Text preview for : 5991-4088EN English _ 2014-04-15 _ PDF 3.82 MB c20140925 [16].pdf part of Agilent 5991-4088EN English 2014-04-15 PDF 3.82 MB c20140925 [16] Agilent 5991-4088EN English _ 2014-04-15 _ PDF 3.82 MB c20140925 [16].pdf
Back to : 5991-4088EN English _ 201 | Home
Keysight Technologies
Evaluating Oscilloscope Signal Integrity
Application Note
Introduction
The term "signal integrity" surfaces regularly in electronic test. Signal integrity is the primary measure
of signal quality, and signal integrity importance increases with bandwidth, the need to view small
signals, or the need to see small changes on larger signals. Why does oscilloscope signal integrity
matter? Signal integrity impacts all scope measurements. The amount of impact signal integrity can
make on signal shape and measurement values might surprise you. Oscilloscopes themselves are
subject to the signal integrity challenges of distortion, noise, and loss. Scopes with superior signal
integrity attributes provide a better representation of signals under test, while scopes with poor
signal integrity attributes show a poorer representation of signals under test. This difference impacts
engineers' ability to gain insight, understand, debug, and characterize designs. Results from
oscilloscopes with poor signal integrity can increase risk in development cycles times, production
quality, and components chosen. To minimize this risk, it is a good idea to evaluate and choose an
oscilloscope that has high signal integrity attributes.
03 | Keysight | Evaluating Oscilloscope Signal Integrity - Application Note
Infiniium DSO9404A (4 GHz) Infiniium DSOS404A (4 GHz)
Eye height Eye height
1.55 V 1.75 V
Figure 1(a). Even scopes from the same vendor rated at equal bandwidths can have varying signal integrity qualities. Here we show an eye diagram from the same
source on two scopes each rated at 4 GHz. The scopes have identical bandwidth, vertical, and horizontal settings. The Infiniium S-Series show a more represen-
tative eye shape and has an eye height measurement 200 mV higher than what the DSO9404A shows. The lower value and more representative shape is a result
of superior signal integrity.
Signal integrity is made of many attributes and this application note There are multiple scope attributes that work in conjunction with
walks through each of them in detail. The principles here can be each other and hence the topic must be considered holistically. There
applied to scopes in all bandwidth ranges. This application note are many banner specifications that are said to give you the `best';
articulates key signal integrity attributes and uses Keysight resolution, noise floor, jitter, etc. But you need to be mindful that
Technologies, Inc. Infiniium S-Series oscilloscopes for examples one good specification will not ensure the best representation of the
in the 500 MHz to 8 GHz bandwidth ranges. signal. Only considering multiple attributes together will solidify what
oscilloscope to choose. Looking at only a single signal integrity attri-
bute in the absence of others; can lead a user to false conclusions.
LeCroy 604Zi (4 GHz, 20 GSa/s) Infiniium DSOS404A (4 GHz, 20 GSa/s)
Rise time std deviation Rise time std deviation
4.0 ps 668 fs.
Figure 1(b). Note the difference in the standard deviation of rise time measurements. Both scopes are rated at 4 GHz bandwidth, are sampling at 20 GSa/s
and set to the same settings. When measuring a fast rise time, the Infiniium S-Series reports a rise time value with standard deviation of 668 fs while the
LeCroy scope reports a standard deviation of 4 ps, six times larger than the standard deviation measured by the Infiniium S-Series. Small standard devia-
tions when measuring rise time on the same signal indicate superior signal integrity and a better horizontal system.
04 | Keysight | Evaluating Oscilloscope Signal Integrity - Application Note
ADC Bits and Minimum Resolution
A key technology block for vertical signal integrity is the analog-to- Let's look at a specific example as shown in Figure 3. Two scopes
digital converter (ADC). The higher the number of ADC bits, the more are both scaled to 800 mV full screen. A scope with an 8-bit ADC has
resolution the scopes has. A 10-bit ADC ideally provides 4 times resolution of 800 mV/(28 = 256 Q levels), or 3.125 mV. A scope with a
the resolution as a scope with an 8 bit ADC. Similarly, a 12-bit ADC 10-bit ADC has resolution of 800 mV/(210 = 1024 quantization levels),
provides 4 more times the resolution that a 10-bit ADC. In the case of or 0.781 mV. In normal mode, each scope can only resolve signals
the Infiniium S-Series there is a 10-bit ADC (shown in Figure 2) that down to the smallest Q level.
enables more resolution than a scope with only an 8-bit.
When excess sample rate is available coupled with an analog front
Resolution is the smallest quantization level determined by the end to prevent aliasing, scopes typically offer another mode called
analog-to-digital converter (ADC) in the oscilloscope. A scope's ADC high-res mode. Oversampling techniques combined with DSP filters
with a resolution of 8 bits can encode an analog input to one in 256 can increase vertical resolution. Vendors often refer to this increase in
different levels, since 28 = 256. We'll refer to these as quantization or terms of "bits of resolution." In the case of Infiniium S-Series, high-res
Q levels. The ADC operates on the scope's full scale vertical value. increases bits of resolution from 10-bits (native ADC resolution), to
For both current and voltage measurements, the Q-level steps are 12-bits of resolution. This technique requires an ADC that has been
associated with the full-scale vertical scope setting. If the user adjusts architected with excess sample rate relative to the hardware band-
the vertical setting to 100 mV per division, full screen equals 800 mV width needed for a particular measurement.
(8 divisions * 100 mV/div) and Q-level resolution is equal to 800 mV
divided by 256 levels, or 3.125 mV. Increased ADC bits and a wider range vertical sensitivity
increases resolution.
Traditional scope S Series ADC