The si5351 manual shows that the internal VOC operates at a maximum of 900MHz and a 4-divide-frequency output with a maximum frequency of 225MHz. In order to output a frequency of 300MHz, the internal VCO needs to be overclocked to 1200MHz. Not every si5351 can be stably overclocked to 1200MHz. As the temperature increases, the internal VCO operating limit frequency of the si5351 will decrease. If you notice a significant spike(>0dB) in your nanoVNA at 300 MHz or 900 MHz, I recommend that you use the 800MHz firmware.
Thank you!
