With the ladder two-element match, the IC consumes ~40 mA current at ~10 dBm power level. At low power levels, due to the lower
current consumption and the required lower PA supply voltage, the built in dc-dc converter can supply the PA through the PA_VDD pin.
The generated supply voltage by the dc-dc converter is 1.8V, which is correct for PA operation up to 13 dBm power levels. Since the
dc-dc converter converts the higher external supply voltages to the optimum 1.8 V with good efficiency, the PA efficiency improves sig-
nificantly. Without the dc-dc converter, the PA unnecessarily burns significant dc power and, thus, has lower efficiency at higher supply
voltages. A second advantage is that, with the dc-dc converter, the PA power is stable independently of external VDD variations. For
these reasons, Silicon Labs suggests supplying the PA_VDD rail from the internal dc-dc converter up to power levels of 13 dBm. The
allowed harmonic level by the US FCC is –41.2 dBm EIRP (or 500 µV/m electric field strength at a distance of three meters), while it is
-30 dBm EIRP by EU ETSI regulation. As shown in the figures, the second harmonic with the ladder two-element match has approxi-
mately 3 dB margin to the more strict FCC limit at a 10 dBm power level. With the ladder four-element match, the margin is much larg-
er, even at the highest (20 dBm) power level.
It should be noted that, in the spectrum measurements, the Spectrum Analyzer is used as a wideband 50 Ω termination. With a real
antenna, the termination at the harmonics can differ significantly from 50 Ω with variations in power delivered to the antenna. Moreover,
the harmonic termination impedance at the matching output depends on the transmission line properties between the matching network
and the antenna. In addition, the radiation gain of the antenna can differ at harmonic frequencies. Due to these facts, the radiated har-
monic power levels can be very different from the conducted measurement results and must be checked with the final antenna.
The different package versions have no significant effect on the output spectrum using both the ladder two-element and four-element
matches. Band edge (2.4 GHz and 2.48 GHz) spectrum variations across the bands are typically less than 0.5 dB, and the harmonics
are also much lower than the allowed limits, independent of package versions. More information about power variation across the band
and the package effect can be found in
Appendix 2. 2.4 GHz RF Network Schematics and Technical Data
.
Figure 3.13. Measured Spectrum Plots of the Dual Band 7x7 mm EFR32 with (a) a Ladder Two-Element Match in 10 dBm Pow-
er State and (b) a Ladder Four-Element Match in 20 dBm Power State
AN930: EFR32 2.4 GHz Matching Guide
2.4 GHz RF Matching Design Steps
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