FM Transmitters

8/11/2019 FM Transmitters

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FM Modulators and Transmitters

Sections: 4-8


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Outline

FM modulators and transmitters

Frequency drifting; ppm

Basic component review


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Angle Modulation Classification

Direct PM Modulation Techniques

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 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

Direct FM

Indirect FM

See notes for dia rams


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FM Transmitters

Direct

Crosby utilizing AFC loop (automatic frequency control loop)

PLL- based

Indirect

Armstrong

FM transmitter using PM modulators


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FM Transmitters/Receiver Key Components (review)

Linear and non-linear devices

Discriminators

Frequency to amplitude

converters

Differentiators

Multipliers

Dividers

Mixers

Phase detectors

Oscillators

Tank circuits (LC)

Varactor diodes

Adders

Bandpass Limiters

Envelop detectors

VCOs

Filters

RC (LPF, HPF)

LRC (Bandpass Filter)

All-pass filters

Amplifiers

PLL

Super-heterodyning

Preemphasis and Deemphasis


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Filters

Devices which take input

waveform and modify its

frequency spectrum content

Use energy storage elements toobtain 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!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!less DRIFT!


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Filter Construction


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Filter ConstructionsLumped LC elementsare impractical above

300MHz Low Q

Active filters using OPAMPS are

limited to 500KHz opamps

have large open-loop gain!

Crystal filters using quartz crystal

elements are good up to 100 MHz, good

stability high Q!very good

performance!low drift!moreexpensive than RC


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FM Transmitters Crosby Direct FM Used for commercial broadcast-band transmitters

Uses an Automatic Frequency Control (AFC) Loop

Characteristics: 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 Frequencymodulator (fc)

Crystal

Oscillator

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

To the antenna


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FM Transmitters - Example Assume fc drift 40 ppm (40 x 5.1 = +/- 204Hz)!3672; thus, 18.36 KHz at the antenna!

In this case the open-loop drift is dfopen= N1.N2.dfc.

Master Frequency

modulator (fc)

CrystalOscillator

Non-linear mixer



to any DRIFT

Note:Kd is in V/Hz

Ko is in Hz/V

To the antennaMax. 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. C.

Example:A drift of 40 ppm at the master oscillator will translate to

[(40ppm x 5.1)/10^6] = +/- 204Hz=!f)

Similarly,!f=204 Hz![(!f/fc)*10^6] = 200 ppm


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FM Transmitters Example w/AFC Assume fc drift 40 ppm (40 x 5.1 = +/- 204Hz)

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. Much less than the previous case!

Master Frequency

modulator (fc)

Crystal

Oscillator

Non-linear mixer



to any DRIFT

Note:Kd is in V/Hz

Ko is in Hz/V



FCC is 2KHz


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FM Transmitters Example w/AFC

Master Frequency

modulator (fc)

CrystalOscillator

Typical Values:

Discriminators: +/- 100 ppm

DC correction voltage is added tothe modulator to adjust the fc due

to any DRIFT

Note:Kd is in V/Hz

Ko is in Hz/V



FCC is 2KHz

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)

Note that had we not used the

Mixer, the drift at the output ofthe discriminator would have

been 100ppm*30.6 = 3060 Hz

as opposed to100ppmx2 = 200Hz!!


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Direct FM Transmitter Using PLL

Generating WBFM(large !F) ; we assume the stability ofthe VCO (carrier) is not very good !we use PLL

The stability of the crystal oscillator is relatively good and

has high Q

Good stability;Lower frequency

ac

Phase detector

DCVoltage Correction

dc

ac

fc


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Indirect WBFM (Armstrong Method)

Uses NBFM to generate WBFM

The NBFM is generated using indirect method


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WBFM Using Indirect Method of Armstrong

Two blocks: Mixer and Modulator

Note that the output of NBFM is !

Utilizes heterodyning and up-conversion


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Heterodyned

Low Freq.Carrier / High Q

Must be 88-108 MHzFor commercial FM

Modulation index:!F/fm

fmMax 15KHz

Low !F=25 Hz

s(t)=Vccos(!ct+"(t))

sPM(t)=Vccos(!ct+Dpm(t))

sFM(t)=Vccos(!ct+ Dfm(#)d! #)

Can leadOr lag

Also called thebalanced modulator


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Heterodyned

Low Freq.Carrier / High Q

Must be 88-108 MHzFor commercial FM

Modulation index:!F/fm

fmMax 15KHz

Low !F=25 Hz

s(t)=Vccos(!ct+"(t))

sPM(t)=Vccos(!ct+Dpm(t))

sFM(t)=Vccos(!ct+ Dfm(#)d! #)

Can lead orlag

Also called thebalanced modulator

Questions:Calculate the min. modulation index.

How do you create NBPM?


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References

Leon W. Couch II, Digital and Analog CommunicationSystems, 8thedition, 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 (http://highered.mcgraw-hill.com/sites/0073380369/information_center_view0/)

SeeNotes

Documents

FM Transmitters