Measurement of Total Harmonic Distortion contained by output of amplifier, inverter

AIM: Measurement of Total Harmonic Distortion contained by output of amplifier, inverter.

EQUIPMENTS:

1) Spectrum Analyzer (Agilent,9KHz-3GHz)

2) BJT amplifier circuit

3) connecting probes

4) power supply (Radial industries,SVP030002D,0-32V,DC-2A)

5) Function Generator (Aplab, FG1MD, 1MHz)

THEORY:

The total harmonic distortion, or THD, of a signalis a measurement of the harmonicdistortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. Lesser THD allows the components in a loudspeaker, amplifier or microphone or other equipment to produce a more accurate reproduction by reducing harmonics added by electronics and audio media.

To understand a system with an input and an output, such as an audio amplifier, we start with an ideal system where the transfer function is linear and time-invariant. 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:

which can equivalently be written as

if there is no source of power other than the signal and its harmonics.

Measurements based on amplitudes (e.g. voltage or current) must be converted to powers to make addition of harmonics distortion meaningful. For a voltage signal, for example, the ratio of the squares of the RMS voltages is equivalent to the power ratio:

where V_n is the RMS voltage of nth harmonic and n=1 is the fundamental frequency.

THD is also commonly defined as an amplitude ratio rather than a power ratio,^[1]resulting in a definition of THD which is the square root of that given above:

This latter definition is commonly used in audio distortion (percentage THD) specifications. It is unfortunate that these two conflicting definitions of THD (one as a power ratio and the other as an amplitude ratio) are both in common usage.

As a result, THD is a non-standardized specification and the results between manufacturers are not easily comparable. Since individual harmonic amplitudes are measured, it is required that the manufacturer disclose the test signal frequency range, level and gain conditions, and number of measurements taken. It is possible to measure the full 20–20 kHz range using a sweep. For all signal processing equipment, except microphone preamplifiers, the preferred gain setting is unity. For microphone preamplifiers, standard practice is to use maximum gain.

Measurements for calculating the THD are made at the output of a device under specified conditions. The THD is usually expressed in percentas distortion factor or in dBrelative to the fundamental as distortion attenuation.

THD+N

THD+N means total harmonic distortion plus noise. This measurement is much more common and more comparable between devices. It is usually measured by inputting a sine wave, notch filtering the output, and comparing the ratio between the output signal with and without the sine wave:

A meaningful measurement must include the bandwidth of measurement. This measurement includes effects from intermodulation distortion, and so on, in addition to harmonic distortion. In Europe, it is preferable to apply a ITU-R BS.468 weighed curve, which is intended to accentuate what is most audible to the human ear, contributing to a more accurate measurement. However, as the weight of the curve adds 12 dB of gain to the critical midband, making THD+N measurements bigger, manufacturers object to its use and have widely prevented its adoption in American and Asian markets.

For a given input frequency and amplitude, THD+N is equal to SINAD, provided that both measurements are made over the same bandwidth.

PROCEDURE:

1.      Connect the Vcc (12V) & ground of the BJT amplifier to the DC regulated power supply.

2.      Give input as 1KHz, 1Vp-p sinusoidal wave at the input side of BJT amplifier.

3.      Connect the output of the amplifier to spectrum analyzer.

4.      Set the spectrum analyzer in “spectrum analyzer” mode.

5.      Set the center frequency as 1KHz by pressing frequency button.

6.      Now set the span as 20KHz for observing the harmonics.

7.      Take the readings of different harmonics & calculate THD.