Question

What is the recommended distance between antennas in an antenna diversity application?

Answer

Antennas in a product that implements antenna diversity have their antennas mounted at a distance of at least ¼ wavelength apart. This amount of spatial separation improves the probability that at least one antenna is NOT in a deeply faded signal condition. Another typical recommendation is ½ wavelength antenna distance, however, it can result quite large board sizes at low frequencies (e.g. for 434 MHz frequency ½ wavelength is 34,5 cm).

Question

Can I use the same matching network for EZRadioPRO and an EZR32 wireless MCU that is based on the same radio? 

Answer

The answer depends on the actual PCB layer stack-up as the distance between the top and the first inner (ground) layer determines the PCB parasitic capacitances, which plays part in the matching network.

If the same PCB layer stack-up is used (or at least the distance between the top and the first inner layer is similar), in that case the same matching network component values should result very similar TX and RX performance for an EZRadioPRO radio and an EZR32 wireless MCU. If the PCB layer stack-up deviates significantly, it is not recommended to use the same matching network as the detuning can cause lower output power, higher harmonics, higher current consumption and sensitivity loss.

Question

How can I use the EFR32 Series 1 (EFR32xG1x) sub-GHz RF ports in single-ended way?

Answer

Both TX and RX sub-GHz RF ports of EFR32 Series 1 wireless geckos (EFR32xG1x) are truly differential ones. However, they can be used in more simple single-ended way as well, which will enable to use more simple matching networks, but, on the other hand, it will sacrifice some RF performance.

RX:

For single-ended RX operation, sensitivity degradation of 6 dB is expected. The unused input pin can be connected to GND. Note, that it also affects the performance of RFSENSE.

TX:

Regarding the TX, 3 dB lower output power is expected. Also, the harmonic performance will be degraded. The unused TX pin can be connected to GND (to save current). 

The single-ended matching network should use halved impedance compared to the differential one, with removed ceramic balun. 

• The single-ended RX and TX matches use the series elements of the original differential match without any change: i.e. the series capacitor in the RX match and the series inductor in the TX match will remain the same.
• The shunt element impedances are halved in the single-ended match and connected to GND: i.e. half shunt inductor value in the RX match and double shunt capacitor values in the TX match are used. As the TX internal capacitor will be small as well an additional external shunt capacitor will be used right at the TX pin as well.
• Remove the balun and connect the single ended match output directly to the single ended LPF.  

Question

At which pins should I apply DC feed for the sub-GHz and 2.4GHz PA of EFR32 Series 1 devices?

Answer

For the 2.4GHz PA, the DC feed should be applied on PAVDD pin.

On the sub-GHz side, the PA DC feed is required through the TX output pins: SUBGRF_OP, SUBGRF_ON.

For more details, refer to the EFR32 Series 1 Radio Board Reference Designs.

Question

In which cases should I apply DC-DC to PAVDD when creating an EFR32 Series 1 design?

Answer

Using the 1.8V output of the internal DC-DC converter of EFR32 Series 1 devices for DC supply of the PA provides better efficiency (i.e. lower PA current for the same output power) compared to the case when the PA is running from the main 3.3V. The recommendation for applying the DC-DC output to PAVDD:

• <= 13dBm for 2.4GHz
• <= 14dBm for sub-GHz

Above these power levels, using the main 3.3V supply for DC supply of the PA results better efficiency.