The envelope function of a rapidly varying signal is a smooth curve outlining its extremes in amplitude. There is a technique to predistort a signal so that the inherent distortion generated by the transmitter will be compensated for and thus the desired signal will be produced after tranmission. This is termed Envelope Feedback transmission.
in a high frequency amplifier, because of non-linear gain, errors are caused. Modulation of one frequency may bring it into the range of another frequency.
THD is used to characterize the power quality of electric power systems. When a signal passes through a non-ideal, non-linear device, additional content is added at the harmonics of the original frequencies. THD is a measurement of the extent of that distortion. When the input is a pure sine wave, the measurement is most commonly the ratio of the sum of the powers of all higher harmonic frequencies to the power at the first harmonic, or fundamental, frequency.
Ts = 1/ fswhere fs is the symbol rate. See Symbol rate
fs = R / N
or put more logically, the gross bit rate, R = fs * N
Now, let's define M as the maximum number of distinct messages passable per Ts. M is also called the alphabet since it represents all the different frequencies we might need to send.
It turns out that M = 2N. For example, if our baud rate is 1000 and N is 3 then our gross bit rate is 3000 and our alphabet of frequencies (M) is 8. Furthermore, in a 64QAM modem, M=64.
By taking information per pulse N in bit/pulse to be the base-2-logarithm of the number of distinct messages M that could be sent, Hartley constructed a measure of the gross bitrate R as:
R = fslog2( M )where fs is the baud rate in symbols/second or pulses/second. M becomes important because it determines the S/N ratio required. The larger our alphabet becomes within a finite channel, the harder it becomes to distinguish between out alphabet letters.