FM CIRCUITS

FM CIRCUITS
➢Many different circuits have been devised to produce FM
and PM signals.
➢There are two different types of frequency modulator
circuits,
1. direct circuits
2. circuits that produce FM indirectly by phase modulation
technique
FM CIRCUITS
➢Direct FM circuits make use of techniques for varying the
frequency of the carrier oscillator in accordance with the
modulating signal.
➢Indirect modulators produce FM via a phase shifter after the
carrier oscillator stage.
➢Frequency demodulator or detector circuits convert the FM signal
back to the original modulating signal.
➢Most FM circuits today are inside integrated circuits, and some
are implemented in software with digital signal processing
methods.
FREQUENCY MODULATORS
➢a circuit that varies carrier frequency in accordance with the
modulating signal.
➢ The carrier is generated by either an LC or a crystal oscillator
circuit, and so a way must be found to change the frequency of
oscillation.
➢In an LC oscillator, the carrier frequency is fixed by the values of
the inductance and capacitance in a tuned circuit, and the carrier
frequency can therefore be changed by varying either inductance
or capacitance.
FREQUENCY MODULATORS
➢The idea is to find a circuit or component that converts a modulating
voltage to a corresponding change in capacitance or inductance.
➢the objective is to find a circuit or component whose capacitance will
change in response to the modulating signal.
➢The component most frequently used for this purpose is a varactor.
➢Also known as a voltage variable capacitor, variable capacitance
diode, or varicap, this device is basically a semiconductor junction diode
operated in a reverse-bias mode
VARACTOR OPERATION
➢A junction diode is created when P- and N-type semiconductors
are formed during the manufacturing process.
➢Some electrons in the N-type material drift over into the P-type
material and neutralize the holes, forming a thin area called the
depletion region, where there are no free carriers, holes, or
electrons.
VARACTOR OPERATION
VARACTOR OPERATION
VARACTOR OPERATION
➢Varactors are made with a wide range of capacitance values,
most units having a nominal capacitance in the 1- to 200-pF range.
➢The capacitance variation range can be as high as 12 :1
VARACTOR MODULATORS
➢a carrier oscillator for a transmitter, shows the basic concept of a
varactor frequency modulator.
➢The capacitance of varactor diode D1 and L1 forms the parallel-
tuned circuit of the oscillator.
➢The value of C1 is made very large at the operating frequency
so that its reactance is very low.
➢As a result, C1 connects the tuned circuit to the oscillator circuit.
Also C1 blocks the dc bias on the base of Q1 from being shorted
to ground through L1.
➢ The values of L1 and D1 fix the center carrier frequency
VARACTOR MODULATORS
VARACTOR MODULATORS
➢The modulating signal derived from the microphone is amplified and
applied to the modulator.
➢As the modulating signal varies, it adds to and subtracts from the
fixed-bias voltage.
➢Thus, the effective voltage applied to D1 causes its capacitance to
vary.
➢This, in turn, produces the desired deviation of the carrier frequency.
➢A positive- going signal at point A adds to the reverse bias, decreasing
the capacitance and increasing the carrier frequency.
➢A negative-going signal at A subtracts from the bias, increasing the
capacitance and decreasing the carrier frequency.
E.G.
The value of capacitance of a varactor at the center of its linear
range is 40 pF. This varactor will be in parallel with a fixed 20-pF
capacitor. What value of inductance should be used to resonate
this combination to 5.5 MHz in an oscillator? Total capacitance CT
= 40 + 20 = 60 pF.
VARACTOR MODULATORS
➢The main problem with the circuit is that most LC oscillators are
simply not stable enough to provide a carrier signal.
➢The LC oscillators simply are not stable enough to meet the
stringent requirements imposed by the FCC.
➢As a result, crystal oscillators are normally used to set carrier
frequency.
➢Not only do crystal oscillators provide a highly accurate carrier
frequency, but also their frequency stability is superior over a wide
temperature range.
FREQUENCY-MODULATING A CRYSTAL OSCILLATOR
➢It is possible to vary the frequency of a crystal oscillator by
changing the value of capacitance in series or in parallel with the
crystal.
FREQUENCY-MODULATING A CRYSTAL OSCILLATOR
➢it is possible to achieve a deviation of only several hundred
cycles from the crystal oscillator frequency, the total deviation can
be increased by using frequency multiplier circuits after the carrier
oscillator.
➢A frequency multiplier circuit is one whose output frequency is
some integer multiple of the input frequency.
