LeCroy DDA 7 Zi series, Руководство оператора

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WavePro 7Zi
431
WP700Zi-OM-E-RevA
Signal Slew Rate and How Vertical Noise Converts to Jitter
In RF electronics, this process is known as AM to PM conversion. The basic principle is that the slew rate of the
signal serves as a mechanism for vertical noise to be confused with horizontal (timing) noise, and vice versa. The
following image illustrates:
It is easy to see how if the slew rate is very high (i.e., very low rise-time) that the effect of vertical noise is low and,
conversely, that if the slew rate is very low, the vertical noise can dominate.
The Relative Impact of Noise on Jitter (Quadratic Addition of Noise)
The “quadrature addition” used to “add” the noise components is really only precise for Gaussian distributions of
variations (even though this works well for many combinations of incoherent phenomena).
What’s important in this application is that (at least for current instruments) the instrument’s contribution to vertical
noise is almost always Gaussian in nature. Furthermore, this can be confirmed by in-situ calibration procedures.
How the Measurement System Noise Is Subtracted from the Jitter Measurement
The timing noise can be very complex, exhibiting periodic jitter ( Pj ) and, for data streams, ISI induced data
dependent jitter DDj . Other forms of bounded uncorrelated jitter ( BUj ) can contribute, as well, to make the jitter
distribution shape quite non-Gaussian.
If, however (as is usually the case), the jitter analysis breaks down the distribution characteristic into Rj and Dj ,
the systematic effects are lumped into the Dj part. The Rj part of this jitter behaves as a Gaussian, and the vertical
noise contribution (from the instrument) can be compensated as follows:
So, if the dv/dt can be well estimated by the instrument, and if the instrument’s contribution to vertical noise is
known (and incoherent with the signal, which is nearly always the case), this compensated Rj figure can be
reported.
As such, the LeCroy SDA software has been equipped to perform this compensation, and applies it to the Rj
figure and not the Dj figure; however, the corresponding effect on Tj is reported as well.