FM Modulators and Transmitters
Sections: 4-8
Outline
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FM modulators and transmitters
Frequency drifting; ppm
Basic component review
Angle Modulation Classification
•  Direct PM Modulation Techniques
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Indirect FM
Phase of the carrier changes according to m(t)
Thus, Indirect FM Modulation –
Advantages of direct PM: Uses stable crystal oscillator
Disadvantages of direct PM: Limited phase deviation\
•  Indirect PM Modulation Techniques
Direct FM
–  Direct FM Modulation - frequency of the carrier changes according to m(t)
–  Advantages of direct FM: easy to obtain high frequency deviation
–  Disadvantages of direct FM: when using LC tanks it is not very stable, thus additional
circuitry is required
–  Approaches to create direct FM:
•  Varactor diode modulators
•  FM reactance modulators
•  IC-based modulators
See notes for diagrams
FM Transmitters
•  Direct
–  Crosby – utilizing AFC loop (automatic frequency control loop)
–  PLL- based
•  Indirect
–  Armstrong
–  FM transmitter using PM modulators
FM Transmitters/Receiver – Key Components (review)
•  Linear and non-linear devices
•  Discriminators
–  Frequency to amplitude
converters
–  Differentiators
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Multipliers
Dividers
Mixers
Phase detectors
Oscillators
–  Tank circuits (LC)
–  Varactor diodes
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Adders
Bandpass Limiters
Envelop detectors
VCOs
Filters
–  RC (LPF, HPF)
–  LRC (Bandpass Filter)
–  All-pass filters
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Amplifiers
PLL
Super-heterodyning
Preemphasis and Deemphasis
Filters
•  Devices which take input
waveform and modify its
frequency spectrum content
•  Use energy storage elements to
obtain frequency discrimination
–  Inductors
–  Capacitors
•  They have different
classifications:
–  Construction
•  LC elements
•  Quartz crystal elements
–  Transfer function response
•  Butterworth, Chebyshev
•  Filters contain energy storage
elements that are physically
imperfect
–  Inductors have resistance
–  Capacitors have shunt
resistance à leakage
•  The quality of these elements
can be measured using Quality
Q of the filter
•  Two ways of calculation:
–  Q = 2pi (maximum energy stored
during on cycle)/Energy dissipated
per cycle
–  Q = fo/B (B is 3-dB BW; and fo is
resonant freq.
•  For LRC circuits we use Q = fo/B
–  The more narrowband the filter the
larger the Q à less DRIFT!
Filter Construction
Filter Constructions
Active filters using OPAMPS are
limited to 500KHz – opamps
have large open-loop gain!
Lumped LC elements
are impractical above
300MHz – Low Q
Crystal filters using quartz crystal
elements are good up to 100 MHz, good
stability high Qà very good
performance à low drift à more
expensive than RC
FM Transmitters – Crosby Direct FM
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Used for commercial broadcast-band transmitters
Uses an Automatic Frequency Control (AFC) Loop
Characteristics:
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Phase deviation of the output is multiple of phase deviation of the modulator
The modulating frequency is unaffected by the multiplication process
The angle modulated carrier is heterodyned through the non-linear mixer
The output of the mixer depends on the passband filter – could be up/down converted
Discriminator generally has high-Q (narrowband)
Master Frequency
modulator (fc)
DC correction voltage is added to
the modulator to adjust the fc due
to any DRIFT
Crystal
Oscillator
To the antenna
Non-linear mixer
Note:
Kd is in V/Hz
Ko is in Hz/V
FM Transmitters - Example
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Assume fc drift 40 ppm/degree (40 x 5.1 = +/- 204Hz) à 3672 Hz at the antenna;
Thus, following 5 degree temp. change à freq. drift will be 18.36 KHz at the antenna!
In this case the open-loop drift is dfopen = N1.N2.dfc.
Master Frequency
modulator (fc)
To the antenna
Max. frequency
deviation allowed by
FCC is 2KHz
Note that frequency drifting can occur due to temperature change.
