| Popis: |
A wideband digital signal acquisition instrument intended primarily for ultrasonic experimentation has been designed and built. It comprises an integrated array of modules which combine to provide: a pulser for transducer excitation, wideband amplification (>80 MHz bandwidth), high speed sampling (>160 MHz), on-board memory (4K x 16 bits capacity), control hardware and real-time coherent averaging. The integration of the modules offers significant benefits over systems consisting of a number of separate items of equipment. The major benefit is a reduction in acquisition time which is accomplished as follows. The time relationship between the sampling clock and the input signal is controlled to minimise the signal repetitions required for interleaving. This is achieved by triggering the input signal using a pulser board locked to the sample clock. In addition, signal averaging is implemented in hardware using two memory modules and an adder module. Since the time relationship between the sampling clock and the input signal is controlled it cannot be tested using a sinewave because it is not possible to produce a sinewave which is correlated with the sampling clock. Instead, a rectangular pulse, triggered by the instrument, is applied to a single-pole low pass filter to provide the test signal. The time constant and start time is ascertained and used to reproduce the single-pole response. The reproduced response is then compared to the sampled signal to produce an error plot from which the system's linearity and effective bits can be deduced. As the system was designed for fast acquisition, and hence has a short aperture time, it is highly applicable to dynamic processes. The dynamic process application chosen was the measurement of absorption and velocity on rapidly flocculating and sedimenting colloids. |
