Page 55 - HydrophoneHandbook_eReader_Ed2_20151208
P. 55

Frequently Asked Questions

Should I get an EOC or preamplifier-loaded calibration?
An End-of-Connector Open Circuit (EOC) sensitivity represents
the sensitivity measured at the hydrophone connector without
the preamplifier. An EOC hydrophone calibration can be
mathematically converted to a preamplifier-loaded calibration.
See the Resources section for the white paper. Onda also can
supply a software utility to combine the standalone calibration
data sets. An EOC calibration offers more flexibility because it
can be paired with different preamplifiers. However, the user
can avoid the conversion by having hydrophones calibrated in a
preamplifier-loaded configuration.
Please note, however, that as the frequency goes up, the
mathematical conversion becomes invalid because cable
transmission line effects become more important. In particular,
cable resonance effects become more prevalent at high
frequencies. In some cases for fairly non-linear pulsed
waveforms, there may be significant waveform distortion
because the pressure signal contains very high frequency
harmonics which excite cable resonances. In general, we
recommend that the cable length be less than a tenth of the
electrical wavelength in the cable for any relevant frequency.
The cable electrical wavelength is given by the cable phase
velocity divided by the frequency. The phase velocity can be
assumed to be 2 x 108 m/s, so for example, at 20 MHz the cable
electrical wavelength will be 1 meter. Just bear in mind that in
many practical applications non-linear harmonics go to much
higher frequencies than 20 MHz.
Note that frequencies above 20 MHz require the calibration to
be preamplifier-loaded to ensure sufficient sensitivity.

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