Bob.

On 2022-07-13 23:32, Bob Martin via time-nuts wrote:

I am in full agreement. While there is a certain pain in doing it, but
it removes a long set of issues relating to not having it. I've had that
standpoint from the start, but felt that Erik may benefit from getting a
lot of the other things going first and learn from it, as he seemed so
negative about it. It is only when one sees the alternative cost that
one can make the correct decisions. However, when trying to fix many
things, it can take time to spend on all the things, so spreading things
out a bit in time can serve it's purposes. This not to be disrespectful
to Erik, rather the opposite. Then again, sometimes there is a different
balance that pans out for it's purpose.

I'm setting up measurement rig myself and it starts of with +5V and +/-
15V. Luckily I have a spare supply delivering exactly that, will just
check that it is reasonably in good condition.

There will be a challenge of getting all the pieces fit together, but
I'll slam a RPi4 to it and see how that works out.

I've realized that I should be doing a input level measure and
programmable damper. Wonder if someone already have nifty small boards
for that.

Cheers,
Magnus

Magnus,
See below

On 14-7-2022 14:50, Magnus Danielson via time-nuts wrote:

No offense taken!!
Unfortunately I'm the type of person that learns most by making
mistakes. So I first wanted to try to get the single supply working as
the PNA is not getting its own permanent supply. Performance at 10kHz
offset is now at the -155dBc/Hz noise level, although still with a lot
of mains harmonics but these could also come from the not so good audio
cable connecting the PNA and the Audio input or a ground loop.
Replacing the single supply with a triple supply is still to be tested.

As long as you want check for any level above +7dBm a simple rectifier
will do in combination with an analog input of the RPi
Erik.

Erik,

On 7/14/22 16:46, Erik Kaashoek wrote:

Great.

OK, great. Then I correctly assumed your modus operandi that I was
trying to respect and support. Also great that you consider testing it.

As for ground-loop, that is somewhat misunderstood, as most often the
ground-loop is not driven by magnetic fields, but different
ground-currents driving up potential differences, and when
cross-connecting causes a current to handle the difference in such
voltage. The two main solutions are: Break away supply currents from
reference voltage/grounding and make the ground-reference using bigger
wires (both to the common point and between the different parts). I've
found that considering it all as an operation of isolation helps to move
away from incorrect analysis.

There is a whole board inbetween with lots of goodies, including RPD-1
mixers. I was considering the input into the RPD-1 mixer from various
sources which comes from completely different boxes in the labs. Some of
them is wheel-around level of boxes.

Cheers,
Magnus

Hi

Die cast boxes are your friend when building this sort of thing. Hammond
makes them and so do a lot of other folks. They are reasonably cheap and
pretty easy to work with. I have accumulated a pretty good pile of them “in
inventory” in a dusty box in a back corner of the basement :).

One nice thing about a fairly heavy metal enclosure is that you can tie things
like BNC’s straight to the case. They give you a pretty low resistance “bonding”
without a lot of crazy effort. There are good reasons to do this and equally
good reasons to isolate depending on what you are doing. Blocking the RF
ground with a 0.1 uf cap is one example of isolation that folks do use.

My vote normally is to tie everything tightly together and then isolate the
“far end” of the sources. This may involve grounding on supplies ( = use
a battery  maybe ) or other “interesting” approaches. Running everything
involved off of a single good isolation transformer is not all that crazy.
(hint: You want the toroidal one with the better magnetic characteristics …).

The other nice thing about the metal box is that when you knock the device
off on the floor, it’s less likely to re-wire itself. They tend to last longer that way :)

Bob

On 7/14/22 8:01 AM, Magnus Danielson via time-nuts wrote:

Well, that's one evil of a ground loop.

But magnetic coupling is also an issue. For instance, the vast majority
of line frequency interference is coupled magnetically, so twisted pairs
and such help a lot.  We also see magnetic coupling from switching
transients on a power supply (where the "transmitting" loop is the
circuit being powered, spread out across a board), and the victim loop
is on another board, or different area of the same board.

On 7/14/22 10:38 AM, Bob kb8tq via time-nuts wrote:

Except, the sides are tapered on most die cast boxes.

I like the rectangular boxes made from an extrusion, with covers. That
way, the sides are at right angles to each other. COMPAC is one brand.
You can get them with machined grooves, gaskets, etc. too (for a price
<grin>)

There are a variety of sellers of extruded aluminum housings with slots
to hold a PCB, etc. and end caps. I'm not so wild about those for
tinkering, because I like being able to take the big lid off and peer in
and probe.  But for a product that's being sold, they're wonderful.

I'll make a pitch here for a company that makes CNC front panels, which
I've used as RF breadboards - As long as what you want can be machined
out of a flat sheet of aluminum, they can make weird cutouts, they've
got just about every connector outline known, and they can do engraved
labels, etc.  But they can also put 4-40 tapped holes on a regular grid,
for instance, if you're bolting down minicircuits flanged parts.

https://www.frontpanelexpress.com/

There are also companies that do cases as well as places that do quick
turn run of the mill CNC machining (e.g. https://protolabs.com).  The
trick then is that you need some design tools and expertise.

On Thu, July 14, 2022 2:15 pm, Lux, Jim via time-nuts wrote:

How do those boxes work for low and mid frequency shielding?  Does the
aluminum oxide that grows on bare aluminum keep the side extrusions and
plates you add to make a box from making good electrical contact?
I assume that at GHz frequencies the oxide is thin enough that you get
capacitive coupling, but that doesn't seem reliable at 10MHz, and
especially not for power line protective earth connection if you put a
power supply inside the box.
I like aluminum because it is easy to drill, but I've always been nervous
about relying on it for electrical connections because of that oxide
coating.

--
Chris Caudle

Nothing some 2" wide adhesive copper tape doesnt fix on joins.

However, they are Aluminium-Zinc and I've never had any oxidization
issues in good environments. Never.

What you can do if you are keen, is bow the lid convex, so that the
middle mates with a very positive contact. (for boxes with just four screws.

glen

On 15/07/2022 9:49 am, Chris Caudle via time-nuts wrote: