Tying Chassis Ground to Signal Ground

Yeah - around here, it looks (and sounds, and smells) like Mexico Jr.

Sigh. Rich

Nah - the Pacific plate is moving north relative to the continental plate - by the year 2525, Los Angeles will be across the bay from Oakland. ;-)

Cheers! Rich

Ya gotta admit, the guy's got an "interesting" thesaurus. ;-P

Cheers! Rich

The function of socialism is to raise suffering to a higher level. - Norman Mailer MikeK

Mount a circuit board with a single spacer. Push it down somewhere else and release it suddenly. It will "twang" mechanically at the cantelevered section's mechanical resonance frequency.

Same thing happens at RF.

John

I did a laser controller [1] a while back. It was mounted to the "optical bench" of the laser, a big vertical aluminum slab. The guys who designed the laser system insisted on floating the ground planes of all circuit boards... then seeding each board with a zillion bypass caps to frame ground! They also wanted to opto-isolate all the RS485 stuff. I hard grounded my board at every PEM spacer and didn't isolate anything. My excuse was "picosecond jitter" which they couldn't argue against.

John

With optical guys you have to word that differently: Femtometer jitter :-)

The one where I was part of the design crew will help people get out of a nasty medical pickle.

But you don't see a way of getting the same result using some combination of resistors and capacitors and preventing the DC current? Or am I just kidding myself that this is an RF ground?

You can achieve a good RF ground using capacitors. Lots of them if the board is large.

Every had any problem with resonances of the entire ground/chassis structure? I.e., the entire thing turns into a patch antenna?

They are in essence all antennas, with varying effectiveness. I have seen resonances in EMC cases. Then it's like a water bed. You suppress something here, works great but increases all the peaks over at 400MHz. Move the grounding, and the noise moves right along. Like a wave. This is often the point where one has to (grudgingly) move from system level fixes to board level measures.

I've done RF sweeps looking for rectification in thermocouple front-ends. What I found were sharp resonances in the 150-400 MHz sort of range, a combination of enclosure, PCB, and cabling. I think.

This helped a lot:

ftp://jjlarkin.lmi.net/Ferrite.JPG

Our competitor's product would mess up if a security guard used his radio in the next room. We were 30 dB better, 50 or so with the bead.

John

Amazing how much a little chunk of ferrite can help!

Certainly, it is. North America, to be more specific. Always has been.

Nice clean assembly there. Could use a tad more paste screeded onto the boards though before reflow ;-)

In the Netherlands they didn't call the dual-holers beads but "varkensneusje", translated "nose of a piglet".

Do you do compliance testing on your products? This sort of stuff falls out there (though it may be in the CE tests).

...and you ragged on me for the chunk of ferrite on our (internal) LAN cable. ;-)

BTW, I got the Kensington monitor adapter. It works as advertised (a little slow, but flawless otherwise). It's great for things like documentation that doesn't get moved around much. I now have three monitors on my ThinkPad. ;-)

You have plenty of competition from NY, IL, and VT, to name only three.

Or as Rush has said, "to spread misery equally".

Did you run out of USB ports? ;-)