Re: AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-low carrier frequency

 

Rich Grise wrote:


No, It doesn't have to be there (the 801 Hz frequency).  If your method
of 'beating' two signals together is by adding them, then there is no 801
Hz tone, only the 400 and 401 Hz tones.

With two function generators and a spectrum analyzer you can see this.

With a scope, you can see that the zero crossings in the summation occur at
a 400.5 Hz rate.

This is exactly what the trig identity earlier in the thread indicates.

If your method of beating is via multiplication, then there will be 0, 400,
401 and 801 Hz signals present (assuming the mixer is not balanced).

When you are discussing 'beating' two signals together you need to indicate
whether you are adding or multiplying the signals. The results are
different.

If you are multiplying two signals to find a zero beat with your ear, that
is difficult as you will be trying to hear tones less than 20 Hz.

If you are adding two signals to find a zero beat, that is easy because you
are listening to a tone that is at the average frequency. In the above
example, at 400.5 Hz.