The metallization ratio of IDTs is 0.33, the length of fingers is 1500 um, width of fingers is 6.5 um. The S21 parameters shows that SAW with fundamental frequency of 92 MHz was generated, but no odd harmonics. What can I do next to get higher harmonic.
I think you may need to make the metallisation thinner and move 1/3 of our array by 1/3 of a period and another 1/3 of the array by 2/3 of the period. This should cancel the fundamental and enhance the odd harmonic. It is probably best not to make it progressive along the array. You need positive bars centred where you expect peaks for the harmonic, and negative bars where you expect troughs. I think the width of the metallisation probably suppresses the harmonic. The harmonic will probably be more than 20 dB down - because the electrodes are 1/6 period wide the excitation is not a square wave but trapezoidal, with lower sidelobes. It may even be there but you can't see it because it is hidden in the noise or floor.
Hello Mr. White,
Thanks for your resonable answer. The IDTs we designed is single finger structrue,as shown in files. The thinner of metallisation ratio is, the weaker of amplitude will become, as shown in files (reference). So sorry, I can not understand that "move 1/3 of our array by 1/3 of a period and another 1/3 of the array by 2/3 of the period". If possible, could you help me to design a IDTs to generate higher harmonic with GHz.
At the moment the gaps are at the peaks and troughs of the standing wave at 92 MHz on the crystal. They are spaced at a half wavelength. If you want a larger 3rd harmonic you need some gaps centred on where the peaks would be for this harmonic. The peaks for that harmonic have 1/3 of the spacing of those of the fundamental. You probably can make some of the gaps 4/3 further apart by making the bars wider in some places. This is the wrong place to excite the fundamental but the right place to excite the 3rd harmonic. You can test this using the Fourier transform function in excel or Matlab for instance, to see which pattern increases the 3rd harmonic content. If they are all spaced by 4/3 you will only get 3/4 the frequency.
Chenbo
I was mistaken in my earlier advice.
I have looked at your problem properly. I think all you need to do is to make your electrodes thicker. At the moment they are half a wide as the gaps, they need to be twice as wide as they are, and the gaps need to be half as wide.
If you do a fourier transform of 1 1 1 1 1 1 0 0 0 -1 -1 -1 -1 -1 -1 0 0 0 repeated then there is no third harmonic!
If you do a fourier transform of 1 1 1 0 0 0 0 0 0 -1 -1 -1 0 0 0 0 0 0 repeated (add a few zeros right at the end if there need to be 2^n points, I used 256 points and had four zeros at the end) then the third harmonic is very strong! The is no field under the metal, the 1s and -1s are in the gaps.
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