Applications for Pico Products

Using the Pico ADC212/100 to Acquire Ultrasonic Echoes

The powerful sampling capability of the ADC212/100 PC oscilloscope is remarkably illustrated in the data acquisition set–up described below, where a piezoelectric polymer transducer is bonded to the underside of a tall cylindrical container filled with water.

A pulser/receiver is used to deliver a very fast high-voltage spike (approx 25 ns wide, -200 V) to the piezo film transducer. The transducer launches an ultrasonic pulse into the wall of the container, which then propagates upwards through the water until it reaches the surface. The water/air interface acts as a reflector, and the ultrasonic echo returns to the base of the container, where the transducer detects it and converts it back to an electrical signal.

Because the liquid depth is, in this case, 0.931 m and the approximate speed of sound in water is 1500 m/s, the ultrasonic signal is expected to take 1.24 ms to traverse the total path length of 1.862 m (plus a little extra time to pass through the container wall twice.)

Due to this long propagation delay, the triggering rate of the pulser/receiver has to be set to a very low frequency. In theory, a rate slower than 800Hz should be sufficient to allow capture of the echo, before re-triggering. In practice, however, multiple echoes can arise in the container, and a PRF of lower than 100Hz is preferable.

The transducer delivers ultrasonic energy in the region of 3.4 MHz, with the initial burst comprising approximately 7 cycles and total duration of around 2 µs. A small repeat echo appears at around 2.4 µs, due to internal echo within the container wall, and further decaying repeats at this delay may be seen.

With the ADC212 set to acquire 128,000 data points at timebase setting of 200 µs/div and 0% pre–trigger delay, the full waveform from time of trigger to well beyond echo arrival time can be captured in a single record:

On the trace shown above, very little detail of the ultrasonic signal can be seen. The great advantage of the deep memory (also with good vertical resolution of 12 bits) is shown in the following plot, which is the same stored waveform but with x–axis zoom of 200X applied:

In this view (spanning just 10µs from the acquired trace of 2ms total), the full detail of the ultrasonic signal is visible. On such a view, the effect of adding or removing as little as 1ml of water to the column (which contained a total of 4l) could be clearly seen.