The crystal or TCXO reference spur is located +/- XO frequency (typ. 38.4 MHz for EFR32 Series 1-based designs) from the RF carrier frequency. This KBA is focusing on the possible HW modifications that can be suggested to enhance the suppression of these XO reference spurs around the carrier frequency. Also, the KBA is based on the measurement results of EFR32 Series 1-based radio boards at the 868/915 MHz frequency bands.
An evident approach could be to apply a high-Q band-pass filter, e.g. SAW filter, in the RF path to filter out the XO reference spurs. This will ensure very low level of spurs in a reliable way, but on the other it will also increase the insertion loss at the fundamental frequency while it is not a desirable solution for cost sensitive applications.
The XO coupling is basically occurred through two main domains within the part: VCO and PA. These blocks are supplied through RFVDD and PAVDD and thus forming two coupling paths. The power supplies can be modulated either by VDD and/or GND. Leakage current paths outside the chip depend on board design through impedance at HFXO frequency between supply domains.
So, a filtering scheme focused for the XO frequency on both RFVDD and PAVDD nets can help suppress these spurs effectively. The table below shows some measurement data with different RFVDD and PAVDD filtering configurations, captured on BRD4164A Rev A02 radio board at 915 MHz / +20 dBm.
|Fund power in dBm||XO spur at -38.4MHz||XO spur at +38.4MHz||subG PAVDD||RFVDD|
|19,48||-57,9||-77,4||-58,5||-78,0||ferrite + 1nF||ferrite + 1nF|
|19,51||-57,6||-77,1||-58,3||-77,8||ferrite + 1nF + 220pF||ferrite + 1nF|
|19,53||-57,7||-77,2||-58,3||-77,8||1nF + 220pF||ferrite + 1nF|
|19,57||-63,8||-83,4||-57,3||-76,9||ferrite + 56pF||1nF|
|19,46||-62,3||-81,8||-56,9||-76,4||ferrite + 1nF||56pF|
|19,45||-52,4||-71,9||-63,2||-82,7||ferrite + 1nF *||ferrite + 56pF *|
|19,48||-60,8||-80,3||-57,2||-76,7||ferrite + 1nF||ferrite + 56pF + 47nF|
|19,48||-60,9||-80,4||-57,2||-76,7||1nF||56pF + 47nF|
|19,5||-60,8||-80,3||-57||-76,5||47nF||56pF + 47nF|
* Note: close to the original BOM of reference radio boards.
Some good performer configurations in terms of spur suppression are highlighted in yellow above.
The RFVDD is filtered at the RFVDD pin, while the subG PAVDD is filtered at the PAVDD pin and at the BIAS pin of the external ceramic balun.
Further layout considerations: