Read Development of Impedance Matching Transformers (IMT) for EMAT Applications text version

392

Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation

NDE 2009, December 10-12, 2009

Development of Impedance Matching Transformers (IMT) for EMAT Applications

A. Vinay Kumara, R. Dhayalana, K.Aruna, Krishnan Balasubramaniama, C.V. Krishnamurthya and T. Naganjaneyulub

a b

Centre for Nondestructive Evaluation, Department of Mechanical Engineering,Indian Institute of Technology Madras, Chennai-36. National Institute of Technology Tiruchirapalli-15

ABSTRACT

Impedance Matching Transformer (IMT) is presented as visible solution to the problem of impedance mismatch between high power RF tone burst generators to the low impedance EMAT. IMT'S are mandatory between source and transducer at frequencies where coupling transformers doesn't work properly. IMT's are designed using transmission line circuit theory. The ferrite cores with different radii and bifilar wires are used for making different impedance ratios of IMT transformers for both transmission and reception. Turn ratio of 2:1 and 1:4 IMT proto type with isolation are designed for transmission and reception to match impedances. The designed IMTs are analyzed and verified for a wide frequency range (500 kHz ­ 5 MHz) for EMAT applications.

1. Introduction

Most of the High frequency circuits use some form of impedance transformers and in many applications wideband transformers are required. Traditionally, in power amplifiers, a large number of inductors and capacitors or transmission line section, or there combinations, are used in conventional ladder network to realize a transformer from very low impedance to 50&! over an octave or large bandwidth. Generally, conventional techniques using aforementioned components having limited bandwidth (one to two octaves) and results in large circuit size. The IMT transmits the energy from input to output by a transmission line mode and not by flux-linkage as in the conventional transformers. As a result the IMT has much wider bandwidth and higher efficiencies than its than its conventional counterpart, with proper core materials and impedance level of 50&! or less bandwidths of about 50MHZ and efficiencies approaching 99% are possible today when matching 5&! up to 50&! and 50&! down to about 3&!.

2. IMT Designing

Figure.1, shows the schematic of 2:1 impedance transformer (ZS = 2ZL ), where ZS and ZL are the source and load impedance respectively. Here transmission lines A and B are not electromagnetically coupled and there length (L) is typically ë/4. This IMT's are designed based on two assumptions. The first one is that transmission line should be kept electrically short, implying that â !w--$%w-- be kept small. The second, if the assumption of perfect transmission line prorogation is made, signifying equally and opposite current are flowing within the line (A & B) segments. The important characteristics of IMT to be obtained are the voltage transformation ratio Ní, the current transformation

Fig. 1 : Transmitter side IMT (2:1 Ratio)

ratio N i , the load transformation ratio N l 2 , the input impedance Zin, and reflection coefficient G.

Ní = Vin / Vl. Ni= Il / Iin. Nl2 = Ní * Ni. (1) (2) (3)

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Or Nl = " Ní * Ni Fi = (109 Rg) / (4ðNp2Al) (4) (5)

The IMT performance is best represented by load transformation ratio since it incorporates both voltage and current characteristics. Ideally, Ní, Ni & Nl2 are constants, if the voltage or current ration from primary to secondary is N: 1, then load transfer ratio will be N2:1. In transmission line theory, these quantities vary in magnitude and phase with frequency, characteristic impedance and length of transmission line. In the designing point of view an ideal transformer should have almost constant magnitude and phase for a wide frequency range. The ratio of the reflected to incident wave voltage is measured in terms of a reflection coefficient, G. These reflections are produced from the material interfaces or equivalently from transmission line loads, for perfectly matched circuit's the reflections coefficient should be zero, which means no insertion losses. The reflection coefficient is given by:

G = (Zin-Zg) / (Zin+Zg) (6)

can establish the low-frequency cutoff frequency (fi) and, using Eq. 5, can calculate the required number of turns (Np) for the primary winding. To determine the high-frequency response, information about the transmission line is required, such as its characteristic impedance (Zo), the propagation velocity (vp), and the phase factor (â), all at the desired operating frequency. Along with the values of the source impedance (Rg) and the load impedance (Rc), the optimum theoretical value of the characteristic impedance (Nl) can be determined by applying Eq. 5.

3. Analysis of IMT's

The frequency response of the IMT with ratio of 2:1 for the transmitter is shown in Figure 3(a) and the IMT with ratio of 1:4 for the receiver side is shown in Fig. 3(b). The bandwidth of IMT for transmission side transformer varies from 500 kHz to 5.5MHz and for the receiver side transformer varies from 500 kHz to 6.8MHz. These IMT's can be used for the EMAT applications of different wave mode generations and receptions such as Lamb waves, Rayleigh waves and Shear waves for a wide frequency range of 500 kHz to 2MHz.

