Freq in real world vs. Freq in the SW

Hi folks,

I wrote a amplitude modulation block and here’s an interesting problem:

It seemed that the Freq showed on the plot of post modulation FFT is
always (1/80) of the freq I set in the SW. For example, If I set the
sub-carrier freq to 2MHz, and use a message signal (constant 1.0)to
modulate that, I can only get a 50KHz sub-carrier showed on FFT plot.
I’ve attached my plots(both in freq and time domain).

Below are the test results:
freq. in post-mod FFT freq. set in SW
12.5k 1M
25k 2M
50k 4M

Is there anyone who has some ideas or comments? Thank you!


Oh, There was a typo in my previous description: The sub-carrier freq I
set in SW is 4MHz not 2MHz

(i.e. 4MHz sub-carrier set in SW results in a 50KHz sub-carrier on
FFT.). Please let me know if you have any comments. Thank you.