Electrical Indicating and Test Instruments 1 Introduction 161

Digital Phosphor Oscilloscope

This newer type of oscilloscope, first introduced in 1998, uses a parallel-processing architecture instead of theserial-processingarchitecturefound in digital storageoscilloscopes. The components of the instrument are shown schematically in Figure 7.13. The amplifier/attenuator and analogue- to-digitalconverter are thesame as in a digital storage oscilloscope. However, the signalprocessing mechanism is substantially different. Output from the analogue-to-digital converter passes into a digital phosphor memory unit, which is, in fact, entirely electronic and not composed of chemical phosphor as its name might imply. Thereafter, data follow two parallel paths. First, a microprocessor processes data acquired at each sampling instant according to the settings on the control panel and sends the processed signal to the instrument display unit. In addition to this, a snapshot of the input signal is sent directly to the display unit at a rate of 30 images per second. This enhanced processing capability enables the instrument to have a higher waveform capture rate and to detect very fast signal transients missed by digital storage oscilloscopes.



Figure 7.13. Components of a digital phosphor oscilloscope.

Digital Sampling Oscilloscope

The digital sampling oscilloscope has a bandwidth of up to 25 GHz, which is about 10 times better than that achieved by other types of oscilloscopes. This increased bandwidth is achieved by reversing the positions of the analogue-to-digital converter and the amplifier, as shown in the block diagram in Figure 7.14. This reversal means that the sampled signal applied to the amplifier has a much lower frequency than the original signal, allowing use of a low bandwidth amplifier. However, the fact that the input signal is applied directly to the analogue-to-digital converter without any scaling means that the instrument can only be used to measure signals whose peak magnitude is within a relatively small range of typically 1 volt peak-peak. In contrast, both digital storage and digital phosphor oscilloscopes can typically deal with inputs up to 500 volts.

Figure 7.14. Components of a digital sampling oscilloscope.