ECE 4371, Fall, 2014 Introduction to Telecommunication Engineering/Telecommunication Laboratory Zhu Han

Department of Electrical and Computer Engineering Class 5 Sep. 10 th , 2014

FM Modulator and Demodulator

  Review of FM FM modulator – Direct FM – Indirect FM  FM demodulator – Direct: use frequency discriminator (frequency-voltage converter) – Ratio detector – Zero crossing detector – Indirect: using PLL    Superheterodyne receiver FM broadcasting and Satellite radio Project 1

Review of last class

      PLL and math LPF Instantaneous frequency FM and PM

i

 1 2 

d



i dt



A c



A

c

 



f t c

 2 

k f

 0

t m

f t

c



p d

  VCO Modulation index Narrow band FM characteristics Carson’s rule

B T f

AM (envelope): max | 1

a

FM (frequency):   max |

f m f

, 2

f m



2( 



1)

f m

ECE 4371 Fall 2008

FM Direct Modulator

 Direct FM – Carrier frequency is directly varied by the message through voltage-controlled oscillator (VCO) – VCO: output frequency changes linearly with input voltage – A simple VCO: implemented by variable capacitor – Capacitor Microphone FM generator

FM Direct Modulator cont.

 Direct method is simple, low cost, but lack of high stability & accuracy, low power application, unstable at the carrier frequency Capacitance changes with the applied voltage: 

C

0  

Cm t

LC oscillator frequency:

i

 2  1

LC

 2     2

f

0

f

0  1

LC

0 

f

0 

C

  2

C

0  1  

C

2

C

0 1

LC

0     

O t

2  Modern VCOs are usually implemented as PLL IC  Why VCO generates FM signal?

Indirect FM

 Generate NBFM first, then NBFM is frequency multiplied for targeted Δf.

 Good for the requirement of stable carrier frequency  Commercial-level FM broadcasting equipment all use indirect FM  A typical indirect FM implementation: Armstrong FM  Block diagram of indirect FM

Indirect FM cont.

 First, generate NBFM signal with a very small β 1 

A c

cos(2 

f t

1 )   1

A c

sin(2 

f t

1 ) sin(2 

f t m

)

Indirect FM cont.

 Then, apply frequency multiplier to magnify β – Instantaneous frequency is multiplied by n – So do carrier frequency, Δf, and β – What about bandwidth?

f i

right 

n f i

left

Analysis of Indirect FM



A c

 

f t

1  2 

k f

 0

t m

( )

f t i f

  max |

W f

2. Nonlinear device outputs frequencies:

nf

1 

v t

 ( )  2 2 ( )  

n n

   ,

f

3. Bandpass filter select new carrier

f c



A c

 

nf

1 

nf t

1  2 

nk f

 0

t m

   

f t i



nf

1 

f

  max |

W f

1

Armstrong FM Modulator

 Invented by E. Armstrong, an indirect FM  A popular implementation of commercial level FM  Parameter: message W=15 kHz, FM s(t): Δf=74.65 kHz.

 Can you find the Δf at (a)-(d)?

Solution: (a)

f

14.4 Hz. (b)

f

(c)

f

1.036 kHz. (d)

f

 74.65 kHz.

FM Demodulator

 Four primary methods – Differentiator with envelope detector/Slope detector 

FM to AM conversion

– Phase-shift discriminator/Ratio detector 

Approximates the differentiator

– Zero-crossing detector – Frequency feedback 

Phase lock loops (PLL)

FM Slope Demodulator

 Principle: use slope detector (slope circuit) as frequency discriminator, which implements frequency to voltage conversion (FVC) – Slope circuit: output voltage is proportional to the input frequency. Example: filters, differentiator freqency in s(t) voltage in x(t) 10

Hz

 20

Hz



FM Slope Demodulator cont.

