Signal Gen

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BENP 2183 ELECTRONICINSTRUMENTATION

CHAPTER 5

Signal Generators and

Analyzers

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Objectives

Describe the basic block diagram of several types of signal

generators and analyzers commonly used.

Explain the function and operation of signal generators such

as function and pulse generator.

Explain the function and operation of signal analyzers such

s ectrum and Fourier anal zer.

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Introduction

A signal generator is very useful and important equipment in

electronictroubleshootingand development.

Applications ofa signal gen.:

checking the stagegain, S/N, bandwidth.

checking the frequency response

checking the alignmentin receivers

Oscillator providessinusoidal signal only

Converts a DC source to an AC energy (no energy is

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create .

Usuallybuilt-in in the instrument.

E.g:1.Fixed freq normally at1000c/s.

2.Variable freq 100kHz - MHz

Introduction

Generator provides several types of waveforms including sine

wave, square wave, triangular wave, pulse trains, as well as

amplitudemodulation (AM) waveform.

Available as a separate instrument.

Provide signals for testing purposes (eg: radio transmitter

&receiver). There areseveral requirements forsignal generator :

The frequencyof thesignal should beknown and stable.

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values.

Thesignal should bedistortion-free.


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Signal Generator

Table 1 shows the band limit of various types of signal provided

byan RF generator.

an pprox ma e ange

AF 20Hz 20kHz

RF Above 30kHz

VLF Very Low Frequency 15 100kHz

LF Low Frequency 100 500kHz

Broad Cast 0.5 1.5MHz

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HF 1.5 30MHz

VHF 30 300MHz

UHF 300 3000MHz

Microwave Beyond 3000MHz (3GHz)

Table 1

Conventional Signal Generator

6Figure 1: Conventional Standard Signal Generator

Signal Generator

7Figure 2: Modern Signal Generator

Signal Generator

Highest freq. ranges are provided by RF Oscillator (34MHz

80MHz).

Lowestfreq. ranges are obtainedbyusing frequency divider.

34MHz 80MHz divided by512 (29) 67kHz 156kHz.

Buffer amplifiers (B1, B2, B3) provide isolation between the

masteroscillator andpower amplifier.

Eliminates frequency effects (signal distortion) between input

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andoutputcircuits.


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Signal Generator

Compared to conventional std signal gen, modern signal gen

uses same oscillator onall bands.

Eliminates range switchingeffects.

Master oscillator is tuned by a motor driven variable

capacitor.

Coarse freq. tuning 7%frequency changespersecond.

Finetuning at0.01%of themaindial.

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Modulationprocess is done atthe power amplifier stage.

Two internally generated signal are used (400Hz &1kHz) for

modulation.

Function Generator

A function generator produces different waveforms of

adjustable frequency.

Capable of producing various outputs at the same time (eg: a square wave

totestan amplifier and a sawtooth todrive a CRO).

The commonoutputwaveforms are the sine, square, triangular,

andsawtooth waves.

The blockdiagramof a functiongenerator is shown inFigure3.

Freq.Control regulates two currentssources (control the freq).

Upper current source supplies constant current to the

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integrator, produces an output voltage that is increasing

linearly with time.

Lower current source supplies a reverse current to the

integratorso that its outputdecreases linearlywithtime.

Function Generator

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Figure 3: Function Generator

Function Generator

Theintegrator outputvoltage is given by:

Freq is controlledbyvaryingupperandlower currents.

=t

dti0

0C

1V

An increase or decrease in the current will increase or

decrease the slope of the output voltage, hence controls the

frequency.

The voltage comparator changes states ata pre-determined

maximumandminimumlevelof the integratoroutputvoltage.

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When the pre-determined level is reached, it changes the

stateandswitches the currentsource.

Produces a square wave.


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Function Generator

The integratoroutput is a triangular waveformwhose frequency

is determined bythe magnitude of the constantcurrentsources.

The comparator output delivers a square wave of the same

frequency.

The resistance diode network produces a sine wave fromthe

triangularwave withless than 1%distortion.

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Function Generator

Square to sineconverter

14Figure 4: Basic Function Gen

Function Generator

Figure 4 shows the circuit for a basic function generator.

Vo1 is the square wave, Vo2 is the triangular wave, and Vo3 is the sine

.

Themaximum amplitude of Vo2 is given by :

CCV

RRV =

2

102

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e requency o oupu vo age s gven y :

=

1

2

R

R

4RC

1f

Example 1: Figure shows a basic function generator. A variable resistor, R is used

to tune the required freq of the waveform and the circuit is supplied by a voltage

of 10V. If R is tuned to the value of five times the R2 to produce a 1kHz

triangular waveform with the amplitude equals to 0.8Vcc, find R1, R2 and R.

Then, draw the output waveform at V01 and V02.

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Pulse Generator

Pulse generators are instruments that produce arectangular waveform similar to a square wave but with adifferent dutycycle.

Duty cycle is given y :

A square wavegenerator has a 50% dutycycle.

The basic circuit for pulse generation is the asymmetrical

periodpulse

widthpulsecycleDuty =

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- . Figure6 shows blockdiagramof a pulse generator.

