DM182017 - Why am I getting harmonics when using a fob transmitter during transmission?

The harmonics are expected because the red fob transmitter is designed to operate at multiple frequencies as there is no band-pass filter in the schematic. You can only see the impedance matching circuitry, DC-block caps, and the pull-up at the RF_out pin. Using a spectrum analyzer, the measured values are:

  • 434 MHz -39 dBm,
  • 868 MHz -58 dBm,
  • 1302 MHz -62 dBm,
  • and 1736 MHz -74 dBm.

Note that the power level difference between the fundamental frequency and the harmonics. It can be noticed that there is a difference of 20 dBm between the fundamental and the second harmonic. Before resolving the harmonics issue, you should first ask how these harmonics affect a product, considering that the power levels are minimal. Do these harmonics violate any government wireless regulations? If the harmonics affect the product or violate wireless regulations, add a low-pass filter or a band-pass filter in your circuitry. The filter is normally added right after the RF_out pin of U1 and before L5. The added filter might slightly affect the impedance matching's scheme of your circuitry.

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Part number 0400LP15A0122 by Johanson Technology Inc. is an example of a Low Pass Filter.

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