Ground Traces and Ground Planes in PCB Eagle

[BlackMelon]

Hello,

I am making an H-bridge motor driver circuit. I want to make a ground plane to be a return path for high frequency signals, and ground traces for low frequency signals. So, I drew ground traces first. After that, I encircled my board with a polygon, set it to GND, and do Ratsnest. But the ground plane that I got merges all ground traces into itself. How can I tell PCB eagle to spare ground traces?

PS. On my schematic, the plane and the traces are the same point, and it is referred to as GND.

Thank You
BlackMelon

 

[Harald Kapp]

But the ground plane that I got merges all ground traces into itself.

Not totally unexpected, is it? all GND traces are treated as one single net and therefore merged with the GND plane.
I'm not experienced with Eagle, from what I know from other similar tools I suggest you change the GND traces to signal traces (e.g. name them GND1), then flood the plane as required with GND. Now that the nets are different (GND <> GND1) there should be no merging. You'll have to manually add the connection between GND plane and GND1 traces where required (you can't simply connect the two in the schematic editor as they then become a single net which can't have two names).

A question on the side: Why do you want to have separate GND traces at all, when you have the comfort of a GND plane? The GND plane with its low impedance will be favourable for low frequency currents, too.

 

[BlackMelon]

Just look at this video at 13:08 :

. The low frequency is going to spread out all over the ground plane. I don't want this frequency mixing with higher ones. That's why I need to provide it paths to return.

 

[Harald Kapp]

As I see it, it's not about low-frequency vs. high frequency. It's about decoupling different signal pathes like analog (audio), digital and power. Decoupling these is indeed good practice. You can easily use different grounds by naming them accordingly, e.g. AGND (analog), DGND (digital) and PGND (power).
You'll have to take care to connect the different grounds at one point to achiev a common potential. This is typically done at a so called star point at the power supply or at the interface from analog to digital (e.g. ADC), if present.
You can then use these different grounds to flood different areas of the PCB with different ground potentials, depending on the subcircuit placed there.