Different types of Digital Oscilloscopes
Digital Oscilloscope
In our previous article we studied about different types of oscilloscopes such as Analog oscilloscope and Digital Oscilloscope.
In this article we are going to discuss about different types of digital oscilloscopes in detail.
As already discussed digital oscilloscope can be classified into:
- Digital storage oscilloscopes (DSOs)
- Digital phosphor oscilloscopes (DPOs)
- Sampling oscilloscopes
Digital Storage Oscilloscopes
A conventional digital oscilloscope is known as a digital storage oscilloscope (DSO). Its display typically relies on a raster-type screen rather than luminous phosphor.
Digital storage oscilloscopes (DSOs) allow us to capture and view events that may happen only once – known as transients.
Because the waveform information exists in digital form as a series of stored binary values, it can be analyzed, archived, printed, and otherwise processed, within the oscilloscope itself or by an external computer.
The waveform need not be continuous; it can be displayed even when the signal disappears. Unlike analog oscilloscopes, digital storage oscilloscopes provide permanent signal storage and extensive waveform processing.
However, DSOs typically have no real-time intensity grading; therefore, they cannot express varying levels of intensity in the live signal.
Some of the subsystems that comprise DSOs are similar to those in analog oscilloscopes.
However, DSOs contain additional data-processing subsystems that are used to collect and display data for the entire waveform.
A DSO employs a serial-processing architecture to capture and display a signal on its screen, as shown in Figure 1. A description of this serial-processing architecture follows.
The serial-processing architecture of a digital storage oscilloscope (DSO).
Serial-processing Architecture
Like an analog oscilloscope, a DSO’s first (input) stage is a vertical amplifier. Vertical controls allow us to adjust the amplitude and position range at this stage.
Next, the analog-to-digital converter (ADC) in the horizontal system samples the signal at discrete points in time and converts the signal’s voltage at these points into digital values called sample points. This process is referred to as digitizing a signal.
The horizontal system’s sample clock determines how often the ADC takes a sample. This rate is referred to as the sample rate and is expressed in samples per second (S/s).
The sample points from the ADC are stored in acquisition memory as waveform points. Several sample points may comprise one waveform point. Together, the waveform points comprise one waveform record. The number of waveform points used to create a waveform record is called the record length.
The trigger system determines the start and stop points of the record.
The DSO’s signal path includes a microprocessor through which the measured signal passes on its way to the display. This microprocessor processes the signal, coordinates display activities, manages the front panel controls, and more.
The signal then passes through the display memory and is displayed on the oscilloscope screen.
Depending on the capabilities of your oscilloscope, additional processing of the sample points may take place, which enhances the display.
Pre-trigger may also be available, enabling us to see events before the trigger point.
Most of today’s digital oscilloscopes also provide a selection of automatic parametric measurements, simplifying the measurement process.
A DSO provides high performance in a single-shot, multi-channel instrument (see Figure 2).
The TDS694C delivers high-speed, single-shot acquisition across
multiple channels, increasing the likelihood of capturing elusive glitches and transient events.
DSOs are ideal for low-repetition-rate or single-shot, high-speed, multi-channel design applications.
In the real world of digital design, an engineer usually examines four or more signals simultaneously, making the DSO a critical companion.
Digital Phosphor Oscilloscopes
The digital phosphor oscilloscope (DPO) offers a new approach to oscilloscope architecture. This architecture enables a DPO to deliver unique acquisition and display capabilities to accurately reconstruct a signal.
While a DSO uses a serial-processing architecture to capture, display and analyze signals, a DPO employs a parallel-processing architecture to perform these functions, as shown in Figure 3.
The parallel-processing architecture of a digital phospor oscilloscope (DPO).
The DPO architecture dedicates unique ASIC hardware to acquire waveform images, delivering high waveform capture rates that result in a higher level of signal visualization.
This performance increases the probability of witnessing transient events that occur in digital systems, such as runt pulses, glitches and transition errors. A description of this parallel-processing architecture follows.
Parallel-processing Architecture
A DPO’s first (input) stage is similar to that of an analog oscilloscope – a vertical amplifier – and its second stage is similar to that of a DSO – an ADC. But, the DPO differs significantly from its predecessors following the analog-to-digital conversion.
For any oscilloscope – analog, DSO or DPO – there is always a holdoff time during which the instrument processes the most recently acquired data, resets the system, and waits for the next trigger event. During this time, the oscilloscope is blind to all signal activity. The probability of seeing an infrequent or low-repetition event decreases as the holdoff time increases.
