STANDARD SIGNAL GENERATOR ,Sweep Generator ,sine wave generator

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                     STANDARD SIGNAL GENERATOR:-

A standard signal generator produces known and controllable voltages. It is used as power source for the measurement of gain, signal to noise ratio (SN), bandwidth standing wave ratio and other

properties. 

It is extensively used in the measuring of radio receivers and transmitter instrument is provided with a means of modulating the carrier frequency, which is indicated by the dial setting

on the front panel. 

The modulation is indicated by a meter. The output signal can be  Amplitude Modulated (AM) or Frequency Modulated (FM). Modulation   may be done by a sine wave,Square, rectangular, or a pulse wave.                                                                                                                                               

The elements of a conventional signal generator :
1) RF Osillator
(2) Wide band amplifier.
(3) External Osillator.
4) Modulation Osillator
(5) Out put attenuator.

The carrier frequency is generated by a very stable RF oscillator using an LC tank circuit, having a constant output over any frequency range. The frequency of oscillations is indicated by the frequency range control and the venire dial setting. AM is provided by an internal sine wave generator or from an external source.

The signal generator is called an oscillator. A Wien bridge oscillator is used in this generator.The Wien bridge oscillator is the best of the audio frequency range. The frequency of oscillations can be changed by varying the capacitance in the oscillator. 

The frequency can also be changed in steps by switching the resistors of different values. The output of the Wien bridge oscillator goes to the function switch. 

The function switch directs the oscillator output either to the sine wave amplifier or to the square wave shaper. At the output, we get either a square or sine wave.The output is varied by means   of an attenuator.                                                                                                                                       

The instrument generates a frequency ranging from 10 Hz to 1 MHz continuously vV (rms).The output is taker through a push-pull amplifier. For low output, the impedance is 6000. The square wave amplitudes can be varied from 0 - 20 v (peak). It is possible to adjust the symmetry of the square wave from 30 -70%. The instrument requires only 7W of power at 220V 50Hz. 

The front panel of a signal generator consists of the following.

l. Frequency selector: It selects the frequency in different ranges and varies it continuously in

a ratio of 1: 11. The scale is non-linear.

2. Frequency multiplier: It selects the frequency range over 5 decades from 10 Hz to 7 MHz

3. Amplitude multiplier: It attenuates the sine wave in 3 decades, x l x 0.1 and x 0.01.

4. Variable amplitude: It attenuates the sine wave amplitude continuously

5. Symmetry control: It varies the symmetry of the square wave from 30% to 70%.

6. Amplitude: It attenuates the square wave output continuously.

7. Function switch: It selects either sine wave or square output.

8. Output available: This provides sine wave or square wave output.

9. Sync: This terminal is used to provide synchronization of the internal signal

with an external signal.

10. On-Off Switch

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RANDOM NOISE GENERATOR:

The spectrum of random noise covers all frequencies the lower density spectrum tells us how the energy of the signal is distributed in frequency, but it does not specify the signal uniquely nor

does it tell us very much about how the amplitude of the signal varies with time

The spectrum does not specify the signal uniquely because it contains no phase information. 


The method of generating noise is usually to use a semi conductor noise which delivers frequencies in a band roughly extending from 80 – 220 KHz The output from the noise diode is amplified and heterodyned down to audio frequency band by means of a balanced symmetrical modulator. The filter arrangement controls the bandwidth and supplies an output signal in three spectrum choices, white noise, pink noise and Usasi noise. 

From the Fig  it is seen that white noise is flat from 20Hz to 20 KHz and has upper cutoff frequency of 50 kHz with a cutoff slope of -12

dbs/ octave. 

Pink noise is so called because the lower frequencies have larger amplitude, similar to red light. 

Pink noise has a voltage spectrum which is inversely proportional to the square root of frequency and is used in band analysis. Usasi noise ranging simulates the energy distribution of speech and music frequencies and is used for testing audio amplifiers and loud speakers.

