Information Technology Reference
In-Depth Information
Figure 2.16:
(a) Conventional description of ZOH. (b) System in (a) does not have linear phase. (c) Linear phase
ZOH system in which the samples are spread symmetrically. (d) Phase response of (c). (e) Flat response can be
obtained using equalizer.
An alternative in the time domain is to use resampling which is shown in
Figure 2.17
.
Resampling passes the zero-
order hold waveform through a further synchronous sampling stage which consists of an analog switch that closes
briefly in the centre of each sample period. The output of the switch will be pulses which are narrower than the
original. If, for example, the aperture ratio is reduced to 50 per cent of the sample period, the first frequency
response null is now at twice the sampling rate, and the loss at the edge of the pass band is reduced. As the figure
shows, the frequency response becomes flatter as the aperture ratio falls. The process should not be carried too
far, as with very small aperture ratios there is little energy in the pulses and noise can be a problem. A practical
limit is around 12.5 per cent where the frequency response is virtually ideal.
Figure 2.17:
(a) Resampling circuit eliminates transients and reduces aperture ratio. (b) Response of various
aperture ratios.
