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Be careful if you want to use ISN for Cinducted Emission or Immunity testing
of Ethernet or any other high speed differential-pair interface. I just could
not resist to reply, because of so poor specifications which allow the ISN to
cause APPARENT failures, and can cost the industry millions to "fix" problems
that do not exist. I tried to talk with a major vendor of these devices,
offering help to make them actually appropriate for use with high-speed
diff-pair interfaces (e.g. Ethernet), but there appears to be absolutely no
interest. It appears there is too much politics and marketing, as opposed to
engineering, that dictates how these devices are constructed (and specified).
The cost is paid by the industry who is then forced to follow the standards. I
have worked on many cases where after a reported emission failure, measured
using ISN, the clamp-method (e.g. VOltage and Current Clamp method) resulted
in very different and passing result.
So here are some of my thoughts on this topic.They are mostly based on the use
of CDN/ISN for Immunity, but they really equally apply to both SLCE and
Immunity.
• The most recent 6100-4-6 standard specifies the common-mode (CM)
impedance
Zcm (Table 3) at the EUT-port of the coupling device
• It also specifies that the Common-to-Differential mode conversion
(defined
by Longitudinal Conversion Loss or LCL) of the CDN should be better than the
conversion of a cable or the equipment connected to the cable, whichever
conversion is lower, i.e. whichever LCL value is larger (Note in 6.2.1.2)
• The standard recognizes that a given method and coupling device may not
be
appropriate for all types of devices under test, so it allows for alternate
test methods. Specifically, in 6.2 it says:
“If CDNs are not suitable or available, other injection methods can be
used”.
• The note in 6.2.1.2, regarding common to differential mode conversion,
recognizes that the CDNs for multi-pair balanced lines (e.g. UTP) may not be
suitable:
“Often, clamp injection needs to be applied to multi-pair balanced cables,
because suitable CDNs might not be available.” *
*What defines “suitable” can be the major limiting factor
• Rules for selecting appropriate method are given in 7.1. The main
deciding
factor for selection of the method is (again): “are the CDNs suitable?”,
which could result in a subjective decision
• Conversion can be a problem with the currently available CDN and can
make
it not-suitable for Ethernet lines
• Relatively well designed RJ45/Ethernet devices can have between
-40 dB and -46 dB conversion (2%-1% impedance balance)
• Ethernet devices, as well as other high-speed devices that utilize
balanced differential pairs, are very sensitive to CM-DM conversion
• It is critical that the conversion characteristics of any coupling
devices
be at least the same if not better than the conversion by the well-designed
signal lines used for the application
• In practice, on multi-pair balanced cables where all wires are coupled
(e.g. 8 wires of a CAT5 UTP), it is not easy to:
– specify how to measure conversion
– measure conversion
– specify required levels of conversion
• Lack of clear and appropriate method for measuring conversion can also
be
a problem with currently pending draft CISPR22 emission standard.
• The problem lies in the fact that the CDN/ISN is inserted into an
otherwise well-balanced line. The design of the CDN/ISN is not balanced (the
standards lack realistic requirement and appropriate verification method) and
it creates mode-conversion, which can create apparent CM emission and
susceptibility. This condition does not resemble the realistic situation were
the cable is fairly balanced and uninterrupted
• Conversion by the CDN/ISN can be well beyond the conversion
characteristics of the cable. The currently specified test method for
measuring conversion (or LCL) is not appropriate for multi-pair interfaces.
• Test equipment should not increase the conversion by introducing
imbalance
considerably higher than that of the cable. Conversion by the test equipment
artificially increases the test levels beyond the characteristics of the
measured device and intended cabling, causing apparent failures
• Therefore, unless better CDN/ISN devices are constructed (and/or
better-defined in the standards), I recommend methods using clamps (e.g.
Voltage and Current Clamp method), which preserve integrity of the used
cabling and allow testing in more realistic environment. The CM-impedance
requirement that can be of some concern with the clamp-methods is not hard to
meet, at least for 4-pair differential interfaces, but that is another topic.
I hope it helps make your decision.
Regards, Neven
-------------- Original message ----------------------
From: <[email protected]>
> Hi,
> this may be easier than you think (or wish).
> To my knowledge there are only two manufacturers at the moment: Teseq
> and Fischer.
>
> Regards,
> Ari Honkala
>
>
> On 5/8/07, wendy <[email protected]> wrote:
>
> Dear All,
>
>
>
> My company intends to purchase the above mentioned. Will
> someone recommend any brand/model that compiles to the latest
> requirements?
>
>
>
> Regards,
>
> Wendy Nya
>
> -
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