Electronics > RF, Microwave, Ham Radio

Via overkill?

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daqq:
Hi,

I've been looking through the datasheet of the ADRF5044 ( http://www.analog.com/media/en/technical-documentation/data-sheets/adrf5044.pdf , page 11) and I noticed the board in the attachment... Isn't this a tad excessive? I mean, isn't this an overkill? How exactly did they determine that that particular area of the PCB should be so flooded with vias?

I mean, sure, stitching, stripline stuff... but it does seem a bit too much.

I'm not criticizing, I'm asking, as the realm of 30GHz is a mystery to me...

Thanks,

David

TheUnnamedNewbie:
I don't think it's overkill. Using two or three rows of vias is something I've seen before, perhaps more under the motto "if it's worth doing it's worth overdoing" but then again, if you are paying for a more expensive stackup like that might as well go all the way.
In the center part I think it's more to keep any odd resonances between input signals at bay. It is a switch so the signals are going to be at exactly the same frequency, so it is easy to imagine that some strange coupling modes start messing up your impedance, especially at lower frequencies.

RadioNerd:
Think about that:
1) How many vias would you put on an RF-Board working at 3 GHz to achieve a solid ground connection? Already quite a few I presume... Now you have to increase the via density by a factor of 10 in each direction to achieve the same effect at 30 GHz... Then you easily end up with this kind of PCB layout.

2) Vias are virtually for free on a PCB, therefore RF engineers usually put a few more than strictly necessary.

3) And last but not least: Esthetic preferences :)

Yansi:
When tinkering with 10+ GHz, one should be aware of not to place vias symmetrically or in any repeated patterns - as that might cause standing waves to exist in between them.

TheUnnamedNewbie:

--- Quote from: Yansi on February 01, 2018, 05:44:42 pm ---When tinkering with 10+ GHz, one should be aware of not to place vias symmetrically or in any repeated patterns - as that might cause standing waves to exist in between them.

--- End quote ---

This is not unique to 10+ GHz. What you should be aware of is that you don't want to place anything repetitively if the order of size of the repeating (Ie, the "period" of the structure) is electrically large. Usually people use as rule of thumb larger than a tenth of the wavelength. It doesn't matter if this is at 100 GHz, 1 GHz, or 1 MHz. It's just that in the latter cases it is much harder to actually have issues with this since 1/10th of the wavelength at 1 GHz is still on the order of a centimeter, but at 10 GHz we are talking about millimeters.

Sometimes this is of course done on purpose, to create the classic "voodoo RF black magic" filter structures based on distributed networks.

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