`Jitter Happens` when a Twisted Pair is Unbalanced

`Jitter Happens` when a Twisted Pair is Unbalanced
Application Note:
HFAN-4.5.4
Rev.1; 04/08
‘Jitter Happens’ when a Twisted Pair is Unbalanced
Functional Diagrams
Pin Configurations appear at end of data sheet.
Functional Diagrams continued at end of data sheet.
UCSP is a trademark of Maxim Integrated Products, Inc.
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AVAILAB
‘Jitter Happens’ when a Twisted Pair is Unbalanced
Deterministic Jitter (DJ) can be caused by Differential-to-Common-Mode conversion (or viceversa) within a Twisted Pair (STP or UTP), usually a result of twist or dielectric imbalance
The good news is that specific cable properties,
such as high common-mode loss, can help mitigate
intra-pair skew despite other asymmetries.
1 Purpose
Despite the title, this is NOT a paper about
psychiatric pathology in couples.
2 Test Case: Digital Video
This paper IS about test methods to differentiate
good quality twisted pair cable, from bad, for data
rates >500Mb/s. Measurement examples will show
that severe intra-pair delay skew introduced by a
twisted pair cable can be a problem that is NOT
recoverable by either equalizers or compensating
skew.
The DVI and HDMI digital video standards refer to
data transmission over long STP (Shielded Twisted
Pair) cables at serial rates up to 1.65Gb/s. Ultralong cables (e.g., 50-100ft) suffer from the
accumulative effects of both skin-effect high
frequency losses, and intra-pair skew.
S-PARAMETERS: 15 meters, 30 AWG STP
0
-20
-30
3950
3800
3650
3500
3350
3200
3050
2900
2750
2600
2450
2300
2150
2000
1850
1700
SDD21
SDC21
SCD21
SCC21
-10
LOSS [dB]
1550
1400
1250
1100
950
800
650
500
350
200
50
MAXIM CONFIDENTIAL
dB
dB
dB
dB
-40
-50
-60
-70
-80
-90
FREQUENCY [MHz]
Figure 1
Application Note HFAN-4.5.4 (Rev.1; 04/08)
1.65Gb/s Data,
MAX3800 EQ Output,
Scope @ 300ps/div
Maxim Integrated
Page 2 of 5
S -P A R A M E T E R S : 1 5 m e te rs , 2 6 A W G S T P
3830
3650
3470
3290
3110
2930
2750
2570
2390
2210
2030
1850
1670
1490
1310
1130
950
770
590
410
230
50
MAXIM C O N FID E N TIAL
0
S DD21 dB
LOSS [dB]
-1 0
S DC21 dB
-2 0
S CD21 dB
-3 0
S CC21 dB
-4 0
-5 0
-6 0
-7 0
-8 0
F R E Q U E N C Y [M H z ]
1.65Gb/s Data,
MAX3800 EQ Output,
Scope @ 200ps/div
Figures 1 and 2 show 1.65Gb/s DVI signaling over
50 feet (15m) samples of economical STP cable,
the first (Fig 1) having excellent performance, and
the second (Fig 2) having poor performance.
Figure 1 demonstrates textbook skin-effect loss
(SDD21) with low differential-to-common-mode
conversion (SDC21).
Note the scope photos in Figures 1 and 2 show the
state-of-the-art MAX3800 Equalizer compensating
1.65Gb/s data for accumulative skin-effect losses:
Superb results are seen in Figure1, but Figure2
clearly shows the deterministic jitter (DJ)
remaining after EQ due to severe differential-tocommon-mode conversion, a.k.a. intra-pair skew.
Figure 2
skew, the fidelity of digital differential signaling is
compromised and may be unrecoverable.
For a “simple” case to examine, consider twin coax
(two separate parallel coax cables) carrying a
differential signal between them, inherently having
NO coupling between the signal pair. In this case,
any intra-pair delay skew shows up as a “pure delay
difference” at the output. Spectrally, nulls occur in
the frequency response, with first differential-mode
null at frequency = 1 / ( 2 * delaydifference). In
other words, the first null occurs at a frequency
where the difference in electrical delay between the
two coax cables is one-half wavelength. At this
same frequency there is a common-mode maximum
(both signals in the pair are now in-phase).
3 “Simple” Intra-Pair Skew
Given a differential swept-sine-wave stimulus over
a differential cable, any amount of intra-pair skew
within the cable pair will manifest as a conversion
from differential-mode to common-mode, and vice
versa. Since conversion to common-mode varies
with frequency, for a given amount of intra-pair
Application Note HFAN-4.5.4 (Rev.1; 04/08)
Maxim Integrated
Page 3 of 5
Of course, this simple case of “pure delay
difference” could be fixed by inserting a matching
“pure delay” in the signal path of the shorter of the
two cables. This may be easy in concept, but is not
a solution for the general case.
4 Coupled Differential Pairs
(STP, UTP, Twinax)
Now let’s focus on the real world of coupled
differential pairs, with or without shielding. These
cables derive a portion of their characteristic
differential impedance from coupling between the
pair, in addition to coupling to ground (e.g., shield).
rate, need only have a 1” asymmetry in electrical
length to have a 0.25 unit interval (UI) error.
