How is RF layout design different from analog layout?

[rajeshkr1979]

Hi All,
I have some question regarding RF Layouts ,
I have worked extensively on analog layout for baseband signal , can someone suggest how RF layout are different, I do have idea about coupling stuffs.BUt while laying out how is it differnt from normal analog. what extra care do we take which is different in normal analog.
regards,
rajesh

 

[septipop]

rf layouts

One basic rule is to design the bias lines far away from the signal signal lines. Better in different planes. A second one is that the transistors should be very good grounded thru vias. Under transistors as much as possible ungaped ground plane. For other things I hope some layouter will help You. ADS has also the capability to develop layout.

 

[rajeshkr1979]

layout rf

Thanks,
Do you mean, noise coupling through substrate is important and this needs to be considered.
I analog designs we take care of this by putting substrate guardring ,is it the same.

 

[rfmw]

rf layout design

In baseband you do not care about signal integrity, in RF is a must: keep all parasitics small, make RF path as short as possible, transmission line theory comes into play, keep impedance-controlled environment, further there are surface waves to consider, etc...

 

[dipak.rf]

rf grounding and layout

Hi,

The very first thing is RF means high frequency that means you have to be extra careful to parasitics.
because at high frequency you will never come to which parasitic will come to picture and kill your chip performance. If its a GaAs substrate then their is no term called as guard ring to take of noise and so on

some brief points

1. RF layout should be simple and less croudy
2. The very important thing is source to ground connection should be very short and wide to avoid ground inductance which will kill your gain
3. RF signal path should be as straight as possible
4. Put enough grounding near to transistor device

Enjoy....................

 

[sharethewell]

Re: rf grounding and layout

Hi,

The very first thing is RF means high frequency that means you have to be extra careful to parasitics.
because at high frequency you will never come to which parasitic will come to picture and kill your chip performance. If its a GaAs substrate then their is no term called as guard ring to take of noise and so on

some brief points

1. RF layout should be simple and less croudy
2. The very important thing is source to ground connection should be very short and wide to avoid ground inductance which will kill your gain
3. RF signal path should be as straight as possible
4. Put enough grounding near to transistor device

Enjoy....................

Are you talking about RF IC design or RF PCB design?

 

[biff44]

Re: RF Layout Design

There are some very fundamental differences in RF vs Analog layouts. An experienced Analog layout designer will often try to apply the same logic to RF circuits...often with poor results.

For one thing, in Analog layouts there is a tendency to try to achieve "single point grounds" for noise and ground loop reasons. In RF layouts, there is no such thing! You instead, in almost all cases, want a "ground plane" that covers most of the board, and you bring each component's ground connection to that ground plane with a via.

You do NOT want to sever the ground plane! I know you will be tempted to run a power line or digital line from point A to point B, and think "it won't matter if I cut the ground plane just a little". Don't do it. The RF currents floating in the ground plane will have to travel around that cut, and in doing so you might inadvertently create a radiating antenna, or make a semiconductor chip start oscillating, etc.

A safe way to do the board stackup would be to have top metal for chip connections, a dielectric layer (perhaps 20 mils thick or so) and then the ground plane. If you need lots of power lines or digital lines, put them on the OTHER side of the ground plane. If you really need to jump RF lines over each other, and are tempted to put a metal later between the top metal and ground plane, make sure it have VERY FEW metal traces on it...just where you absolutely need them. Otherwise you will have RF coupling/leakage that causes big trouble.

Do NOT have long distances between a component ground pad and the via hole going to the ground plane. At higher frequencies, these act as transmission lines instead of good ground vias, and cause trouble. Do not be afraid of using two or more ground vias if you can fit them in CLOSE to the component--the more the better, (especially if you are above 3 Ghz).

 

[sharethewell]

Re: RF Layout Design

There are some very fundamental differences in RF vs Analog layouts. An experienced Analog layout designer will often try to apply the same logic to RF circuits...often with poor results.

For one thing, in Analog layouts there is a tendency to try to achieve "single point grounds" for noise and ground loop reasons. In RF layouts, there is no such thing! You instead, in almost all cases, want a "ground plane" that covers most of the board, and you bring each component's ground connection to that ground plane with a via.

You do NOT want to sever the ground plane! I know you will be tempted to run a power line or digital line from point A to point B, and think "it won't matter if I cut the ground plane just a little". Don't do it. The RF currents floating in the ground plane will have to travel around that cut, and in doing so you might inadvertently create a radiating antenna, or make a semiconductor chip start oscillating, etc.

A safe way to do the board stackup would be to have top metal for chip connections, a dielectric layer (perhaps 20 mils thick or so) and then the ground plane. If you need lots of power lines or digital lines, put them on the OTHER side of the ground plane. If you really need to jump RF lines over each other, and are tempted to put a metal later between the top metal and ground plane, make sure it have VERY FEW metal traces on it...just where you absolutely need them. Otherwise you will have RF coupling/leakage that causes big trouble.

Do NOT have long distances between a component ground pad and the via hole going to the ground plane. At higher frequencies, these act as transmission lines instead of good ground vias, and cause trouble. Do not be afraid of using two or more ground vias if you can fit them in CLOSE to the component--the more the better, (especially if you are above 3 Ghz).

Thank you very much for your reply. It's very helpful. But all you talk about is RF board level design. What about RF integrated circuit design? That's what I am working on.