It should be noted that it is impossible to determine the probability of capture by simply looking at the display update rate. If you rely solely on the update rate, it is easy to make the mistake of believing that the oscilloscope is capturing all pertinent information about the waveform when, in fact, it is not.
The digital storage oscilloscope processes captured waveforms serially. The speed of its microprocessor is a bottleneck in this process because it limits the waveform capture rate.
The DPO rasterizes the digitized waveform data into a digital phosphor database.
Every 1/30th of a second – about as fast as the human eye can perceive it – a snapshot of the signal image that is stored in the database is pipelined directly to the display system.
This direct rasterization of waveform data, and direct copy to display memory from the database, removes the data-processing bottleneck inherent in other architectures.
The result is an enhanced “live-time” and lively display update.
Signal details, intermittent events, and dynamic characteristics of the signal are captured in real-time.
The DPO’s microprocessor works in parallel with this integrated acquisition system for display management, measurement automation and instrument control, so that it does not affect the oscilloscope’s acquisition speed.
A DPO faithfully emulates the best display attributes of an analog oscilloscope, displaying the signal in three dimensions: time, amplitude and the distribution of amplitude over time, all in real time.
Unlike an analog oscilloscope’s reliance on chemical phosphor, a DPO uses a purely electronic digital phosphor that’s actually a continuously updated database.
This database has a separate “cell” of information for every single pixel in the oscilloscope’s display.
Each time a waveform is captured – in other words, every time the oscilloscope triggers – it is mapped into the digital phosphor database’s cells. Each cell that represents a screen location and is touched by the waveform is reinforced with intensity information, while other cells are not. Thus, intensity information builds up in cells where the waveform passes most often.
When the digital phosphor database is fed to the oscilloscope’s display, the display reveals intensified waveform areas, in proportion to the signal’s frequency of occurrence at each point – much like the intensity grading characteristics of an analog oscilloscope.
The DPO also allows the display of the varying frequency-of-occurence information on the display as
contrasting colors, unlike an analog oscilloscope.
With a DPO, it is easy to see the difference between a waveform that occurs on almost every trigger and one that occurs, say, every 100th trigger.
Digital phosphor oscilloscopes (DPOs) break down the barrier between analog and digital oscilloscope technologies. They are equally suitable for viewing high and low frequencies, repetitive waveforms, transients, and signal variations in real time.
Only a DPO provides the Z (intensity) axis in real time that is missing from conventional DSOs.
A DPO is ideal for those who need the best general-purpose design and troubleshooting tool for a wide range of applications (see Figure 4).
Some DPOs can acquire millions of waveforms in just seconds,
significantly increasing the probability of capturing intermittent and exlusive events and revealing dynamic signal behavior.
A DPO is perfect for communication mask testing, digital debug of intermittent signals, repetitive digital design and timing applications.
Digital Sampling Oscilloscopes
When measuring high-frequency signals, the oscilloscope may not be able to collect enough samples in one sweep. A digital sampling oscilloscope is an ideal tool for accurately capturing signals whose frequency components are much higher than the oscilloscope’s sample rate (see Figure 5).
Time domain reflectometry (TDR) display from a TDS8000
digital sampling oscilloscope and 80E04 20-GHz sampling module.
This oscilloscope is capable of measuring signals of up to an order of magnitude faster than any other oscilloscope.
It can achieve bandwidth and high-speed timing ten times higher than other oscilloscopes for repetitive signals.
Sequential equivalent-time sampling oscilloscopes are available with bandwidths to 50 GHz.
In contrast to the digital storage and digital phosphor oscilloscope architectures, the architecture of the digital sampling oscilloscope reverses the position of the attenuator/amplifier and the sampling bridge, as shown in Figure 6.
The architecture of a digital sampling oscilloscope.
The input signal is sampled before any attenuation or amplification is performed.
A low bandwidth amplifier can then be utilized after the sampling bridge because the signal has already been converted to a lower frequency by the sampling gate, resulting in a much higher bandwidth instrument.
The tradeoff for this high bandwidth, however, is that the sampling oscilloscope’s dynamic range is limited.
Since there is no attenuator / amplifier in front of the sampling gate, there is no facility to scale the input.
The sampling bridge must be able to handle the full dynamic range of the input at all times.
Therefore, the dynamic range of most sampling oscilloscopes is limited to about 1 V peak-to-peak.
Digital storage and digital phosphor oscilloscopes, on the other hand, can handle 50 to 100 volts.
In addition, protection diodes cannot be placed in front of the sampling bridge as this would limit the bandwidth. This reduces the safe input voltage for a sampling oscilloscope to about 3 V, as compared to 500 V available on other oscilloscopes.
