Page 43 - HydrophoneHandbook_eReader_Ed2_20151208
P. 43


8.4 Methodology
Onda's calibrations are accomplished through the use of
reference equipment calibrated at a National Laboratory such as
National Physical Laboratory (NPL) in the UK or Physikalisch-
Technische Bundesanstalt (PTB) in Germany. The particular
reference equipment and national laboratory used for any
calibration is referenced on the calibration certificate.
Calibration is obtained by comparison techniques to reference
hydrophones. For calibration below 20 MHz, the stepped single
frequency comparison technique is used, and for calibration
above 20 MHz, the harmonic comparison technique is
employed. The procedures are as follows:

Stepped Single Frequency Comparison Technique:
1) The reference hydrophone is placed at the focus of a

      broadband source [1], which is driven by a sinusoidal tone
      burst. The reference hydrophone is aligned by maximizing
      the hydrophone signal with the source driven at the
      maximum end of its frequency range (20 MHz for a 1-20
      MHz calibration, 5 MHz for a 0.25-1 MHz calibration). The
      voltage driving the source is then swept in across the
      frequency range (i.e., 1 to 20 MHz, or 0.25 to 1 MHz). At
      each frequency, the output of the reference hydrophone is
      measured and digitally filtered to separate the primary
      frequency component from any harmonics that may be
      present in the signal. The reference hydrophone has
      previously been calibrated by NPL, so that the voltage it
      detects can be translated into a pressure value, hence
      calibrating the pressure output of the source as a function
      of frequency, at the source’s focal position.


                                                                                                                        0NDA Corporation
   38   39   40   41   42   43   44   45   46   47   48