Engineering instruments for analysis of signals or waves

Overview of signal analyser

In various electrical and electronic instruments it is required to analyse the signal produced by them, there are numerous devices serving the purpose depending upon the signal they deal with. Some of them can be listed as:

• Signal analyser
• Distortion analysers
• Spectrum analyser
• Audio analyser
• Modulation analyse

These all analysing instruments may use different technique but do the same basic functioning of measuring the basic frequency of the signal the virtue of their analysis. For example in case of an spectrum analyser the signal frequency band is swept and the output is displaces by it in form of a graph between amplitude and frequency and it is capable of working in a frequency range of 0.002Hz to 250 GHz which is a very wide frequency range. On the other hand when we talk about an wave analyser is can be considered to be a voltmeter with the capability of taking measurement of the voltage of only a particular frequency in a collection of signals of different frequencies i.e. from a frequency band it measures the amplitude of one frequency and discards all other with the help of band pass filter circuits. It working frequency range is 10 Hz to 40 MHz we can take distortion analyser as another example of signal analyser which is used for measurement of the strength energy of the signal ling outside of a given frequency band. It is basically tells how much energy is present there in the signal which is not being taken under used that is why it's given the name distortion analyser. The working range of standard distortion analyser is from 5 Hz to 1MHz. An audio analyser can be studied as an extension of distortion analyser because is performs in the same way as a distortion analyser with an added quality of measuring the noise present in the original signal.

In order to study a more sophisticated and advanced analyser se can take modulation analyser which performs the function of recovering the whole envelope by tuning to a required signal. Here the modulation may:
• Amplitude modulation
• Frequency modulation
• Phase modulation

Each of the above analysers can have different form and type according to the complexity of their working and designing. So in order to have a insight of these device we need to explain each one of them separately.

Wave analysers

Here first thing, very important to understand, is that

What is wave form analysis?
The analysis of a waveform consists of measurement/determination of the basic parameters regarding an electrical harmonic signal which are:
1. Amplitude
2. Frequency
3. Phase angle

Here, it is well known that any periodic waveform can be thought to be a combination of two distinct parts that construct the whole waveform as their summation.
• A DC component and
• Series of sinusoidal harmonics

Analysis of these parameters can be carried out with the help of graphical of mathematical means (i.e. manually) but this all stuff becomes very tedious and also reduces the accuracy of the output, so in order to make it practically feasible we take electrical means under use. The most important section is band pass filter which actually filters out different components and leaves them for their individual analysis i.e. a very narrow band of frequencies is obtained from the output of the filter while all the other are attenuated to such a extent that only a requires portion remains remarkable.

Usually there is collection of signals in which each one is required to be measured separately that requires selection of required one from the cluster. According to this statement we can have a proper definition of a wave analyser as

"An instrument that as a frequency selective voltmeter tuned to the frequency of one desired signal and rejecting all other frequencies".

In actual practice the instrument comes with a dial fixed upon it which can be at any position to select a particular frequency this dial is generally referred to as frequency calibrated dial and thus produces signal can be displayed with help of a suitable display device such as voltmeter or CRO.

The frequency to be dealt with decides the type of wave analyser i.e. depending upon the frequency there are two types of wave analysers
1. Frequency selective
2. Heterodyne wave analyser 

Frequency selective wave analyser

When it is requires dealing with the frequency in audio range, the analyser, in this case, is referred to as frequency selective wave analyser.
Here analysis is performed in several different steps such that the output of one stage is fed to the input of next one. In this way in order to study functioning and interconnection of each stage we will be discussing them separately.

1. Attenuator: Firstly, the input signal is fed to the input attenuator in order to bring the signal strength within measurable range, in case it's caring larger energy than required. The attenuator is set by the range switch mounted upon the front panel of the instruments which provides the external control to the input signal strength.

2. Driver amplifier: Thus attenuated signal may or may not be of a specific required strength and form so. To meet the requirement this attenuated signal also has to be amplified that is performed by the driver amplifier.

3. High Q-active filter: This section is employed with the purpose of selection of the desired frequency from the complex waveform. It comprises of a cascaded arrangement of RC resonant sections and filter amplifiers. The capacitor used in RC band pass filters is generally of polystyrene capacitor type. And for tuning of the signal at a desired frequency, precision potentiometer is brought in use that proves to be very precise for frequency selection.

4. Final stage amplifier: The selected frequency signal is, now, supplied two stages i.e.
• Meter circuit
• Un-tuned buffer amplifier

Meter circuit involves amplifier that amplifies selected signal to s desired level so as to display it with the help of some display device and also calibrates it. While buffer amplifier involves a recorder or an electronic counter

It is also very important to emphasise that frequency selective analyser has a narrow band width which has typical value of about 1 % of selected frequency

Heterodyne wave analyser

This category of wave analysers works upon the signals in range of megahertz in which , again the input signal is applied to the device through an attenuator for the same purpose as in frequency selective wave analysers i.e. to bring the signal strength under control.

It also employs some certain steps while attenuation, amplification and filtration. After attenuation the signal is then passed to a amplifier stage that feeds the attenuated signal to the first mixture stage, also called local oscillator whose frequency is in order of the band pass filter of i.f. amplifier that is the next section of the system which gets the output from mixture ,furthermore frequency of local oscillator is adjustable . i.f. amplifier than feeds to the next mixture that mixes the signal to a crystal controlled oscillator such its frequency is so adjusted to produce an output of frequency centered at zero frequency.

