Communication Systems, 5ebazuinb/ECE4600/Ch07_02.pdfCommunication Systems, 5e Chapter 7: Analog Communication Systems A. Bruce Carlson ... block diagram (b) amplitude response Figure

Communication Systems, 5e

Chapter 7: Analog Communication Systems

A. Bruce CarlsonPaul B. Crilly

© 2010 The McGraw-Hill Companies

Chapter 7: Analog Communication Systems

• Receiver block diagram design• Image frequency bands that may cause

spurious responses (more filter requirements)• Signal Multiplexing

– Frequency division (FDM) and – Time division (TDM)

• Phase-Lock Loops (PLL)


Dual Downconversion Receiver

• Classical Analog Radio Processing• Digital Processing

– 1st Generation – ADC after Demod– 2nd Generation – ADC before Demod– 3rd Generation – ADC 1st IF– Future – ADC earlier? 3

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Figure 7.1-5

Dual Conversion Superhet

Both LOs are typically high-side LOs

4


Figure 7.1-2

RF to IF-1 Output Waveforms

Image

High Side LO

Image Considerations

• Real mixing always places the signal of interest band and an image band into the IF bandwidth.

• To avoid interferences:– (1) Filter out energy in the image band before mixing– (2) Use a receiver technique that cancels or nulls energy

in the image band. (complex or quadrature mixing)

5

6

High and Low Side LOs

• Low Side Local Oscillator (LO < RF)

• High Side Local Oscillator (LO>RF)

LOfcfIFf

imagefIFfLOf cfimagef

IFLOimage fff

LOf cfIFf

imagefIFfLOfcf imagef

IFLOimage fff

Yellow – Freq BandBlue – SOIRed – ImageGreen – IF

7

Example 7.1-1a• IF @ 500 kHz• Fc/RF Band: 4.0 to 4.5 MHz• Low Side LO mixing 3.5 to 4.0 MHz

– 3rd harmonic present in LO (mixer more likely)(10.5 to 12 MHz)

• 1st Image Band: 3.0 to 3.5 MHz

• LO 3rd Harmonic: 10.5 to 12 MHz• 3rd Image Band Low: 10.0 to 11.5 MHz• 3rd Image Band High: 11 to 12.5 MHz

8

Example 7.1-1b• IF @ 500 kHz• Fc/RF Band: 4.0 to 4.5 MHz• High Side LO Mixing 4.5 to 5.0 MHz• 3rd harmonic present in LO (mixer more likely)

– (13.5 to 15 MHz)

• 1st Image Band: 3.5 to 4.0 MHz

• LO 3rd Harmonic: 13.5 to 15MHz• 3rd Image Band Low: 13.0 to 14.5 MHz• 3rd Image Band High: 14 to 15.5 MHz

Comparing High and Low Side LOs• High Side LOs can simplify filters.

– LOs at higher frequencies and LO harmonics further away– Images above band, not between band and IF

9

Receiver Techniques

1. Eliminating Images2. Keep all LOs “out of band” (and harmonics)

– not a frequency that is part of the RF band or IF band

3. Eliminate harmonic mixing products from being in-band or creating images

10

The concept:Eliminate any possible signal that could be strong enough to interfere with the “Signal-of-Interest” (SOI)

11


Figure 7.1-3

Direct conversion receiver

tf2costff2cosAtff2cosAty c2ccic1cc

tff22costf2cos2

A

tff22costf2cos2

Aty

2c2

cic

1c1c

tffAtffAtx ccicccc 21 2cos2cos

tfAtfAtycicc

LPF 21 2cos2

2cos2

One of the two sidebands is not wanted

txc

With upper and lower sidebands

ty tyLPF

Direct conversion receiver with opposite sideband (image) rejection

12Copyright © The McGraw-Hill Companies, Inc. Permission required

for reproduction or display.

Figure 7.1-4

Image Cancellation

13

Image Rejection Direct Conversion

tftffAtffAty ccciccc 2cos2cos2cos 21cos

tfAtfAtycicc

LPF 21cos 2cos2

2cos2

tffAtffAtx ccicccc 21 2cos2cos

tfAty csum 12cos

tftffAtffAty ccciccc 2sin2cos2cos 21sin

tfAtfAtycicc

LPF 21sin 2sin2

2sin2

tfAtfAtycicc

LPF 2190sin 2cos2

2cos2

tfAty cicdiff 22cos

Image Rejection Receiver

• Receive Mixer with 35dB Image Rejection• Adjustable-Gain LNA• Up to +2dBm Combined Receiver Input IP3• 4dB Combined Receiver Noise Figure• Low Current Consumption:• 23mA Receive• 9.5mA Oscillator• 0.5μA Shutdown Mode• Operates from

Single +2.7V to +4.8V Supply

14https://www.maximintegrated.com/en/products/comms/wireless-rf/MAX2440.html

Obsolete Part!

15


Figure 7.1-5

Double conversion receiver

Dual Conversion Superhet, typically using high side LOs

Double Conversion Receiver

• Additional IF stage, first stage with high IF for better image rejection.