➢A frequency multiplier that multiplies a frequency by 2 is called a
doubler, a frequency multiplier circuit that multiplies an input
frequency by 3 is called a tripler, and so on.
➢Frequency multipliers can also be cascaded.
FREQUENCY-MODULATING A CRYSTAL OSCILLATOR
➢When the FM signal is applied to a frequency multiplier,
both the carrier frequency of operation and the amount of
deviation are increased.
➢Typical frequency multipliers can increase the carrier
oscillator frequency by 24 to 32 times.
➢Frequency modulation of the crystal oscillator by the
varactor produces a maximum deviation of only 200 Hz
FREQUENCY-MODULATING A CRYSTAL OSCILLATOR
VOLTAGE-CONTROLLED OSCILLATORS
➢Oscillators whose frequencies are controlled by an external input
voltage are generally referred to as voltage-controlled oscillators
(VCOs).
➢Voltage-controlled crystal oscillators are generally referred to as
VXOs.
➢Although some VCOs are used primarily in FM, they are also used
in other applications where voltage-to-frequency conversion is
required
VOLTAGE-CONTROLLED OSCILLATORS
VOLTAGE-CONTROLLED OSCILLATORS
➢The Schmitt trigger circuit is a level detector that controls the
current source by switching between charging and discharging
when the capacitor charges or discharges to a specifi c voltage
level.
➢A linear sawtooth of voltage is developed across the capacitor by
the current source.
VOLTAGE-CONTROLLED OSCILLATORS
VOLTAGE-CONTROLLED OSCILLATORS
PHASE MODULATORS
➢Most modern FM transmitters use some form of phase
modulation to produce indirect FM.
➢The reason for using PM instead of direct FM is that the
carrier oscillator can be optimized for frequency accuracy
and stability.
➢Crystal oscillators or crystal-controlled frequency
synthesizers can be used to set the carrier frequency
accurately and maintain solid stability.
PHASE MODULATORS
➢The output of the carrier oscillator is fed to a phase
modulator where the phase shift is made to vary in
accordance with the modulating signal.
➢Since phase variations produce frequency variations,
indirect FM is the result.
➢Some phase modulators are based upon the phase shift
produced by an RC or LC tuned circuit.
PHASE MODULATORS
➢It should be pointed out that simple phase shifters of this type do
not produce linear response over a large range of phase shift.
➢The total allowable phase shift must be restricted to maximize
linearity, and multipliers must be used to achieve the desired
deviation.
➢The phase shift is computed by using the formula
PHASE MODULATORS
PHASE MODULATORS
PHASE MODULATORS
E.G.
PHASE MODULATORS
PHASE MODULATORS

➢To eliminate this effect and to generate real

FM, the audio input frequency must be applied
to a low-pass filter to roll off the signal
amplitude at the higher frequencies in a low-
pass filter
E.G.
FREQUENCY DEMODULATORS
➢Any circuit that will convert a frequency variation in the
carrier back to a proportional voltage variation can be
used to demodulate or detect FM signals.
➢Circuits used to recover the original modulating signal
from an FM transmission are called demodulators,
detectors, or discriminators.
SLOPE DETECTORS
➢The simplest frequency demodulator, the slope detector, makes
use of a tuned circuit and a diode detector to convert frequency
variations to voltage variations
SLOPE DETECTORS
➢To use the circuit to detect or recover FM, the circuit is tuned so
that the center or carrier frequency of the FM signals is
approximately centered on the leading edge of the response curve
➢As the carrier frequency varies above and below its center
frequency, the tuned circuit responds
➢If the frequency goes lower than the carrier frequency, the output
voltage across C1 decreases.
SLOPE DETECTORS
➢If the frequency goes higher, the output across C1 goes
higher.
➢Thus, the ac voltage across C1 is proportional to the
frequency of the FM signal. The voltage across C1 is
rectified into dc pulses that appear across the load R1.
➢These are filtered into a varying dc signal that is an
exact reproduction of the original modulating signal.
SLOPE DETECTORS
➢The main difficulty with slope detectors lies in tuning them
so that the FM signal is correctly centered on the leading
edge of the tuned circuit.
➢In addition, the tuned circuit does not have a perfectly
linear response.
➢It is approximately linear over a narrow range
SLOPE DETECTORS
SLOPE DETECTORS
➢The slope detector is never used in practice, but it does
show the principle of FM demodulation, i.e., converting a
frequency variation to a voltage variation.