A controllable band width active filter is employed in the next section that allows a selected frequency component to pass to the meter circuit.

Here it is to be noticed that the whole system works upon mixing of the original signal which other frequencies having frequency in range of megahertz. The accuracy of the whole system can be improved by introduction of devices capable of producing the frequency accurately. Frequency synchronizers and automatic frequency control having higher resolution and good accuracy, serves the purpose well. Where automatic frequency control (AFC) makes the system to get attached with the signal frequency and avoids the possibility of drifting between them.

Applications of wave analyzer

Wave analyser being capable of performing analysis upon gives complex waveform it proves to be applicable in a number of measurement systems like:
• Electrical measurement
• Sound measurement
• Vibration measurement

Industrially machines like motor rotors and cutters generate a large amount of noise due to vibrations produced in them, which is obviously an undesired output from them. In order to minimize them it is, first, needed to analyse their pattern with the help of suitable wave analyser.

Discrete frequency pattern and resonance related to the motion of the machine can be analysed swith the help of the wave analyser. After recognition of source of vibration and the way it is being produced an appropriate way can be obtained to eliminate that.

Harmonic distortion analyser

A sinusoidal waveform should be generated when a sinusoidal signal is applied to some electronic processing devices like an amplifier. But in most of the cases it is not seen to be with so perfection, the output does not comes out to be the exact copy of the one fed to its input i.e. some distortions always remains there at the output of the device. The components used in the electronic circuit may come out to exhibit non linear characteristics that become a reason of distortion in the output signal. Here, non linear characteristic refers to the behaviour of the component when it responds to the different external conditions differently e.g. a transistor has different gain factor at different frequencies that, definitely, leads to asymmetry when the input signal has a wide band width.

Harmonics are generated in the output signal due to this non-linearity of the components used, and thus, the distortion produced is called harmonic distortion.

Categories of distortions

Different devices and the components, used in it, become a source of distortion produced at the output waveform. But here, we will be discussing only those distortions that are generated by the amplifiers as amplifier are the most important part of any device and behaves in a different way with different frequencies. Some of the distortions produced by the amplifier circuits are as under:

Frequency distortion
As is discussed earlier, amplifier attains a different amplification factor for different frequencies. This non-linearity of the amplifier circuit results into frequency distortion.

Phase distortion
Some components like capacitors and inductors stores energy in form of electric field and magnetic fields respectively while their operation in circuit. These energy components leads to the shifting the output waveform by a specific angle with respect the input signal, this results into phase dissimilarity in output and input waveforms are generates phase distortion in the signal. It is to be emphasised that if phase shift is same in the entire signal i.e. if each components of the input waveforms is shifted by the same angle the resulting shift will not be remarkable. But generally it is seen that the shift does not come out to be same throughout the signal different frequency components have gains different phase shift leading to a noticeable distortion.

Amplitude distortion
Due to formation of the harmonics of the supplied signal by the amplifier of the device it leads to generation of the harmonic distortion in it. Harmonic in the signal always results into an amplitude distortion of the input signal. The best example to illustrate amplitude distortion is that when the amplifier is overdriven it leads to overshooting the operating point that results into clipping off a portion of the original waveform which is a type of amplitude distortion.

Intermediate distortion
The waveform, generally taken under analysis, occurs to be a complex waveform containing signals of more than one frequency thus coexistence of different frequencies leads to their interaction. As a result at the output some new signals are generated with frequency equal to sum and difference of the parental frequencies.

Cross-over distortion
This type of distortion is generally seen in push-pull amplifiers because of incorrect boas levels.

Total Harmonic distortion

For the purpose of studding total harmonic distortion of a given system we initially take the input to be in sine waveform. The input sine wave attains harmonics at the output due to non-linearity of the system such harmonics occurs with frequencies equal to a multiple of input signal. The total harmonic content of the wave comprises Total Harmonic Distortion (THD). Mathematically it is given by the square root of the sum of square of all the harmonics divided by the actual or fundamental signal.

Inter-modulation distortion

When two signals of different frequencies, one of high frequency while other one having low frequency, are mixed together it gives rise to only two frequencies at the output, in case of linear circuit. But the scenario changes when these frequencies are fed to an non-linear circuit as in this case four frequencies occurs at the output are
• Original high frequency
• Original low frequency
• Sum of original frequencies
• Difference of original frequencies
Many harmonics also occurs there along with their sum and difference.

Working of inter-modulation distortion analyzer

The system requires two different input frequencies which are made to fix. According to SMPTE (Society of Motion Picture and Television Engineers) these are taken as
f1= 7 kHz and
f2= 60 kHz
While there is standardization, DIN 45403, which set its own standard of frequencies as
f1=8 kHz and
f2=250 kHz

In both of cases the amplitude of frequencies is kept in ratio of 1:4 i.e. the signal of lesser frequency is made to have greater amplitude than that of higher frequency in order to expect a lower degree of non linearity. The mixture of these to frequencies is further fed to high pass filter whose outcome comprises of high frequency carrier and a low frequency modulation. The output thus obtain can be seen with help of an SRC.