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Waveform measurements

Not all waveform measurements of optical signals are performed with a

reference receiver. The filtering can be switched out to provide a wider

bandwidth measurement system. The unfiltered properties of the wave-

form are accurately observed. The transmitter output may be viewed as an

unfiltered eye, or as a pulse train depending on how the DCA is triggered.

A DCA can be placed in ‘pattern lock’ mode to view the individual bits of a

digital communications signal allowing a simple analysis of the waveform

quality including parameters such as rise and fall times, pulsewidth and

overshoot. In ‘pattern lock’ mode a complete single-valued waveform re-

cord, up to 2^23 bits long, can be recorded for off-line analysis. Advanced

signal processing is available with the 86100D (see pages 31 to 36).

The “equivalent time” sampling oscilloscope, with configurations having

over 80 GHz of bandwidth and extremely low levels of intrinsic jitter, is

the most accurate tool available for jitter measurements at high data

rates.

In many communications systems and standards, specifying jitter involves

determining how much jitter can be on transmitted signals. Jitter is

analyzed from the approach that for a system to operate with very low

BER’s (one error per trillion bits being common), it must be characterized

accurately at corresponding levels of precision. This is facilitated through

separating the underlying mechanisms of jitter into classes that represent

root causes. Specifically, jitter is broken apart into its random and

deterministic components. The deterministic elements are further broken

down into a variety of subclasses. With the constituent elements of jitter

identified and quantified, the impact of jitter on BER is more clearly

understood which then leads to straightforward system budget allocations

and subsequent device/component specifications. Breaking jitter into its

constituent elements allows a precision determination of the total jitter on

a signal, even to extremely low probabilities.

Applications:

Communications Waveform Measurements

Individual bits can be observed in a ‘pattern lock’ display

Typically an external timing reference is used to synchronize the oscillo-

scope to the test signal. In cases where a trigger signal is not available or

when required for a standards compliance measurement, clock recovery

modules or clock recovery instruments are available to derive a timing

reference directly from the waveform to be measured. Clock recovery

not only provides a convenient method to synchronize the oscilloscope, it

can also control the amount of jitter displayed. Clock recovery effectively

creates a high-pass effect in the jitter being observed on the oscilloscope.

The clock recovery system loop bandwidth defines the filtering range (see

Agilent Product Note 86100-5).

Jitter analysis

Every high-speed communications design faces the issue of jitter. When

data are jittered from their expected positions in time, receiver circuits

can make mistakes in trying to interpret logic levels and BER is degraded.

As data rates increase, jitter problems tend to be magnified. For example,

the bit period of a 10 Gb/s signal is only 100 picoseconds. Signal

impairments such as attenuation, dispersion and noise can cause the

few picoseconds of timing instability to create eye closure that can mean

the difference between achieving or failing to reach BER objectives.

The problem is further aggravated by the difficulty presented in making

accurate measurements of jitter. A variety of measurement approaches

exist but there has been frustration within the industry around the

complexity of setting up a measurement, getting repeatable results and

the inconsistency of different techniques.

Advanced analysis identifies sources of jitter

Time domain reflectometry and transmission

Most optical devices have high-speed electrical input and output

paths. High signal integrity is achieved with well designed signal paths.

DCA’s can also be configured as time domain reflectometers (TDR) to

easily determine the transmission and reflection properties of electrical

channels. This information can be presented as a function of time or

frequency as S-parameters. Most new circuit designs are differential

to improve crosstalk and interference performance. Circuits need to be

characterized in single-ended, differential signal and common signal

configurations.

The TDR module sends a fast edge along the transmission line, then

analyzes the reflected signal and displays voltage or impedance versus

distance. This information can also be converted into the frequency

domain to display return loss, VSWR or reflection coefficient versus

frequency. Any selected portion of the trace can also be assessed for the

excess inductance or capacitance, allowing the designer to estimate the

amount of required compensation in that region.

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Agilent Technologies N4970A Manuale Utente Pagina 11

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