If two tones f1 and f2 are amplified by a non-linear amplifier, which frequency components would be present in output?
If two tones f1 and f2 are amplified by a non-linear amplifier, which frequency components would be present in output? Correct Answer nf1 ± mf2, where n and m are integers
When two frequency signals (say f1 and f2) are fed as input to any nonlinear device such as an amplifier, it will generate amplified output at f1 and f2. In addition, it will generate IMD (Intermodulation distortion) products at other unwanted frequencies (+/-n*f1+/-m*f2).
Frequencies at f1-f2, f2-f1, 2f1, and 2f2 are referred to as second-order IMD products while frequencies at 2f2-f1 and 2f1-f2, 3f1 and 3f2 are referred as third-order IMD products.
e.g. we use square law modulator in generation of AM which is a Non linear device. Square law device output is in form of
V0 = a1 vi + a2 vi2 …… where vi = input signal
Let vi = As cos 2πf1 t + B cos 2π f2 t
Then SLD output v0
= a1 (A cos 2π f1t + B cos 2πf2t) + a2 {A cos 2πf1t + B cos 2πf2t}2
V0 = a1 {A cos 2 π f1t + B cos 2πf2t} + a2 {A2 cos2 2πf1t + (B2 cos2 2πf2 + 2AB cos 2πf1 t cos 2 πf2t }
A2 cos2 2πf1t → cos2 2πf1t = 1 + cos 2π (2f1)t
2AB cos 2πf1 t cos 2 πf2t → cos 2πf1t cos 2πf2t = cos 2π(f1 + f2) t – cos 2π (f1 – f2)t
So we can see in second order 2f1, 2f2, f1 – f2, f1 + f2 harmonics present.