In applications as diverse as cellular phone, wireless LAN, and modem design, enormous amounts of information now are routinely condensed into waveforms using complex modulation to efficiently deliver voice, video, and data. Because of the advanced nature of these waveforms, you would expect engineers involved in designing and manufacturing these products to rely on only the latest in test and measurement tools.
So why are they still using that old workhorse—the analog oscilloscope? Well, it has a lot to do with the intricate nature of digital transmission.
Digital Transmission Gets Complicated
The rise in microprocessor speeds and higher densities in IC design are enabling the merger of data, voice, video, and networking in single systems and even on individual ICs. At the same time, the sudden dominance of the Internet and widespread installation of sophisticated LAN and WAN networks are producing the need for higher data throughput. As a result, design bandwidths and clocks are moving into the hundreds of megahertz, and new bus and networking architectures are being developed to accommodate the accelerated flow of data.
Similar exponential increases in digital transmission are taking place on a regular basis in computer, computer peripheral, datacom, telecom, and wireless arenas. The principal behind all of these advances is complex modulation, allowing more data to be packed into smaller time sequences. Sophisticated bandwidth compression schemes compact data into dense, modulated waveforms to deliver voice, video, and data quickly and efficiently through copper wires, over coax, down fiber-optic cable, and across space via wireless transmission.
The Test and Measurement Challenge
Although complex modulation has helped fuel spectacular advancements in computer, data, and telecommunications, it has led to some major headaches for designers¾ especially in the area of test and measurement.
Testing and measuring digitally transmitted signals usually involve two tasks. First, engineers need to view signals at the point of transmission. Here, it is especially important to capture and analyze the phase noise and distortion.
Then, it is vital for engineers to see the effects of interference on the received signal, since transmission does not occur in a perfect environment. For example, in wireless transmission, the engineer must assess the impact on a signal of impulse noise, co-located transmitters, and multipath interference as it travels through the air. Similarly, when a signal is transmitted through a copper wire or coax, the engineer needs to know how sensitive the signal is to impedance mismatches, reflections, ingress, and coupled interference.
Regardless of the medium, to efficiently track down and debug these problems in digital transmission, engineers need tools that reliably capture and display complex, phase-modulated signals. Until now, no one tool could do the job.
Today, engineers grappling with modulated signals rely on two different types of oscilloscopes to do their job: the analog real-time (ART) and digital storage oscilloscope (DSO). They depend on the YT and XY display capabilities of the ART to faithfully reproduce these fast-varying signals in real time. And the gray-scaling capability of an ART, because of the chemical phosphor in the display, reveals crucial information about a signal’s frequency of occurrence.
But seeing a signal is not enough. Alongside these analog capabilities, engineers need the measurement and analysis strengths of the DSO as well as the ability to store and print the display.
The full complement of automatic measurements is an absolute necessity when characterizing and examining complex data waveforms, including advanced functionality such as statistics and histograms. In addition, engineers count on the capabilities supplied by a DSO to detect subtle differences in waveform shapes. For example, complex triggering helps detect runts and glitches.
And the activity before the trigger can be captured and displayed with the pre-trigger capabilities in a DSO. The list of reasons why an engineer wants to use a DSO includes computer analysis, color displays, zooming, and downloading the data to a computer for additional analysis.
A New Architecture
Working with two types of oscilloscopes is hardly an ideal situation. For example, as digital transmission gets faster and more complex, the ART simply cannot keep up because of bandwidth limitations.
To move beyond this dilemma of analog and digital, a new architecture has been developed. Based on a digital phosphor technology that digitally emulates the intensity decay of the analog scope, this oscilloscope enables engineers to see modulated signals and all their nuanced details while supporting complete storage, measurement, and analysis capabilities.
