Re: OT: sound speed depends on frequency on mars

On 1/09/2024 5:31 am, john larkin wrote:

On Sun, 1 Sep 2024 02:37:22 +1000, Bill Sloman <bill.sloman@ieee.org>
wrote:
 

On 1/09/2024 12:18 am, john larkin wrote:

On Sat, 31 Aug 2024 16:17:39 +1000, Bill Sloman <bill.sloman@ieee.org>
wrote:
>

On 31/08/2024 3:10 am, john larkin wrote:

On Sat, 31 Aug 2024 01:23:19 +1000, Bill Sloman <bill.sloman@ieee.org>
wrote:
>

On 31/08/2024 12:34 am, john larkin wrote:

On Fri, 30 Aug 2024 11:13:05 GMT, Jan Panteltje <alien@comet.invalid>
wrote:
>

NASA's Mars rover Perseverance has found that sound travels much more slowly on the Red Planet than it does on Earth
and behaves in some unexpected ways that could have strange consequences for communication on the planet.
https://www.space.com/nasa-mars-rover-perseverance-speed-of-sound#main
     At frequencies above 240 Hertz, "the collision-activated vibrational modes of carbon dioxide molecules do not have enough time to relax, or return to their original state,"
     the researchers said, which results in sound waves at higher frequencies traveling more than 32 feet per second (10 m/s) faster than the low-frequency ones.
     That means that if you were standing on Mars, listening to distant music, you would hear higher-pitched sounds before you would hear the lower-pitched ones.
>
paper:
https://www.hou.usra.edu/meetings/lpsc2022/pdf/1357.pdf
>
So...
Music from far away may sound funny?
>
For Mars we will need compensation headphones with distance measurement and variable delays....
;-)
>
Better use radio.. and earplugs/ headphones...

>
Funny, I just delivered a lecture on transmission lines and noted that
microstrips have dispersion from  the unbalanced dielectric constants
and skin effect. Rising edges get sloppy at the and of a long trace.

>
I hope you pointed out that buried strip-line isn't dispersive. I have
pointed this out here from time to time.

>
Of course it's dispersive, maybe a bit less than microstrip.

>
Why do you think that?

>
Because dielectrics are imperfect, especially FR4, and because there
are lots of papers online that analyze dispersion in stripline.

>
But you can't cite any of them.

 Can't you google?

Can't you? You know where you found the papers (if you did) and are much better placed to define the search terms that would throw them up.

You wouldn't use FR4 around a stripline

if you wanted a low-dispersion transmission line. There are better
substrates his frequency work.
>

It's hard to keep up decent impedances on stripline in a multilayer
board, especially 8 or 10 layers.

>
Stripline is buried between two ground planes. The only tricky part of
impedance control is the thickness of the dielectric in the two layers
above and below the strip-line. In a ten layer board this is thinner
than it would be in a board with fewer layers.

>
And eventually the trace has to be skinnier than PCB houses are
willing to etch.

>
Why?

 Run the Saturn program. More layers make the dielectrics thinner, so
to maintain a useful impedance the traces have to get narrower.

I can do that with a handheld calculator. What you seem, to have missed is that low dielectric constant substrates give you wider traces for a given impedance. I got a 150R line on the surface of teflon-alumina substrate.

10 layers gets nasty.

If you don't think about what you are doing.

Standard pricing seems to be around 5 or maybe 4 mils
width these days. We do a lot of 5, to sneak between BGA balls, but
sometimes even 5 is too big.
>
Thin dielectrics have tolerance issues too. I'm talking about real
PCBs here, not ideal theoretical stuff.

>
Printed circuit board are always real.

 Exactly.
 

Pay enough for close-tolerance substrates in the two relevant layers and
you should be okay.

>
"Pay enough" can get crazy fast. I don't want to pay hundreds of
dollars for a smallish PCB.

>
An eight or ten layer PCB isn't going to be small. You only need lots of
layers when you have to connect lots of stuff.

 >>

https://www.wevolver.com/article/stripline-vs-microstrip
>

I wonder if anyone has added surface-mount Heaviside loading coils to
a PCB trace.
>
https://en.wikipedia.org/wiki/Loading_coil

>
It would be a bit silly.

>
John snipped the rest of that sentence, without marking the snip.
>

You can make lumped constant transmission lines by linking a
series of capacitors with discrete inductors, if you want a high
impedance transmission line - people sold them as thick film hybrid
assemblies, and I even used a few, a very long time ago.

>
Most ideas seem silly to people who are by nature hostile to ideas.

>
Not a problem I've got.
>

Dismissing is easier than thinking.

>
Thinking about what a loading coil might be doing to the impedance of a
PCB trace isn't something that you seem to have managed to do.

>
I certainly had the idea.

>
In a remarkably half-baked way.

 Ideas start out fuzzy, or at least they should. I tell my kids, stay
confused for a while.

You seem to have chosen to stay confused for decades.

I might Spice a bunch of ltlines with
inductors between, just for fun. It's unlikely that I'd use such an
arrangement in real life, but it's just possible, especially if analog
quality of a fast edge matters, like in a laser modulator maybe. It's
preferable to just keep all the traces very short, but that's not
always possible.

>
You seem to be intent on re-inventing the lumped constant delay line,
without being aware that they were commercially available some thirty
years ago, back when I used them. They may still be available.

 The Tek 545 30 MHz scope had a gigantic, lumped, tunable, differential
delay line up to the CRT, so you could see the edge that you triggered
on.
 https://w140.com/tekwiki/images/thumb/1/10/Tek_545a_delay_close.jpg/231px-Tek_545a_delay_close.jpg
 A lossy pcb trace with periodic loading coils is not a lumped-constant
tx line.

The discrete loading coils are lumped elements.

In fact, lumped lines are nasty. The number of sections goes
as Td/Tr squared, which can get awkward fast.

All true, which doesn't make them any less useful.

Meander-line sections connected by loading coils could be interesting.
One product that I'm considering now is a programmable delay line, and
that idea might help.

Like the MC100EP195?
https://www.onsemi.com/pdf/datasheet/mc100ep195-d.pdf
You do seem to spend a lot of time re-inventing the wheel, and congratulating yourself on the originality of your re-invented concepts.
--
Bill Sloman, Sydney