这是手册上的要求。
permissible oscillation frequency. If the required L.O. is beyond
oscillation limits, or the system calls for an external L.O., the
external signal can be injected at Pin 6 through a DC blocking
capacitor. External L.O. should be 200mVP-P minimum to 300mVP-P
maximum.
虽然之前用1000mVp-p也能工作。不过还是改了。至于过大有什么问题我还不清楚。