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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