A-Scan from the Ultrasonic Starter Kit

[Brueck]

Hallo,

I've a problem concerning the USonic_A_scan.vi from the "Ultrasonic Starter Kit" downloadable at the VI-Website
(http://sine.ni.com/apps/utf8/niepd_web_display.display_epd4?p_guid=F248370C3F141D78E0340003BA7CCD71)

My task is to measure and detect the amplitude and delay of an ultrasonic signal, for this purpose the functinality provided within the "USonic_A_scan.vi" would serve perfectly, but yet I'm not able to determine how the ampltuide measurement exactly works.

Here's my problem:
When a change the vertical position of my gates, the value returned for the amplitude changes (The horizontal position and the width of the gate stay constant). Why? Is there any other (comfortable) possibility to measure the amplitude within a certain "gate"?

Thanks in advice!

Benjamin Brück

[Odd_Modem]

The backwall echo measured amplitude is stable WRT gate height in the example. The front wall amplitude jumps around a lot. Assuming that you are looking for a single echo then there is a lot of noise in the signal in relation to the peak detector.vi width setting. The width is set to the default value of three which means that it will look for inflections based on three sample points. If you set it to 8 then the front wall echo amplitude stabilizes, even for changes in gate height. Whether that makes sense in terms of an ultrasonic measurement is another issue as the peak detector width, the transducer frequency and the sample rate are all tied together here. The peak detector vi is not going to return the sample within the gate that has the maximum amplitude, it is going to return an interpolated value based on a curve fit over a certain number of points, where the first derivative of the fitted curve is zero, indicating an inflection.