Bear in mind that the recommendations by cable vendors are not taking
into account that the Ethernet device in this case is a 100W RF signal
generator. The same RF that can flow down the shield of a Firewire
cable due to common mode RF currents that can lock up a PC can also flow
down the shield of a STP Ethernet cable and lock up an Ethernet switch
too. A different dynamic you need to considerwhen visualizing this system.
Tim Ellison
On 12/3/2012 3:45 PM, Stan Williams wrote:
Thanks Tim for your response. I am glad to hear that the 6000 series uses
shielded RJ-45 connectors.
We agree that the newer CAT6 and 6A cables have better alien signal
rejection due to their tighter and quality controlled twist structure, as
well as the balanced nature of the Ethernet physical units. We also agree
that grounding only one end of a shielded cable will avoid ground
differential common mode signals.
I do question, however, your assertion that a STP or SSTP cable will inject
alien signals into the cable's inner pairs when grounded at both ends.
Studies and practice demonstrate that the alien signals injected, below
30Mhz where they will penetrate the shield, are substantially lower than if
there were no shield (20-40db).
Sample reference:
http://www.gocsc.com/UserFiles/File/Siemon/WPShieldedMythCUS.pdf is written
by a STP cable manufacturer.
My shack has an isolated ground so I would choose to use SSTP cable between
everything inside that ground perimeter and ensure that the connection to
the backbone is through an unshielded RJ-45 connector. The backbone is also
shielded but again it doesn't cross different ground systems.
-----Original Message-----
From: Flexedge [mailto:[email protected]] On Behalf Of Tim
Ellison
Sent: Sunday, December 02, 2012 11:30 AM
To: [email protected]
Subject: Re: [FlexEdge] Type of RJ45 connector on 6000 series, Stan Williams
VE3FLW
The RJ-45 connectorshell on the FLEX-6000 is grounded, but we are not
recommending the use of shielded Ethernet cables with the FLEX-6000.
There are two concerns here; emitted RF from the Ethernet cable and its
susceptibility to RFI ingress. Let's address the later first.
UTP (unshielded twisted pair) cable is by its very nature, balanced and very
immune to RFI and has very good noise rejection characteristics due to the
differential signaling methodology used for twisted pair Ethernet.
Shielded twisted pair (STP) CAT cable is only really effective if physical
connection does not create a ground loop, a difference in voltage potential
between the chassis grounds (a DC path between the two), which can cause
excessive common mode currents on the shield and provide an ingress point
for RFI. The only place I have seen it be effective is in telco COs where
EVERYTHING is at the same ground potential and the equipment is designed to
be used in that type of grounding system (NEBS compliance)
In the shack, you would need to ensure that everything is connected to the
same ground potential, there are no ground loops via the AC safety ground
and that you are connecting the right ground plane in the equipment to the
grounding system. Since creating the ideal grounding infrastructure involves
a lot of careful engineering, it is easy to miss something, therefore it is
better to use UTP and not induce a new problem where one did not exist in
the first place. STP network cable can be useful in demanding electrical
environments, such as environments are where the network cable is located in
parallel with electrical mains supply cables or where large inductive loads
such as motors. This is usually not the case in the shack.
Additionally, the physical Ethernet interface (port) is transformer coupled
for DC isolation, which eliminates ground loops between the connected
equipment. Adding STP into the equation could negate that inherent
isolation. You could take a STP cable and replace one end with a
non-conducting connector and make sure you connect the connected connector
to equipment that has a very good RF ground, but I digress.
From the standpoint of your Ethernet cabling emitting RFI, the second
concern listed above, if the network device/hardware is emitting RFI, this
is because the equipment it is connected to is very poorly engineered and
does not have sufficient filtering. It probably is not FCC Class B
compliant (any more). The FLEX-6000 will be compliant for CE emission
standards, which are more stringent than the FCC. In these cases where the
equipment is radiating, RF interference suppression is best effected by
employing ferrite beads at the cable end connected to the offending
equipment or on the power cord. Consumer network equipment that use "wall
warts" are especially guilty of being RFI emitters. STP cabling can also be
effective in reducing RFI emissions, but only if they are grounded properly
and the RFI isn't coming from the equipment's ground plane itself. In this
later case, STP will make to problem worse by the cable shield becoming a
very effective radiator.
So, I as initially said, using STP vs. UTP Ethernet cabling can be
problematic and actually make issues worse, which is why it is not
recommended for use with the FLEX-6000.
Tim Ellison
On 12/1/2012 10:54 PM, Stan Williams wrote:
The best network cables in high RF environments are shielded, for
example CAT6A shielded (STP) or double shielded (SSTP) with shielded
RJ45 type connectors. For these to be most effective at least one end
should be connected to a box that is grounded.
Do the 6000 series have shield continuing RJ45 connectors so the
station ground may protect the CAT cables?
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This is the FlexRadio Systems e-mail Reflector called FlexEdge. It is used
for posting topics related to SDR software innovation and other technical
SDR topics.
_______________________________________________
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This is the FlexRadio Systems e-mail Reflector called FlexEdge. It is used for
posting topics related to SDR software innovation and other technical SDR
topics.
_______________________________________________
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[email protected]
http://mail.flex-radio.biz/mailman/listinfo/flexedge_flex-radio.biz
This is the FlexRadio Systems e-mail Reflector called FlexEdge. It is used for
posting topics related to SDR software innovation and other technical SDR
topics.
