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It is extensively used in the testing of radio receivers and transmitters. This is basically

a radio frequency (RF) signal generator. The standard signal generator produces known

and controllable voltages. - --( -----

6.5.1 Principle of Working

The output of the generator is amplitude modulated or frequency modulated. The

frequency modulation is possible using a carrier signal from RF oscillator. The amplitude

modulation can be done using internal sine wave oscillator. The modulation may be done

by a sine wave, square wave, triangular wave or a pulse also. The setting on the front

dane1 indicates the carrier frequency to be used for modulation.

'7 6.5.2 Block DiagramThe block diagram of conventional standard signal generator is shown in the

Fig. 6.11.

Range

~\. .

Frequency

~\. .

RF

oscillator

Wide

band

amplifier

Output

attenuator

Externaloscillator

Modulationoscillator

II

Modulation -'

frequency

/

-' % Modulation

The LC tank circuit is very stable RF oscillator. It is used to generate the carrier

frequency with a constant output over any frequency range. The amplitude modulation 1

done by an internal sine wave generator or by external source. The frequency of

oscillations is indicated by the frequency range control and vernier dial setting. Themodulation is done in the output wideband amplifier. The output of amplifier is

modulated carrier and it is given to an attenuator. This attenuator helps in selecting proper

range of attenuation and the output signal level is controlled.

The master oscillator is LC tank circuit; so the frequency stability is limited. The

switching of frequency in various ranges is achieved by selecting appropriate capacitor.

This upsets circuit design and requires some time to stabilize at new resonant frequency.

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It is extensively used in the testing of radio receivers and transmitters. This is basically

a radio frequency (RF) signal generator. The standard signal generator produces known

and controllable voltages.( _._---

6.5.1 Principle of Working

The output of the generator is amplitude modulated or frequency modulated. The

frequency modulation is possible using a carrier signal from RF oscillator. The amplitude

modulation can be done using internal sine wave oscillator. The modulation may be done

by a sine wave, square wave, triangular wave or a pulse also. The setting on the front

danel indicates the carrier frequency to be used for modulation.

~ 6.5.2 Block Diagram

The block diagram of conventional standard signal generator is shown in the

Fig. 6.11.

Range~. . .

. . .

Frequency

~\. .

RFoscillator

Wideband

amplifier

Outputattenuator

External

oscillator

Modulationoscillator

If

Modulation i (j'% Modulation

frequency

The LC tank circuit is very stable RF oscillator. It is used to generate the ca "'iter

frequency with a constant output over any frequency range. The amplitude modulation is

done by an internal sine wave generator or by external source. The frequency ofoscillations is indicated by the frequency range control and vernier dial setting. The

modulation is done in the output wideband amplifier. The output of amplifier is

modulated carrier and it is given to an attenuator. This attenuator helps in selecting proper

range of attenuation and the output signal level is controlled.

The master oscillator is LC tank circuit; so the frequency stability is limited. The

switching of frequency in various ranges is achieved by selecting appropriate capacitor.

This upsets circuit design and requires some time to stabilize at new resonant frequency.

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Automatic

control

; _ _ ._ _ ._ - - - - - - - _ . - - _ . - - - _ ._ ,

: .-------...,:! RF Oscillator ii 34 MHz -- 80 MHz i

i [ ]= i; :i ~

: '\,._ __ __ __ J

,- - _ - - _ . _ _ - - - _ _ - - - - _ .._ - --- - - - - - - - - - - _ .._ - - - _ ..-._ .-. --- -.---- ---_.-.----- -- ,

i Power amplifier i

I""mbly i

-'B>-Auto

Manual ~

Coarse

freq.

tuning

Carrier

level

Audio

oscillator

400 Hz /1 kHz

Audio

oscillator

assembly

Fig. 6.12 Modern signal generator

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 level of modulation can be adjusted upto 95% by using

control devices.

The lowest frequency range obtained by using frequency divider is the highest

frequency range divided 29 or 512. Thus, frequency stability of highest range is imparted

to the lowest frequency range. The effects of frequency range selection is eliminated as

same oscillator is used for all frequency bands. The master oscillator is tuned automaticallyor 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 corresponding to 0.01 % of main dial setting.

