Digital Read out Oscilloscope

The digital read out oscilloscope instrument has a CRT display and a counter display. The diagram shown is of an instrument where the counter measures the time (Fig. 9.1)

  

 

The input waveform is sampled and the sampling circuit advances the sampling position in fixed increments, a process called strobing. The equivalent me between each sample depends on the numbers of sample taken per cm and the sweep time/cm, e.g. a sweep rate of 1 nano-sec/cm and a sampling rate of J samples/cm gives a time of 10 pico-sec/sample.  

Figure 9.1 shows a block diagram of a digital read out oscilloscope when measuring voltage. Two intensified portions of the CRT trace identify 0% and 100% zones position. Each zone can be shifted to any part of the display. The voltage divider Bps between the 0% and 100% memory voltage are set for start and stop timing, The coincidence of any of the input waveforms with the selected percentage point is sensed by this voltage comparator. The numbers of the clock pulse which correspond to the actual sample taken are read out digitally in a Nixie display tube in ns, µs, ms or seconds. Figure 9.2 shows a block diagram of a digital readout CRO when used for voltage to time conversion. 
The CRT display is obtained by sampling the 0% reference voltage as chosen by the memory circuit. A linear ramp generator produces a voltage; when the ramp voltage equals the 0% reference the gate opens. When the ramp equals 100% reference the gate closes. The number of clock pulses that activate the counter is directly proportional to the voltage between the selected references and is read out in mV or volts by the Nixie tube display.

Digital Storage Oscilloscope 

Storage cathode ray tube has several limitations. They are as follows, 1. There is a short duration of time, in which it can preserve a stored waveform, so the waveform may lose. 2. Trace of storage tube is not as fine as that of a normal CRT. 3. Writing rate of the storage tube is less than that of a conventional CRT which in turn limits the speed of the analog storage oscilloscope. 4. It is more expensive than a conventional CRT and requires additional power supplies. 5. Only one image can be stored. For comparing two traces they are to be superimposed on the same and displayed together. Digital storage oscilloscope is used to limit these limitations. In DSO, the waveform to be stored is digitized, stored in a digital memory and retrieved for display on the storage oscilloscope.

Stored waveform is continuously displayed by repeatedly scanning it. Therefore a conventional CRT can also be used for the display. The stored display can be displayed continuously as long as the power is applied to the memory which can be supplied from a small battery.

Digitized waveform can be analyzed by oscilloscope or by reading the contents of the memory into the computer. Display of the stored data is possible in both amplitude versus time and x-y modes. In DSO, fast memory readout is used for CRT display in addition to this a slow readout is also possible which is used for development of hard copy externally. Figure shows the block diagram of DSO which consists of, 1. Data acquisition 2. Storage 3. Data display.    Data acquisition is earned out with the help of both analog to digital and digital to analog converters, which is used for digitizing, storing and displaying analog waveforms. Overall operation is controlled by control circuit which is usually consists of microprocessor.

  Data acquisition portion of the system consist of a Sample-and-Hold (S/H) circuit and an analog to digital converter (ADC) which continuously samples and digitizes the input signal at a rate determined by the sample clock and transmit the digitized data to memory for storage. The control circuit determines whether the successive data points are stored in successive memory location or not, which is done by continuously updating the memories. When the memory is full, the next data point from the ADC is stored in the first memory location writing over the old data.  The data acquisition and the storage process is continues till the control circuit receive a trigger signal from either the input waveform or an external trigger source. When the triggering occurs, the system stops and enters into the display mode of operation in which all or some part of the memory data is repetitively displayed on the cathode ray tube. 

 In display operation, two DACs are used which gives horizontal and vertical deflection voltage for the CRT Data from the memory gives the vertical deflection of the electron beam, while the time base counter gives the horizontal deflection in the form of staircase sweep signal. The screen display consist of discrete dots representing the various data points but the number of dot is very large as 1000 or more that they tend to blend together and appear to be a smooth continuous waveform. The display operation ends when the operator presses a front-panel button and commands the digital storage oscilloscope to begin a new data acquisition cycle.