches.
Another characteristic which simplifies implementation, is that the relay does not switch the full resonator voltage or current; only a portion of the full system energy is commutated by the modulation mechanism. Therefore, relatively low-voltage and slow mechanical relays can be effective for DAM.
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Electrical breakdown as well as crystal fracture strength limits can bound the attainable dipole moment with a piezoelectric transmitted. As such, crystal defects are minimized, surface electric fields are reduced through field shaping, and the chamber is filled with hexafluoroethane an electrically insulating gas. In addition, a portable system open to the ambient environment is used to measure the electric and magnetic field versus range see supplementary Fig.
Input lead lengths and effects from RFI are minimized. A fit to the electric field data gives the measured value of dipole moment, 7. Discrepancies between the measured and calculated dipole moments occur due to effects of nearby structures, deviations of the resonator from an ideal dipole, and drifts of the Q during operation. Measured E and B field versus range. The data points show each individual measurement and the lines are fits to the data. These results illustrate only one embodiment of this technique. Geometry optimization will result in a wider modulation frequency separation.
Different carrier frequencies are attainable by varying the length of the piezoelectric element. Further, higher mode excitement will enable operation at higher harmonic frequencies. Piezoelectric arraying is straightforward, particularly due to strong coupling and the ability to phase-lock. The piezoelectric material is not limited to LN and can be tailored to the application.
Varying operation in air, vacuum, or other background gasses can help balance between heat removal, high-field operation, and vibration damping. The LN crystals are grown with the standard Czochralski process. The congruent composition produces crystals with uniform composition and therefore minimal property variations see US patent 5,, The coatings are applied prior to finishing and polishing the crystal OD so masking is not required.
After coating, the LN is ground to rough shape using a rotating grit then a grit diamond sintered plate. Next, a lathe is used to grind with successively finer grit sizes using wet silicon carbide sand paper. Each metalized end of the LN rod has a 0. On one end, the thin wire attaches to the input signal. On the other end, the wire attaches to a field shaping toroid. Using this common high-voltage design technique, the toroids are at or near the potential of the LN rod corners and therefore spatially distribute the equipotential lines. This decreases the peak surface electric field on the LN for a given dipole moment, thereby increasing the achievable dipole moment prior to high voltage flashover.
The toroids on either end of the LN are mechanically supported by alumina posts and do not contact the LN rod. The LN is suspended by two fused silica rods located at the longitudinal center of the LN rod. The fused silica rods are supported by vertical alumina rods see supplementary Fig.
In laboratory tests, the input signal is supplied to the crystal via a Tektronics AFGC arbitrary function generator. Waveforms are post-processed in Matlab. For a given input voltage and frequency, the frequency domain response is used to calculate parameters such as peak electric fields, stress within the LN, dipole moment, velocity, and input impedance.
skCUBE very-low-frequency radio waves detector and whistlers - IEEE Conference Publication
To model time domain problems, an equivalent circuit model is used However, in essence we want to study peculiarities in all detected VLF spectrums regardless of type of lightning together with properties of Earth's ionosphere. The VLF detector consists of air-core loop antenna, trans-impedance amplifier and microcontroller capable of signal processing directly on-board. It works in two modes, slow and fast. Article :. DOI: Need Help? GPS signals are generally unable to penetrate water, building walls, or soil. This limits their uses, particularly for soldiers, mariners, and surveyors.
However, this new device could solve all those problems, allowing people to work safely even in inhospitable conditions. Scientists are studying low-frequency magnetic radio. Very low frequency VLF digitally modulated magnetic signals are capable of traveling through water, soil, and various building materials.
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Traditional electromagnetic communications signals, which operate at higher frequencies, are simply unable to achieve these levels. VLF technology has already proved useful for underwater submarine communications. Unfortunately, it lacks the data-carrying capacity for audio or video. One-way texts are the only real option.