The internal calibration is provided by 1 MHz crystal oscillator. The small power

consumption of the instruments makes output with very low ripple. The supply voltage of

the master oscillator is regulated by temperature compensated reference circuit. The output

of the main amplifier is given to an output attenuator. The attenuator controls the

amplitude level and provides the required stable RF output.

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Electronic Instrumentation 6 - 18 Signal Generators

~] Audio Frequency Sine and Square Wave Generator

The block diagram of an AF sine-square wave generator is a s shown in the Fig. 6.13.

Wien

bridge

oscillator (

Function - = -

switchSquare

waveshaper

Amplifiersquarewave

As per our previous discussion, 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 Wi en bridge oscillator is fed to square wave

shaper circuit which uses schmitt trigger circuit. The attenuators in both the sections are

used to control output signal level. Before attenuation, the signal level is made very high

using sine wave amplifier and square wave amplifier.

/t/6.7.1 Front Panel ControlsThe front panel controls of typical AF signal generator are as follows,

1. Frequency Selector: This helps in selecting the frequency in different ranges. This

varies frequency in the ratio 1 : 11 which is nonlinear scale.

2. Frequency Multiplier : It selects the frequency ranges more than 5 decades from

10 Hz to 1 MHz.

3. Amplitude Multiplier

XO.1, XO.01.

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

5. Symmetry Control : It varies the symmetry of square wave from 30 'Y o to 7 0% .

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.

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9. Sync : This terminal provides synchronisation of the internal signal with external

signal.

10. ON-OFF Switch

6.7.2 Specifications

The specifications of typical AF 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 ( rms).

3. The amplitude of square wave output can be varied from 0 - 20 V (peak).

- 1 - . The square wave symmetry is adjustable from 30% to 70%.

5. The output is taken from push-pull amplifier with low output ilnpedance of 600 n

6. At 220 V, 50 Hz, AF signal generator requires 7 W of power only.

The square wave generator and pulse generator are generally used as measuring

devices in combination with the oscilloscope. The basic difference between square wave

generator and pulse generator is in the duty cycle. The duty cycle is defined as the ratio of

average value of a pulse over one cycle to the peak value. It is also defined as ratio of the

pulse width to the period of one cycle.

D IPulse width

uty cyc e = .Pulse penod

0, T/2Period

- ---- ------

}T

Peak

value

Fig. 6.14 Square wave waveform

The average value is half of peak value. Both the average value and peak value are

inversely proportional to time duration. The average value of a pulse is given as,

1Average value = " 2 Peak value

1 ' / 2Duty cycle of square wave = T= 0.5

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A verage value

Peak value

I" 2 Peak value

Peak value

Thus square wave generator produces an output voltage with equal ON and OFF

periods as duty cycle is 0.5 or 50% as the frequency of oscillation is varied. Then we can

state that irrespective of the frequency of operation, the positive and negative half cycles

extend over half of the total period.

Consider any general pulse as shown in Fig. 6.15.

~ Pulse widthI I

--

TPeak value

--- ---- ----- ---- -- -----

tAveragetrT x Pea

value =k value

I Period T I: , I ' :

On Offperiod period

Fig. 6.15 Pulse waveform

Total period of one pulse is T. This one pulse can be splitted into two parts namely

ON period and OFF period. The ON period and OFF period when combined together,

gives period of one pulse. The pulse width is t.

Pulse widthDuty cycle for a pulse =

Pulse period

Average value

Peak value

tT Peak value

Peak value

t tON

T tON + tOFF

Thus depending on the 'ON' period of pulse, the duty cycle o f a pulse may vary

between 50% to 95%. Generally the pulse generator can supply more power than square

wave generator during ON period of pulse. Because comparing pulse waveform and

square wave, we can make tON greater than tOFF of pulse only and not of square wav(

Also the short duration pulses reduce power dissipation in the components under test.

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The square wave generators are used when the system is ana lysed for low frequency

characteristics, testing of audio system.

