Part Number: LMX2820EVM
Other Parts Discussed in Thread: LMX2820
Hi!
I have rather strange and annoying issue with LMX2820 RF synthesizer.
We have a rather basic schematics mainly lifted from the evaluation board (including PLL filter). The problem is that some units works fine but some fail to lock (no instant calibration involved).
Interfacing failing units with TICS Pro indicates that the problem is with VCO calibration routine - it always falls to VCO7 for any frequency I choose (and if the frequency is such that VCO7 happens to be the correct choice, the unit would lock and operate correctly). Moreover some units fail to lock when they are just powered up, but will work normally after warm up or if they are heated up (literally, by a heat gun) beforehand.
It seems to me that some component values are such that they put LMX2820 on the edge of an instability, so that random variations or changes with temperature push it over.
Unfortunately, documentation that I was able to find online does not go into details how the VCO calibration happens and if it depends on any of the external components (and why it is necessary at all; are not VCO ranges known from the beginning?).
I would greatly appreciate any additional input on the topic or any suggestion.
P.S.: I've seen alternative set of bypass capacitors suggested in this forum and tried them; they do not seem to make any difference.
