Choosing Absorber shields

[mw_rookie]

Hello,

I am looking for RF absorber shields for my PCB and have come across:

RF Absorber Shields

For a clearer understanding, if my board is operating at say 600 MHz-800 MHz, can I use PS800EMC24 from the catalog? It's peak attenuation is at 800 MHz, so, it should absorb any radiated emissions up to 800 MHz. But my operating frequency falls within the absorber frequency range. I will be using a microstrip/CPWG line on the board. So in any scenario will my required frequency see any attenuation?

Also, when I select an absorber sheet for the PCB, then the return loss of the sheet for my frequency has to be low. In case it is about 2 or 3 dB only, most of the radiation will be a reflected/heat loss. In that case, the shield is not serving any purpose. Is this understanding correct?

Any info on the above is greatly appreciated.

Thank you for interest and help.

 

[jiripolivka]

Hello,

I am looking for RF absorber shields for my PCB and have come across:

RF Absorber Shields

For a clearer understanding, if my board is operating at say 600 MHz-800 MHz, can I use PS800EMC24 from the catalog? It's peak attenuation is at 800 MHz, so, it should absorb any radiated emissions up to 800 MHz. But my operating frequency falls within the absorber frequency range. I will be using a microstrip/CPWG line on the board. So in any scenario will my required frequency see any attenuation?

Also, when I select an absorber sheet for the PCB, then the return loss of the sheet for my frequency has to be low. In case it is about 2 or 3 dB only, most of the radiation will be a reflected/heat loss. In that case, the shield is not serving any purpose. Is this understanding correct?

Any info on the above is greatly appreciated.

Thank you for interest and help.

From your explanation I do not see the exact purpose why you use the absorber. Typically such absorbers are used to suppress unwanted resonance in- and out-of the desired band and thus any instability where active devices are used. If you rather need to attenuate the desired signal it is better to install a fixed attenuator in the line.
Absorbers may be needed if a shielding case above the microstrip PCB is used and may resonate in-band. Small strips are glued in sensitive locations and tested when the device is adjusted.
Try to use the absorber you have, more than one layer may be needed. Absorber manufacturers often offer kits so you can select the best for your particular device.

 

[Bob60]

Hello,

if my board is operating at say 600 MHz-800 MHz

Typically @ 600 MHz-800 MHz absorber not necessary. For instance mobile phones works without absorber. Problems appear @ 5GHz and above. On the picture my design 600 MHz-800 MHz, absorber doesn't present.

 

[biff44]

those do look interesting. They took standard leader tech emi shields and glued some eccosorb into the cover. In certain applications, that would help.

One application might be if you have a transmitter on one half of a board, and a receiver on the other half of the board. You might put such a cover over the receiver, trying to keep down the leakage into it bypassing an input filter. You might put a cover over the transmitter part to meet regulatory leakage requirements.

YES, the material itself has a poor return loss to a plane wave incident to it. But that is NOT its purpose. The absorber is there the "de-Q" any rectangular waveguide resonances that might form when you put a big rectangular metal box over your circuit. If a resonance occurs, that specific frequency's energy goes back and forth in the housing many times (maybe hundreds of times). If each time the energy passes near the absorber, say 20% is absorbed...then very quickly the "resonating" energy is mostly absorbed and no longer big enough to leak out of the cover.

if you actually needed a low return loss from the first wave to be incident on the surface, you would need a much different structure. Pyrimidal absorbers (of some length) would work, or multiple absorber layers, of increasing dielectric constant forming quarter wave transformers, would be needed.

 

[mtwieg]

Typically @ 600 MHz-800 MHz absorber not necessary. For instance mobile phones works without absorber. Problems appear @ 5GHz and above. On the picture my design 600 MHz-800 MHz, absorber doesn't present.

Okay I've got to ask: what is that component at the top of the image, feeding the connector? Cavity resonator? Circulator?

 

[mw_rookie]

Thank you everyone for your patient replies.

@jiripolivka: My PCB has multiple frequency bands spread over it and the shields are to reduce leakage.

@biff44 : Thanks for your explanation about the purpose of the absorber inside the structure.

However, shouldn't the sheet have a good return loss at the frequency of operation? Let's say I have an up-converter to convert L Band into S Band, then the shield should be able to operate well at the L Band, the IF and should be able to absorb any radiated S Band frequencies. In that case, it should have a good return loss at all the 3 frequencies. If it doesn't then it will only reflect those frequencies back and in that case, is it serving the purpose? A similar working should hold true for any abosrber sheet I directly place on the PCB. Is my understanding correct? If it is so, will the desired in band signal on the Tx line face any attenuation because the shield has a good return loss at that frequency?

Thank you for your interest and time.

 

[jiripolivka]

Thank you everyone for your patient replies.

@jiripolivka: My PCB has multiple frequency bands spread over it and the shields are to reduce leakage.

@biff44 : Thanks for your explanation about the purpose of the absorber inside the structure.

However, shouldn't the sheet have a good return loss at the frequency of operation? Let's say I have an up-converter to convert L Band into S Band, then the shield should be able to operate well at the L Band, the IF and should be able to absorb any radiated S Band frequencies. In that case, it should have a good return loss at all the 3 frequencies. If it doesn't then it will only reflect those frequencies back and in that case, is it serving the purpose? A similar working should hold true for any abosrber sheet I directly place on the PCB. Is my understanding correct? If it is so, will the desired in band signal on the Tx line face any attenuation because the shield has a good return loss at that frequency?

Thank you for your interest and time.

As I explained, absorbers are not used for screening on a PCB- for this screening covers and enclosures are better. Metal enclosures sometimes resonate or guide unwanted modes to sensitive places. Amplifiers can oscillate and spurs get somewhere they do not belong. Then such guides and resonating volumes are made attenuating by gluing pieces of absorbers as needed. Some antenna designs are not perfect and side lobes appear too high. Again a chance to add some absorbers.

Absorbers are also used around connectors and cables, tuning screws, all to suppress unwanted leakage. A good design requires no absorbers.

Absorbers are also good emitters of thermal noise, with emissivity close to one. To calibrate microwave radiometers for instance.

If your system integrates several frequency bands, make a good design in carefully separating each from others. Do not allow many signals to share one line or a PCB. No absorbers then can help a poor designer.

 

[Bob60]

Okay I've got to ask: what is that component at the top of the image, feeding the connector? Cavity resonator? Circulator?

Yes BPF, almost cavity. Ceramic .

 

[mw_rookie]

Thank you everyone for the replies.