6.8.1 Pulse Characteristics and TerminologyThe characteristics of a general pulse are shown in Fig. 6.16.

Amplitude50% width (W)

Fig. 6.16 General pulse characteristics

The base line is the d.c. level. At this level pulse starts and finishes. The shift or offsetof this base line from zero value or expected value is called base line offset. The

amplitude of the pulse is measured from base line upto the steady state valu.e o f pulse.

1. Pulse Rise and Fall Time : The pulse rise time is the time needed for the pulse to

go from 10% to 90% of its amplitude. The fall time is the time for the trailing edge

to go from 90% to 10%. These are also called leading edge and trailing edge

transition times.

2. Linearity: The linearity of the pulse is the deviation of the edge from the straight

line drawn through the 10% and 90% points expressed as a percentage of

amplitt1--le of pulse.

3. Pulse PI ,hoot: The pulse preshoot is the deviation prior to reaching the base line

at the start of the pulse. The overshoot is the maximum height following leading

edge.

4. Ringing: It is the positive and negative peak distortion excluding overshoot.

5. Settling Time : It is the period needed for pulse ringing to be within a specified

percentage of the pulse amplitude, measured from 90% point of the leading edge.

6. Pulse Droop or Sag: It is the fall in pulse amplitude within time. Pulse rounding

is the curved portion of the pulse at the leading and trailing edges.

7. Pulse Width: The width of the pulse is measured between the 50% points on the

leading and trailing edges.

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8. Pulse Repetition Rate The pulse repetition rate is reciprocal of pulse period and

it is measured in units of frequency.

9. Duty Cycle : The duty cycle is the ratio of pulse width to the pulse period. It is

generally expressed as a percentage of time period.

10. Pulse Jitter: It is the measure of short term instability of one event with respect toother event.

6.8.2 Requirements of Pulse Generator

1. The pulse should have minimum distortion.

2. The pulse must have sufficient maximum amplitude if greater output power is

required. Also the attenuation range should be enough to produce small amplitude

pulses which prevents over driving of circuits.

3. The pulse repetition rate must be sufficient to have range of frequency control.

4. The pulse generator can be used to trigger signals.

5. Some pulse generators may be triggered by externally applied triggers.

6. The output impedance of the pulse generator plays important role. In the fast

pulse system, the generator should be matched with the cable and cable should be

matched with test circuit. The mismatch in impedance reflects energy back to

generator by the test circuit giving distortion in the pulses.

/TO maintain d.c. bias leveL d.c. coupling of output circuit is needed.

6.9 Laboratory Type Square Wave and Pulse Generator

Constant i1

currentsource

: - - - - - - - - - - - - - - - - - - ,

: :I Schmitt:

trigger

Constantcurrentsource i2

~

:~ :ec~f--Period--j

T

Fig. 6.17 Basic generating loop

The circuit consists of two current sourcesJ a ramp capacitor, and schmitt trigger circuit

as well as curr~wit~ht!!g 9,rcuit. The two current sources provide a constan Cllr.ITn~to

aJamp c:apaci tor for charging and discharging. The ratio of tnese charging and discharging

current is' determined by setting of symmetry control. The symmetry control determines

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duty cycle of output waveform. In the current source, an appropriate control voltage is

applied to current control transistors which controls the frequency i.e. sum of two currents.

The multiplier switch provides decade switching control output frequency. While

frequency dial provi.des continuous vernier control of output frequency.

The block diagram of laboratory type square wave and pulse generator is as shown in

Fig. 6.18.

