Microwave PCB ground boarder ring

[ctzof]

Hello,

Maybe this is a silly question....but I was wondering was the use of ground boarder ring full with vias in Microwave circuits like microwave filters. An example of what I mean can be seen in the attached picture. Is a bottom ground plane not good enough?

 

[volker@muehlhaus]

Discontinuities in microstrip lines generate some amount of surface waves, depending on the substrate. The ground rings are for shielding these surface waves.

 

[BigBoss]

Hello,

Maybe this is a silly question....but I was wondering was the use of ground boarder ring full with vias in Microwave circuits like microwave filters. An example of what I mean can be seen in the attached picture. Is a bottom ground plane not good enough?

View attachment 107024

SMD Components need GND connection and this GND will obviously be on Top Layer.
Consequently Top Layer GND must be tied well with Bottom Layer GND.These VIAS are for that..
Second target is to Guide some Microstrip lines to prevent radiation that will disturb the other components.Top Layer GND planes create a EM wall around the MS structures and this guides the waves without any reflection or antenna effect.

 

[ctzof]

So for example regarding the filter these walls protect other components by don't letting radiation from the filter interfere with them right? What are the benefits for the filter itself?

 

[volker@muehlhaus]

So for example regarding the filter these walls protect other components by don't letting radiation from the filter interfere with them right?

Correct.

What are the benefits for the filter itself?

For the single filter, it makes no difference. It only reduces crosstalk to other regions on the PCB, which might leak across the filters.

 

[ctzof]

I´ve designed a microstrip filter board that uses two identical filters. One for the receiver and one for the transmitter. Although I put a ground boarder ring and seems to be good enough for the other external components in the board, I don't know if the two filters are enough isolated with each other. I´ve attached a picture of this filter can you take a look and tell me what you think?

 

[volker@muehlhaus]

I don't know. If this is for TX/RX, many designers would add some shielding boxes. But you already have the PCB, so maybe you can measure the crosstalk and compare to your requirements?

 

[vfone]

Your concern in the initial post: "ground boarder ring full with vias in Microwave circuits like microwave filters" make sense and I've seen series of vias acting as a microwave filter with particular group delay, which finally (being part of a closed feedback loop in the circuit) makes the circuit oscillate at a particular microwave frequency.
Always in my designs I try to avoid placing vias with a particular pattern, or equal distance between them, and I place them random as possible and with different diameters if are available.

There are many discussions regarding closed (or not) ground rings around a microwave PCB circuit.
Myself, if I don't have option for a very solid, uniform, and wide ground plane (for entire length) I prefer to do not close the ground ring.

 

[D.A.(Tony)Stewart]

Horizontal stray leakage crosstalk is suppressed by shunting to a ground pour. Many microvias in parallel reduce the inductive coupling of each via. At least 20 per 1/4 wavelength minimum.