Amplitude Modulation Reception

RF Sectio

n

Bandpass Filter

Mixer/converter section

Bandpass Filter

IF Sectio

n

Bandpass Filter

AM detector

Bandpass Filter

Audio Section

Receive antenna

Speaker

RF Section – also called receiver front end used for detecting, bandlimiting & amplifying the received signalsMixer/converter – down converts the received RF frequencies to Intermediate

frequencies (IF) IF – frequencies that fall somewhere between RF and information freq.

IF section – amplifies and select the signalAM Detector – demodulates the AM wave and converts it to the original info. SignalAudio section – amplifies the recovered information.

Receiver Parameters:1. Selectivity – measure the ability of the receiver to accept a given band of frequencies

and reject all others - give the bandwidth of the receiver at the -3dB points or at two levels of attenuation such as -3 dB and -60 dB which ratio is known as shape factor shape factor - ratio between the -3dB and -60dB, measure the skirt steepness

dB

dB

B

BSF

3

60

Where SF – shape factor (unitless) - bandwidth 60dB below max signal level - bandwidth 3dB below max signal level

dBB 60

dBB 3

1 – ideal value of SF (if bandwidth at 03dB and -60dB points are equal

Example 1: -3dB bandwidth = 10kHz -60dB bandwidth = 20 kHz

210

20

kHz

kHzSF

Note: to achieve ideal SF, use more expensive and sophisticated circuits

12

34ffactor

ff

fshape

43

12factor shape

shape factor – measure of skirt steepness or skirt selectivity

example 2: 60 dB bandwidth = 12 kHz; 6 dB bandwidth = 3 kHz

the lower the shape factor, the better the skirt selectivity

ideal shape factor = 1

R

XQ L

Q

fBW r

MHz

xxLCf r 5

104.10110102

1

2

1126

7.15

20

10101052

20

2 66

xxfL

R

XQ L kHz

x

Q

fBW r 318

7.15

105 6

1251040

1053

6

x

x

BW

fQ r

R

XQ L 512.2

125

314

Q

XR L

- using tuned circuit, LC, determined by

• Bandwidth of a tuned circuit is measured by its selectivity

with fr – resonant frequency

ex. L = 10 μH, R = 20 Ω, C = 101.4 pF, find BW.

Solution:

upper and lower cut-off frequency = 318 kHz / 2 = 159 kHzupper fc = 5 + 0.159 = 5.159 MHz = f2

lower fc = 5 – 0.159 = 4.841 MHz = f1

BW = f2 – f1 = 5.159 – 4.841 = 318 kHz

To improve selectivity, let BW = 40 kHz

to increase Q, lower coil resistance larger wire gaugeso with

should be careful not to cut-off the sideband since they contain the information although we wanted to increase Q to increase selectivity

selectivity curve of a tuned circuit ideal receiver selectivity curve Practical

better selectivity can be achieved by:i.cascading tuned circuitii.using crystal or mechanical filters

2. Bandwidth Improvement: - increasing the bandwidth also increase thermal noise, so to improve the performance, decrease the bandwidth improve SNR difficult to construct narrow-band filter

Bandwidth improvement,

Hz)bandwidth( B and

)( B

IF

RF

IF

HzbandwidthRFwhereB

BBI

IF

RF

Noise Figure Improvement, BINF timprovemen log10

Example: Determine the improvement in the noise figure for a receiver with an RF bandwidth equal to 200kHz and an IF bandwidth equal to 10kHzSolution:

2010

200

kHz

kHzBI and dBNF timprovemen 1320log10

3. Sensitivity – minimum RF signal level that can be detected at the input to the receiver and still produce a usable demodulated information signal - also called receiver threshold

Usable information signal is arbitrary, a. For AM broadcast-band receivers:

section audioat )27(5.0P with 10 output dBmWdBSNR

b. For broadband microwave receivers:

power signal )7(5P and 40 output dBmmWdBSNR

Typical sensitivity:a. AM commercial broadcast:b. Two-way mobile radio receiver:

