Measurement of the insertion loss

The insertion loss characterize the sensitivity of a transducer, and is given by :

In order to measure the insertion loss of a transducer, an ultrasonic burst is emitted by the transducer. The same transducer receives an echo issued from a plane perpendicular target, which acoustic impedance differs very much from the liquid used as a transmitting media. The voltage ratio between the echo and the emitted burst, expressed in logarithm, is called Insertion Loss (IL).

Signal Processing's transducers have an IL between 6 and 20 dB, depending on the type of transducer.

To measure the Insertion Loss of a transducer, you need the following equipment:

• 1 burst generator
• 1 amplifier (in option)
• 1 oscilloscope
• 2 coaxial cable (50 W) + 1 T
• 1 water container, with a transducer holder
• 1 target (stainless steel preferred)

Burst generator setup

• Generated burst
  Amplitude = 10 Vpp on 50 ohm
  4 cycles
  Space between burst = 10000 Cycles
  

Connect the transducer, the burst generator and the oscilloscope as shown in the figure above.

Caution

To prevent any damage to the burst generator, we recommend to check the specifications of the instrument in order to verify that it can support the connection of a very low impedance (<50 ohm) during a long time.

Measuring procedure

1. Be sure to have a good coupling between the transducer and the liquid(no air bubbles).
2. Be sure that the transducer is perpendicular to the target (max echo voltage).
3. Measure the voltage of the emitted burst.
4. Measure the voltage of the echo.
5. Compute the Insertion Loss.

Important notices

• The distance between the transducer and the target must be enough in order to avoid the disturbances generated by the ringings, but not too long to be sure to collect all the emitted energy (divergence of the ultrasonic beam).
• The thickness of the target must also be considered.
  To avoid any interference generated by the second echo, coming from the far wall of the target, a minimum thickness is required. As the amount of the energy that penetrates inside the target depends on the difference in acoustic impedance between the target and the transmitting media. It is recommended to use a target with an acoustic impedance as high as possible.
  
  The following table gives recommended values for the thickness of the target (Tk) and the distance between the transducer and the target (D) for different transducers.

F [MHz]	D [mm]	Tk[mm]
0.5	60	20
1	40	10
2	40	10
4	30	10
8	30	5
10	30	5

Sound velocity of 1500 m/s (water).