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Sheilded CATx Cable CharacteristicsJack MacDougall & Hossein Shakiba

AMS Systems Engineering & AMS Principal Engineer, Gennum Corporation

IntroductionThe purpose of this document is to define the limitations present in the category 6/5e STP (shielded) cabling assemblies in an HDMI 1.3b compliant A/V connectivity solution.

ContentsIntroduction.........................................................................................................................................................2

Insertion Loss.......................................................................................................................................................2

STP Notch Cable Variation..............................................................................................................................3

STP Notch Jitter Analysis.................................................................................................................................4

Conclusions..........................................................................................................................................................5

STP CATx System Usage ..................................................................................................................................5

Insertion LossThe plot shown in Figure A shows the insertion loss resultant from a single differential pair, through a variety of standard TIA/EIA-568-B approved STP cable assemblies. Each of the tested STP cables reveal significant notch filtering effects very close to the in-band HDMI DTV 1080p60 12-bit Deep Colour data rates (1.125GHz Insertion Loss Frequency or 2.25Gb/s). These notch effects do not occur within unshielded cable (UTP) assemblies.

Figure A: Insertion Loss versus Frequency

50ft Cat 650ft Cat 675ft Cat 6100ft Cat 6100ft Cat 6100ft Cat 6125ft Cat 5e

In-bandnotches

Frequency [GHz]

0.5

Inse

rtio

n L

oss

[d

B]

-1001 1.5 2 2.5 3

-90

-80

-70

-60

-50

-40

-30

-20

-10

01080 p60 8 bit - 1.485 Gbps1080 p60 10 bit - 1.856 Gbps1080 p60 12 bit - 2.228 Gbps

50ft Cat 650ft Cat 675ft Cat 6100ft Cat 6100ft Cat 6100ft Cat 6125ft Cat 5e

Sheilded CATx Cable Characteristics

STP Notch Cable VariationTwo cables of the same characteristics were chosen to compare notch filtering effects on an HDMI in-band signal over the same differential pair. The cables have the same overall electrical characteristics, manufacturer, and TIA/EIA568.2-1 qualification markings.

As a side by side comparison, a sinusoidal single tone frequency, similar to the in-band fundamental frequency of a typical 1080p60 12-bit Deep Colour signal, was swept through the aforementioned notch frequencies of each cable.

The plots shown in Figure B and Figure C depict the signal attenuation across each of the 50ft' CAT6 STP cables, labelled Cable A and Cable B. In both cables, the signal was attenuated significantly at the lowest points of the notch and did recover once the frequency was increased beyond the notch band. In the case of Cable B (Figure C), the notch completely filtered out the 1.19GHz signal.

Figure B: Cable A: 50ft CAT6 STP Figure C: Cable B: 50ft CAT6 STP

Gennum Corporation

STP Notch Jitter AnalysisUsing the same 50ft CAT6 STP cable tested in Figure C (Cable B), an industry standard PRBS7 signal ranging from 2.2Gb/s to 2.6Gb/s was injected into an HDMI/CATx extender application with a typical quality equalizer. Figure D shows three eye diagrams at data rates that correspond to frequencies just below, right at, and just above the notch frequency of the cable. Note that the absolute value of the jitter is not the primary focus of this exercise and no attempt was made to optimize the equalizer for this specific application, nor was the jitter residue due to the test assembly compensated and calibrated out. Rather, the practice was to show the relative change in the eye diagram quality as the data rate approaches the notch frequency of the channel. For data rates below this frequency, the equalized jitter increase follows the expected trend versus data rate. But once the data rate sufficiently approaches the frequency of the notch, a catastrophic degradation takes place in the quality of the recovered eye, leading to a practically closed eye. Once the data rate passes the notch frequency, the eye quality starts to recover; however, a full recovery is not expected as the signal may still contain information within the lost bandwidth. From now on the operation of the link becomes data pattern dependent, ranging from a normally open eye with acceptable jitter to a completely closed eye.

Figure D: STP Notch Jitter

0

1

2.2GbpsJitter (UI): 0.47

2.4GbpsJitter (UI): 1 - CLOSED

2.6GbpsJitter (UI): 0.58

-10

Frequency [GHz]

S21

[dB

]

0.5

-30

1.5

-40

-20

-50

-60

Sheilded CATx Cable Characteristics

Conclusions• The measured notch frequencies fall directly within the 2.25Gb/s, 1080p60 12-bit

Deep Colour bandwidth and to a lesser extent, the 1.85Gb/s (10-bit) bandwidth.

• The notch filtering effect can lie anywhere between 800MHz and 1.25GHz.

• For lower data rates such as 480p (270Mb/s), 720p and 1080i (750Mb/s), 1080p60 8-bit mode (1.485Gb/s), the notch was not apparent.

• The notch location and depth varies, but is not completely unpredictable. Considering the amount of variations in CATx cable types and lengths, the frequency of the notch does seem to be predictable.

• The existence of the deep notch does make the overall system performance pattern dependent. Furthermore, seeing as all HDMI compliant designs are required to pass a PRBS7 pattern (does transmit a small instance of consecutive ones and zeros) the system may be prone to accumulative system bit errors.

STP CATx System UsageAlthough STP CATx cabling usage is never guaranteed for operation, a combination of a properly screened STP CATx cabling and cable equalization may make it possible to use all of the measured CATx STP cabling at the 1080p60 8bit mode (1.485Gbps). Although close, the notches primarily do not occur at this particular in-band location.

The 10-bit and 12-bit modes of operation should be considered practically unusable when using STP CATx cabling. This is due to the fact that even though in certain installations the system may work, but reliable operation that is required to accommodate drifts in the overall system conditions cannot be guaranteed.

Furthermore, there are no physical specifications which define or regulate the construction of the CATx shield, and therefore, no STP cabling could be guaranteed to operate under many of the 1080p HDMI DTV data rates.

As mentioned previously, the aforementioned notch frequencies are not strongly cable length dependent. However, in some instances, if sufficient equalization were to be applied to the system, shorter cables may survive the CATx notch characteristics at the 1080p,10 and 12-bit bands as long as the equalizer is able to recover the signals attenuated by the deepest point of the attenuation plot.

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