VySensitivit 50V10 to0.1between is VySensitivit

-To improve sensitivity, reduce noise level through decreasing temperature or reduce the bandwidth of the receiver

4. Dynamic Range – the difference between the minimum voltage and the maximum voltage level

- input power range over which the receiver is useful

For a single frequency input signal, the high power input signal limit is 1-dB compression point

1-dB compression point – output power when the RF amplifier response is 1 dB less than the ideal linear-gain response

DR = 100 dB highest possible value -Low DR can cause a desensitizing of the RF amplifiers and result in severe intermodulation distortion of the weaker input signals

5. Fidelity – measure of the ability of the communication system to produce, at the output of the receiver, an exact replica of the original source information

distortion – frequency, phase and amplitude variations from the original signal

3 forms of distortion:a. Amplitude – amplitude vs frequencies characteristics of a signal at the

output is different from those in the inputb. phase – not so much a problem in voice

2 kinds:i. absolute phase shift – total phase shift of the signal

- can be tolerated as long as all frequencies undergo the same amount of phase delay

ii. Differential phase shift – when different frequenciesundergo different phase shift- detrimental in cases such as PSK

c. frequency – resulting from harmonics and intermodulation distortion2nd order harmonic – problem in broadband 3rd order harmonic – caused by cross-product frequencies

ex.- can be reduced using square-law devices (can only

produced 2nd order harmonics and can be filtered out)

2121 2,2 ffff

6. Insertion Loss (IL) – ratio of the power transferred to a load with a filter in thecircuit to the power transferred to a load without the filter

in

outdB P

PIL log10 Where

outP Output power of the filter

inPInput power for frequencies that fall within the filter’s passband

7. Noise Temperature and Equivalent Noise Temperature

(hertz)bandwidth B

)/10 x (1.38constant sBoltzmann' K

(watts)power noise N

(kelvin) re temperatutalenvironmen T where

23-

KJ

KB

NT

Equivalent noise Temperature - indication of the reduction in the SNRas a signal propagates through a receiver.Typical values: for cool receivers

for noisy receivers

eT

oeT 20

oeT 1000

(unitless)factor noise F

(kelvin) re temperatutalenvironmen T

(kelvin) re temperatunoise equivalentT where)1( e

FTTe

AM Receivers

2 Types:1. Coherent – also called synchronous

- frequencies generated in the receiver and used for demodulation

are synchronized to oscillator frequencies generated in the

transmitter2. Noncoherent - also called asynchronous

- either no frequencies are generated in the receiver or the frequen-

cies used for demodulation are completely independent from the

transmitter’s carrier frequency- envelope detector

2 Kinds:

1. Tuned Radio-Frequency (TRF) - simple

-3 stage TRF receiver-The three RF amplifiers are used to filter and amplify the received signal, the detector converts RF signal to information, audio amplifiers amplify the information signals to a usable level

Disadvantages:a. Inconsistent bandwidth – unstable frequency selectivity is affectedb. Instability cause oscillation can be corrected by stagger tuning, different frequencies for each amplifiersc. Nonuniform gain nonuniform L/C ratios of the transformer-coupled tank circuits

Example: For an AM commercial broadcast-band receiver (535kHz to 1605kHz) with an input filter Q-factor of 54, determine the bandwidth at the

low and high ends of the RF spectrum.Solution:

kHzkHz

Q

fB 10

54

540

For low-end of AM, center frequency = 540kHz

HzkHz

Q

fB 630,29

54

1600

For high-end of AM, center frequency = 1600 kHz

Note: the 10kHz bandwidth at low end is the desired value but the 29,630Hz is not at the high-end since it will select 3 stations for an almost 30kHz bandwidth at high end. To find for the Q factorthat can make the high-end selective, chose B=10kHz and solvefor Q factor:

16010

1600

kHz

kHz

B

fQ

Q

fB However at Q=160, the low-end

bandwidth will beHz

kHz

Q

fB 375,3

160

1600

This is too selective which will block some info. signal

2. Superheterodyne receiver

- Above is a noncoherent superheterodyne receiver- has superior quality in terms of gain, selectivity and sensitivity