The digital phosphor oscilloscope (DPO) displays, stores, and analyzes three dimensions of signal information in real time: amplitude, time, and the distribution of amplitude over time. Similar to the actual phosphor on an ART display, the DPO captures and remembers the frequency of events, resulting in the 3-D array that retains information for hundreds of millions of samples (see sidebar).
Applications for DPOs
By providing a third dimension of information, the DPO gives insights into the behavior of complex signals. This helps designers interpret signal dynamics, including instantaneous changes in the signal and the frequency of occurrence of signal phenomena.
The DPO is useful for measuring different types of spread-spectrum signals used in wireless applications such as cellular phones, cordless phones, and wireless LANs. For example, it could be used to directly assess peak average power vs number of calls for an IS95-compliant, PCS-standard, CDMA cellular site. Using the DPO in the time domain, the designer first captures the 2.0-GHz CDMA signal to examine the escalation in noise as calls are added to the transmission.
The DPO’s display shows where most of the signal activity is occurring. Also, the DPO’s histogram function graphically quantifies the changes in power spectral distribution, depending on the number of calls (Figure 1).
After viewing the timing relationships and examining any glitches or other aberrant behavior, the designer can re-acquire the signal in the XY or phase domain. The incoming PCS signal would need to be demodulated with an external RF demodulator to display the in-phase and quadrature phase signals¾ often referred to as the I and Q signals¾ as a constellation diagram (Figure 2).
Now, as calls are added, the designer can see how the power is distributed around the different symbols for the modulated signal. Moreover, coherent interferers are displayed immediately. This is especially important if the cell site is located near some sort of intermittent interference, such as a police or fire transmitter.
In another example, a designer might need to examine a fast-moving signal like a disk-drive output. When acquiring a signal like this, a DSO could give false information due to aliasing.
The DPO minimizes the problem of aliasing by:
Sampling at rates high enough that sufficient detail can be gathered to represent even the most active waveforms.
Responding to signal changes in real time due to its fast waveform capture rate.
About the Author
Thomas Brinkoetter is the marketing manager for high-performance oscilloscopes at Tektronix. Before coming to Tektronix five years ago, he spent five years at Anritsu-Wiltron and 13 years at Hewlett-Packard’s test and measurement organization. Mr. Brinkoetter received a B.S.E.E. degree from the University of Illinois and an M.S. degree in industrial engineering from Stanford University. Tektronix Measurement Business Division, P.O. Box 500, M/S 39-729, Beaverton, OR 97077-0001, (503) 627-4819.
Sidebar
How Digital Phosphor Works
The chemical phosphor in an analog oscilloscope creates a gray scale because of the decay in its light output over time. Digital phosphor replicates this decay in intensity by digitally controlling the replacement of data in a 3-D array.
Using ASIC technology, the DPO accumulates multiple images of signal information in an array of integers. Each integer represents a pixel in the display of the DPO and is used to control intensity. As a signal is acquired over time, this array is continuously updated with image after image of the signal. The DPO uses all the incoming acquisition samples to draw the image.
To retain information about each snapshot, the integers in the array are adjusted. If the signal traverses one point again and again, that integer will be modified repeatedly to reflect that fact.
With the accumulated updating, the array eventually contains a detailed map of the signal intensity similar to an ART, except with better horizontal resolution. Like the actual phosphor that coats the CRT screen of an analog oscilloscope, digital phosphor captures and remembers the frequency of events, resulting in a 3-D array that retains information for all samples.
The acquisition engine in the DPO continuously samples at the maximum rate
with minimal dead time between acquisitions. Once a sufficiently detailed array is created, that information is sent to the DPO’s display while a new array is being generated. A new snapshot of the digital phosphor is sent to the display every 1/30th of a second.
As a result, DPOs can display, store, and analyze three dimensions of signal information: amplitude, time, and the distribution of amplitude over time. This 3-D data base can be downloaded to a computer for off-line analysis or used to construct a 3-D topography map (Figure 3).
Copyright 1998 Nelson Publishing Inc.
August 1998