We have an issue with lmx2492:
We used to install 25MHz reference oscillators on our boards. With doubling reference frequency to 50MHz we obtained good phase noise.
Problems started when there was a need to use external reference frequency 10MHz.
In first experiments we used quartz crystal filter and transistor amplifier to filter and amplify reference frequency. With internal frequency doubling we didnt see even pll lock.
When we programmed chip without frequency doubler, we seen lock but phase noise performance was terrible(10dB lower than calculated and spurs). Reference level on input was 5dBm.
Then we installed smidt trigger on reference path. We even obtained pll locking with reference frequency doubling. Noise performance was perfect, but when we connected boards in working construction performance degraded by 7dB in loop and we seen many spur noise. On shmidt trigger output we see oscillation with 3V level and 40-60% duty cycle. Chip reference input is AC coupled and loaded by 220Ohm.
Problem is that we dont see any connection between pll behavior and reference oscillation level and its duty cycle.
Did anyone tested chips with 10MHz reference frequency(with or without doubling)?
What is optimal reference path for system with external reference with input reference level +-5dBm?
What is predicted phase noise performance with reference frequency 10MHz with phase noise -150dBc/Hz on 1kHz offset(with and without doubling)?