Heterodyne – mixing of two frequencies in a linear device or to translateone frequency to another using nonlinear mixing

Sections:1. RF section – has a preselector and amplifier

preselector – broad-tuned BPF with an adjustable center frequency tunedto the desired carrier frequency

- block image frequency, an unwanted frequency from enteringthe receiver

- reduces noise bandwidth - determines the sensitivity and noise figure of the receiver

amplifier – has several advantages:a. greater gain, thus better sensitivityb. improved image-frequency rejectionc. better signal-to-noise ratiod. better selectivity

2. Mixer/converter section –includes RF oscillator (local oscillator) and mixer/converter (first detector)

mixer – nonlinear device that convert RF to IF, performs heterodyningmost common IF (intermediate frequency) = 455 kHz

3. IF section – consists of IF amplifiers and BPF called IF stripIF < RF to attain high-gain stable amplifiers

4. Detector section - convert IF to original information signal (such as AF)- also called audio detector or second detector- can be a single diode or a PLL or a balanced demodulator

5. Audio amplifier section – amplifies the AF signal

Receiver Operation

Conversion of RF to IF to AF

RF in AM commercial broadcast: between 535 kHz to 1605 kHzIF in AM broadcast band: 450 kHz and 460 kHzIF in FM broadcast band: 10.7 MHz

Frequency conversion

RF signals are combined with LO frequency in a nonlinear device harmonics & cross-product frequencies sum & difference frequencies

IF filters tuned to the difference frequencies Preselector & LO frequencies adjustment are gang tuned two adjustments

are mechanically tied together so that a single adjustment will changethe center frequency of the preselector at the same time changethe LO frequency

For high-side injection (LO freq. above RF): where

For low-side injection (LO freq. below RF):

IFRFLO fff

IFRFLO fff )(

)(f

(Hz) freq. LO

RF

HzIFf

HzRF

f

IF

LO

Using high-side injection tuning the preselector to channel 2 (550kHz carrierfrequency), with 30-kHz passband allows chan 1, 2 & 3 (each with 10-kHzBW) and mixed it with LO freq=1005kHz to produce 455kHz IF

* although 3 channels are preselected, but since the bandwidth of the IF filteris only from 450 kHz to 460 kHz, only chan 2 can pass through IF filters

Example: For an AM superheterodyne receiver that uses high-side injection and has a local osc. frequency of 1355 kHz, determine the IF carrier, upper side freq. & lower freq. for an RF wave that is made up of a carrier and upper and lower side freq. of 900 kHz, 905 kHz & 895 kHz, respectively.

Referring to the figure below:

Solution:

kHzkHzkHzfff

kHzkHzkHzfff

kHzkHzkHzfff

usfRFLOlsfIF

lsfRFLOusfIF

RFLOIF

4509051355

4608951355

4559001335

)()(

)()(

*sideband inversion - RF upper side frequency translated to IF lower freq. and RF lower side freq. translated to IF upper freq.

Local Oscillator Tracking – the ability of the LO to oscillate either above or belowthe selected radio frequency carrier by an amount equal to the IF throughout the entire RF band

Image Frequency – any frequency other than the selected radio frequency carrier that, if allowed to enter a receiver and mix with LO, will producea cross-product frequency that is equal to the IF- equivalent to a second RF that will produce an IF that will interfere with the IF from the desired RF

IFRFim

IFLOim

fff

fff

2

IF RF LO Image

IFf IFf

IFf2

frequency

Image-Frequency Rejection Ratio (IFRR) – numerical measure of the abilityof the receiver to reject image frequency

imRFRF fffQIFRR im22 f where)1(

Example: For an AM broadcast-band superheterodyne receiver with IF, RF and LOfrequencies o 455kHz, 600kHz and 1055 kHz, respectively, determinea. Image frequencyb. IFRR for a preselector Q of 100

Mixer/converter

RF=600kHzImage=1510kHz

LO-RF=IF 1055-600=455 kHzImage-LO=IF 1510-1055=455 kHz

Local Oscillator1055 kHz

Solution: a.