Figure 2 shows the schematic of 1:4 IMTs, with terminal 4 connected to the input terminal 1, thus raising this transmission line by the input voltage V. This is called the "Bootstrap Effect." The output voltage V1 is delayed as it travels the transmission line. In order to determine a transformer's low-frequency response, the characteristics of the ferrite core must be known since the inductance factor Al is determined relative to a specific frequency. Knowing this as well as the source's internal impedance (Zg), a designer

4. Experimental Procedure

Figure 4 shows the schematic diagram of the Experimental setup which has been used for generating acoustic waves in samples. In order to get a high power RF

Fig. 2 : Receiver side IMT (1:4 Ratio)

Fig. 3 : (a) Transmitter IMT and (b) Receiver IMT

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Kannajosyula et al. : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation

Fig. 4 : Schematic diagram of the experimental setup

Fig. 5 : Measurement set-up for generation of Lamb waves on 2mm thick plate

tone burst, a GA -2500 RITEC gated amplifier is used in conjunction with an Agilent 33250A waveform generator and MATEC 5011 Gating Modulator, was supplied to the transmitting EMAT coil through impedance matching transformer. The waveform generator was used for generation of CW sine wave transmitting signal at amplitude of 1 Vpp which is fed to the RF IN connector of the gated amplifier which is amplified only during the gating pulse duration. By, adjusting the dial setting present in the gated amplifier the output power to the coil was controlled. A RITEC Broad band receiver was used to amplify the signal from the receiving transducer. The output from the broadband receiver was fed to Agilent 54621A Digital storage oscilloscope, which was interfaced with PC for data acquisition. 50-Ohm cables were used for interconnecting different instruments that are

used to minimize the losses. Before starting the actual experiment, the variations of power level of GA-2500 RITEC with the dial setting were calibrated.

Experimental setup

The approach of the experimental work is to use the Impedance matching transformer (IMT) with Meander coil EMAT to generate Lamb wave modes at 500 kHz in thin plate. To verify the importance of the IMT's, the measurements have been done using EMAT with IMT and without IMT for different wave modes at different frequencies. The technique used for this experiment was through transmission method. Figure 7(a) shows the measurement setup for generating Rayleigh waves (R-waves) on thick Aluminum block at 1MHz. The transmitter EMAT was located on the middle of the flat surface of the specimen and the receiver was placed 100mm away from the centre of the transmitter and oriented along the axis of the wave front generated by the source EMAT. Figure 7(b) shows the comparison of Rayeligh (R-waves) wave signals with IMT and without IMT on Aluminum plate. Figure 8 shows the measurement set up for generating vertically polarized (SV) shear waves on Aluminum block. At 1.6 MHz, the Meander coil EMAT generates shear (SV) wave at angle of 40degree inside the test material. The receiver EMAT was kept 84 mm away from the source EMAT exactly to receive the back reflected shear wave. Fig 9 shows the comparison of angle Shear (SV-wave) wave signals with IMT and without IMT on Aluminum block.

Fig. 6 : Lam wave modes in 2mm thick aluminium plate at 500 kHz

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Fig. 9 : Shear (SV) waves on Aluminum block with and without IMT

Fig. 7 : (a) Measurement set up for Rayleigh wave generation (b) Rayleigh waves on 50mm thick aluminium block at 1MHz

IMT (2:1) has the frequency range from 500 kHz to 5.5 MHz and the reception side IMT (1:4) has the frequency range from 500 kHz to 6.8 MHz. The developed IMT's for both transmission and reception transformers have been used for matching the impedance of the EMAT excitation coil and the high power system. It has been shown that the IMT's can be used with EMATs for different sound generation such as Lamb waves, Rayleigh waves and Shear waves for a wide frequency range.

References

1. 2. Ruthroff C L, Some broadband transformers, Proc. IRE, 47(8) (1959 pp. 1337­1342. Pablo Gómez-Jiménez and Pablo Otero, Analysis and Design Procedure of Transmission-Line Transformers, IEEE Transactions on microwave theory and techniques, 56(1). Guanella G, New Method of Impedance Matching in RadioFrequency Circuits, The Brown Boveri Review, (1944) pp. 327329. The ARRL Handbook for Radio Communications, 82 ed., The American Radio Relay League Inc., Newington, CT, (2005). Advanced Design System (ADS) 2004 C, Agilent Technologies (www.agilent.com), 2004. Dong M and Salvy H D S, Analyzing 4:1 TLTs for Optical Receivers, Microwaves & RF, 45(1) (2005). Chris Trask / N7ZWY, A Tutorial on Transmission Line Transformers.

Fig. 8 : Measurement set up for Shear (SV) wave generation on Aluminum block at 1.6 MHz

3.

Conclusion

The prototype wideband Impedance Matching Transformers (IMT'S) for both transmission and reception have been developed and analyzed for different EMAT wave generation. It has been also explained the designing and application of IMT'S and experimental results shows the importance of IMTs for matching the impedance in EMAT systems. It has been analyzed that the transmission side

4. 5. 6. 7.

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