Sweep Generator

It provides a sinusoidal output voltage whose frequency varies smoothly and continuously over

an entire frequency band, usually at an audio rate. The process of frequency modulation may be

accomplished electronically or mechanically. It is done electronically by using the modulating

voltage to vary the reactance of the oscillator tank circuit component, and mechanically by

means of a motor driven capacitor, as provided for in a modern laboratory type signal generator.

Figure shows a basic block diagram of a sweep generator. The frequency sweeper provides a

variable modulating voltage which causes the capacitance of the master oscillator to vary. A

representative sweep rate could be of the order of 20 sweeps/second. A manual control allows

independent adjustment of the oscillator resonant frequency. The frequency sweeper provides a varying sweep voltage synchronization to drive the horizontal deflection plates of the CRO. 

Thus the amplitude of the response of a test device will be locked and displayed on the screen.

To identify a frequency interval, a marker generator provides half sinusoidal waveforms at any

frequency within the sweep range. The marker voltage can be added to the sweep voltage of the CRO during alternate cycles of the sweep voltage, and appears superimposed on the response curve. 

The automatic level control circuit is a closed loop feedback system which monitors the

RF level at some point in the measurement system. This circuit holds the power delivered to the

load or test circuit constant and independent o frequency and impedance changes. A constant

power level prevents any source mismatch and also provides a constant readout calibration          with frequency

SQUARE AND PULSE GENERATOR:-

These generators are used as measuring devices in combination with a CRO. They provide both

quantitative and qualitative information of the system under test. They are made use of in

transient response testing of amplifiers. The fundamental difference between a pulse generator

and a square wave generator is in the duty cycle.

Duty cycle =

A square wave generator has a 500/o duty cycle.

Requirements of a Pulse

1. The pulse should have minimum distortion, so that any distortion, in the display is solely due

to the circuit under test.

2. The basic characteristics of the pulse are rise time, overshoot, ringing, sag, and undershoot.

3. The pulse should have sufficient maximum amplitude, if appreciable output power is required

by the test circuit, e.g. for magnetic core memory. At the same time, the attenuation range should be adequate to produce small amplitude pulses to prevent over driving of some test circuit.

4. The range of frequency control of the pulse repetition rate (PRR) should meet the needs of the experiment. For example, a repetition frequency of 100 MHz is required for testing fast circuits

Other generators have a pulse-burst feature which allows a train of pulses rather than a

continuous output.

5. Some pulse generators can be triggered by an externally applied trigger signal; conversely,

pulse generators can be used to produce trigger signals, when this output is passed through a

differentiator circuit.

6. The output impedance of the pulse generator is another important consideration. In a fast pulse system, the generator should be matched to the cable and the cable to the test circuit. A mismatch

would cause energy to be reflected back to the generator by the test circuit, and this may be rereflected by the generator, causing distortion of the pulses.

7. DC coupling of the output circuit is needed, when dc bias level is to be maintained.

The basic circuit for pulse generation is the asymmetrical multi-vibrator. A laboratory type

square wave and pulse generator is shown in Fig 6.1

The frequency range of the instrument is covered in seven decade steps from 1Hz to 10 MHz,

with a linearly calibrated dial for continuous adjustment on all ranges.

The duty cycle can be varied from 25 - 75%. Two independent outputs are available, a 50Ω

source that supplies pulses with a rise and fall time of 5 ns at 5V peak amplitude and a 600Ω

source which supplies pulses with a rise and fall tme of 70 ns at 30 V peak amplitude. The

instrument can be operated as a freerunning genenrator or, it can be synchronized with external

signals.

The basic generating loop consists of the current sources, the ramp capacitor, the Schmitt trigger and the current switching circuit as shown in the fig

                                                         

The upper current source supplies a constant current to the capacitor and the capacitor voltage

increases linearly. When the positive slope of the ramp voltage reaches the upper limit set by the internal circuit components, the Schmitt trigger changes state. The trigger circuit output becomes negative and reverses the condition of the current switch. The capacitor discharges linearly, controlled by the lower current source. 


