I am working on a design that requires 4 channels coming onto a PCB. The inputs are SMA connectors and are spaced 15mm apart from each other. These signals are routed with co-planar wave guides on the board to a SP4T RF switch. My operating frequency is 300Mhz - 900Mhz. The incoming signals have an amplitude range from -30dBm to 10dBm.

I am curious to know what kind of isolation can be expected in this situation. On my first experiment I was only able to obtain about 20dB of isolation between signals. From what I have read so far it seems that the best you can get is 40dB with a 2 layer board and a CPWG. Does anyone have any experience on how to improve this?

My next test board will involve all the connectors mounted so the signals are coming in at oblique angles to one another. However this does take up more space.

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  • \$\begingroup\$ Can you show the layout? Did you stitch vias to GND? Can you link to where it said 20dB of isolation is the maximum attainable? \$\endgroup\$ – mkeith May 28 '18 at 23:52
  • \$\begingroup\$ I think wave guide is not the right word for your setup. A wave guide is kind of a pipe, where the wave is propagating in air and the wave guide defines the bounds. I think you have common pub traces on a plane in your setup. If this is true, a lot more than 20dB of isolation is possible, even for 15mm spacing. \$\endgroup\$ – Stefan Wyss May 29 '18 at 4:28
  • \$\begingroup\$ @StefanWyss The definition of waveguide is widely debated, some authors considering transmission lines as a subset (eg, Orfanidis). There are also other types of waveguides aside from the "pipe" type, like SIW, dielectric ribbons or dielectric rodes (optical fiber is an example). Regardless of that, "coplanar waveguide" is a correct name for certain types of transmission lines. \$\endgroup\$ – Joren Vaes May 29 '18 at 6:37
  • \$\begingroup\$ To the OP: one of the limiting factors will be the space you have to place them apart on the board. Switching to CPWG can give you some improvements as you then tie both ends of the CPW together through a ground plane, which serves as a form of isolation. I would also look into the footprints of your SMA connectors - if you have even a small amount of radiation there it might ruin whatever isolation you have (as isolation is limited by the weakest link, if you have 100 dB isolation on your CPW lines but only 15 dB at the connectors...) \$\endgroup\$ – Joren Vaes May 29 '18 at 6:40
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    \$\begingroup\$ Yes 50Ohm, its standard FR4 1.6mm. Top and bottom planes are ground. \$\endgroup\$ – Meozaa May 29 '18 at 19:33

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