➢Numerous practical designs based upon these principles
have been developed.
➢These include the Foster-Seeley discriminator and the
ratio detector, neither of which is used in modern
equipment
PULSE-AVERAGING DISCRIMINATORS
➢The FM signal is applied to a zero-crossing detector or a clipper-
limiter that generates a binary voltage-level change each time the
FM signal varies from minus to plus or from plus to minus.
➢The result is a rectangular wave containing all the frequency
variations of the original signal but without amplitude variations.
➢The FM square wave is then applied to a one-shot (monostable)
multivibrator that generates a fixed-amplitude, fixed-width dc
pulse on the leading edge of each FM cycle.
PULSE-AVERAGING DISCRIMINATORS
➢The duration of the one shot is set so it is less than
one-half the period of the highest frequency
expected during maximum deviation.
➢The one-shot output pulses are then fed to a simple
RC low-pass filter that averages the dc pulses to
recover the original modulating signal.
PULSE-AVERAGING DISCRIMINATORS
PULSE-AVERAGING DISCRIMINATORS
➢At low frequencies, the one-shot pulses are widely spaced; at higher
frequencies, they occur very close together.
➢When these pulses are applied to the averaging filter, a dc output
voltage is developed, the amplitude of which is directly proportional to
the frequency deviation.
➢When a one-shot pulse occurs, the capacitor in the filter charges to the
amplitude of the pulse.
➢When the pulse turns off, the capacitor discharges into the load. If the
RC time constant is high, the charge on the capacitor does not decrease
much.
PULSE-AVERAGING DISCRIMINATORS
PULSE-AVERAGING DISCRIMINATORS
➢When the time interval between pulses is long, however, the
capacitor loses some of its charge into the load, so the average dc
output is low.
➢When the pulses occur rapidly, the capacitor has little time
between pulses to discharge; the average voltage across it
therefore remains higher.
➢the filter output voltage varies in amplitude with the frequency
deviation.
PULSE-AVERAGING DISCRIMINATORS
➢The pulse-averaging discriminator is a very high-quality
frequency demodulator.
➢In the past, its use was limited to expensive telemetry and
industrial control applications.
➢Today, with the availability of low-cost ICs, the pulse-
averaging discriminator is easily implemented and is used
in many electronic products.
QUADRATURE DETECTORS
➢uses a phase-shift circuit to produce a phase shift of 90° at the
unmodulated carrier frequency
➢The frequency-modulated signal is applied through a very small
capacitor to the parallel-tuned circuit, which is adjusted to resonate
at the center carrier frequency.
➢At resonance, the tuned circuit appears as a high value of pure
resistance.
➢The small capacitor has a very high reactance compared to the
tuned circuit impedance.
QUADRATURE DETECTORS

The term quadrature refers to a 90º phase shift between two signals
QUADRATURE DETECTORS
➢When frequency modulation occurs, the carrier frequency
deviates above and below the resonant frequency of the tuned
circuit, resulting in an increasing or a decreasing amount of phase
shift between the input and the output
➢The two quadrature signals are then fed to a phase detector
circuit.
➢The output of the phase detector is a series of pulses whose width
varies with the amount of phase shift between the two signals.
QUADRATURE DETECTORS
➢Normally the sinusoidal FM input signals to the phase detector are at a
very high level and drive the differential amplifiers in the phase
detector into cutoff and saturation.
➢The differential transistors act as switches, so the output is a series of
pulses.
➢No limiter is needed if the input signal is large enough.
➢The duration of the output pulse is determined by the amount of phase
shift.
➢The phase detector can be regarded as an AND gate whose output is
ON only when the two input pulses are ON and is OFF if either one or
both of the inputs are OFF
QUADRATURE DETECTORS
PHASE-LOCKED LOOPS
➢a frequency- or phase-sensitive feedback control circuit used in
frequency demodulation, frequency synthesizers, and various fi
ltering and signal detection applications.
1. A phase detector is used to compare the FM input, sometimes
referred to as the reference signal, to the output of a VCO.
2. The VCO frequency is varied by the dc output voltage from a
low-pass filter.
3. The low-pass filter smoothes the output of the phase detector
into a control voltage that varies the frequency of the VCO.
PHASE-LOCKED LOOPS
➢The primary job of the phase detector is to compare the two input
signals and generate an output signal that, when filtered, will
control the VCO.
