Symmetricom 5115A, Руководство по эксплуатации и обслуживанию

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5120A/5115A Operations and Maintenance Manual 69
8: Optional Functionality
The PNTS include optional DOCSIS functionality.
8.0.1 DOCSIS Overview
DOCSIS stands for “Data Over Cable Service Interface Specification”. PNTS with the DOCSIS
option enabled can be used to easily test jitter for cable modems and supporting equipment. The
goal of these tests is to ensure that the master clock and symbol clock are properly locked via the
M/N ratios as per the DRFI Specification Section 6.3.6. The DOCSIS option is included with the
PNTS.
The test works on the assumption that locking between the master clock and the symbol clock may
be verified by comparing the master clock from a DTI server test port to that of the symbol clock
from the EQAM RF output in alternating binary sequence mode. All measurements must be
performed with worst case common mode noise on the DTI client input from the DTI server. The
test run by the option covers 100% of the locking parameters on a single channel or the single test
port. Other channels are assumed to behave in the same manner.
The jitter calculation uses frequency counter’s reference frequency, 100 second frequency
average, integrated phase noise with a low limit of 1 mHz and the user supplied DRFI ratio as
inputs. It computes jitter resulting from frequency error (
J1 ) and jitter resulting from random
variations ( J2 ), and finally combines those values to compute Total Jitter . Table 5 explains in
detail how Total Jitter is calculated.
Table 5: Calculating Total Jitter
Value Units Definition or Calculation Notes
M DRFI specification ratio numerator see step 3 below
N DRFI specification ratio denominator see step 3 below
reference_frequency Hz reference frequency see step 4 below
desired_freq Hz M / N * reference_frequency
meas_freq Hz measured input frequency 100 s frequency average
meas_random_jitter_RMS radians
Φ
RMS(1 mHz) integrated phase noise
con s 50 / SQRT(3) constant
J1 ns con * | meas_freq - desired_freq | / desired_freq jitter from frequency error
J2 ns meas_random_jitter_RMS / (2 *
Π
* desired_freq) jitter from random variations
Total Jitter ns
SQRT(J1
2
+ J2
2
)
pass/fail criterion