8: Optional Functionality; Docsis Overview - Symmetricom 5120A Operation And Maintenance Manual

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.
detail how Total Jitter is calculated.
Table 5: Calculating Total Jitter
Value
M
N
reference_frequency
desired_freq
meas_freq
meas_random_jitter_RMS radians Φ
con
J1
J2
Total Jitter
5120A/5115A Operations and Maintenance Manual
Units Definition or Calculation
DRFI specification ratio numerator
DRFI specification ratio denominator
Hz reference frequency
M / N * reference_frequency
Hz
Hz measured input frequency
RMS(1 mHz)
s 50 / SQRT(3)
ns con * | meas_freq - desired_freq | / desired_freq
meas_random_jitter_RMS / (2 * Π * desired_freq) jitter from random variations
ns
ns
2 2
SQRT(J1 + J2 )
Table 5 explains in
Notes
see step 3 below
see step 3 below
see step 4 below
100 s frequency average
integrated phase noise
constant
jitter from frequency error
pass/fail criterion
69