2019-07-08
1. Theory:
The TV IF sound table filter is a transmission type filter composed of an overlap-weighted transducer and an equal-length transducer. The total frequency response of the S-function model can be written as follows: H(f) =H (input) (f) + H (output) (f)
Therefore, when H(f) is known (ie the frequency response required by the user), the required logarithm N of H(output)(f) can be found according to the required bandwidth, then H(output)=sin(wπx)/(nπx ), x=(f-f0)/f0, f0 is the center frequency, so you can find H (input) (f) and perform Fourier transform to find the impulse response, and sample the impulse response (usually 4f0 sampling) can determine the shape of the input converter.
2, design considerations:
What is said above is the theoretical situation. In fact, we must consider some effects to make the analog frequency response more accurate.
A. Diffraction: The aperture of the transducer can be estimated from the output impedance of the user, typically 8λ0~15λ0. Since the input transducer is an overlap-weighted transducer, the taps sampled by the impulse response have a small overlap, which causes the theoretical calculation to be different from the actual measurement. The actual contribution must be considered. This is described in various publications. The angular spectrum wave (A SOW) model can be better simulated.
B. Reflection: According to the theory of the line of the world, the wave will produce reflection at the discontinuity of the impedance, and the surface acoustic wave has some different sound velocity on the free surface of the crystal and the surface of the metallized film, and the mass load of the metallized film affects the transducer. The edge of the interdigitated finger produces reflections. Although these reflections are small, since the input transducer has a large number of fingers, the reflection generated by it has an influence at the center frequency. Therefore, the input transducer is usually sampled at 4f0, so that The reflection peak shifts from f0 to 2f0, thereby reducing the usual effect.
C. Body Wave: The interdigital transducer also excites the body wave, which reaches the output transducer after being reflected inside the alarm. It also affects the performance of the device. We can slap or slot the back of the alarm to make the impact very small. .
D. Other effects: Surface acoustic waves will be reflected when they are transmitted to the edge of the crystal. It can be removed by applying acoustic ink at both ends. The influence of line resistance and the influence of external circuits and charge distribution can also be used in the angular spectrum. Simulation in the wave formula. The end effect can be eliminated by a special structure, and the use of a single finger on the output transducer can reduce the impact of three echoes by about 6 db.