 Block diagram of direct method (slope detector = slope circuit + envelope detector) 

A c

 

f t c

 2 

k f

 0

t m

 

f t i



f c

 Let the slope circuit be simply differentiator: 1 ( )  

A c

2 

f c

 2 

f

  

f t c

 2 

k f

 0

t m

( ) 2 

f c

 2 

o

 

A c f

s o (t) linear with m(t)  

f

Slope Detector

Magnitude frequency response of transformer BPF.

Bandpass Limiter

 A device that imposes hard filter limiting on a signal and contains a that suppresses the unwanted products (harmonics) of the limiting process.

 Input Signal

v i

(

t

) 

A

(

t

) cos  (

t

) 

A

(

t

) cos(

w c t



k f



t

 

m

(

a

)

da

)    Output of bandpass limiter

v o

(

t

)  4  cos  (

t

)  Bandpass filter

e o

(

t

)  4  cos(

w c

Remove the amplitude variations 1 3

t

cos 3  (

t

)  

k f

 

t

 1 5 cos 5  (

t

)  

m

(

a

)

da

)

Ratio Detector

 Foster-Seeley/phase shift discriminator – uses a double-tuned transformer to convert the instantaneous frequency variations of the FM input signal to instantaneous amplitude variations. These amplitude variations are rectified to provide a DC output voltage which varies in amplitude and polarity with the input signal frequency. – Example  Ratio detector – Modified Foster-Seeley discriminator, not response to AM, but 50%

Zero Crossing Detector

FM Demodulator PLL

 Phase-locked loop (PLL) – A closed-loop feedback control circuit, make a signal in fixed phase (and frequency) relation to a reference signal 

Track frequency (or phase) variation of inputs



Or, change frequency (or phase) according to inputs

– PLL can be used for both FM modulator and demodulator 

Just as Balanced Modulator IC can be used for most amplitude modulations and demodulations

PLL FM

 Remember the following relations – Si=Acos(w c t+  1 (t)), Sv=A v cos(w c t+  c (t)) – Sp=0.5AA

v [sin(2w c t+  1 +  c )+sin(  1  c )] – So=0.5AA

v sin(  1  c )=AA v (  1  c )

Phase and Frequency Acquisition

Superheterodyne Receiver

 Radio receiver’s main function – Demodulation  get message signal – Carrier frequency tuning  select station – Filtering  remove noise/interference – Amplification  combat transmission power loss  Superheterodyne receiver – Heterodyne: mixing two signals for new frequency – Superheterodyne receiver: heterodyne RF signals with local tuner, convert to common IF – Invented by E. Armstrong in 1918.

Advantage of superheterodyne receiver

 A signal block (of circuit) can hardly achieve all: selectivity, signal quality, and power amplification  Superheterodyne receiver deals them with different blocks  RF blocks: selectivity only  IF blocks: filter for high signal quality, and amplification, use circuits that work in only a constant IF, not a large band

FM Broadcasting

  The frequency of an FM broadcast station is usually an exact multiple of 100 kHz from 87.5 to 108.5 MHz . In most of the Americas and Caribbean only odd multiples are used. f m =15KHz,  f=75KHz,  =5, B=2(f m +  f)=180kHz  Pre-emphasis and de-emphasis – Random noise has a 'triangular' spectral distribution in an FM system, with the effect that noise occurs predominantly at the highest frequencies within the baseband . This can be offset, to a limited extent, by boosting the high frequencies before transmission and reducing them by a corresponding amount in the receiver.  Block diagram and spectrum  Relation of stereo transmission and monophonic transmission

FM Stereo Multiplexing

Fc=19KHz.

(a) Multiplexer in

transmitter of FM stereo.

(b) Demultiplexer in

receiver of FM stereo.

Backward compatible For non-stereo receiver

TV FM broadcasting

 f m =15KHz,  f=25KHz,  =5/3, B=2(f m +  f)=80kHz  Center f c +4.5MHz

 Eye cells structure

XM vs. Sirus

 Project 1 – AM/FM/Real voice – Due 9/29/14

Project 1