ISymmetry I" ,,

4 I l Amplitude\\

r,I "

I "

---~ A ."'~-II ... I

~ I

Ramp I

capacitor :

. .---------

Output

amplifier

600 nOutput

4 I l Vernier

\4 I l Amplitude

\\

50 nOutput

amplifier

Step 50naUenuator Output

Triggeroutputcircuit

Trigger

output

The upper current source supplies a constant current to ~he ramp capacitor. This

charg;s capacitor at a constant rate as voltage across capacitor increases linearly. When the-=---~---- ------- ------_-:.-

positive ramp reaches the maximum upper limit set by the circuit components, the schmitt

trigger changes its state. The tri$ger Q.rollLQutpULb~()mes negatiY.e. The trigger ~ircuit

negative output changes the condition of the current control switch. Now the capacitor

st~dischargmg linearly, The discharge rate is linear and.;t' is controi'ledby the 'lower------( ~- -- -- -----'-"--------===~-

cur~~. When n~gative ramp reas:hes the l()wer . limit, the schmitt_ trigger comes

bac to its original state: This 'now provides positive output. This changes condition of the

current controlSw'ttch again b y cutting off the Iowercullent source while turning on theupper current source. This gives one cycle o f operation. Then such a process is repeatative

giving positive and negative pulses at a constant rate.

The output of schmitt trigger is passed to the trigger o~t circuit and 500 ancL6QO.o

ampl~fiers.The trigger output circuit differentiates square wave output, inverts resulting

pulse and provides positive trigger pulse. The 50 .0 amplifier 'is provided with step

attenuator which allows a ver~er control of signal output voltage. The gener;;Jor can be

synchronized to an external signal by triggering the circuit by an external synchronization

pulse.

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6.9.1 Specifications

i) The frequency range is f rom 1 Hz to 10 MHz.

ii) The duty cycle can be varied from 25% to 75%.

iii)Two independent outputs are available.

a) 50 Q source supplying pulses with 5 nsec rise and fall times at 5 V peaK

amplitude.

b) 600 Q source supplying pulses with 70 nsec rise and fall times at 30 V peaK

amplitude.

iv) The generator can be operated as free running generator.

v) This can be synchronised with external signal.

vi) To synchronise external circuits, trigger output pulses are available.

~ 0 Function Generator

T e f unction generator is an instrument which generates different types of waveforms

The frequency of these wave orms can be vane over wide range. le most required

common waveforms are sine wave, sawtooth wave, triangular wave, square wave. These

various OlltputS of the generator are available simultaneously. We may require square

wave for testing linearity measurements in audio system. At the same time, we mal'

require!"~awtooth output to drive horizontal deflection amplifier of an oscilloscope which

gives visual display of the measurements. The purpose of providing simultaneous wavesis

flllfilled by the function generator.

Another useful f eature of the function generator is that it can be phase locked to an

external signal source. One function generator can be phase locked with second function

generator, the two output signals can be displaced in phase by an adjustable amount.In addition to that, the fundamental frequency of one generator can be phase lockedto

a harmonic of another generator. By adjusting the amplitude and phase of the harmonic,

almost any waveform can be generated.

The function generator can be phase locked to a standard frequency of the source

Then aJJ the output waveforms of the generator will have same accuracy and stability as

that of standard source.

c

c

6.10.1 Block Diagram

The block diagram of a typical function generator is as shown in Fig. 6.19.

The function generator can supply output signals at very low frequencies. Normally

the frequency is controJJed by varying the capacitor .in LC circuits or R-C circuits. But the

lower frequency possible using R-C circuits is limited, so different ;-lethod is used to

control frequency. In the function generator, the frequency is controJJed by varying the

magnitude of current which drives the integrator. The function generator generates sine- - = - - - - - - - - - - -

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wave, triangular wave and square waves with a frequency varying from Qj)1 Hz to-100 kHz.

Freq.

~\control

Freq.

control

N/W

Upperconstant

currentsource

Voltage

comparatorM IV

Outputamplifier

#1

Externalfreq.

controlLower

constant

currentsource

Resistance

diodeshaping

circuit )/\/\V

Outputamplifier

#2 AAV

Fig. 6.19 Typical function generator

The frequency controlled voltage i s 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.Ifthe current, charging the capacitor increases or decreases, the slope of output voltage

increases or decreases respectively. Hence this controls frequency. The voltage comparator

multivibrator circuit changes the state of the network 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 andswitching ON the upper current source.

