Silicon Labs' sub-GHz reference designs for EFR32 chip family utilize an external ceramic balun in the RF-FE matching network. This matching approach is documented and well-detailed in the application note AN923.
However, mostly due to cost reasons, full discrete matching designs might be more desirable. The KBA shows several options about how to apply a matching network for EFR32 with utilizing SMD discrete components only.
Since the EFR32 wireless Gecko has differential TX and RX ports, the matching circuit has to have a balun function too, upon the impedance matching, so the standard 4-element matching balun approach can be applied here as well as shown in the application notes AN369/643 (however, these are discussed with other radios). The impedance goals for the matching network for EFR32 can be found in AN923.
Some extra details for simulations: TX bonding wire inductance is around 2 ... 2.5 nH; RX bonding wire inductance is around 1 ... 1.5 nH. LNA capacitance is around 1 ... 1.1 pF; PA capacitance is adjustable but it is recommended to use the min. value of it for the best efficiency which is around 2.5 pF. Impedance goals are e.g. 125 ohms in TX mode for +20 dBm running at 3.3 V and for +13/14 dBm too running from the on-chip DCDC converter (1.7 ... 1.8 V); and 500 ... 600 ohms in RX mode.
Here is a recommended schematic topology for a full discrete match in TX-RX direct-tie configuration:
- differential-to-differential L-C match for the first section of RX path [LGATE; CSER-1; CSER-2]
- 4-element matching balun approach [L1-1; L1-2; L2; C1; C2] + common mode suppressor [CH] applied on the TX path (+ rest part of RX match) + LPF [CHF0/1/2; LHF0/1]
Here is a recommended schematic topology for a full discrete match in split (separate TX and RX paths) configuration:
- TX path: 4-element matching balun approach [L1-1; L1-2; L2; C1; C2] + common mode suppressor [CH] applied + LPF [CHF0/1/2; LHF0/1]
- RX path: 4-element matching balun approach [LR1; LR2; CR1; CR2]
The split matching configuration can easily be re-used for designs with external SAW filter, FEM or RF switch utilized.
The common mode suppressor (CH) improves the balun function of circuit and can also be tuned for a specific even harmonic (typically 2nd or H2) where enhanced suppression can be achieved by the given notch filters composed by the L1-1 -- CH and L1-2 -- CH series resonances to GND.
The component values of LDC (RF choke inductor), CC (RF bypass - DC block capacitor), LGATE, CSER and LPF elements can be found in the application note AN923.
The simulated component values (simulated only, so bench tuning can likely be required) of the discrete matching balun networks - applicable for both schematics shown above - are summarized here:
TX path: (L1 here below is shown as total value, so L1 = L1-1 + L1-2; while L1-1 = L1-2)
These designs shown above are single- and typically narrow-band solutions, especially in the RX path due to the higher-Q impedance transformation needed. For dual-, multi- or wide-band matching solutions please contact Silicon Labs' Technical Support (and make sure there is an NDA in-placed with Silicon Labs).