[Elecraft] OT Cat 7 Ethernet Cable

[Jim Brown]

On Mon,3/7/2016 8:56 PM, Clay Autery wrote:
> On 3/7/2016 9:50 PM, Jim Brown wrote:
>>
>> The Ethernet birdies/carriers I listed are from the data stream, NOT
>> from the PSU. Of course the PSU often generates trash, but it's
>> DIFFERENT trash. :)
> That's a very definitive statement...  while an unlikely cause of
> multiple HF region emissions, switch-mode PS frequently switch at
> frequencies in/around HF frequencies.

Actually, switch-mode power supplies run in the range of 10-50 kHz, NOT 
at HF.  It is the unfiltered high order harmonics that we hear at HF.

>
>>> Wireless radios that are improperly designed or shielded can also be the
>>> culprit.
>> I can't see any way that a 900 MHz or 1.8 GHz radio can cause RFI at
>> HF. It is the baseband digital signals that have HF components.
> A wireless "router" radio is a radio just like any other...  they take
> an input signal, massage it and send out a signal...  they are subject
> to the SAME design issues as any other radio.  IF frequency(ies),
> mixers, oscillators, et al.  Subject to the same mixing products and
> improperly shielded can radiate any product, harmonic, etc.

But the IF and mixing products are NOT at HF. Trash from WiFi systems is 
produced by the baseband digital electronics and the switch mode power 
supplies, NOT from any part of the RF system.

>>> Not having a separate logical and chassis ground can be a problem...
>> WRONG -- that is a CAUSE of problems, not a solution. See Henry Ott's
>> classic text on EMC.
> Perhaps I should have been more precise...  logical ground and a chassis
> shield.  Although, there ARE in fact many electronic devices where the
> device itself has an isolated logical "ground" and the entire device is
> located WITHIN a totally separate shield from end to end essentially
> floating the devices on each end and the signals inside their own "space".

As Ott has observed, there are holes in that design logic.

>
> What may have confused you is that I was considering both well-designed
> devices with chassis designs that act as or contribute to the shielding
> vs. the majority of consumer grade devices which do NOT have a shielding
> chassis...  e.g. plastic case with venting, etc.

Yes, I did miss that.

> MOST of this consumer
> grade stuff has 8P8C modular jacks that are designed to accept UTP RJ-45
> connectors made of plastic thus they do not provide any means for a
> metal shielded connector (a la STP CAT 5/5e/6+ or all CAT 7 and above.
> IF you didn't do anything else, the STP cable and connectors would do
> little to assist because they are NOT connected to the chassis/supplemental shield and have no path to ground.

Exactly right.

> On a device with a plastic case providing no overall shielding and/or if
> the radio shield or PS shield, or any other potential "trash" maker
> (common OR differential) circuit is not properly designed and/or
> shielded, and/or filtered, or other mitigating method, you're peeing
> into the wind by simply adding shielded cables.  You can connect a drain
> to ground from the cable/connector housing and that will help

I don't get "drain to ground."

It IS possible to make unshielded equipment RF-quiet with proper circuit 
layout and construction. The return for current in a circuit trace over 
a continuous "ground" layer flows in the very small area of the "ground" 
trace directly under the circuit trace, forming a transmission line. In 
effect, the "ground" layer shields the conductor. Thia single "ground" 
layer technique is called "microstrip; a second "ground" layer on the 
other side of the trace provides much more shielding, and that 
construction is called stripline.

With this sort of construction and no chassis, shield return would need 
to be to an island of copper at the perimeter of the board, and all 
cable shields and external "ground" paths (like power supply) would need 
to be terminated to that perimeter shield. As with a chassis, shield 
current stays "outside the box." But if that "ground" layer is broken 
under the signal trace, return current follows whatever path left to it 
by the whim of the PC board layout "artist," which often forms a large 
loop that includes the chassis or some long return path on the board. 
Now, it's no longer a transmission line, it's an antenna (and a magnetic 
loop), so signal escapes both by antenna action and magnetic coupling.

> ...  BUT
> what you really have to do to clean up a non-FCC compliant device
> putting out harmful/unwanted IF is to either/both/hybrid:
>
> 1) Construct an internal shield that will block the radiation from
> leaving the enclosure AND connects to the signal cable shields and
> thence to ground...  OR, if you can't do it internally,...
> 2) create and EXTERNAL shield with the same electrical characteristics.
> 3) Some hybridization of the 1 and 2.
>
> Bottom line...  we said the exact same thing...

Not quite. What I've observed is that MOST of the noise we hear on HF is 
common mode current on cables connected to the device, NOT due to the 
device being poorly shielded. Yes, SOME noise is radiated directly from 
poorly shielded and/or equipment with the PCB layout issues I noted, but 
that part of the noise is rarely dominant because the antennas inside 
the box are too small to be efficient at HF, whereas the antennas 
carrying the common mode current (the interconnecting cables are usually 
much longer (including path to other equipment, power supply, facilities 
ground, etc.)

I would NEVER waste my time shielding a poorly shielded box that was bad 
enough for me to hear. I would dump it and buy something without that 
problem.

73, Jim K9YC