The output voltage of the integrator has triangular waveform. The frequency of this

triangular waveform is determined by the magnitudes of the currents supplied by upper

current source and lower current source. To get square wave, the output of the integrator

is passed through comparator. The voltage comparator delivers square wave output

voltageof same frequency as that of input triangular waveform. The sine wave is derived

from triangular wave. The triangular wave is synthesised into sine wave using diode

resistance network. In this shaper circuit, the slope of triangular wave is changed as its

arr'plitude changes. This results in a sine wave with less than 1% distortion.

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The two output amplifiers provide two simultaneous, individually selected outputs of

any of the waveform functions.

The function of a signal generators is to supply signals of known amplitude an d

known frequency. The signal generators are used to supply signal levels at very low levels

for the testing of receivers. But it is very difficult to measure and calibrate a signal at a

very low level. Thus attenuators are used in function generators. It is a device whichreduces power level of a signal by fixed amount.

The attenuator reduces the power of an input such that the ratio of the input power to

the output remains constant. It is expressed in decibel as follows,

A (in dB) = 10 loglO[~:JIn general, we have two switches for attenuator such as 20 dB and 20 dB. If we press

either of a switch we can get fixed attenuation of 20 dB.

If two attenuators are used, then the attenuation is given by,

A (. dB) 0 1 [P in J [P ~ 1 J 10 1 P in 10 1 P inin = 1 OglOP-,-= OglOP+ OglO-, -out POllt out POllt

Thus in dB notation,

A = A] +A2 in dB

Hence the total attenuation in dB of two cascaded attenuators is the sum of the decibel

attenuation of each attenuator.

Hence when both the switches are pressed, the total attenuation achieved is 40 dB.

The various features of a function generator are,

1. The frequency range is 0.01 Hz to 100 kHz.

2. Can produce various waveforms such as sinewave, sawtooth wave, triangular

wave, square wave etc.

3. The accuracy is within 1%, in low frequency range.

4. The distortion is less than 1% for the sine wave.5. Can be phase locked to another external signal source.

6. Can be phase locked to standard frequency, so all the output waveforms of

generator will have same accuracy and stability as that of standard source.

7. A continuous adjustable d.c. offset is available between - 5 V to + 5 V.

6.10.3 Specifications of Function Generator

The typical important specifications of a function generator are as follows

i) frequency range - 0.001 Hz to 20 MHz

ii) frequency stability - 0.05 %

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iii) Distortion - -55 dB below 50 kHz, -40 dB above 50 kHz

iv) output amplitude (open circuit) and impedance - 10 Vp--p, 50 Q

v) output waveforms - sine, square, triangular, ramp, pulse, AM and FM modulated,

arbitary.

6.11 Sweep-Frequency Generators

The sine wave generator discussed in earlier sections generates output voltage at a

known and stable frequency. But in some applications such as measuring frequency

res.onse of am.plifiers, filters and other networks, a v~e frequency source is required.

In such cases, sweep frequency

generators are used.Movable

plate

Permanent(2-magnet \5

50 HzA.C.Sweep

width

Fixedplate

Fig. 6.20 Electro-mechanical system for variablefrequency

D.C. Biasrectifier

50 HzA.C.

Sweep ---jwidth

In the early days, the method

for varying frequency electronically

was not invented. Some other

methods were used to get variable

frequency source. Reactance tube

modulator used was providing

very little frequency variation, so

most of the times,

electro-mechanical systems such as

motor driven capacitors were used.

This is shown in the Fig. 6.20

OSC

tankcoils

Fig. 6.21 Saturable reactor sweep modulator

But in this system, the reliability of system performance was poor. Also sweep width

obtained was really very less. The main measurements were made by point to point

technique using conventional single frequency signal generators.

Then saturable reactor sweep modulator was invented. In this system, the major

advantage is that there are no moving parts.

In this system tank coils are wound on ferrite core and permeability of core is varied

by 50 Hz supply and magnetic field from control winding.

The B-H curve has maximum linearity at medium flux density. Hence d.c. blilS current

is pa~ oed through control winding in addition to 50 Hz a.c. current.

