Journal of Magnetism and Magnetic Materials 23 (1981) 209-210 North-Holland Publishing Company
MICROWAVE MAGNETOSTATIC HAN Shi-ying, LIU Gong-qiang, Department
DELAY LINE *
LIU Ji-zhe, YANG Gui-lin and ZHAI Hong-ru
of Physics, Nanjing University, Nanjing, People’s Republic or China
Received in final form 21 January 1981
A magnetostatic delay line (MSDL) composed of a longitudinally magnetized YIG single crystal rod is constructed. The multiple round trip echoes observed, especially at short delay times, are successfully suppressed by changing the sample shape. At S-band the useful delay time controlled magnetically ranges from 0 to 0.5 yF,The untuned insertion losses at 2 and 4 GHz are <26 dB.
2. Experimental results
It is known theoretically that the group velocity of bulk wave MSDL approaches zero and thus its delay time becomes infinity when the bias field approaches the turning point (Hi = w/A) [l ] . But the infinite delay coexists with infinite delay dispersion (at&w + -) and dispersion always leads to distortion of the delayed pulse signal. So the useful delay time is determined to a great extent by the degree of distortion permitted. Our experiment indicates that when the delay time td > 1 ~_lsthe delayed pulse obviously is distorted. For short delay times, e.g. td < 0.5 ~_ls,the distortion is very slight. Nevertheless, a series of signals appears due to round trip echoes. As the bias field increases toward the cut-off value, td increases and the echo signal disappear gradually due to large attenuation and dispersion. Actually a single delayed signal remains when td > 1 vs. In order to put this kind of DL to some practical use we suppress successfully the echo signals in the short delay time region. A MSDL with a single clean delayed signal having td = O-O.5 DS and a very slight distortion was constructed, at S-band frequencies the untuned insertion loss being 26 dB.
YIG single crystals were grown in our laboratory by the flux method. In order to grow comparatively large crystals of good quality, the accelerating rotation crucible and local cooling techniques were used. The FMR linewidth AH of the crystals at X-band frequencies is 0.5 Oe. The length of YIG rods is about 9-l 2 mm with a square or round cross-section, and the diameter d = l-3 mm. The sample holder is shown in fig. 1. Input/output isolation is 70 dB and the leakage signal due to direct radiation between input and output terminals is suppressed efficiently. By utilizing coupling coils as transducers, as shown in fig. 2, the transducing efficiency is improved . The delayed pulse is amplified by a heterodyne intermediate receiver and is displayed by an oscilloscope. The delay time and insertion loss are measured by conventional methods. The photographs of fig. 3 show the continuous variation of the delayed signal when the bias field changes from zero to cut-off value
* Was part of the Proceedings of ICM’79, Munich.
1/OOOO-0000/$02 SO 0 North-Holland
Fii. 1. Sample holder.
Han Shi-ying et al. / Micronme
Table 1 Losses of a magnetostatic delay line, oi = insertion loss, 01~= conversion loss
f (CDs) Fig. 2. Coil coupler. 2
Fii. 3. Delayed signal with round trip echoes.
Fig. 4. Inclined end face to suppress echoes.
Fig. 5. Delayed signal with echoes suppressed.
Loss with echoes unsuppressed (delay time td = 0.5 /.s)
Loss after suppressing the echoes (td = 0.5
CYi= 22 dB
at = 26 dB CY~ = 23 dB
ai = 21 dB czc = 15 dB
“J = 24 dB a,=lSdB
at a fixed frequency (4 GHz). In photograph 3 of fig. 3, the round trip echoes can be seen; in photograph 6 the echoes almost disappear. The first signal is delayed by about 1 /JS. Thereafter the delay time of the signal increases abruptly with the field and the wave form distorts heavily. The signal disappears in the noise as the delay time increases to 3 ps. Several attempts have been made to test and suppress echo signals. Polishing or roughening of the end faces has no effect on the echoes. It seems that the reflection does not take place right at the end surfaces, or the wavelength of the magnetostatic wave at the end face is long enough that it is not scattered appreciably by the irregularities there. Grinding the end to make the end surface inclined by an angle 0 (fig. 4) does affect the echo signals. When 0 = 45”, the echoes are suppressed effectively, as shown in fig. 5, but the insertion loss increases to some extent. Different transducing techniques, i.e. single wire coupler, thin strip coupler and coil coupler, were investigated. The last shows the highest efficiency. The optimum diameter of coupling coils is sensitive to frequency. The dependence of the propagation loss c+, on frequency is given in fig. 6, which agrees well with the empirical formula . As an example, the insertion losses (Yiand conversion losses CX,of the magnetostatic delay line before and after suppressing the echoes are listed in table 1.
Fii. 6. Propagation loss as a function of frequency (o empirical formula, X experiment).
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