RF signal sources separation distance

[a8811323]

Hi, I am getting troubled with making connection between separation distance between two rf input port and wavelength of signal.

I know the two rf input ports cannot be placed to each other, otherwise there will be an EMI. However what is the minimum separation distance in order to have a smaller size. Is that related to wavelength of input rf power?
Let's say, we have a rf input power of frequency 200MHz, what is the minimum separation distance between two port?

Thanks in advance!

 

[niccoh]

Assuming your doing a PWB/PCB design it will depend on a number of factors.

What kind of isolation do you want between the paths/sources? 70dB, 80dB, 90dB, 100dB... The more isolation the harder it will be as different sources of RF crosstalk will rise. Then you will have to figure out where it's coming from. You can add physical grounds between the paths or physical ground walls for improved performance.

What is your method of distributing your sources? Microstrip, Stripline, Co-planar waveguide? Co-Planar or Stripline are more complicated but would provide better isolation to adjacent RF paths without the need of a lot of walls in your design if this is in an enclosure.

A simple EM simulation can give you an answer too just by looking at a pair of ports separated by a distance on a PWB/PCB. I've noticed >80dB of isolation at that frequency between adjacent microstrip lines separated by ~170mils. But this was only about 300mils of a section of the microstrip parallel to the other. If you have the RF paths in parallel following for longer distance your isolation will degrade further.

As for the relationship to isolation, distance and wavelength or frequency I'm not 100%... I'd say as you increase in frequency isolation will degrade. But up to a certain point losses due to the PWB/PCB technology will take over.

Best of luck!

 

[a8811323]

Hi,
Thanks for your reply.
One question is that why the more isolation will causes RF crosstalk? From my understanding, isn't longer separation distance will reduce the possibility of nearby EMI?

 

[FvM]

I believe you need to ask a clearer question to get reasonable answers. Particularly what kind of ports are asking about? Under circumstances, screening allows to place them quite near without much crosstalk.