0. Update

• Feb. 4 2021: I am just circling around again to this project. Its been a very busy year for me and I haven't been focusing much on side projects. I was looking at the connector I chose, when I started this project, and looking at my simulations and initial measurements, I have decided to move towards a more "RF friendly" connector. I have tightened my design goals and pushed the frequency higher (see the spec tables below) to give myself more of a challenge.

1. Intro

At higher frequencies, one must pay more attention to the geometry of the SMA footprint - often referred to as the "transition" since the electromagnetic fields are transitioning from a coaxial distribution in the SMA connector to a microstrip, stripline, etc. distribution on the PCB. The transition on the PCB aids the fields while they change distribution ensuring the process is as smooth as possible.

The consequence of a poor transition at RF is a lower Return Loss (RL), and higher Insertion Loss (IL). With a poor transition, we won't be able to deliver power to the load (the PCB and subsequent RF circuitry), and thus we won't be able to do useful work. The RF transition, while not very glamourous, is an incredibly important aspect of RF design.

I have been using OSH Park for a while, and I wanted to take the time to design a nice RF transition for their 4-layer process that everyone could potentially use.

I am using CST electromagnetic software to do all my simulation work. I wouldn’t consider myself a power user, but I am proficient with the software. This is by no means a CST tutorial, nor should you blindly trust any of the CST results herein

2. Specifications

A. Simulation Specifications

Table 1 lists the desired specifications of the simulated transition. Note that due to imperfections in the model that I will build (using CST), we will not achieve these results on PCB. These are goals to shoot for in the model and we will get what we get in real life – such is the nature of RF design.

B.    Frequency Range

Typical 3.5mm SMA connectors "work" from DC - 18 GHz / 26.5 GHz depending on the quality of the connector. I use the word "work" very loosely, because SMA connectors will work to much higher frequencies; the upper frequency is really the "moding frequency" - the upper frequency where other modes (field distributions) are excited in the SMA connector. We typically want to avoid moding, so we only use the connector to its specified max frequency.
When we need to go to higher frequencies, we move to smaller coaxial connectors which go to higher frequencies before they start moding. e.g. 2.92 mm, 2.4mm, 1.85 mm, 1 mm, and smaller.

C. Insertion Loss

Any real RF device has Insertion Loss (IL) - the loss we must account for when we insert an element into our circuit. Good connectors and transitions are virtually transparent - you see very little loss by using the connector and transition. This means we deliver most of the power to the RF circuitry while wasting very little in the connector and transition.
The lossy-ness of the transition is dependent on the geometry as well as the PCB substrate material. In this case we are using the OSH Park 4-layer process which is an FR408 material with a high loss tangent of ~0.011 at 1 GHz and ~0.013 at 10 GHz. Nice RF substrates have very low loss tangents (~ 0.001 to 10s of GHz).
In this project, we are shooting for 0-1.5 dB of loss across frequency. Naturally, we will always have some loss around DC (milli-dB) and higher loss at 18 GHz (~3-4 dB on FR408). Some of the higher loss will be due to imperfections in my model, but most of it will be due to the lossy FR408 substrate. There is a reason that the datasheet doesn't specify loss tangent higher than 10 GHz - it’s not meant to be used up there. 

D.    Return Loss

In our project, the Return Loss (RL) describes how well matched the 50 Ohm SMA connector is to the PCB. RL is equivalent to |S11|, and we often specify RL at min & max frequencies. We want RL to be as high as possible (S11 is as small as possible), which means that we are delivering most of the RF power to the load! The RL depends highly on the geometry of the transition to the PCB, and I will spend most of my energy trying to improve the RL in my model. Excellent transitions have RL figures of 30+ dB across frequency. An RL of 20+ dB is considered good and will be what we shoot for at 26.5 GHz in our physical measurements.

3. CST MODEL

We need some basic information about the OSH Park 4-layer stackup, the datasheet of the substrate material, and the SMA connector datasheet. 

A.    Modelling the PCB

The PCB model is a simple sandwich of metal and substrate layers distributed according to the OSH Park 4-layer stackup on their website. I have disregarded the solder mask layers for simplicity as I don’t want to include those in the model. The white looking layers are the FR-408 material, and the dark gray layers the metal. For the substrate material, I started with traditional FR-4, and then modified the dielectric constant (DK) and the loss tangent (DF) according to the linked datasheet above.
 

 
Image 1. PCB model in CST showing the various dielectric and metal layers.

B.    Modeling a CoPlanar Waveguide (CPW) transmission line

The first thing we want to do with our pcb material is model a basic transmission line to see the loss characteristics of the board. I have chosen

Updated content coming soon.