# Electronic – Contradicting ideas about differential signal route, confining fields vs Increasing impedance

differentialelectromagneticpcbpcb-designRF

By initial sense I think we must:

1. keep pair intra-distance as close as possible for field cancellation.
2. Fill surround of pair with GND plane and have via near to it to confine the fields, prevent incoming interference and making it.

The source have 200 ohm differential impedance and the load is 1k differential. I've decided to terminate the line with end termination. Now I must raise the trace impedance to 200/100 ohm to terminate it by 200/100 ohm by:

1. Increase intra-space as more as I can.
2. Remove surrounding GND plane.
3. Reduce trace width.

And these are contradicting each other can you explain why?

My working frequency is 12.2GHZ. (With respect to @ThePhoton)

2 helpful link: Differential impedance and Differential Trace Design Rules – Truth vs Fiction .

keep pair intra-distance as close as possible for field cancellation.

No! You need to adhere to a defined distance to get a defined wave impedance. What you describe is a coupled microstrip line.

Fill surround of pair with GND plane and have via near to it to confine the fields, prevent incoming interference and making it.

... in a defined distance to get a defined impedance.

In fact, you don't need a surrounding GND plane on the same layer – practically all field will be between the two differential conductors; what would be good would be a plane below!

The source have 200 ohm differential impedance and the load is 1k differential.

So, that's a high-impedance load and not really a low-impedance source. I'd recommend having two matching networks: one at the source to match the source to the transmission line impedance, and one at the sink.

You could then use an arbitrary transmission line impedance, e.g. the microwave-typical 50 Ω or the 75 Ω. In theory, 200 Ω should work (and would save you the source matching), too, but it might be hard to build using your PCB materials – it depends, can't tell without knowing with what you're working.

And these are contradicting each other can you explain why?

They are not contradicting. A perfect transmission line does not radiate, so your "as close as possible" simply isn't right – yes, close, but not "as close as possible".

Use a specific calculator to calculate the right dimensions for a coupled microstrip line on a PCB substrate of your PCB's thickness, with your PCB's $$\\varepsilon\$$, and on the frequency you work on.