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#### jackielin007

##### Newbie level 2
hi guys

when calculating the THD of a certain circuit, are you do a one tone(amplitude=V) simulation and calculate it. And how about get THD from two-tone(V1,V2 = 1/sqrt(2) V) sim?

it looks like the result is different. coz two-tone sim will generate intermodulation terms. So the THD will be lower

I am confused. How is THD exactly defined???

Thx

#### renwl

normally,THD is defined in one tone.

#### iamxo

##### Full Member level 4
it's interesting question!
maybe calculate THD to each signal!

#### safwatonline

i am not sure but i think when they say HD they mean the harmonic(mul;tiples of fundemental) distortions and when they say IMD they mean non-tones distortion (non-harmonic distortion)

#### bbbb

##### Member level 2
That seems right , in two-tone test, results are usually described by IM2, IM3, ... . Depending on the circuit and application, THD or IM3 may be improtant. for example in a Low-pass filter it is common to describe distortion in terms of THD whereas in high-Q bandpass filter IM3 is a better figure for linearity.

#### maharshi_qis

##### Full Member level 5
THD is defined for signal which is distorted by nonlinearity in the components of a ckt to which the signal can a o/p..
THD =sqrt(sum squares of all harmonics)/fundamental
gen is <2%

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