FM Modulators and Transmitters - Sonoma State · PDF file• FM modulators and transmitters ... • Uses an Automatic Frequency Control ... Direct FM Transmitter Using PLL

FM Modulators and Transmitters

Sections: 4-8

Outline •  FM modulators and transmitters •  Frequency drifting; ppm •  Basic component review

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 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 •  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

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 Lumped LC elements are 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 Qà very good performance à low drift à more expensive than RC

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 Frequency modulator (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

FM Transmitters - Example •  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)

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 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. C.

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

[(40ppm x 5.1)/10^6] = +/- 204Hz=Δf) Similarly,

Δf=204 Hz à [(Δf/fc)*10^6] = 200 ppm

FM Transmitters – Example w/AFC •  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

Master Frequency modulator (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 Max. frequency deviation allowed by FCC is 2KHz

FM Transmitters – Example w/AFC

Master Frequency modulator (fc)

Crystal Oscillator

Typical Values: Discriminators: +/- 100 ppm

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 Max. frequency deviation allowed by 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 of the discriminator would have been 100ppm*30.6 = 3060 Hz as opposed to 100ppmx2 = 200Hz!!

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

Good stability; Lower frequency

ac

Phase detector

DC Voltage Correction

dc

ac

fc

Indirect WBFM (Armstrong Method) •  Uses NBFM to generate WBFM •  The NBFM is generated using indirect method

WBFM Using Indirect Method of Armstrong •  Two blocks: Mixer and Modulator •  Note that the output of NBFM is à •  Utilizes heterodyning and up-conversion

WBFM Using Indirect Method of Armstrong

Heterodyned

Low Freq. Carrier / High Q

Must be 88-108 MHz For commercial FM

Modulation index: ΔF/fm

fm Max 15KHz

Low ΔF=25 Hz

s(t) =Vc cos(ωct +θ(t))sPM (t) =Vc cos(ωct +Dpm(t))

sFM (t) =Vc cos(ωct + Dfm(τ )d∫ τ )

Can lead Or lag

Also called the balanced modulator

WBFM Using Indirect Method of Armstrong

Heterodyned

Low Freq. Carrier / High Q

Must be 88-108 MHz For commercial FM

Modulation index: ΔF/fm

fm Max 15KHz

Low ΔF=25 Hz

s(t) =Vc cos(ωct +θ(t))sPM (t) =Vc cos(ωct +Dpm(t))

sFM (t) =Vc cos(ωct + Dfm(τ )d∫ τ )

Can lead or lag

Also called the balanced modulator

Questions: Calculate the min. modulation index. How do you create NBPM?

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 •  (http://highered.mcgraw-hill.com/sites/0073380369/information_center_view0/)

See Notes