➢If there is a phase or frequency difference between the FM input
and VCO signals, the phase detector output varies in proportion to
the difference.
➢The filtered output adjusts the VCO frequency in an attempt to
correct for the original frequency or phase difference. This dc
control voltage, called the error signal, is also the feedback in this
circuit.
PHASE-LOCKED LOOPS
PHASE-LOCKED LOOPS
➢When no input signal is applied, the phase detector and
low-pass filter outputs are zero.
➢The VCO then operates at what is called the free-
running frequency, its normal operating frequency as
determined by internal frequency-determining components.
➢When an input signal close to the frequency of the VCO
is applied, the phase detector comparesthe VCO free-
running frequency to the input frequency and produces an
output voltage proportional to the frequency difference
PHASE-LOCKED LOOPS
➢The phase detector output is a series of pulses that vary
in width in accordance with the amount of phase shift or
frequency difference that exists between the two inputs.
➢The output pulses are then filtered into a dc voltage that
is applied to the VCO.
➢This dc voltage is such that it forces the VCO frequency to
move in a direction that reduces the dc error voltage.
PHASE-LOCKED LOOPS
➢The phase detector output is a series of pulses that vary
in width in accordance with the amount of phase shift or
frequency difference that exists between the two inputs.
➢The output pulses are then filtered into a dc voltage that
is applied to the VCO.
➢This dc voltage is such that it forces the VCO frequency to
move in a direction that reduces the dc error voltage.
PHASE-LOCKED LOOPS
➢Although the input and VCO frequencies are equal, there is a
phase difference between them, usually exactly 90°, which
produces the dc output voltage that will cause the VCO to produce
the frequency that keeps the circuit locked.
➢If the input frequency changes, the phase detector and low-pass
fi lter produce a new value of dc control voltage that forces the
VCO output frequency to change until it is equal to the new input
frequency.
PHASE-LOCKED LOOPS
➢Any variation in input frequency is matched by a VCO frequency
change, so the circuit remains locked.
➢ The VCO in a PLL is, therefore, capable of tracking the input
frequency over a wide range. The range of frequencies over which a
PLL can track an input signal and remain locked is known as the lock
range.
➢The lock range is usually a band of frequencies above and below the
free-running frequency of the VCO. If the input signal frequency is out
of the lock range, the PLL will not lock.
➢When this occurs, the VCO output frequency jumps to its free-running
frequency
PHASE-LOCKED LOOPS
➢If an input frequency within the lock range is applied to the PLL, the
circuit immediately adjusts itself into a locked condition.
➢The phase detector determines the phase difference between the free-
running and input frequencies of the VCO and generates the error
signal that forces the VCO to equal the input frequency. This action is
referred to as capturing an input signal.
➢Once the input signal is captured, the PLL remains locked and will track
any changes in the input signal as long as the frequency is within the lock
range.
➢The range of frequencies over which a PLL will capture an input signal,
known as the capture range, is much narrower than the lock range, but,
like the lock range, is generally centered on the free-running frequency
of the VCO
PHASE-LOCKED LOOPS
➢The characteristic that causes the PLL to capture signals within a
certain frequency range causes it to act as a bandpass fi lter.
➢Phase-locked loops are often used in signal conditioning
applications, where it is desirable to pass signals only in a certain
range and to reject signals outside of that range.
➢The PLL is highly effective in eliminating the noise and interference
on a signal.
➢The ability of a PLL to respond to input frequency variations
makes it useful in FM applications.
PHASE-LOCKED LOOPS
➢The PLL’s tracking action means that the VCO can operate as a
frequency modulator that produces exactly the same FM signal as
the input.
➢In order for this to happen, however, the VCO input must be
identical to the original modulating signal.
➢The VCO output follows the FM input signal because the error
voltage produced by the phase detector and low-pass filter forces
the VCO to track it.
PHASE-LOCKED LOOPS
PHASE-LOCKED LOOPS
PHASE-LOCKED LOOPS
➢The capture range fo of a phase-locked loop is
smaller than the lock range.
➢Once the input frequency is captured, the output
frequency will match it until the input frequency
goes out of the lock range.
➢Then the phase-locked loop will return to the free-
running frequency Of the VCO.
PLEASE READ ELECTRONICS
COMMUNICATION SYSTEM(CHAPTER
6) BY: FRENZEL AND ELECTRONICS
COMMUNICATION (ANGLE
MODULATION) BY WAYNE TOMASI
THANK YOU
AND
GOD BLESS!!!!