HB simulation problem(is it a bug)?

Hi guys
I did a simulation for mixer using HB simulator, the weird thing is the spectrum output does not agree with time domain output at all. I snap the graph, can anyone give me some explaination? For the same sigal, spectrum is correct, but the time domain inpy a 1/(20e-9)=50Mhz singal, it is so weird.
Can anyone give some suggestions?
Thanks a lot

Could be many reasons, difficult to determine without a description of your design. Archive your ADS project files using WinRAR and post here, someone may take a look.

It is a simply test, here is the graph, there has two signal, signal 1 and signal 2, siganl 1 a sin wave with frequency 1G hz, but the output for signal 1 and signal 2 are both weird, signal 2 is extremely weird. Please have a look and give me some suggestions.
Thanks