Amplitude Modulation with a Ring Modulator

Ring Modulators are circuits with the ability to combine two separate signal frequencies into a single output signal.   Passive ring modulators (what we'll be using) are typically constructed with four diodes in a ring configuration with one transformer connected to each side.    

In the case of amplitude modulation the general equation which defines the amplitude modulated signal is:

 f(t) = (1 + m sin ωt)sin ω't   
Where (m) is an attenuation coefficient and( ' )denotes the angular frequency of the carrier signal.

By using a handy trigonometric identity, we can re-write this equation as:

f(t) = sin ω't + (m/2)cos[(ω' - ω)t] - (m/2)cos[(ω' + ω)t]

As we can see, this output is a combination of the original carrier wave and two side bands.
When the signal frequency (ω) varies, the amplitude of the carrier sinusoid is modified accordingly. 
Upon recieving this sigal at the base unit, the original signal can be reconstructed with a simple envelope detector circuit.

For our application, we will be connecting the signal sinusoid across the primary winding of the first transformer and the carrier sinusoid to the center tap of the primary winding to the second transformer.

For additional information, check out this site.
http://www.du.edu/~etuttle/electron/elect61.htm