The digital phosphor oscilloscope takes its name from the old analog phosphor scope, but the resemblance is superficial. It was difficult to see the trace on the CRT screen in an analog scope because the electron beam dissipated instantly. To solve this problem, early engineers came up with the idea of coating the inner surface of the glass screen with one of the many phosphor compounds that fluoresce for a couple seconds so the image persists long enough to be seen. As the technology developed, it became more elaborate, and the amount of persistence could be controlled by the user.
The modern digital phosphor (so-called) oscilloscope operates according to entirely different, non-chemical principles and the goals are not at all the same. The purpose of the digital phosphor oscilloscope is to process and display signals that are much higher in frequency than is implied by the instrument’s bandwidth rating. In a conventional oscilloscope, over-bandwidth signals are attenuated to
the extent that, due to low amplitude, they are lost below the noise floor of the instrument. That is because capacitive reactance, in parallel with the signal path, is lower and inductive reactance, in series with the signal path, is higher at high signal frequencies.
Capacitive and inductive reactance is present in all conductors and components, particularly semiconductor devices. Instrument designers battle these reactances at enormous expense, which is why end-users pay so much for high bandwidth in an oscilloscope or spectrum analyzer. The digital phosphor oscilloscope is a largely successful solution to this problem. It can process signals more quickly than a conventional oscilloscope of equivalent bandwidth. As a consequence, the digital phosphor oscilloscope can display higher frequencies.
This capability is made possible by its unique architecture. The processor is based on parallel rather than serial technology. Additionally, a virtual Z-axis is added, permitting the instrument to display a third dimension of information that appears as intensity and corresponds to the number of occurances of the waveform at a given point in time. The bottom line, then, is that transition errors, glitches and spurious pulses may be displayed even as they happen.
In the conventional digital oscilloscope, there is a time interval during which signal activity cannot be displayed. The instrument is waiting for the next triggering event. In the digital phosphor oscilloscope, parallel processing overcomes this restriction.
However, the digital phosphor oscilloscope is for well-heeled serious users. Tektronix offers four models in its 7000 series, all with four analog channels:
The DPO7054C has a 500 MHz analog bandwidth rating. The sample rate is 5 – 20 GS/sec. The record length is 25M to 250M points. The list price is $19,200.
The DPO7104C has a 1-GHz analog bandwidth rating. The sample rate is 5 – 20 GS/sec. The record length is 25M to 250M points. The list price is $24,000.
The DPO7254C has a 2.5-GHz analog bandwidth rating. The sample rate is 10 – 40 GS/sec. The record length is 25M to 500M points. The list price is $32,300.
The DPO7354C has a 3.5-GHz analog bandwidth rating. The sample rate is 10 – 40 GS/sec. The record length is 25M to 500M points. The list price is $41,300.
Persistence in a digital non-phosphor oscilloscope is an entirely different matter. In the Tektronix MDO3000 Series instrument, for example, to see the effect of persistence, first display a sine wave by feeding the default 100 kHz waveform from the internal AFG by running a BNC cable from AFG Out on the back panel to an active channel input on the front panel. Press AFG so that the default sine wave is displayed.
Now, disconnect either end of the BNC cable. Notice that the sine wave no longer displays. Then, reconnect the BNC cable so that the sine wave reappears. Once again, it is synthesized in the AFG at 100.00 kHz, 500 mV peak-to-peak.
To see persistence in this non-phosphor oscilloscope, press the Acquire button, which is located on the front panel just to the right of the large Wave Inspector concentric pan and zoom knobs. The horizontal Acquisition menu appears below the display. Press the soft key associated with Waveform Display, which causes the vertical Waveform Display menu to appear on the right. Notice that Persistence can be toggled On or Off by pressing the applicable soft key.
Persist Time is adjusted by turning Multipurpose Knob a or by using the number pad to put values and units into the field. To demonstrate persistence, set the Persist Time to ∞ seconds. Then, once again, disconnect either end of the BNC cable. Now, the sine wave remains in the display even though it is no longer being fed to the active channels. It does not matter if you toggle Off AFG, and changing waveform, for example to Ramp, has no effect. Presumably, the sine wave would be displayed indefinitely, as long as you did not press Default Setup or power-cycle the oscilloscope.
A naïve assumption would be that you could use Infinite Persistence to create an eye pattern. But the sad reality is that this is usually not possible. Creating an accurate eye pattern or any other eye pattern is far more problematical for a variety of reasons.
Eye patterns are primarily applicable in the telecommunications field, where digital signals are transmitted across some medium. Accelerating data rates are mandating that our current serial links maintain high levels of signal integrity at ever-higher gigahertz frequencies. Problems arising include impedance mismatches and termination and grounding errors, either introduced in the initial design and installation or acquired due to external stress or interference. These problems can be detected and interpreted by means of eye patterns created by a properly configured oscilloscope.
The oscilloscope constructs an eye pattern by combining segments of the data stream. The instrument superimposes successive cycles in a single display. Because they are driven by a single clock, a high degree of rising and falling edge and voltage level alignment may be expected, and voltages for logic low and logic high should coincide when distortion is not present. Such distortion may be afflicting the signal under investigation or it may be intrinsic to the oscilloscope used to create the eye pattern.
To be capable of displaying an eye pattern, the oscilloscope must be of sufficient bandwidth. In digital signal analysis, the high and low logic levels are similar to square waves, but rather than invariably having 50% duty cycle, they are more accurately described as pulse waves. In either case, the rising and falling areas are quite nearly vertical. This fast frequency component generates powerful harmonics which can best be seen in the frequency domain. Because these harmonics are not merely incidental but actually defining components of the waveform, the oscilloscope must be of sufficient bandwidth to acquire them if an accurate eye pattern is to be created. At contemporary telecommunications frequencies, this bandwidth requirement is substantial.
An eye pattern that perfectly represents a signal that is ideal to begin with will closely conform to a two-dimensional rectangle. The bottom and top should be sharp, horizontal lines representing the positive, zero or negative voltage rails, and the vertical lines should likewise be sharp and straight with square corners and minimal ringing.
If there is a high level of signal integrity, successive waveforms should not appear as separate, irregular lines. Even a good signal, however, will exhibit a degree of irregularity, and of course, this depends a lot on the resolution of the instrument used to create the eye pattern.
Besides variation in voltage levels, there are likely to be timing errors, such as jitter when rising and falling times go out of synch, which can be cumulative or the result of isolated random events. Either way, rising and falling edges deviate from the ideal temporal location. They will manifest as a decrease in the horizontal eye size along with data errors in the signal.
Poor terminations cause data reflections, collisions and loss of signal integrity, and they show up as increased restriction of the eye. Together with bit error rate, the appearance of eye patterns can be used to detect incipient signal integrity problems before they become critical.
The bathtub curve is another diagnostic tool that can be applied to a digital signal. It indicates bit-error rate and is generated by a bit-error rate tester. Because jitter is typically Gaussian, it is inevitable that the eye would eventually close completely after a sufficient amount of time elapsed. This unfortunate outcome can be avoided by using the bathtub curve, which takes advantage of a user-defined confidence level to put the brakes on complete eye closure.