What is the function of RF plane in radio module?

[fuzzy_boy]

What for RF plane is used is an radio module,i came across a psb design where,it has 4 layer stack up,1 for signal layer,1 for ground and one for power ,and the other is RF plane, i am confused what for the RF plane is used and where is it connected. please help.

Regards,
Nafis

 

[Azulykit]

Re: RF Plane Issues

I would suspect that you have come across a PCB with microwave frequency transmission lines like microstrip or coplanar waveguide. These often use a separate rf groundplane and sometimes signal plane.

 

[fuzzy_boy]

RF Plane Issues

What is the need for RF planes,does the microstrip antenna use the RF plane

 

[Azulykit]

Re: RF Plane Issues

Yes, often.

 

[Rame]

Re: RF Plane Issues

Hi,

Check the site called **broken link removed** might be answered here.

Regards,

Ramesh

 

[zandyp]

Re: RF Plane Issues

I'll try to answer with one caveat: I still consider myself a novice in this area even though I've been working on RF pcb's for 3+ years.

In designing RF boards we're trying to minimize losses from either reflections (due to impedance mismatch) or 'resistive' loss.

Most RF systems are designed with an impedance of 50 ohms. To 'match' the impedance you need to adjust the width of the track and if possible its height above a solid plane (grounded or otherwise). In a perfectly matched line the RF wave see's little difference and continues on with less loss.

In the four layer board the typical stackup I use is:

Top Layer - RF Tracks
Interior 1 - Ground
Interior 2 - Power
Bot Layer - non-RF tracks

The advantage with a standard 4-layer board is that the height of the RF track above the ground plane will be 14mil (approx) which means that a 50ohm trace will need to be about 25mil thick. In a two layer 60mil board of FR4 it would have to be around 100mil - a real pain when you're working with fine pitch components.

I'm willing to bet that the board you described is using the 'RF Plane' for the RF tracks. Although I'm not certain why they would use the power plane instead of the ground plane.

There's a number of different types of microstrips that can be used.

Cheers!
Andy