• Put “adjacent channel” selectivity in the second IF stage BPF

• Additional gain• Add a frequency converter to an existing receiver• Can be subject to more spurious inputs

– Harmonics from 2 LOs– Image bands from both LOs must be considered


17

RFID Receiver Downconversion

• ISM Band Downconversion (902-928 MHz)– Only mixing and filters

shown

• High-side LOs– Synthesizer provides center

frequency selection

• IF filter sets bandwidth• LPF for ADC anti-aliasing• Convert to fs/4 for post-

ADC complex processing– Fs > 4 x fmax

18

Cascaded Gain

• Multiply the linear gain (loss) of each stage together– If gain in dB, add the gains (in dB) and subtract the losses

(in dB)

– If the mixers have loss instead of gain (passive mixers)

dBGdBGdBGdBGdBGdBG 2IFndMixer21IFstMixer1RFmodprede

dBGdBLdBGdBLdBGdBG 2IFndMixer21IFstMixer1RFmodprede

19

Noise Figure (Appendix A)

• The noise figure is a measure of the additional noise that is added by any circuit element.– Effective additional input noise …

ampin

Sin

in

Sin

out

in

NNGPG

NP

SNRSNRF

tx ty

in

amp

in

ampin

out

in

NN

NNN

SNRSNRF

1

Note: Noise occurs in the amplifier and shows up at the output, but for computations it is “easier” to define it all as additional, additive input noise.

20

Cascaded Noise Figure

• The noise figure is a measure of the additional noise that is added by any circuit element.– Effective additional input noise …

1

212

11

2

1

1 1111111G

FFFG

FN

NGN

NF

in

amp

in

amp

2amp1ampin12

Sin21

in

Sin

out

in

NNNGGPGG

NP

SNRSNRF

tx ty

in

ampampin

in

ampampin

out

in

NG

NNN

NGGNNNGG

SNRSNRF 1

21

21

2112

21

Basic Receiver

• RF Filter removes images• Low Noise Amplifier• Mixer to IF• IF BPF sets the system BW• Mixer to baseband• Baseband LPF to remove

mixing products

dBGdBGdBGdBGdBGdBGdBG LPFndMixerBPFIFstMixerstAMPBPFRFeD 211Pr

ndMixerBPFIFstMixerstAMPBPFRF

LPF

BPFIFstMixerstAMPBPFRF

ndMixer

stMixerstAMPBPFRF

BPFIF

stAMPBPFRF

stMixer

BPFRF

stAMPBPFRFeD

GGGGGF

GGGGF

GGGF

GGF

GFFF

21111

2

111

11Pr

11

111

tf2cos 1LO

tx c

Bandpass Filter

tf2cos 2LO

tx M

Amplifier Lowpass Filter

Demod

Bandpass Filter

Tuning

tx eDPr

22

Thermal Noise Power

• Modeled as additive white Gaussian noise (AWGN)

– Where N is the noise power– κ is Boltzmann’s constant– T is absolute temperature in degrees Kelvin– B is the bandwidth in Hertz

BTN

HzK/dBW6.228

refIEEEK290T0

21e00.429023e38.1TN 00

Hz/dBm174Hz/dBW204N0

Section 9.3, p. 412

23

Receiver Operating Characteristics

• Sensitivity – minimum input value• Dynamic Range – usable signal range • Selectivity – filter out adjacent noise and

interference• Adjacent Channel Interference (ACI) Rejection

and Image Rejection• Noise Figure

Building a performance diagram for a software radio

RF Input to ADC input

24

FM Radio Design Diagram• FM receiver (88-108 MHz)• 200 kHz BW• 12-bit ADC with 10-bit

performance• Multiple signal environment• SOI detection threshold• ROC

– Sensitivity -103 dBm– Dynamic Range 41 dB– Gain 63 dB– NF 10 dB– Selectivity: based on IF filter– ACI: filter attenuation at n

channels away (n x 200 kHz)

25


Scanning Spectrum Analyzer

a) block diagram (b) amplitude responseFigure 7.1-6

26

Spectrum Analyzer Considerations

• Resolution Bandwidth – The bandwidth of the signal power measurement– Change it and the noise power must change!– IF Filter setting defines the RB

• Video Bandwidth – The filter prior to displaying the scan on the screen– Baseband filter setting

• Sweep time must support the filters!– typically adjusts with changes in RB

27

Multiplexing

• Combining multiple signals into a wider bandwidth system for transmission– Typically multiplex in time or frequency

• TDM time division multiplexing• FDM frequency division multiplexing

– For time multiplexing, “PAM sampling”, bandwidth based on PAM pulse periods

– For frequency multiplexing, bandwidth is the sum of all the frequency stacked elements plus their guard bands

Multiplexing Methods

• Frequency division multiplexing (FDM)• Time division multiplexing (TDM)

• Quadrature-carrier multiplexing or quadrature amplitude modulation (QAM) – complex signals

• Code division multiplexing (see Chap. 15)• Spatial multiplexing

– Antenna direction– Signal polarization


Time-Division Multiplexing (TDM)