When the negative ramp reaches a predetermined lower level, the Schmitt trigger switches back to its original state. The entire process is then repeated.

The ratio i1/i2 determines the duty cycle, and is controlled by symmetry control. The sum of i1

and i2 determines the frequency. The size of the capacitor is selected by the multiplier switch.

The unit is powered by an intenal supply that provides regulated voltages for all stages of the

instrument.

The Basic difference between a signal generator and an oscillator are:-

Signal generators are the sources of electrical signals used for the purpose of testing and operating different kinds of electrical equipment. A signal generator provides different types of waveforms such as sine, triangular, square, pulse etc., whereas an oscillator provides only sinusoidal signal at the output.

The AF oscillators are divided into two types. They are as follows,

1. Fixed frequency AF oscillator

2. Variable frequency AF oscillator.

1. Fixed Frequency AF Oscillator:

Many instrument circuits contain oscillator as one of its integral parts to provide output signal

within the specified fixed audio frequency range. This specified audio frequency range can be 1

kHz signal or 400 Hz signal.

The 1 kHz frequency signal is used to execute a bridge circuit and 400 Hz frequency signal is

used for audio testing. A fixed frequency AF oscillator employs an iron core transformer. Due to

this a positive feedback is obtained through the inductive coupling placed between the primary

winding and secondary winding of the transformer and hence fixed frequency oscillations are

generated.

2. Variable Frequency AF Oscillator:

It is a general purpose oscillator used in laboratory. It generates oscillations within the entire

audio frequency range i.e. from 2O Hz to 20 kHz. This oscillator provides a pure, constant sine

wave output throughout this AF range. The examples of variable AF oscillators used in

laboratory are RC feedback oscillator, beat frequency oscillator.

Sine Wave Generator:-

The circuit configuration of a sine wave generator consists of Wien bridge oscillator, sine wave

amplifier and attenuator. The block diagram of a sine wave generator is shown in figure.

Wein bridge oscillator produces an oscillating output which is usually a sinusoidal (sine) wave.

Thus, half of the operation of a sine wave generator is done by the Wein bridge oscillator. The

frequency of oscillations of this oscillator can be varied by varying its capacitance and thus a

sine wave of desired frequency can be generated. The remaining elements of sine wave generator

i.e., amplifier, and attenuator are used as signal conditioners to condition the output of Wien

bridge oscillator in order to obtain a sine wave of desired amplitude

Square Wave Generator:-

The circuit configuration of a square wave generator consists of the basic elements of a sine

wave generator (i.e., Wien bridge oscillator, attenuator) and square wave shaper and square wave amplifier. Figure  shows the block diagram of a square wave generator.

A

A square wave is obtained by feeding the sinusoidal output of the Wein bridge oscillator to the

square wave Shaper circuit. The square wave shaper is usually a sine-to-square wave converter.

The square wave is further processed through square wave amplifier and attenuator in order to

obtain a square wave of desired amplitude. The frequency of the square wave can be varied by

varying the oscillation frequency of Wein bridge oscillator

The precautionary measures to be taken in a signal generator application:-

A signal generator is an instrument, which can produce various types of wave forms such as sine wave, square wave, triangular wave, saw tooth wave, pulse trains etc. As it can generate a variety of waveforms it is widely used in applications like electronic troubleshooting anti development,testing the performance of electronic equipments etc. In such applications a signal generator is used to provide known test conditions (i.e., desired signals of known amplitude and frequency

Hence, the following precautionary measures should be taken while using a signal generator for an application.

1. The amplitude and frequency of the output of the signal generator should be made stable and well known.

2. There should be provision for controlling the amplitude of signal generator output from very small to relatively large values.

3. The output signal of generator should not contain any distortion and thus, it should possess very low harmonic contents.

4. Also, the output of the signal generator should be less spurious.

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