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The development of solid state

variable capacitance diode (varicap

diode) helps in building sweep

frequency genera tors. These are

extensively used than any other

electronic devices. These varicapdiodes provide the method of

electronically tunning an oscillator.

The block diagram of simple

sweep frequency generator is as

shown in Fig. 6.22.

The sweep generator is very

much similar to the simple signal

generator. Tn the simple signal

genera tor, an oscilla tor is tuned to

fixed single frequency. Tn the sweepgenerator, an oscillator is electronically tuned and by using voltage controlled oscillator

variable frequency is obtained. As name indicates, a sweep voltage generator provides

voltage, known as control voltage, to the voltage controlled oscillator (VeO). The function

of voltage controlled oscillator is to provide various frequency sweeps according to voltage

provide by sweep voltage generator.

But the relationship between

sweep voltage and frequency is

nonlinear. To obtain linearity, a

compensation circuit is provided

between sweep frequency voltageand oscillator tunning voltage. The

compensation circuit is called

linearizing circu it. A typical

linearizing circuit for sweep

generator is a s shown in Fig. 6.23(a)

Generally there is a limit of 2:1

of maximum to minimum frequency

of any sweeping oscillator.

The linearizing circuit is mainly

used to ma tch the transfer

~ti_c w~. The

slopes are adjusted by resistors in

the circuit. The gain of the circuit

shown is a function of feedback resistor R f and the net resistance of parallel combinations

of R1 through R4. Tnitially when input sweep voltage is very low, the diodes can not

Voltage-controlledOscillator RF

Output

RFOut

Sweep

VoltageGenerator

SweepVoltageOutput

/1 / Sweep

/ V voltagein

Fig. 6.23 (a) Linearizing circuit for a sw~epgenerator

m

cO

isre

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conduct and the gain of op-amp circuit equals (1 + R fiR,). When sweep voltage

approaches V;, the first diode conducts and the gain of amplifier increase and it becomes,

. Rf R fGam = 1 + R

A= 1 + (RJiIR

2)

Fig. 6.23 (b)

6.12 Frequency Synthesizers

When the sweep voltage input reaches

R2 , D, and D2 both conducts and gain

increases to (1+ R r I R B ) where R B IS

parallel combination of R " R 2 and R 3.

When the sweep voltage reaches v"the gain still increases and becomes (1 +

RrlRc) where Rc is paranel combination

of R l' R 2 , R 3 and ~. The net result is a

non-linear relationship made of straight

line segments as shown in Fig. 6.23 (b).

The frequency generators are of two types.

1. One is free running f requency generators in which the output can be tuned

continuously either electronically or mechanically over a wide frequency range.

The generators discussed uptill now are of this type.

2. The second is frequency generator with frequency synthesis technique. The

synthesis means to use a fixed frequency oscillator called reference oscillator or

clock and to derive the wide frequency range in steps from the ou tpu t of the

reference oscilla tor.

The stability and accuracy of free running frequency generator is poor while frequency

synthesizers provide output which is arbitrarily selectable, stable and accurate frequency.

The reference oscillator used in frequency synthesizers is generally precision crystal

oscillator with an output at some cardinal frequency such as 10 MHz. Various signal

processing circuits then operate in synchronism to provide a large choice of the output

frequencies.

Every possible output frequency is derived from the reference oscillator frequency by

multiplying its frequency by a fraction mln where m and n are int~ers. The front panel

controls are provided to select m and n values. Many times out of m and n, one variable

is constant and other is varied to obtain the required frequency. For example if the

reference oscillator frequency is 10 MHz and n is constant 10000 then varying m the user

can generate output in a range of frequencies which are spaced 1 kHz apart.

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The frequency synthesizer effectively synthesize f requency using two methods:

1. Direct synthesis

2. Indirect synthesis.

Let us see in detail these two techniques of synthesis.

6.12.1 Direct Synthesis

The direct synthesis use a technique of directly deriving the output frequency from the

reference frequency. For this, it uses frequency dividers, multipliers, mixers and bandpass

filters. With the help of all these devices, an output which is m/n times reference can be

generated. This configuration is used to avoid low level, non harmonically related

sinusoidal spurious signals to exist at output. Hence such method produces much accurate

and stable output.

