[time-nuts] SMD TADD-1 distribution amplifier - seeking comments and suggestions?

[Magnus Danielson]

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
> <magnus at rubidium.dyndns.org <mailto:magnus at 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.wallin at gmail.com <mailto:anders.e.e.wallin at 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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