7/31/2019 L22-FSK

1/37

Frequency shift keying

7/31/2019 L22-FSK

2/37

Outline

Introduction to FSK

Bit rate and baud rate

Transmitter

Bandwidth considerations Receiver

7/31/2019 L22-FSK

3/37

Voltage controlled oscillator

VCO

InputGiven as

voltage

OutputGot as change infrequency

7/31/2019 L22-FSK

4/37

Frequency shift keying

Consider a Voltage controlled oscillator

If the controlling voltage to VCO is binary input thenVCO output is FSK wave form

7/31/2019 L22-FSK

5/37

7/31/2019 L22-FSK

6/37

FSK wave form

7/31/2019 L22-FSK

7/37

Expression FM with message as

binary input

( )[ ]tftvfVtv mccfsk += )(2cos)(

Peak carrier

Amplitude (volts)

Carrier

Frequency

(Hertz) Message(1)

Frequency

Deviation (Hertz)

7/31/2019 L22-FSK

8/37

( )[ ]tftvfVtv mccfsk += )(2cos)(When modulating signal takes 1

( )[ ]

( )[ ]tffVtvand

tffVtv

ccfsk

ccfsk

=

+=

2cos)(

2cos)(

7/31/2019 L22-FSK

9/37

( )[ ]

( )[ ]tffVtv

and

tffVtv

ccfsk

ccfsk

=

+=

2cos)(

2cos)(

Carrier shifts between two frequencies : (fc+f) and (fc-f)

Also known as mark and space

Mark and space frequencies are separated by 2.f

7/31/2019 L22-FSK

10/37

Bit rate and baud rate

7/31/2019 L22-FSK

11/37

FSK transmitter Use VCO

Carrier frequency is chosen such that itfalls halfway between mark and spacefrequencies

A binary input changes VCO frequency Logic 1 changes the frequency to mark

Logic 0 changes the frequency to space

As input changes between logic 1 and 0,VCO output shifts between mark andspace frequencies

7/31/2019 L22-FSK

12/37

If VCO sensitivity is kl (hertz per volt)

Then f=vm(t)kl vm(t) takes only two values

7/31/2019 L22-FSK

13/37

Bandwidth

7/31/2019 L22-FSK

14/37

7/31/2019 L22-FSK

15/37

2smff

f

=

Can bandwidth be equal to 2.f?

It only tells how far frequency changes?

It does not tell how fast the frequencies shift

from one to another?

It is decided by modulating signal

7/31/2019 L22-FSK

16/37

Spectrum of sine wave

time

Time=0

Spectrum of a pureSine wave

frequency

7/31/2019 L22-FSK

17/37

Spectrum of pulsed sine wave

time

Time=0

Real world sine wave

Spectrum of a pulsedsine wavesinx/x function

frequency

7/31/2019 L22-FSK

18/37

Band width = |fs-fm|+2fb=2(f+f

b)

7/31/2019 L22-FSK

19/37

Bandwidth B= 2(f+fb)

Depends onfrequency deviation

Difference between

space and markfrequencies

And timing of inputdata bit

7/31/2019 L22-FSK

20/37

Problem Determine (a) peak frequency deviation

(b) minimum band width (c) baud rate foran FSK signal with mark frequency of 49KHz, space frequency of 51 KHz and an

input bit rate of 2 Kbps

7/31/2019 L22-FSK

21/37

Answer

2000

6)21(2

)(2

1

2

51492

==

=+=

+=

=

=

=

BitRatebaud

KHz

ffB

Bandwidth

KHz

fff

b

sm

7/31/2019 L22-FSK

22/37

Comparison with Carsons law For FM/PM

B=2(f+fm) For FSK

B=2(f+fb)

Difference

FM modulating signal frequency ischanged by FSK bit rate

7/31/2019 L22-FSK

23/37

Modulation index Peak frequency

deviation to modulatingsignal frequency ratio

f/fa

b

ms

b

ms

f

ff

f

ff

=

=

2

2

7/31/2019 L22-FSK

24/37

Problem What is the modulation index of the previous

problem?

12

5149

=

=

=b

ms

f

ff

7/31/2019 L22-FSK

25/37

FSK receiver Non-coherent

No carrier needed

Coherent

Local carrier needed

7/31/2019 L22-FSK

26/37

Non-coherent FSK demodulator

7/31/2019 L22-FSK

27/37

Operation principle Apply to two band pass filters

One BPF responds to mark frequency Another filter responds to space frequency

Find out the average power at the output

of filters Find out the DC level

Envelope detectors can be used to look for

DC level Compare DC levels using a comparator

7/31/2019 L22-FSK

28/37

Coherent FSK demodulator

Multiply the incoming signal by locallygenerated mark and space frequenciesi.e. carrier-1 is space and carrier-2 is mark

One of the multiplier will produce a strong DC

Signal (apply cos2t identity)

Difficult to produce local carrier

-1

-2

7/31/2019 L22-FSK

29/37

PLL FSK demodulator

7/31/2019 L22-FSK

30/37

7/31/2019 L22-FSK

31/37

Applications Low cost asynchronous modems

Not for high performance system

7/31/2019 L22-FSK

32/37

Transition between mark and

space frequencies If fs and fm are chosen randomly then we may

not have smooth phase transition from markto space or space to mark frequencies

Errors due to phase discontinuities

7/31/2019 L22-FSK

33/37

Continuous-phase FSK

Choose fs and fm in such a way so that when

input bit changes its state fs and fm waves are

at their zeroes

fb= 1 kbps(Fundamental

frequency is 0.5 kbps)

If fs and fm are chosento equal n(fb/2) with n

as odd number, thenwe will have smoothtransition between twofrequencies

7/31/2019 L22-FSK

34/37

Comments on CPFSK Bit error rate reduced

Disadvantage Transmission frequency may deviate

We need synchronization circuits

More expensive

7/31/2019 L22-FSK

35/37

Minimum shift keying When mark and space frequencies are

separated one-half of bit rate i.e. fs-fm=0.5fb it is called MSK

Modulation index=(fs

-fm

)/fb

=0.5fb

/fb

=0.5

7/31/2019 L22-FSK

36/37

1

0Tb

7/31/2019 L22-FSK

37/37