# Relationship between frequency domain and time vs spectral

### LearnEMC - Time/Frequency Domain

The Importance Of The Time/Frequency Domain Relationship . or altering the phase relationship between the spectral elements of the signal. A signal can be converted between the time and frequency domains with a pair y1, are practically the same (the norm of their difference is on the order of 1e). . FFT and normalizes the output to obtain a power spectral density, PSD, or a. In the time domain, voltage or current is expressed as a function of time as illustrated in Which of the following equations describes the relationship between the .. The frequency domain representation of a periodic signal is a line spectrum.

Removing high frequency components of a signal is referred to as lowpass filtering. You can still hear the melody but it sounds like if you had covered your ears you filter high frequency sounds when you do this. Even though guitars produce notes that are between and 1 kHz, as you play a note on a string, the string also vibrates at multiples of the base frequency.

These higher frequency components, referred to as harmonics, are what give the guitar its particular tone. When you remove them, you make the sound seem "opaque". To illustrate the importance of phase on the audio signal, remove the phase information completely by taking the magnitude of each frequency component. Note that by doing this you keep the magnitude response unchanged. You cannot recognize the original sound at all. The magnitude response is the same, no frequency components have been removed this time, but the order of the notes has disappeared completely.

The signal now consists of a group of sinusoids all aligned at time equal to zero. In general, phase distortions caused by filtering can damage a signal to the point of rendering it unrecognizable. For instance, frequency-domain analysis becomes useful when you are looking for cyclic behavior of a signal.

Analyzing Cyclic Behavior of the Temperature in an Office Building Consider a set of temperature measurements in an office building during the winter season. Measurements were taken every 30 minutes for about Look at the time domain data with the time axis scaled to weeks. Could there be any periodic behavior on this data? However, the cyclic behavior of the temperature becomes evident if we look at its frequency-domain representation.

Obtain the frequency-domain representation of the signal. This makes sense given that the data comes from a temperature-controlled building on a 7 day calendar. The first spectral line indicates that building temperatures follow a weekly cycle with lower temperatures on the weekends and higher temperatures during the week. The second line indicates that there is also a daily cycle with lower temperatures during the night and higher temperatures during the day.

You compute the power spectrum by integrating each point of the PSD over the frequency interval at which that point is defined i. The units of the power spectrum are watts. You can read power values directly from the power spectrum without having to integrate over an interval. Note that the PSD and power spectrum are real, so they do not contain any phase information. Measuring Harmonics at the Output of a Non-Linear Power Amplifier Load the data measured at the output of a power amplifier that has third order distortion of the formwhere is the output voltage and is the input voltage.

The data was captured with a sample rate of 3. The input consists of a 60 Hz sinusoid with unity amplitude. Due to the nature of the non-linear distortion, you should expect the amplifier output signal to contain a DC component, a 60 Hz component, and second and third harmonics at and Hz.

Load samples of the amplifier output, compute the power spectrum, and plot the result in a logarithmic scale decibels-watts or dBW. It also shows several more spurious peaks that must be caused by noise in the signal. Note that the Hz harmonic is completely buried in the noise. Measure the power of the visible expected peaks: The spectrum looks very noisy.

### Practical Introduction to Frequency-Domain Analysis - MATLAB & Simulink Example

The reason for this is that you only analyzed one short realization of the noisy signal. Repeating the experiment several times and averaging would remove the spurious spectral peaks and yield more accurate power measurements. You can achieve this averaging using the pwelch function. This function will take a large data vector, break it into smaller segments of a specified length, compute as many periodograms as there are segments, and average them. As the number of available segments increases, the pwelch function will yield a smoother power spectrum less variance with power values closer to the expected values.

Load a larger observation consisting of e3 points of the amplifier output. The spectral component at Hz that was buried in noise is now visible. Averaging removes variance from the spectrum and this effectively yields more accurate power measurements. For the amplifier output signal, y, the total average power is computed in the time domain as: The value of pwr1 consists of the sum of all the frequency components available in the power spectrum of the signal.

The value agrees with the value of pwr computed above using the time domain signal: You can use the bandpower function to compute the power over any desired frequency band.

### Frequency domain - Wikipedia

You can pass the time-domain signal directly as an input to this function to obtain the power over a specified band. In this case, the function will estimate the power spectrum with the periodogram method. The same waveform then can be represented in the frequency domain as a pair of amplitude and phase values at each component frequency. You can generate any waveform by adding sine waves, each with a particular amplitude and phase.

The following figure shows the original waveform, labeled sum, and its component frequencies. The fundamental frequency is shown at the frequency f0, the second harmonic at frequency 2f0, and the third harmonic at frequency 3f0.

## Differences between Frequency Domain and Time Domain

In the frequency domain, you can separate conceptually the sine waves that add to form the complex time-domain signal.

The previous figure shows single frequency components, which spread out in the time domain, as distinct impulses in the frequency domain. The amplitude of each frequency line is the amplitude of the time waveform for that frequency component. The representation of a signal in terms of its individual frequency components is the frequency-domain representation of the signal.

The frequency-domain representation might provide more insight about the signal and the system from which it was generated.

The samples of a signal obtained from a DAQ device constitute the time-domain representation of the signal.