Tim,
There is two major strategies as you build a system and needs to figure
out how your ground bonding network (often just referred to bonding
network or grounding) should operate.
The isolation strategy says that the various equipments should only be
power grounded, as required for personal safety, and then have all other
grounding paths "galvanically separated" (thus, DC and power frequencies
separated in common mode).
The mesh strategy says that you extend the grounding of the power ground
with additional grounding with every cable and additional grounding
cables. This strategy strengthen with every cable you pull, as the
conductance increases. Any potential difference produce a current that
is current shared by the shunted conductors. It is common to explicitly
add main grounding conductors to take the main current and reduce the
current on other cables.
The difficulty with the isolation strategy is that if you happens to
make contact with ground, you can now be the source of a large current
to average out the potential, and that may not be what you want to see
on your coax cable for instance. We have pointed out that it may not be
so nice that we have sparks jumping the connectors on broadcast
equipment, as we saw once.
Another difficulty with the isolation strategy is that it makes the EMC
aspect harder to do, as you want the shield of your cable to extend the
shield of your box, and not become a source of energy emitting out on
the cable and thus be a conducted source of RF emission (and reception).
The RF choke with associated capacitance (or conductance, the important
being low-impedance path) on both sides is the way to go to achieve good
common mode RF rejection.
In the mesh strategy, you can skip transformers most of the time and
only use RF chokes, and then mostly to decouple the chassi and PCB
RF-wise on common mode.
Measurement instruments is most of the times built for the mesh
strategy, with BNC/SMA/N connectors hard-tied to the chassi. There is
the braindead idea to cut the chassi to ground connection, which do make
some measurements easier, but kills the safety. If you follow the mesh
strategy, you have the star-ground of the power-system complemented with
additional grounding wires of the racks etc.
So, you wire your instruments together, and then provide multiple ground
connections to your DUT. You need to figure out how to avoid common mode
to differential mode conversions, but that is not unique to the mesh BN
strategy, it's even more important in the isolation BN strategy as you
have higher potentials to isolate.
Even when having the majority of the rig being mesh BN style, for some
measurement connections it can be beneficial to do DC separation of
common mode, in which case transformers or capacitors can provide
isolation. I prefer to use differential amps when possible, and for some
reason the 1 GHz differential probe is often used in the lab.
As a reference, Ethernet is designed to work in an isolation BN setup,
because the Ethernet connections often span over an office building,
between different branches of the power distribution for which the
grounding wires can have quite different potential and hence there being
a potential difference that can produce a sizeable current. It also runs
in environments where typical does not understand grounding issues, and
where by local code and design of equipment, they have a star grounding
network and no concept of interconnection between consumers (think of
lamps, radiators, kitchen stoves and ovens and similar "simple" devices).
Also recall that the first rule of thumb for electrical safety is that
the first connection you make to a box is ground, and the last
connection you remove is to ground. Thus, a box shall at least be
grounded for safety, and only when grounded it will receive power, from
anywhere.
There is more war-stories to be told.
Cheers,
Magnus
On 12/19/2015 03:23 PM, Tim Shoppa wrote:
I think there is a valid heritage in transformer isolation in time and
frequency distribution, and it goes back to when telephone wiring was
used to distribute audio-type IRIG signals around a campus or other
facility. Even if a bunch of 60Hz or a local AM station was leaking
through the IRIG signaling was quite impervious to it. (Heh, the
aircraft VHF radio getting into Spinal Tap's lead guitar was hardly
noticeable at that air force base, for that matter!!!)
But something feels "off" with lifting grounds on coax if the
environment is just a test lab.
CAT 5/6 and Ethernet transformers work great at 10MHz but most all test
equipment is expecting coax and a BNC.
Tim N3QE
On Sat, Dec 19, 2015 at 8:29 AM, Magnus Danielson
<mag...@rubidium.dyndns.org <mailto:mag...@rubidium.dyndns.org>> wrote:
Transformer isolation isn't helping much at RF, as you will
capacitively couple through the transformer. I've been bitten by
that in real life, as I was called in to solve issues in someone
elses design. It was only when I introduced an RF choke that we got
conducted noise battled. It's also not enough, as the RF choke needs
an RF path to ground in order to start rejecting effectively, which
was the issue another time, so you want an RF choke with caps to
ground on the inside.
The galvanic isolation can be done using transformer or capacitors
after that.
There is an over believe in isolation, as it only takes one mistake
to break the system. Another approach is to ground everything,
cross-ground etc. and bring the DC/power-spurs down through
conduction. It have proven itself easier to ensure RF properties
when shield and chassi is tied hard to each other, as it provides
good RF conduction and the cable does not act like an antenna
against the shield for the RF power being unbalanced. The RF choke
then acts to separate the chassi RF from that of the board,
assisting in the balance.
Transformers can provide RF shielding, if they have double shields
between the coils, and where the shield of each side is connected to
it's ground. That way each coil will capacitively terminate in it's
own shield, and the remaining capacitive coupling will mainly be
between the shields and hence grounds. I rarely see people doing this.
I've been bitten multiple times by the capacitive coupling in
transformers, and only when I found a way to handle it things have
started to work. It's not all magnetics.
Cheers,
Magnus
On 12/19/2015 12:33 AM, Tim Shoppa wrote:
All the inputs and outputs were deliberately transformer
isolated. Why
break the isolation by using capacitor from coax shield to
chassis ground?
I do realize that some isolation transformers have "extra
floating turns"
to give transformer action that cancels stray capacitive
coupling. I don't
think the capacitors tying coax shield to chassis ground can
serve that
purpose.
Tim N3QE
On Mon, Nov 30, 2015 at 3:02 PM, Anders Wallin
<anders.e.e.wal...@gmail.com <mailto:anders.e.e.wal...@gmail.com>>
wrote:
HI all,
I need to build a few distribution amplifiers (>90% for
10MHz, sometimes
maybe 5MHz) and instead of reinventing the wheel I decided
to try to
modernize the TADD-1 into an all (almost) SMD design. Here
are some draft
sketches:
http://www.anderswallin.net/2015/11/frequency-distribution-amplifier-plans-a-k-a-smd-tadd-1/
Does this sound/look reasonable or crazy?
Any suggestions for op-amps to try and/or compare to the AD8055?
What causes the extra phase-noise below 1 Hz offset in John
A's result:
https://www.febo.com/pages/amplifier_phase_noise/amplifier_phase_noise.png
Suggestions for a low noise DC-regulator circuit? The
12-24VDC supplied to
this board will most likely come from a switched-mode PSU,
so filtering of
common-mode noise is mandatory.
I found the TI LP38798 shown in the schematic by googling -
if someone has
a proven a measured design that would be a safer choice. In
any case more
filtering (e.g. ferriites) is probably a good idea.
This design will be available on my blog or on github when
it is done - if
anyone is interested.
Thanks,
Anders
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