Chemistry Reference
In-Depth Information
the backscattered Doppler signal were explained by means of a backing
dimension and a particle dimension.
Measurement techniques for the backscattering are described by
Sigelmann and Reid (1973), Madsen
et al.
(1984), Ueda and Ozawa
(1985), Insana
et al.
(1986), Berger
et al.
(1991), Madsen (1993) and
Chen and Zagzebski (1996).
3.5
ELECTRONICS, SIGNAL PROCESSING AND SOFTWARE
3.5.1
Electronics
The electronics is responsible for the generation of the electrical signal
sent to the transducer and the registration (in most cases digitalisation) of
the signal received by the transducer. The working principle is described
in several textbooks from the medical field (Jensen, 1996; Evans and
McDicken, 2000; Hill
et al.
, 2004; Cobbold, 2007).
The details of the signal processing of the available devices are dif-
fering, for example some devices use an analogue demodulation and
digitise afterwards, while other recent devices digitise the original sig-
nal from the transducer at a high sampling rate and do the demodulation
with the digital signal. Parameters such as pulse length, gate (or channel)
distance, emission signal amplitude and amplification of the received
signal are adjustable for most devices.
3.5.2
Signal processing and profile estimation
There are several different signal processing approaches described in
the literature from the medical field to extract the actual flow velocity
profile from the echo signal received by the transducer. If and how the
signal processing can be customised depends on the hardware. Some
devices work rather as a black box and only allow access to the estimated
profile. It can be of advantage to have at least access to the complex
demodulated signal (
I
and
Q
), which allows to obtain the distribution
of the Doppler frequency for the individual channels via FFT (fast
Fourier transformation). This simplifies the identification of artefacts
influencing the profile shape (Jones, 1993). In addition, it is possible to
use different velocity estimation approaches such as central or maximum
frequency (Tortoli
et al.
, 1995, 1996, 2006).
3.5.3
Software
For the data acquisition (flow profiles and pressure, maybe temperature
and flow rate), the rheological calculations and visualisation of the data,
