hmsheng
Full Member level 4
I measured a fractinal-N PLL. The in-band phase noise is -80dBc/Hz on integer mode. But when it works at fractional-N mode the in-band phase noise will be -60dBc/Hz. Mash1-1-1 ΣΔ modulator was used for fractional-N. The ΣΔ modulator is always on at both interger-N and fractionan-N mode. But at integer-N mode only integer part was connnected to the multi-module divider.
I checked all the circuits and they seems all right. What may be the cause of the 20dB phase noise degeneration at fractional-N mode?
I checked all the circuits and they seems all right. What may be the cause of the 20dB phase noise degeneration at fractional-N mode?