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RF Shielding - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


davorak
Post subject: RF Shielding Posted: Tue Apr 14, 2009 4:00 pm

Lieutenant

Joined: Tue Apr 14, 2009 2:56 pm
Posts: 3
My question is weather my recent purchase, Rose & Bopla 01234023 85 http://www.rose-bopla.com/Prod_Pgs/Basic_Boxes/Prod_01_Aluminum_Unpainted.htm with rmi-efi gasket, was overkill for my project. What other shielding options make sense?

I have recently started a research project and purchased a box to shield my electronics
I will be making noise measurements ~-215dBm/hz. Frequency range:50Mhz to 3 Ghz, Though the measurement currently focus on f<1Ghz.

I have a similar set up working in a small Hammond aluminum diecast box.

For large rf projects what shielding options do I have?

Thank you.


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Kirt Blattenberger
Post subject: Re: RF ShieldingPosted: Wed Apr 15, 2009 9:54 am

Site Admin


Joined: Sun Aug 03, 2003 2:02 pm
Posts: 308
Location: Erie, PA
Greetings davorak:

Are you really going to be making noise measurements down to -215 dBm/Hz?

Thermal noise in a 1 Hz BW at room temp (25 C) is -174 dBm/Hz, and near absolute zero (1 K), thermal noise in a 1 Hz BW is -198 dBm/Hz. At room temp (25 C), thermal noise is at -174 dBm/Hz. So, in order to obtain -215 dBm/Hz, even at 1 K you would have to be interested in noise in a 0.02 Hz BW. A temperature of about 0.05 K at 1 Hz BW also gets you near the -215 dBm/Hz number.

Thermal noise power (in dBm/Hz)
= 10 * log (k * T * B *1000)
where k = Boltzmann's constant = 1.3806504e-23 J/k
T = temperature in K
B = bandwidth in Hz

So, either you're pulling our legs here, or you might need to revisit your thermal noise number.

_________________
- Kirt Blattenberger
RF Cafe Progenitor & Webmaster


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davorak
Post subject: Re: RF ShieldingPosted: Wed Apr 15, 2009 12:48 pm

Lieutenant

Joined: Tue Apr 14, 2009 2:56 pm
Posts: 3
The bulk of my question is about enclosures for large RF projects, specifically what options do I have so that my next purchase will be a more informed one. I would appreciate and answer on this.

My first calculation and post was sloppy I apologize if you got the chance to read it. Hopefully this is a little better.

You may be right I will tell you how I reached this number.
I am measuring shot noise.
Pn = 2 e I B *R (current noise in a 50 ohm terminated system)
at one micro amp, a bandwidth of ~400 Mhz, 50 termination gives ~-112 dBm and
~-198dbm/hz.

Thermal noise should be
pn = kb T B
Gives ~-88 dBm and ~-174 dBm/hz

The difference between those two should allow for a measurement even though the shot noise even though it is at ~-198dbm/hz right? I can certainly run the numbers though the specs for my amps and log power meter and I get a result that can easily be measured and corresponds to what I do measure fairly closely.

(-215dBm from before was current noise 2 e I, without accounting for the resistance)

edit 2:pn = 4*kb T B/R -> pn = kb T B


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yendori
Post subject: Re: RF ShieldingPosted: Thu Apr 16, 2009 6:13 pm

General


Joined: Thu Sep 25, 2003 1:19 am
Posts: 50
Location: texarcana
Wrong!

Quote:
A temperature of about 0.05 K at 1 Hz BW also gets you near the -215 dBm/Hz number.


A temperature of about 0.05 K at .5Hz BW also gets you near the -215 dBm/Hz number.

Not so smart now are ya, hot shot?

I choose the RFCAFE t-shirt.


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Kirt Blattenberger
Post subject: Re: RF ShieldingPosted: Fri Apr 17, 2009 11:10 am

Site Admin


Joined: Sun Aug 03, 2003 2:02 pm
Posts: 308
Location: Erie, PA
Greetings davorak:

The calculation showing that shot noise will be below thermal noise is just an indication that you can ignore it.

Regarding choice of an acceptable enclosure, it really comes down to how low of a signal you need to detect and how high the ambient noise levels are to determine the required isolation. My approach would be to calculate the inband system noise levels and the required minimum detectable signal (MDS) level. From there, you have to have an idea of the test environment's ambient noise levels in the detection band (as well as any that might translate into the band due to frequency conversion and/or sampling) to calculate how much isolation the enclosure needs to provide.

I see from the spec on the gasket for your box that it is supposed to provide 100 dB or so. That number was probably achieved under conditions of a perfectly mating gasket and seating surfaces, with a previously unused gasket. Let the gasket be slightly misaligned, or the cover screws not be tightened properly, or the mating surface be a bit deformed or contaminated, and the isolation at some frequencies can drop significantly. That is motivation for using the best box you can afford for making measurements. Bigger enclosures need lots of screws and carefully machined surfaces on robust castings to achieve high levels of isolation.

Many moons ago, I had to design a production test fixture for a wireless PDA device that needed around 80 dB of isolation up to about 2 GHz. The company I worked for had a commercialized fixture with a hinged clamshell configuration with a latchable handle to allow operators rapid open/close motions. It was spec'ed at 100 dB, so no problem, right? Well, it turned out that achieving 50-60 dB consistently over many cycles without careful operation was the best it could do in real-world environments. The fixture was useable, but operators had to be trained to ascertain certain conditions were met for enclosure alignment prior to running the tests.

So, this does not necessarily answer your question about a source for an inexpensive large enclosure, but does offer a caveat about cutting corners without being aware of the potential for underperformance. If you have dot yet done so, I recommend looking on my "Cabinets, Enclosures, Equipment Racks & Packaging" vendor page.

http://www.rfcafe.com/vendors/component ... osures.htm

_________________
- Kirt Blattenberger
RF Cafe Progenitor & Webmaster


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davorak
Post subject: Re: RF ShieldingPosted: Fri Apr 17, 2009 3:12 pm

Lieutenant

Joined: Tue Apr 14, 2009 2:56 pm
Posts: 3
Thanks Kirt, thank you for sharing your practical experience with large rf enclosures. It would make my life easier if I could ignore the shot noise, but the purpose of the setup up is to measure it.









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