Post on 23-Mar-2020
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 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