Fortunately, there are tactics in the construction of
economical cable to mitigate inherent intra-pair
skew. One approach is to increase common-mode
loss, as compared to differential-mode loss (while
maintaining EMI shielding performance). Note, in
Figure 1, that the common-mode loss is somewhat
greater than the differential-mode loss. Other lowcost cables have been observed with even higher
common-mode loss and predictably excellent
differential-mode behavior. The construction of the
foil shield and dielectric are the root of this
behavior.
5 Cable Measurements:
Differentiating Good from Bad
Several methods have been used to measure intrapair skew.
A frequency domain method is
suggested here, in preference to typical step-delay
methods.
Any imbalance in the differential pair, such as
asymmetry of length or twist or dielectric
environment, results in some intra-pair skew.
Note, however, in coupled differential pairs, the
intra-pair skew usually does not behave as simple
skew, as was the case with the “ideal” predictable
skew of dual coax in the previous section. Unlike
the “ideal” dual coax case, coupled differential
pairs suffer from dissimilar loss and velocities for
the differential-mode and common-mode! For
instance, in Figure 1, notice the different loss
characteristics for the differential mode (SDD21)
vs. the common-mode (SCC21). Since intra-pair
skew progressively converts signal energy between
these two modes, the end result is no longer an
“ideal” delay difference, or skew, at cable end!
Of course, intra-pair skew can be minimized by
attention to equal length and twist symmetry, plus
balance in the dielectric and shield environment.
This can be a difficult requirement for economical
consumer-grade cables. For instance, a 100 ft
(30m) DVI digital video cable, at 1.65Gb/s data
Application Note HFAN-4.5.4 (Rev.1; 04/08)
Determining intra-pair skew by measuring the
difference between two single-ended step-delay
measurements can be misleading. A single-ended
step stimulus (stimulating only one side of
differential pair at a time) launches both
differential-mode and common-mode energy. Due
to the different propagation velocities and loss
between the modes, the resulting delay numbers do
not apply to the case with a differential-mode-only
source! Further, long cable performance cannot be
predicted from these short cable measurements.
5.1
FREQUENCY DOMAIN METHOD:
DVI Example
For the DVI application, the testing of a given
“long” cable assembly can be made at 2 or 3
selected frequencies (e.g. ½-max-bit-rate: 800MHz
sine, and ¼-max-bit-rate: 400MHz sine), sensing
either the differential-to-common-mode (SCD21)
or
common-mode-to-differential
(SDC21)
conversions. Experience shows that measuring
only at a few spot frequencies is permitted because
“poor” SCD21 or SDC21 occurs over a broad
frequency range (i.e, hard to miss). Either SCD21
or SDC21 response will give the same result and
Maxim Integrated
Page 4 of 5
the choice can be made by equipment availability
(See Table 1).
SDD21 should not have to be measured as it is a
known quantity (skin-effect loss) at these
frequencies, for a given cable AWG.
The objective is the SCD21, or equivalently the
SDC21, should be at least 12dB below the primary
differential-to-differential response (SDD21) at all
frequencies up to bit-rate. You will note this to be
true of the 30AWG cable (Fig 1), but violated
miserably by the 26AWG cable (Fig 2). The
6
Stay Tuned for More …
As MAXIM introduces more solutions for these
cable challenges, this topic will be developed
further in the form of articles and application notes.
Table 1
RECOMMENDED CABLE QUALIFICATION FOR LONG DVI CABLES:
1) Measure common-mode-to-differential conversion of a DVI cable assembly:
a. Apply Single-Ended sine-wave generator for frequencies 800MHz and 400MHz,
and a 50ohm-Power-Splitter, so that the (+) and (-) of the differential STP cable
can be driven together (in common-mode).
b. View Differential Scope or Power Meter to measure cable conversion output at these two
frequencies, where the (+) and (-) of the differential STP cable are sensed differentially.
c. “PASS” a cable demonstrating it has output conversion measurement (SDC21) at least
12dB below the expected differential skin-effect loss (SDD21) at both 800MHz and 400MHz,
respectively. (i.e., note Figure 1 passes easily, and Figure 2 fails)
OR
2) Measure differential-to-common-mode conversion of a DVI cable assembly:
a. Apply Differential sine-wave generator for frequencies 800MHz and 400MHz,
where the (+) and (-) of the differential STP cable are driven differentially.
b. View Single-ended Scope or Power Meter to measure cable conversion output at these two
frequencies, and a 50ohm-Power-Splitter, so that the (+) and (-) of the differential STP cable
can be sensed for the common-mode signal.
c. “PASS” a cable demonstrating it has output conversion measurement (SCD21) at least
12dB below the expected differential skin-effect loss (SDD21) at both 800MHz and 400MHz,
respectively. (i.e., note Figure 1 passes easily, and Figure 2 fails)
Application Note HFAN-4.5.4 (Rev.1; 04/08)
Maxim Integrated
Page 5 of 5
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