I see high radiated harmonics with an EM35x design. How can I get more suppression on the critical harmonics?
If high radiated harmonics are observed the first thing should be to find the main sources of these unwanted radiated harmonics. We can split this - where the radiations could be coming from - into two options:
1. From the RF front-end, antenna
2. From the PCB itself, other (i.e. non-RF) traces
Reducing the conducted harmonics as much as possible always makes sense, since antennas can have quite large gain on the harmonics too. So, the first step has to be to get good and quite clean conducted spectrum in any case.
Regarding the non-RF traces radiation the main suspicious one could be the VDD trace itself in most cases. The RF pins of the EM35x chip must be pulled up to VDD for biasing the PA, so there is connection between the RF and VDD and some RF harmonic leakage can appear to VDD from the RF.
Here is a list what includes some tuning tips on how to get further suppression on the unwanted harmonics:
1. Use a proper 5th order low-pass filter before the antenna, see our reference designs.
2. Make sure whether the ground pads of the filtering capacitors are placed far from each other as possible (should be followed especially for the same-valued capacitors).
3. If extra antenna matching network is required at the antenna input port for the proper antenna impedance at the fundamental, make sure to apply a low-pass kind of matching network.
4. If the design uses FEM, please make sure to do not overdrive it, i.e. do not use the EM35x chip in a too large TX power state (typically set to -5).
5. Please make sure whether many grounding vias are used on the exposed pads of the EM35x chip and FEM. See our reference designs.
6. In more-layer designs, please make sure to put as many traces onto an inner layer as possible and create large and continuous ground reference planes on the top and bottom layers what can work as a natural shield. The top copper pouring is not suggested around the EM35x chip, XO and ceramic balun, but recommended around the RF filtering section.
7. In FEM designs, if the critical harmonic falls quite far from the fundamental (e.g. 5th harmonic) an extra notch filter can be applied between the FEM and EM35x chip. This extra filtering element is just an extra parallel capacitor between the RF pins what can be insert to the design in parallel with the single matching parallel inductor between the FEM and EM35x chip. For example, for the 5th harmonic suppression the suggested capacitor value is 0.4pF what is a short circuit at the 5th harmonic, but practically does not affect the fundamental operation.
8. The VDD trace(s) should be filtered by capacitors and inductors too if necessary (this could be the case when the conducted spectrum looks good, but the radiated harmonics are still too high). In a single EM35x design the 1.8V RF VDD trace, meanwhile in designs with FEM the 2.1 - 3.6V VDD traces too (connected to the supply pins of the FEM) should be filtered. This filter can include parallel capacitors from the VDD trace(s) to GND and/or series blocking inductors on the VDD trace(s) what are suggested to be placed as close to the VDD pins as possible. The recommended capacitor values for the 2nd and 3rd harmonic suppression are 2.4pF and 1pF, respectively. The suggested indcutor values for the 2nd and 3rd harmonic suppression are 7.5nH and 3.3nH, respectively.
9. Please make sure about the layout routing, suggested to follow our reference designs.