I'm using the cofiguration LMX2572, buffer, I/Q mixer, PA, all stages located in extra compartments. Nevertheless I have incedental FM on the LO from the modulation envelope. By searching the leakage I was touching pin by pin of the LMX with a tweezer and looking to the phase noise of the LO signal. To my surprise almost no change when touching the LO output (pin22). When touching pin 24 (chip select for SPI) the phase noise increases dramatically (the pin is connected to a controller output via a LC filter). Is this a known effect? Any explanation?
Thank you
Holger