# What is neper and neper frequency

The neper (Np) is a logarithmic unit that is used to express the ratio of two values of the same physical quantity, such as power, voltage, or current. The neper is defined as the natural logarithm of the ratio of the two values, where the natural logarithm is the logarithm to the base e (the mathematical constant approximately equal to 2.71828).

The unit "neper" is written in lowercase as "neper". It is named after John Napier, the Scottish mathematician who invented logarithms, and the term "neper" is not an acronym or abbreviation, so it is not written in uppercase or with periods between the letters.

The symbol for the neper unit is "Np". However, it is important to note that the neper is not an SI unit and is not commonly used in scientific and engineering applications. The decibel (dB) is a more commonly used unit for measuring ratios of power or amplitude in signals and systems, and is defined in terms of a logarithmic ratio of the powers or amplitudes being compared.

The neper frequency is a concept related to the neper unit that is commonly used in signal processing and communication engineering. It is defined as the frequency at which the power of a signal has been attenuated by one neper (i.e., a factor of e) due to transmission through a system or medium. The neper frequency is also known as the e-folding frequency or the 3 dB frequency, because a one-neper attenuation corresponds to a reduction in power by a factor of e, which is approximately equal to a reduction in amplitude by 3 dB.

In practice, the neper frequency is often used to characterize the frequency response of a system or filter. For example, in an electrical circuit, the neper frequency of a low-pass filter is the frequency at which the power of the output signal has been attenuated to e^(-1) (about 37%) of the power of the input signal. Similarly, the neper frequency of a high-pass filter is the frequency at which the power of the output signal has been attenuated to e^(-1) of the maximum power of the output signal.

Here's an example of how the neper unit and neper frequency can be used in practice:

Suppose we have a signal with a power of 1 watt, and we want to transmit it through a medium that attenuates the signal. If the power of the signal is reduced to 0.5 watts after transmission, we can calculate the attenuation in nepers as:

Attenuation in nepers is, $$L= ln(\frac{P_{out}}{ P_{in}}) = ln(0.5 / 1) = -0.693 Np$$

This means that the power of the signal has been attenuated by approximately 0.693 nepers, or a factor of $$e^{-0.693} ≈ 0.5$$.

Now, let's say we want to find the neper frequency of a low-pass filter that attenuates signals above a certain frequency. Suppose the filter has a cutoff frequency of 1 kHz and the input signal has a power of 1 watt. At the neper frequency of the filter, the power of the output signal would be attenuated by a factor of $$e^{-1} ≈ 0.37$$. So we can calculate the neper frequency of the filter as:

Attenuation in nepers at neper frequency = 1 Np Attenuation in decibels at neper frequency = $$20 log_{10}(e^{-1}) ≈ -8.686 dB$$

Now we can use the formula for the frequency response of a RC low-pass filter in the Laplace domain to find the neper frequency of the filter:

$$H(s) = \frac{1}{1+s RC}$$

using $$RC = \frac{1}{2Ï€f_c}$$  we get,

$$H(s) = \frac{1}{1 + s/2Ï€f_c}$$

where $$f_c$$ is the cutoff frequency of the filter.

Setting the magnitude of H(s) equal to $$e^{-1}$$, we get:

$$e^{-1} = \frac{1}{1 + \frac{jÏ‰}{2Ï€f_c}}$$

Solving for the frequency Ï‰, we get:

$$Ï‰ = 2Ï€f_c e^{-1}$$

Substituting $$f_c = 1 kHz$$, we get:

$$Ï‰ = 2Ï€(1 kHz) e^{-1} ≈ 1.84 kHz$$

Therefore, the neper frequency of the low-pass filter with a cutoff frequency of 1 kHz is approximately 1.84 kHz. At this frequency, the power of the output signal would be attenuated by a factor of $$e^{-1}$$, or about 0.37 times the power of the input signal.