2.2. Quantization

Now let's construct VI ADC to consider on its example effect of quantization. We will set an analog signal

y (t) = 5.2exp ( - 10t) sin (20) + 2.5.

Pay attention that the maximum and minimum values of amplitude of a signal are located in a range from 0 to 7, that is it is possible to present them by means of three bits. On the obverse panel the quantization error, the histogram of an error of quantization, and also quantization target signal are shown.

At construction of the block diagram of the converter (the Figure 5 see) use VI Formula Waveform (the oscillogram Formula) (Functions → Programming Waveform → Analog Waveform → Waveform Generation → Formula Waveform (Functions → Programming → the Signal → the Analog signal → signal Generation → the oscillogram Formula)). VI receives on an input the line constant setting the formula of a signal, amplitude, frequency and the information on readout. Values of a target signal, Y - making, are taken by means of function Get Waveform Components (Reception of components of the oscillogram).

Figure 5. The block – the diagram of quantization of an analog signal by means of 3 – bit ADC.

To show quantization process, we will transform a signal with double accuracy in unsigned an integer signal by means of function That Unsigned Byte Integer (To byte of an integer without a sign) (Functions → Mathematics → Numeric Conversion → To Unsigned Byte Integer (Functions → the Mathematician → Numerical → Transform → In byte of an integer without a sign)). We will assume that the quantization permission equals to 3 bits as we know that the amplitude of a signal lies between 0 and 7. The discrete signal is formed by replacement of values of an analog signal on квантованные values. For this purpose quantized values of function incorporate to function Build Waveform (to Create the oscillogram), and all other properties remain same, as for an analog signal.

Now we will find a difference between entrance values and quantized output values, having used function Subtract (Subtraction). This difference also is a quantization error. Besides, it is possible to receive the histogram of an error of quantization, using express train VI Create Histogram (to Create the Histogram) (Functions Express → Signal Analysis → Create Histogram (Functions → the Express train → the signal Analysis → to Create the histogram)). At placing of it VI there is a dialogue window of a configuration. The maximum and minimum value of an error of quantization. 0.5 and—0.5 accordingly. Hence, to distribute an error on 10 levels in an interval from 0.5 to—0.5, 10 sections is required. Besides, for option Amplitude Representation (amplitude Representation) choose Sample Count (Quantity of readout) to generate the histogram.

To create the schedule of oscillogram Waveform Graph, click the right button on knot Histogram (Histogram) from express train VP Create Histogram and choose Create Indicator (to Create the Indicator).

Return on the obverse panel and change properties of the schedule so that to make display of a discrete signal by more evident. Add the panel of editing of schedules (plot legend) for the schedule of oscillogram Waveform Graph and change its size so that two signals were displayed. Rename an analog signal on Input signal (the Entrance signal), and discrete — in Output Signal (the Target signal).

To display a discrete signal, open dialogue window Properties (Properties) from the menu of a fast call, click Plot (Schedule) and choose Output Signal (the Target signal). Then choose an interpolation method stepwise horizontal (step - by - step horizontal), designated by a badge. Now VI it is finished (the Figure 5) see.