It is often given in ppm per deg.Non-linear
C. mixer
Example:
A drift of 40 ppm at the master oscillator will translate to
DC correction voltage is added to
the modulator to adjust the fc due [(40ppm x 5.1)/10^6] = +/- 204Hz=Δf)
to any DRIFT
Similarly,
Note:
Δf=204 Hz à [(Δf/fc)*10^6] = 200 ppm
Kd is in V/Hz
Ko is in Hz/V
Crystal
Oscillator
FM Transmitters – Example w/AFC
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Assume fc drift 40 ppm (40 x 5.1 = +/- 204Hz) & Assuming KdKo=3.83
In this case the closed-loop drift is dfclosed = dfopen/(1 + N1.N2.Kd.Ko).
Thus, the total drift at the antenna will be 153 Hz (51 Hz before the antenna). Much less than before
To the antenna
Max. frequency
deviation allowed by
FCC is 2KHz
Master Frequency
modulator (fc)
Non-linear mixer
DC correction voltage is added to
the modulator to adjust the fc due
to any DRIFT
Note:
Kd is in V/Hz
Ko is in Hz/V
Crystal
Oscillator
FM Transmitters – Example w/AFC
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What if the discriminator and crystal reference oscillator drift as well?
In this case the closed-loop drift is dfclosed = dfopen/(1 + N1.N2.Kd.Ko).
The total open-loop drift will be:
dfopen = N1.N2(dfc + .Kd.Ko.dfd + Kd.Ko.N4.dfo )
Master Frequency
modulator (fc)
To the antenna
Max. frequency
deviation allowed by
FCC is 2KHz
Typical Values:
Discriminators: +/- 100 ppm
Note that had we not used the
Mixer, the drift at the output of
the discriminator would have
been 100ppm*30.6 = 3060 Hz
as opposed to
100ppmx2 = 200Hz!!
DC correction voltage is added to
the modulator to adjust the fc due
to any DRIFT
Note:
Kd is in V/Hz
Ko is in Hz/V
Crystal
Oscillator
Direct FM Transmitter Using PLL
•  Generating WBFM (large ΔF) ; we assume the stability of
the VCO (carrier) is not very good à we use PLL
•  The stability of the crystal oscillator is relatively good and
has high –Q
ac
ac
Phase detector
dc
Good stability;
Lower frequency
DC
Voltage Correction
fc
Indirect WBFM (Armstrong Method)
•  Uses NBFM to generate WBFM
•  The NBFM is generated using indirect method
WBFM Using Indirect Method of Armstrong
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Two blocks: Mixer and Modulator
Note that the output of NBFM is à
Utilizes heterodyning and up-conversion
WBFM Using Indirect Method of Armstrong
s(t) = Vc cos(ω c t + θ (t))
sPM (t) = Vc cos(ω c t + Dp m(t))
Low ΔF=25 Hz
fm
Max 15KHz
Modulation index:
ΔF/fm
sFM (t) = Vc cos(ω c t +
∫ D m(τ )dτ )
f
Can lead
Or lag
Low Freq.
Carrier / High Q
Heterodyned
Must be 88-108 MHz
For commercial FM
Also called the
balanced modulator
WBFM Using Indirect Method of Armstrong
s(t) = Vc cos(ω c t + θ (t))
Questions:
Calculate the min. modulation index.
How do you create NBPM?
fm
Max 15KHz
Low ΔF=25 Hz
sPM (t) = Vc cos(ω c t + Dp m(t))
Modulation index:
ΔF/fm
sFM (t) = Vc cos(ω c t +
∫ D m(τ )dτ )
f
Can lead or
lag
Low Freq.
Carrier / High Q
Heterodyned
Must be 88-108 MHz
For commercial FM
Also called the
balanced modulator
References
•  Leon W. Couch II, Digital and Analog Communication
Systems, 8th edition, Pearson / Prentice, Chapter 4
•  Signal Conditioning: An Introduction to Continuous Wave
Communication By Apurba Das, Chapter 5
•  Contemporary Communication Systems, First Edition by M
F Mesiya– Chapter 5
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See
Notes
(http://highered.mcgraw-hill.com/sites/0073380369/information_center_view0/)