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Measure antenna ring down time for medical apps

18 Aug 2014  | Punithavathi Duraiswamy

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The ring down of the antennas is a considerable bottleneck in spectroscopic measurements involving relaxation time (T2) measurements. The ring down of the antenna determines the dead time of the spectrometer. To measure the relaxation time T2, the dead time of the spectrometer has to be greatly reduced. Examples of such spectrometers include nuclear magnetic resonance (NMR) and Electron Paramagnetic Resonance (EPR) spectrometers. The relaxation time (T2) measured is used to identify pathological tissues, like tumor or edema in the field of medical imaging.

A spectrometer consists of a RF source, a sample placed in a static magnetic field, and a detector. The RF pulse is applied to the sample placed in the static magnetic field and the response signal from the sample is captured in the receiver. In such systems, loop antennas are mostly employed to excite the sample and receive the response signal from the sample.

These antennas, even after switching off the RF pulse, takes some time to de energise. This is called the antenna ringing time. This energy should be quickly de-energized to measure the relaxation time T2 as they are very small – on the order of 0.1s to 1s in NMR, and 0.1µs to 1µs in EPR systems. In other words, the dead time of the spectrometer should be smaller than the relaxation time T2. This imposes difficulties in designing the receiver. To design the receiver, a knowledge on the antenna ringing time is essential. Here, a simple measurement idea is presented to measure the antenna ring down time. This is done with the help of a power splitter.

Figure 1: Measurement setup for antenna ring down time.

A power splitter is a passive device which accepts an input signal and delivers multiple output signals with specific phase and amplitude characteristics. The output signals theoretically possess equal amplitude, 0° phase relationship between any two output signals, and a high isolation between each output signal.

In a normal measurement setup that uses a power splitter, the outputs are taken to different components. Here, the antenna is the only component used, with a single port through which the input and the output has to be taken. For this purpose, theoretically a circulator will fit the measurement setup. A circulator is a passive non-reciprocal three- or four-port device, in which microwave or radio frequency power entering any port is transmitted to the next port in rotation (only). It is used to isolate the transmitter and receiver. But they are expensive. In this measurement, the pulse generator is the transmitter and the oscilloscope is the receiver. Instead of a circulator, a power splitter can also be used provided the instrument used for generating the pulses has an isolation circuit at the output port. The ports of the power splitter can be interchangeably used. The measurement setup is shown in figure 1. A pulse generator generates the RF pulses and it is applied to the loop antenna through port2. Once the pulse stops, the reflection from the antenna enters both port1 and port3. Port 1 is connected to the instrument and hence requires high isolation to prevent instrument damage. Port 3 is connected to the oscilloscope to measure the reflection from the antenna.

Figure 2: Measurement showing a ring down time of 350ns for a loop antenna.

To measure the ring down time, a loop antenna with a Q of 26.5 and a bandwidth of 10MHz is used. The antenna has a centre frequency of 265MHz and is matched to 50Ω. A three port power splitter (11667L (DC to 2GHz)) from Agilent Technologies is used for the antenna ring down time measurement. One of the power splitter outputs is connected to the loop antenna. The other output is connected to the oscilloscope. Figure 2 shows the ring down time of the loop antenna measured. An RF pulse of 100 ns width, with a power of 0 dBm, is applied to the loop antenna. The measured ring down time of the antenna is 350 ns.

About the author
Punithavathi Duraiswamy is with Nitte Meenakshi Institute of Technology in Bangalore, India.

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