"Crossing" a microstrip line

rf microstrip bridge

Hello all!


I stumped on a problem with microstrip transmission lines. Yes, I did check the usual theory books, handbooks and cookbooks with no answer at all so far. And yes!, I firs made a search on EDAboard befor opening this new thread. Hope someone can give a tip here.


My problem:

I'm working on a somewhat complex PCB at 5.8 GHz and I am limited to 2 layers (one of them, of course, will be the ground plane). For microwave traces I used microstrip.

When routing power supply traces and control/low-frequency traces I realized I wouldn't be able to route them without crossing microwave traces (I must stress I am limited to two layers).

How to overcome this problem?

- Solution 1 - interrupt the low-frequency traces and solder a piece of wire over the microstrip, suficciently apart from it to avoid coupling. While this solution is enough for a single prototype, it is a bad idea for mass production. What do you think?

- Solution 2 - put two vias in the low-frequency trace so I can route it under the microstrip line for a few millimeters. I would prefer this solution since it is neat for production, but I would be creating a small slot in the ground plane just under the microstrip line, and I have no idea how this would afect line (and circuit) performance. What do you think?

- Solution 3 - I will gladly read other ideas from you.

You are right to hold the ground planes as sacred.


Assuming the current you're tying to pass is reasonable and your design rules allow it, I would recommend just routing a thin trace (5-10 mils) under a coupling cap or other series element in a non-critical area -- the lower the impedance the better in some ways (don't zip through a high-Q/high-z circuit).

You may wish to plan for adding additional bypasses or filtering on this supply, so that you can ensure that it is "cold" to RF when you make this underpass.

While this is undesirable in many ways, it can be done successfully and is done in high volume production.

Lance

The problem of crossovers in microwave circuits is fairly common. So much so, that commercial components exist to handle it.

Take a look at:
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There's also an article regarding designing your own solution:

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Depending on your design/freq/etc. you may be able to run the DC/low freq on the RF line, then just use RF and DC blocks to tap of the DC where you need it. That has worked decently for me on higher frequency boards where I can put quarterwave shorts, etc. Never the less, it is another idea.

Good Luck!

Thank you all for your suggestions - they all make perfect sense.

I will explore those ideas on my designs.

Hi,

One of the pcb the I design I had that problem.
What I did was:
RF is going in top side without problems,
other signal (DC/power not RF signal) go below the by via to the bottom side
and go up thru via to top.
Important cross MUST 90 angle!!!!!!

in bottom side if gnd without solder mask you have to "paint" this trace with solder or make release in mechanic.

David

DDavid said:

Hi,

One of the pcb the I design I had that problem.
What I did was:
RF is going in top side without problems,
other signal (DC/power not RF signal) go below the by via to the bottom side
and go up thru via to top.
Important cross MUST 90 angle!!!!!!

in bottom side if gnd without solder mask you have to "paint" this trace with solder or make release in mechanic.

David

Click to expand...

So you gap the ground plane below the microstrip line?

like I said
Not RF signal goes below RF in bottom side
between two lines you have to save 90 deg angle

david

Simply use a 0 ohm resistor as the bridge over the microwave trace. A pad on both sides of the microwave trace and solder the resistor down. The microwave trace runs under the body of the resistor. Most of my microwave boards I can only use the top layer so I only have 1 layer to use. This works in mass production.

toonafishy said:

Simply use a 0 ohm resistor as the bridge over the microwave trace. A pad on both sides of the microwave trace and solder the resistor down. The microwave trace runs under the body of the resistor. Most of my microwave boards I can only use the top layer so I only have 1 layer to use. This works in mass production.

Click to expand...

Seems like a good idea. I will keep that in mind.

toonafishy said:

Simply use a 0 ohm resistor as the bridge over the microwave trace. A pad on both sides of the microwave trace and solder the resistor down. The microwave trace runs under the body of the resistor. Most of my microwave boards I can only use the top layer so I only have 1 layer to use. This works in mass production.

Click to expand...

It seems good idea,
But tell me toonafishy it will work at 5.8GHz??

David

Of course will be a little of coupling between the two traces, but if you use it only for low frequency signal the way is good.

Mandi

I don't mean low freq signal
I am afraid that will be problematic to RF signal!!!
Ecpesially if one off the traces is RF and other is DC!!!
It's my hunch!

David