SIGNAL GENERATORS
INTRODUCTION:
A signal generator is a vital component in a test setup and in electronic trouble shooting and
development, whether on a service bench or in a research laboratory. Signal generators have a variety
of applications, such as checking the stage gain –frequency response, and alignment in receivers and
in a wide range of other electronic equipment and it also produce several output waveforms.
There are various types of signal generators but several requirements are common to all types.
1. The frequency of the signal should be known and stable.
2. The amplitude should be controllable from very small to relatively large values
3. Finally, the signal should be distortion –free.
AF SINE AND SQUARE WAVE GENERATOR:
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The block diagram of an AF sine-square wave audio oscillator is illustrated in the below
figure.
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Wein bridge oscillators are most commonly used as they provide good frequency stability and
distortion free waveform in the audio range.
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Wien bridge oscillator is the heart of an AF sine-square wave generator. Depending upon the
position of switch, we get output as square wave output or sine wave output.
The Wien bridge oscillator generates a sine wave. Depending upon the position of switch, It is
switched to either circuit.
In the square wave generation section, the output of the Wien bridge oscillator is fed to square
wave shaper circuit which uses Schmitt trigger circuit.
The attenuators in both the sections arc used to control output signal level. Before attenuation,
the signal level is made very high using sine wave amplifier and square wave amplifier.
The instrument generates a frequency ranging from 10Hz to 1MHz continuously variable in 5
decades with overlapping ranges. The output sine wave amplitude can be varied 5mV to
5V(rms).
The square wave amplitudes can be varied from 0-20V(peak)
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SIGNAL GENERATORS
Front Panel Controls of Audio Frequency Oscillator (A F O):
The front panel of a AFO consists of the following:
1. Frequency Selector: This helps in selecting the frequency in different ranges
2. Frequency Multiplier: It selects the frequency ranges more than 5 decades from 10 Hz to 1
MHz.
3. Amplitude Multiplier: It attenuates the sine wave in 3 decades namely Xl, X0.l, X0.01.
4. Variable Amplitude: It attenuates the sine wave amplitude continuously.
5. Symmetry Control: It varies the symmetry of square wave from 30 % to 70 %.
6. Amplitude: It attenuates the square wave amplitude continuously.
7. Function Switch: It selects the mode required either sine wave output or square wave
output.
8. Output Variable: It provides actual sine wave or square wave output.
9. Sync: This terminal provides synchronisation of the internal signal with external signal.
10. ON-OFF Switch
Specifications of AFO:
The specifications of typical AP signal generator are as follows.
1. Frequency range is from 10 Hz to 1 MHz The frequency is variable over almost 5 decades
continuously.
2. The amplitude of square wave output can be varied from 5 mV to 5 V (r.m.s)
3. The amplitude of square wave output can be varied from 0 - 20 V (peak).
4. The square wave symmetry is adjustable from 30 % to 70 %.
5. The output is taken from push-pull amplifier with low output impedance of 600 Q.
6. At 220 V, 50 Hz, F signal generator requires 7 W of power only.
FUNCTION GENERATOR:
A function generator is a versatile instrument that produces a choice of different waveforms
whose frequencies are adjustable over a wide range. The common output waveforms are the sine,
square, triangular and saw tooth waves. The frequency may be adjusted from a fraction of a Hertz to
several hundred KHz.
The block diagram of a function generator is illustrated in the figure. Usually the frequency is
controlled by varying the oscillator capacitance or resistance.
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But in function generator frequency is controlled by varying the magnitude of the current
produced by the current sources which drives integrator.
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The frequency controlled voltage is used to regulate two current sources namely upper current
source and lower current source.
The upper current source supplies constant current to an integrator.
The output voltage of integrator then increases linearly with time.
If the current, charging the capacitor increases or decreases, the slope of output voltage
increases or decreases respectively. Hence this controls frequency.
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The voltage comparator multivibrator circuit changes the state when the output voltage of
integrator equals the maximum predetermined upper level. Because of this change in state, the
upper current source is removed and the lower current source is switched ON.
This lower current source supplies opposite current to the integrator circuit. The output of
integrator decreases linearly with time.
When this output voltage equals maximum predetermined upper level on negative side, the
voltage comparator multivibrator again changes the condition of the network by switching
OFF the lower current source and switching ON the upper current source.
The output voltage of the integrator has triangular waveform.
To get square wave, the output of the integrator is passed through comparator.
The triangular wave is synthesised into sine wave using diode resistance network.
APPLICATIONS OF AF OSCILLATOR:
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To test an amplifier.
Used in Audio systems.
Provides signal check in servicing.
Used in Electronic lab for wave shaping circuits.
APPLICATIONS OF FUNCTION GENERATOR:
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Testing, Designing and Troubleshooting of electronic equipment
Used in cascade accumulation.
For alignment (adjustment of various tunable circuits) of electronic equipment’s.
Generates AM / FM / PM waveforms.
RF SIGNAL GENERATOR:
The block diagram of a RF signal generator is shown in fig.
Block diagram:
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 The RF oscillator consists of LC tank circuit. It gives frequency range from 34 MHz to 80 MHz
 This signal is connected to un tuned buffer amplifier B1.
 The output of this amplifier is given to frequency divider circuit the circuit shown above consists
of 9 frequency dividers.
 The LC tank circuit and frequency divider provides a carrier signal.
 The buffer amplifiers B2 and B3 are used for isolation.
 The use of buffer amplifiers provides very high degree of isolation between master oscillator stage
and power amplifier stage.
 Signal for modulation is provided by an audio oscillator.
 The frequency given by this oscillator is in the range of 400 Hz to 1 kHz.
 The modulation takes place in main amplifier, in power amplifier stage.
 The master oscillator is tuned automatically or manually. In automatic controller for tuning master
oscillator, a motor driven variable capacitor used.
 This system is extensively used in programmable automatic frequency control devices.
 The oscillator can be fine-tuned by means of a large rotary switch with each division
 The internal calibration is provided by 1 MHz crystal oscillator.
 The output of the main amplifier is given to an output attenuator which controls the amplitude
level and provides the required stable RF output.
Specifications of RF signal generator:
• Output frequency
: 0.15 MHz to 50 MHz
• Frequency error
: < 1%
• Output impedance
: 75 ohms
• Output voltage
: 50 mV
• Amplitude modulation : depth of 30 %
Applications of RF signal generator:
• Testing of amplifier response.
• Alignment of radio and television receivers.
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• Trouble shooting.
• Checking various instruments in laboratory.
• Generation of AM, FM waves.
Importance of shielding:
• Radio frequency leakage from a signal generator can create practical difficulties in measuring
receiver sensitive, particularly if the receiver itself is not well shielded.
• Precautions should be taken in the generator to ensure that radiation levels are sufficiently low
to permit accurate measurements at the minimum output level specified for the signal
generator.
• Avoids signal interference from the neighboring circuits.
• Avoids external signal to circuits.
• Interference occurs due to radiation from the wire system.
• Proper magnetic shield is necessary.
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