How the phone in your pocket is helping inspire the next generation of humanitarian demining equipment

Quadrupolar Resonance (QR) has long been recognised as one of the most promising methods for the detection of explosives [1]. Its potential stems from the fact that, unlike most other methods, QR can discriminate between different chemical compounds or even different structures of the same compound. Nitrogen (14N) is found in most commercial and improvised explosives which makes 14N QR a perfect method for use in detection devices.

Unfortunately, today’s commercially available equipment is composed of various bulky – and expensive – components, most notable being the QR spectrometer and the power amplifier. The spectrometers were in the recent years successfully made smaller and a lot cheaper by using FPGA chips, leaving the RF power amplifier as the key component which needs improvements to allow for building a truly portable system. The main drawback of the current, class AB power amplifiers is their power efficiency, which is in practice limited to below 50%. As a consequence, large heavy heatsinks must be used to provide for adequate heat dissipation while high power consumption makes the device unsuitable for battery-powered use.Quadrupolar Resonance (QR) has been long time ago identified as one of the most promising methods for the detection of explosives [1]. Its potential stems from the fact that, unlike most other methods, QR can discriminate between different chemical compounds or even different structures of the same compound. Nitrogen (14N) is found in most commercial and improvised explosives which makes 14N QR a perfect method for use in detection devices.

In the recent decades, class D amplifier technology has seen significant progress due to its widespread use in audio signal amplification. Compared to classical (A, B, or AB) topologies, class D amplifiers feature 100% theoretical power efficiency and can attain over 90% in practice. It is therefore no surprise to find class D chips from major semiconductor companies in portable music players and mobile phones. Unfortunately, the requirements for audio amplification differ significantly from the ones for NQR pulse amplification. Besides the different frequency range which is 0.02 – 20 kHz for audio and 0.1 – 5.5 MHz for 14N NQR, the key feature of audio amplifiers is exact reproduction of input waveforms whereas for NQR the main focus is on exact shaping and repeatability of pulses, e.g., fast rise and fall times, low pulse overshoot and tilt, long term amplitude and phase stability.

Taking the inspiration from the portable audio amplification technology, we have set out to build a drop-in replacement for the current power amplifier in our portable QR detector. There are several approaches to class D amplification, the most common being the full H-bridge (4 transistor switches) and half-bridge (2 transistor switches) topologies.

Usually, a technique called pulse width modulation is employed to drive the switches. However, for accurate signal reproduction, the reference frequency has to be much higher than the maximum frequency of the signal with factors 5-50 typically used in practice. For NQR pulse amplification, a simpler modulation scheme was proposed [2]. Since in standard pulse sequences, the amplitude of the transmitted pulses does not vary, the original signal can be approximated with just 2 square pulses per period. In this way, the operational frequency of the driving circuit does not need to be increased.

We have designed a prototype half-bridge stage and verified its behaviour with SPICE-based computer simulations. In the following months, the device will be manufactured and tested on printed circuit boards with the goal of achieving fully working class D amplification at frequencies up to 5MHz and power levels up to 300W. In the future, we will incorporate the class D amplifier into our portable QR detector. This will allow for higher pulse power levels while sustaining or even extending the autonomous operational time running on a smaller, more lightweight battery. 

The phone in your pocket.  Helping to inspire the next generation of humanitarian demining equipment.

References

1. A. N. Garroway, M. L. Buess, J. B. Miller, B. H. Suits, A. D. Hibbs, G. A. Barrall, R. Matthews and L. J. Burnett, Remote Sensing by Nuclear Quadrupole Resonance, IEEE Trans Geo. Remote Sens. 39, 1108–1118 (2001).

2. Mandal, S., Utsuzawa, S., Cory, D. G., Hürlimann, M., Poitzsch, M., Song, Y., An ultra-broadband low-frequency magnetic resonance system. Journal of Magnetic Resonance, 242, 113–125 (2014).