The 10 MHz signal from

the reference oscillator is

directly applied to the

mixer. This is 10 M Hz

signal. Using a divider and

the multiplier combinations,the S1 ~I of 1 MHz,

2 MHz, 3 MHz can be

genera ted. As per the

requirement any of these signals can be selected and given to the mixer. In this case a

3 MHz signal is given to the mixer. The mixer adds the two signals to generate 13 MHz.

Actually output of mixer is sum and difference of the f requencies i.e. 13 and 7 MHz signal.

Using 13 MHz bandpass filter, required frequency is obtained. Infact using another

bandpass filter of 7 MHz, 7 MHz output also can be obtained, if required.

The Fig. 6.25 shows the block diagram of practical direct synthesizer.

It uses the master oscillator, for stability purpose. The spectrum generator provides all

the harmonics of the frequency fed to i t. A set of 10 narrowband filters is used to select

any of the harmonics by switching. Also the balanced. mixer and a set of 10 bandpass

filters is also selected by switching.

The advantage of direct synthesis is its speed with which output frequency can be

changed.

13 MHzBandpass

filter

basic action in

synthesis which

generation of 1;

ou tpu t from I()

reference.

direct

shows

M H z

M Hz

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Multiplier

X 10

1MHzX10Master Osc.

10 MHz

Spectrumgenerator

1 MHzSpectrum

generator

Divider10 : 1

100 kHzSpectrum

generator

100 HzSpectrum

generator

'------.--_ . . . . ./ '-----~--_./MHz

~Hz

Against this advantage, there are number of disadvantages of this method such as,

1. Due to switching frequencies, phase continuity is lost.

2. The method is very prone to spurious signals in the output. The problem of

spurious frequency always increases with increase in the output frequency range.

3. It suffers from wideband phase noise.

4. Due to a lot of hardware the circuit is expensive.

VCOFrequency contml

Modulus

(13)~

reference is used. But by placing an

oscillator in phase locked loop, its

frequency can be controlled so

that the output is m/n times the

13 MHz reference frequency.

Let us see basic phase locked

loop action to generate 13 MHz

from 10 MHz. The technique is

shown in the Fig. 6.26.

These are counters whose

count modulus i.e. number they

reach before starting over IS

externally programmable. The

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reference is divided to 1 MHz which is applied to loop phase detector. T he variable

modulus divider is programmed to 13. The loop will stabilise when output of voltage

controlled oscillator becomes 13 MHz. Programming the modulus divider to any other

number will lock the loop to that number times 1 MHz. Thus in such method, the

available output frequency spacing is equal to the loop reference frequency i.e. 1 MHz in

this case.

Generalised block diagram of indirect frequency synthesis method is shown in the

Fig. 6.27.

Controlvoltage

Crystaloscillator

(Referencefrequencysource)

Squarewavecircuit

Phase

detector

Loop

filter

Voltagecontrolledoscillator

J U l I U l

1\1\1\1\

VVV

Programmablefrequency divider(Divide by N)

Square

wavecircuit

Fig. 6.27 Block diagram of indirect frequency synthesis using phase locked loopsystem

There are five main blocks in indirect frequency synthesizer which are,

i) Voltage Controlled Oscillator (VCo)

ii) Programmable divider

iii)Phase detector

iv) Reference frequency source and

v) Loop filter

The output frequency is given by the voltage controlled oscillator (VCO). The YCa can

be tuned electronically by applying generally variable voltage. For higher frequencie",

tuning is done electronically using current instead of voltage.

The programmable divider is a logic element. It divides the frequency of the VCo by

an integer. The factor by which the frequency of the VCO is to be divided is entered in the

element using programming switches or microprocessor.

The phase detector provides an analog output. This is function of phase angle between

reference source and programmable divider output.

The reference source is a quartz crystal oscillator giving very accurate and stable

reference frequency. The overall accuracy of the frequency synthesizer is totally dependent