Pulse Generator

18Figure 6: Block diagram of a Pulse Generator

Pulse Generator

The duty cycle can be varied from 25-75%.

Two independent outputs:

50 - supplies pulses with a rise and fall time of 5ns at 5Vp.

600 - supplies pulses with a rise and fall time of 70ns at 30Vp.

The instrument can operate as a free-running or can be

synchronized with external signal or circuit.

Basic generating loop consists of the current sources, the

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ramp capacitor, the Schmitt trigger, and the current switching

circuit as shown in Figure 7.

Pulse Generator

20Figure 7: Basic Generating Loop


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Pulse Generator

Upper current source supplies a constant current to theramp capacitor andthe capacitor voltage increases linearly.

When the positive slope of the ramp reaches the upper limit,Sc mittTriggerwi c ange its state.

Reverses the conditionof the currentswitch.

Capacitordischarges linearly. (lower current source takes part)

When the negative slope of the ramp reaches the lower limit,uppercurrentwill control the circuit.

The process is repeated.

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1 2 ,symmetry control.

The sumof i1 and i2 determines thefrequency.

The size of the capacitor is selected by the multiplierswitch.

Sweep Generator

Sweep frequency generators are instruments that provide a

sine wave in theRF range.

Its frequency can be varied smoothly and continuously over an

entire frequency band.

Figure 8 shows the block diagramof the sweep generator.

The frequency sweeperprovides a varying sweep voltage for

synchronization to drive the horizontal deflection plates of the

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CRO.

A sweep rate can beof theorder of20sweeps/sec.

Figure 9 shows the modulated sinewave by a voltage-

controlled oscillator(VCO).

Sweep Generator

23Figure 8: Sweep Generator

Sweep Generator

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Figure 9: RF signal modulated by an audio-frequency ramp voltage.


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Radio Frequency Generator

Radio frequency generators are designed to provide anoutput signal over a wide range of frequencies fromapproximately30kHz to nearly3000MHz.

Contain a precision output attenuator networ t at permitsselectionof output voltages from1 uV to3V inprecisesteps.

Outputimpedance=50.

Figure 10 shows a block diagram for a basic RF signalgenerator.

The frequency range is selected with the band selectorand

exactfre . is selectedwith thevernier fre . selector.

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Broadband amplifier provides buffering between theoscillator andthe load connected to the outputterminal.

The output of the attenuator is monitored by the outputmeter.

Radio Frequency Generator

26Figure 10: Basic RF Signal Generator

Distortion Analyzer

Distortion the alteration of the original shapeofa waveform.

Function of distortion analyzer: measuring the extent of distortion (the

o/p differs from the waveform at the i/p) introduced by the active or

.

An amplitude distorted sine wave is made up of pure sine wave

components, including the fundamental frequency, f of the input

signal, and harmonic multiples of fundamental frequency, 2f, 3f, 4f,

etc.

Harmonic distortion can be measured accurately using harmonic

distortion analyzer, generally calleda distortion analyzer.

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rmslfundamenta

rms)(harmonicsTHD

2=

Distortion Analyzer

The total harmonic distortion (THD) can also bewrittenas :

whereTHD =thetotalharmonicdistortion

Ef =the amplitudeof fundamental frequency

f

n32

E

...THD

+++=

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E2,E3 ,En =the amplitudeof the individual

harmonics


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Distortion Analyzer

Example 1:

Compute the THD of a signal that contains a fundamentalsi nal of E =10V harmonics E =3V E =1 .5V rms rms rms and E4 =0.6Vrms .

Solution :

10V

(0.6V)(1.5V)(3V)THD

222 ++=

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%34.07=

Distortion Analyzer: Examples

1. A sinusoidal signal is applied to a circuit that introduces the second,

fourth and sixth harmonics. If the amplitude of the primary signal has an

rms value of 5V and the second, fourth and sixth harmonics have rms

values of 1V, 0.4V and 0.1V respectively, what is the total harmonic

distortion?

2. A wave analyzer is used to determine the harmonics present in a signal.

Only the fifth harmonic was found to be present and its amplitude was

0.4Vrms. If the fundamental frequency had an amplitude of 4V rms, what

was the total harmonic distortion?

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3. The total harmonic content o a signal is 20% and contains only the

fundamental, first, third and fifth harmonics. If the rms values of the third,

fifth andseventh harmonics are 2 V, 0.8V and0.16V respectively, what is

the rms value of the fundamental frequency?

Wave Analyzer

A harmonic distortion analyser measures the total harmonic content in

a waveform.

Any complex waveform is made up of a fundamental and its

harmonics.

Wave analyzeris used tomeasure the amplitude of each harmonic or

fundamental frequency individually.

Wave analyzers are also referred to as frequency selective voltmeters,carrier frequency voltmeters, andselective level voltmeters.

The instrument is tuned to the frequency of one component whose

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.

Some wave analyzers have the automatic frequency control which

tunes to the signal automatically.

Wave Analyzer

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Wave Analyzer

Figure 9.1(textbook)shows a basic wave analyzer.

