6.3.2 AM Modulators

Product modulator. As Equation (6.5) suggests, the modulation of an AM waveform can be completed through the process of multiplication and addition. Figure 6.5 shows a product modulator for AM.

Square-law modulator. Signal multiplication at higher frequencies can be accomplished by the square-law modulator shown in Figure 6.6 in which the non-linear element approximates the square-law function:

v_{o u t} = a_{1} v_{i n} + a_{2} v_{i n}^{2}

(6.12)

so that with vin = sin ωmt + sin ωct:

v_{o u t} = a_{1} \sin (ω_{m} t) + a_{2} \sin^{2} (ω_{m} t) + a_{2} \sin^{2} (ω_{c} t) + a_{1} [1 + \frac{2 a_{2}}{a_{1}} \sin (ω_{m} t)] \sin (ω_{c} t)

(6.13)

where the last term is the desired AM wave which can be filtered out.