Low Pass Filter

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Due to the internal structure of the transmitter IC and non-linear nature of the power amplifier, it is necessary to add a low pass filter to the output of the device to reduce any harmonics and spurious emissions above the intended frequency of operation. For this design, a 5th order pi-type Chebyshev filter with a cutoff frequency of approximately 150MHz was designed and simulated in LTSpiceIV.


Component values were first determined using Elsie. The components were then chosen from Coilcraft and Digi-Key. Using Coilcraft's "Highest Q" design tool, an 56nH 1812SMS air-core inductor was chosen for its power handling capability and quality factor. The 36pF and 56pF capacitors were found from Digi-Key, both selected for RF applications. Using data from Coilcraft and the capacitor datasheets, an approximate non-ideal model was determined and input into an LTSpiceIV schematic for simulation. The resulting circuit and frequency plot are shown below.

Non-ideal filter model schematic with power dissipation.
Magnitude plot (100-500MHz) showing maximum insertion loss.
Magnitude plot (10-3000MHz) showing spurious responses.

The filter has good harmonic rejection, with >46.9dB rejection at the second harmonic and >83.2dB rejection at the third harmonic. The filter maintains >83dB rejection until >2.35GHz, well beyond any significant harmonics generated by the transmitter/amplifier. The filter has a maximum insertion loss (with nominal values) of 0.55dB at 148MHz. Using a transient simulation with the source set to 5W output power (at 146MHz), the power dissipation through the ESR of the inductors and capacitors was calculated as shown in the schematic above. This indicates that the selected capacitors must be able to dissipate >100mW during transmit.