The analyzer consists of a primary detector, which is a simple

.

The LC circuit is adjusted for resonance at the frequency of the

particularharmonic componenttobe measured.

It passes only the frequency to which it is tuned and provides

a high attenuationtoall otherfrequencies.

The full wave rectifier is used to get the average value of the

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inputsignal.

The indicating device is a simple dc voltmeter that is calibrated

to readthepeakvalue of thesinusoidal input voltage.

Wave Analyzer: Examples

1. The distortion caused by the third harmonic is found to be

3.33% by using a wave analyzer. The total harmonic distortion

when measured with a distortionanal ze is found to be 3.5%. If

the rms value of the fundamental is 18V and if only the thirdand

fifth harmonics are present, what is the rms value of the fifth

harmonic?

2. Using a spectrumanalyzer, we observe a signal to contain the

third, fifth andseventh harmonics in addition to the fundamental.

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e r armonc equas . mes e un amena , eharmonic equals 0.3 times the third harmonic, and the seventh

harmonic equals 0.24 times the fifth harmonic, what is the total

harmonicdistortion?

Heterodyne Wave Analyzer

Wave analyzers audio frequency range measurementonly.

Heterodyne: To mix two or more signals which produce sum and

difference.

e ero yne wave ana yzers are use o ana yze s gna n e

range andabove (MHz range).

In this analyzer, the input signal is mixed with the internal signal to

producea higher IF frequency.

Figure 9.4 (textbook) shows the block diagram of a RF heterodyne

wave analyzer.

The local oscillatoris tunable to get all the frequency components of

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e npu s gna .

Attenuator is used tomodifythe amplitude of the inputsignal.

The first mixer stage produces an output of 30Mhz which is a

difference between the inputandoscillator signal.


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This 30MHz signal will be amplified by IF amplifierand

fed to thesecond mixer.

The second mixerwill roduce a 0 Hz si nal which is the

differencebetween IF andcrystal oscillator signal.

This signal will then be filtered by the active filter of a

bandwidth less than 1500Hz.

The amplitude of the selected frequency component can

beread fromtheoutputmeter inVoltordB.

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18MHz.

Spectrum Analyzer

Oscilloscope is used to displayand measure signal ina time-

domain.

It is also useful to dis la si nal in the fre uenc domain.

The instrument providing this frequency domain view is the

spectrum analyzer.

A spectrumanalyzer display signalon its CRT with frequency

on the horizontal axis and amplitude (voltage) on the vertical

axis.

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Spectrum analyzers use either a parallel filter bank or a

swept frequency technique.

Spectrum Analyzer

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Spectrum Analyzer

In a parallel filter bank analyzer, the frequency range is

covered by a series of filters whose central frequencies and

bandwidth areso selected thattheyoverlap eachother.

See Figure 9.9(a) textbook.

For the RF or microwave signals, the swept technique is

preferred. Figure 9.9(b) shows the block diagram of a spectrum

analyzer using sweptreceiver design.

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In the figure, the sawtooth generatorprovides the sawtooth

voltage which drives the horizontal movement of the scope

and the frequency controlled element of the voltage tuned

oscillator.


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Spectrum Analyzer

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Spectrum Analyzer

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Spectrum Analyzer

The voltage tuned oscillatorwill sweep fromfmin to fmax of

its frequency bandata linearrecurring rate.

The fre uenc com onent and volta e tuned oscillator

frequency beats together to produce a difference frequency,

i.e. IF (intermediate frequency).

This IF will be amplified anddisplayed on the CRT screen ofthe spectrumanalyzer.

Figure 9.10, 9.11, and 9.12 (textbook) shows the spectrum

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pro uce e npu wave s a s nge one . . s gna.

Digital Fourier Analyzer

The basic principle of a digital Fourier analyzer is shown in

Figure 9.14(textbook).

The digital Fourier analyzer converts the analogue waveform

overtime periodT into N samples.

A Fourier analyzer is based on the calculation of the discrete

Fourier transform using an algorithm called the fast Fourier

transform.

This algorithm calculates the amplitude and phase of each

si nal com onent from a set of time-domain sam les of the

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inputsignal.

Figure 9.15 (textbook) shows the block diagram of a digital

Fourier analyzer.


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The block diagram is divided into 3 sections input,

controland display section.

,

conditioned and passed through two or more anti-aliasing

filters.

A 12-bit ADC is used to convert the signal into a digitalform.

The output from the ADC is connected to a multiplier and

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a digital filter.

The processing section of the analyzer provides FFT

processing on the input signal to produce a magnitude

and phase of input signals components.

Logic Probe

A logic probe is a tool used in digital-circuitry

troubleshooting.

integrated circuitry to determine whether the point under

testis logic high, low, bad level, opencircuited, orpulsing.

When the probe is touched to a high level point, a brightlightappears aroundthe probes tip, and the lightgoes out

when it is touchedona low level point.

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Figure 14.18 (Larry textbook) shows a block diagram of

the logic probe circuitry and its response to different

inputs.


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Logic Probe

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