Reference design

AN3206

6/14

Doc ID 17406 Rev 3

To minimize network loss, the matching topology should try to keep Smith chart loci towards

the center of the Smith chart. To do that, the following impedance transformation options are

available:

●

Series inductance followed by Shunt capacitance

–

This option favors lower balun ratios.

–

It leads to comparatively large inductance values.

–

It requires either long PCB traces or two discrete inductors (balanced network)

which means inductor Q becomes an issue.

●

Series inductance followed by Shunt inductance

–

Similar to auto-transformer action.

–

Sweet-spot for 100

Ω balun if series inductance fabricated from PCB traces since

neither too long nor too short.

●

Shunt inductance followed by series capacitance

–

Not allowed because DC feed to PA is required from balun centre-tap.

●

Shunt inductance followed by Series inductance

–

Favors lower balun ratios, but even at 1:1 the required series inductance is

comparatively large.

–

It requires either long PCB traces or two discrete inductors (balanced network)

which means inductor Q becomes an issue.

PCB parasitic elements play a part in the impedance transformation and even with the

tightest of layouts, traces between the matching elements will add significant reactance at

2.4 GHz. In particular, there will be some series inductance between the package pins and

the first matching element. This is significant when that element is shunted because it

cannot be absorbed into that reactance. Since this effect cannot be avoided, it is best to take

advantage of it.

The option with the lowest loss and least complex arrangement is series inductance

followed by shunt inductance. To make use of the PCB traces, it must be recognized that

they operate more like transmission lines which travel slightly differently on the Smith chart

compared with real inductors and this affects the shunt inductance value.

Traces of 100

Ω which equate to 150um wide on a 0.4mm thick FR-4 substrate should be

used. In ST's simple single-ended representation in the Smith chart, the balanced pair must

be modeled as a single 200

Ω transmission line. Figure 3 illustrates the impedance

transformation.