RF PCB guideline

[Nbj123]

Hello,

I am doing my first RF project. i want the help regrading trace width calculations. I have to design 6 Layer PCB in FR 4 material which my first layer contains RF signals.

My layer stack up is like Signal, GND , Inner layer, Power , GND, Bottom.

I have query regarding the RF PCB design.

1) RF signal contains in 1st layer so which calculator used? co planner or Micro strip?
2) How RF Trace length important? any calculator for RF length calculation?


Please guide me as soon as possible as this is my first project and i have to submit it.

Thank you so much.

 

[FvM]

A single ended transmission line in the outer layer above a ground plane is elementarily a microstrip, but it can be turned into a coplanar line with ground if you add ground "fences".

Transmission lines have a characteristic propagation speed according to effective permittivity which relates mechanical length to propagation delay.

 

[Nbj123]

A single ended transmission line in the outer layer above a ground plane is elementarily a microstrip, but it can be turned into a coplanar line with ground if you add ground "fences".

Transmission lines have a characteristic propagation speed according to effective permittivity which relates mechanical length to propagation delay.

Dear sir,


Thank you so much for your prompt reply.

My doubt regrading transmission line impedance clear with your prompt reply.

Can you please help me regarding length,when i have to calculate this.

Thank u so much.

 

[vfone]

There are few options for planar transmission lines. Stripline, microstrip, coplanar (CPW or CPWG), SIW. Each one have pros and cons mainly based on substrate material used, circuit performance and frequency range.
Microstrip lines are the planar structures least affected by the PCB manufacturing process tolerances, so in most of the cases this is the chosen option.
Freeware programs available to characterize planar transmission lines :

http://www.awr.com/awr-software/options/tx-line

AppCAD

www.hp.woodshot.com

 

[ZoroZeng]

Hi,


Here are some suggestions for you to design RF project.


In the PCB design of low-power RF, the standard FR4 material is mainly used (good insulation characteristics, uniform material, and dielectric constant ε=4, 10%). Mainly use 4-layer to 6-layer boards. In the case of very sensitive cost, you can use a double-sided board with a thickness of less than 1mm. Make sure that the opposite side is a complete stratum. At the same time, because the thickness of the double-sided board is above 1mm, the ground layer and the signal layer The FR4 medium between is relatively thick. In order to make the impedance of the RF signal line reach 50 ohms, the width of the signal trace is often about 2mm, making the spatial distribution of the board difficult to control. For a four-layer board, under normal circumstances, the top layer only takes the RF signal line, the second layer is a complete ground, the third layer is the power supply, and the bottom layer generally takes the digital signal line that controls the state of the RF device (such as setting the clk of the ADF4360 series PLL, data, LE signal lines.) The power of the third layer should not be made into a continuous plane, but the power traces of each RF device should be star-shaped, and finally connected to a point. The power traces of the third layer RF device should not cross the bottom digital lines.

For a mixed-signal PCB, the RF part and the analog part should be far from the digital part (this distance is usually more than 2cm, at least 1cm), and the ground of the digital part should be separated from the RF part. It is strictly forbidden to use a switching power supply to directly power the RF part. The main reason is that the ripple of the switching power supply will modulate the signal of the RF part. This modulation often severely damages the RF signal, leading to fatal results. Under normal circumstances, for the output of the switching power supply, you can pass through a large choke and π filter, and then through a linearly regulated low-noise LDO (Micrel's MIC5207, MIC5265 series, for high-voltage, high-power RF circuits, Can consider using LM1085, LM1083, etc.) to get the power supply to the RF circuit.

Hope these can help you.