Tektronix TDSDDM1, User Manual, Page: 71 / 102

Measurement Descriptions
TDSDDM1 Disk Drive Measurement Application User Manual
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The NLTS measurement has the following requirements:
H
The waveform under test must have at least 1000 samples.
H
There must be at least eight samples per bit.
H
The oscilloscope must capture at least three periods of the pattern. This
depends on the waveform length, the oscilloscope sample rate specified, the
pattern duration specified, and the number of bits per period. The number of
bits per period for a pseudo-noise sequence depends on the polynomial used.
The NLTS measurement algorithm assumes that the acquired signal consists of a
periodic signal and independent additive noise. The noise is assumed to be
responsible for all the nonperiodic behavior of the acquired waveform.
The NLTS measurement is based on a time domain cross-correlation method, as
detailed in the IDEMA Test and Measurement Committee PRML Working
Group Working Paper Nonlinear Distortion Measurement Techniques, Septem-
ber 1995.
The application performs the measurement according to the following algorithm:
1. Acquire three or more cycles of the waveform.
2. The user specifies the polynomial used to generate the waveform, the
approximate duration of one period of the periodic signal captured in the
waveform, and the tolerance of the approximation.
NOTE . The specified polynomial must generate a pseudo-noise (maximum length)
sequence, or the measurement will not proceed.
3. (This step is not performed if the user specifies a Duration Tolerance of 0%.)
Compute the actual cycle time and delay values. NLTS guarantees that the
real cycle time is determined by the measurement if the initial user estimate
for the cycle time is within the specified tolerance (up to 30%) of its real
value.
The NLTS measurement uses several cycles of the waveform to improve the
accuracy of the measurement, averaging over multiple single-waveform-
cycle NLTS values.
NLTS values for the auto-correlation method are linearly related to the
autocorrelation of the acquired waveform at different time delays.
Measurement
Requirements
Test Methodology