AN3206

Reference design

Doc ID 17406 Rev 3

9/14

using a vector network analyzer (VNA), it is easy to figure out with the help of a Smith chart

the value needed to compensate the de-adaptation. In our practical case, a serial capacitor

(C8) and inductor (L1) have been selected.

Figure 5.

Antenna matching circuit

2.2

RF layout guidelines

At 2.4GHz it does not take much for a poorly designed PCB to build up enough parasitic

reactance to noticeably compromise RF performance. The following notes describe how the

PCB design was implemented and some of the pitfalls that could arise in transferring the

design to a product.

2.2.1

PCB tolerance

Volume manufacturing requires the least expensive materials to be used while meeting an

acceptable level of performance. The effects of component choice have already been

discussed, but the PCB also plays a major part in the matching network and so you must

consider its performance.

For the types of cost-sensitive applications targeted, a low-cost PCB substrate is mandatory,

which means using a standard FR-4 epoxy. But within the FR4 classification there are cost

trade-offs to be made against:

●

●

Dielectric Constant, DK (also known as relative permittivity,

ε

Generally, materials are split into 'standard loss' and 'low loss' categories, with a

corresponding cost penalty. Other product considerations such as flammability rating and

lead-free assembly will narrow these choices. These are beyond the scope of this

document.

Modern PCB manufacturing achieves very accurate etching and so this is not considered as

a performance variable. Z-axis expansion depends on the FR-4 material and does not alter

much between vendors' material.

2.3

Other design considerations

Please refer to Section 3 for the non-RF requirements of the STM32W-based design.