29Stallings, Wireless Communications & Networks, Second Edition, 2005 Pearson Education, Inc. ISBN: 0-13-191835-4

• Interleaved signals in times that occupy assigned time slots– 6 time slots shown;

therefore, 6-TDM

Frequency-Division Multiplexing (FDM)

30Stallings, Wireless Communications & Networks, Second Edition, 2005 Pearson Education, Inc. ISBN: 0-13-191835-4

• Signal frequency bands stacked together, but transmitted as one wider bandwidth signal– 6 frequency bands

shown– Similar to 6 adjacent

radio stations using one transmitter

31


Figure 7.2-1

FDM Transmitter

Guard Bands: allow for filter transition bands

• Transmitter

• Receiver

32

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Figure 7.2-2

FDM Receiver

Guard Bands: allow for receiver filter transition bands

Cross Talk: unwanted energy from adjacent FDM channels

Post-De-FDM Baseband Bandwidths:• Signal Passband + both guard bands

33


Figure 7.2-3

FDMA satellite repeater system

Nominal 36 MHz BW Links• 1200 Voice Channels or• 400 channels of 64 kbps or• 16 channels of 1.544 Mbps each or• One 50 Mbps data stream

34


(a) transmitter (b) baseband spectrumFigure 7.2-4

FM stereo multiplexing

FDM-FM• FDM baseband

• FM Signal

kHzBRF 250

MHzfMHz C 10888

35


Figure 7.2-5

FM stereo multiplex receiver

See problems 6.1-4 & 6.1-5

kHzBIF 250

36


Figure 7.2-6

Quadrature-Carrier Multiplexing

tftxtftxAts ccc 2sin2cos 21

A special case of FDM“Orthogonal” Signals at the same frequency

37

Quadrature Carrier Math

tftftxAtftxAty cccI 2cos2sin2cos 21

tftxAtftxAtx ccc 2sin2cos 21

tftftxAtftxAty cccQ 2sin2sin2cos 21

0

121 2sin

2cos

2

txAtxAtxAtyILPF

0

221 2cos

2sin

2

txAtxAtxAtyQLPF

Two signals multiplexed on the same frequency.

What happens if the receiver carrier is not perfectly synchronized or, for example, 90 deg off?

38

Quadrature FDM Channels

• Each of the frequency bands or channels may have signal that are in quadrature. – Doubling the channel capacity– Phase synchronization of the receiver with the received

waveform is required!

– A precursor to Orthogonal Frequency Division Multiplexing (OFDM)

39

Time-Division Multiplexing (TDM)

Stallings, Wireless Communications & Networks, Second Edition, 2005 Pearson Education, Inc. ISBN: 0-13-191835-4

40


Analog signal and corresponding PAM signal: Figure 6.2-1

TDM – Insert more signals!

Signal #1

Signal #2

41


(a) block diagram (b) waveformsFigure 7.2-7

TDM system

PAM inputs multiplexed in time

PAM outputs for reconstruction

42


Figure 7.2-9

TDM synchronization markers

43


(a) TDM transmitter with baseband filtering (b) baseband waveformFigure 7.2-10

TDM Continuous Transmitter

44


Cross talk in TDM

Sufficient time must be available for the receiver to transition from one PAM level to the next. Otherwise adjacent signals effect each other – defined as cross talk.

dBinTBAAk g

ctct

5.54log10 10

Guard time

B = 3dB Bandwidth

gct TBAA 2exp

Figure 7.2-12

B = 3dB Bandwidth

TDM/PPM with guard time

• Time-division-multiplexing multiple pulse-position-modulated signals.– Pulse center +/- t0

45

time

2

0t 0t

gT

MTs

0t2 gT

0t

2

2

0t 0t

gT

MTs

46

TDM/PPM with guard time

Forward and backward maximum locations, t0

22 00

tTtMT

gs

g

s TMTt

21

0

4th ed. Figure 7.2-13 is wrong: Fix - Tg is between the signal pulses

5th ed. Figure 7.2-13 is wrong: to is on opposite side of pulses

47

Comparing TDM and FDM

• TDM based on time slots– Overlap in frequency domain– Bandwidth of total signal defines cross talk– Well supported by digital circuitry, multiple mux rates– Synchronization concerns– Time based receiver, therefore less dependent upon

filter performance or ripple

• FDM based on frequency slots– Overlap in time domain– Guard bands and filtering determine cross talk– Simple frequency assignments

48

TDM, FDM, TDD and FDD

• TDM: Time division multiplex• FDM: Frequency division multiplesFor Two-way Communications :• TDD: Time division duplex

– One side talks and then the other side talks– Note that only one transmission can happen at a time on

the signal frequency being used

• FDD: Frequency division duplex– Two different frequencies are used so both sides can

talk simultaneously

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Communication Systems, 5ebazuinb/ECE4600/Ch07_02.pdfCommunication Systems, 5e Chapter 7: Analog Communication Systems A. Bruce Carlson ... block diagram (b) amplitude response Figure