8cube said:
after the signal come out from modulator, need to pass through a low pass filter to remove the distortion , is it correct?
The i.c. MC1374 is the modulator.
The signal comes out of pin 9.
The components which I highlighted in my 2nd post (L3, L4, C11, C12, and C13), and mentioned again by Ron H in his last post, form the low-pass filter which you are so anxious to locate yet so reluctant to accept.
It attenuates the harmonics and other high-order modulation products.