High radiated harmonics on loop antenna with Si4010/11/12
10/289/2014 | 02:33 PM
I see high radiated harmonics with a Si4010 keyfob using loop antenna. How can I reduce the unwanted harmonics?
To get better suppression on the radiated harmonics the followings are suggested to be tried.
Reduce the top antenna capacitor value. This capacitor is responsible for the proper impedance transformation. If its value is higher the chip termination impedance is also higher what can cause higher harmonic contents. The typical top antenna capacitance is between 1.5 and 2pF for a 25x25mm loop antenna at 434MHz. With a same-sized loop it is lower than 0.5pF at 868MHz. Larger loops require lower capacitor values.
Make sure of the antenna symmetry.
Use copper pour on both top and bottom layers connected to each other with many vias, where there is at least 2-3mm keep-out between the copper pours and antenna traces.
Filter the VDD trace with parallel capacitors at the critical frequency.