b.

kHzkHzkHzf

orkHzkHzkHzf

im

im

1510)455(2600

15104551055

3.211113.21001

113.21510

600

600

1510

22

IFRR

kHz

kHz

kHz

kHz

Note: the closer the RF is to the IF, the closer the RF is to the image frequency

Example: For a citizens band receiver using high-side injection with an RF carrierof 27MHz and an IF center frequency of 455 kHz, determine:a. LO frequency b. image frequencyc. IFRR for a preselector Q of 100d. preselector Q required to achieve the same IFRR as that of the previous

example for RF carrier of 600 kHz Solution:

31670663.0

177.61IFRRQ .

77.60663.01001Q1IFRR

0663.09673.00337.191.27

27

27

27.91MHz .

91.27455455.27f .

455.2745527 .

2

2

2

2

2222

im

d

MHz

MHz

MHzc

MHzkHzMHzb

MHzkHzMHzfa LO

Note: for the two example, it is more difficult to prevent image frequencies for

high RF than low IF For higher IFRR, this will require high Q – difficult to achieve use high IF

Double Spotting – when receiver picks up the same station at two nearby pointson the receiver tuning dial

AM Receiver Circuits

RF Amplifiers Circuit

Characteristics of good RF:a. Low thermal noiseb. Low noise figurec. Moderate to high gaind. Low intermodulation and harmonic distortion (i.e. linear operation)e. Moderate selectivityf. High IFRR

(Hz)bandwidth - B

290 temp.absolute - T

/1.38x10constant sBoltzmann'-K

resistance load -R

V where4

o

23-

N

K

Kjoules

voltagenoiseRKTBV

o

N

Comparing the three kinds of RF amplifier configurations:1. Bipolar transistor – more nonlinear distortion2. DEMOS-FET – square law device which offers only second-order harmonic

- less nonlinear distortion3. Cascoded - high gain and less noise

Low-Noise Amplifiers (LNA) – generally includes 2 stages with impedance matching networks

1st stage moderate gain and minimum noise2nd stage high gain and moderate noise

Silicon BJT or FET Up to 2 GHzGaAs FETs more than 2 GHz

uses MESFET (Mesa Semiconductor FET), a metal-semiconductor junction at the gate called Schottky barrier

Example of LNA – IC RF Amplifier NE/SA5200

Mixer/Converter Circuits

From RF amplifier: From the LO:

Output of the mixer:

)2sin( tfRF)2sin( tfLO

tfftfftftfV LORFLORFLORFout 2cos2

12cos

2

12sin)2sin(

•Uses FET•Conversion loss – IF output signal amplitude lower than RF input signalKinds:1. Self – excited2. Separately excited mixer3. Single-diode mixer4. Balanced diode mixer5. IC mixer/oscillator – NE/SA602A

IF Amplifier circuit - operate at lower frequency - advantage: easy to design stable circuit

Inductive Coupling - coupling IF amplifiers

ps MIE Where - voltage magnitude induced in the secondary windings (volts) - angular velocity of the primary voltage wave (radians per second)

M - mutual inductance (henrys) - primary current (amperes)

sE

pI

2 kinds of transformers:1. single-tuned transformers2. Double-tuned transformers

Single - tuned Double-tuned

Gain for single-tuned = 0.707two-tuned = (0.707 x 0.707) = 0.5three – tuned = (0.707 x 0.707 x 0.707) = 0.353

12

1

1n

n BB

Over-all bandwidth of n single-tuned stages

Where - bandwidth of n single-tuned stages (Hz) - bandwidth of one single-tuned stage (Hz)

n - number of stages (any positive integer)

nB

1B

Bandwidth of n double-tuned stages

41

1

1 12

n

dtndt BB Where - bandwidth of n double-tuned stages (Hz) - bandwidth of one double-tuned stage (Hz)

n - number of stages (any positive integer)

ndtB

dtB1

Bandwidth reduction

AM DETECTOR CIRCUITS

AM Detector:

demodulate the AM signal recover or reproduce the frequencies of original signal relative amplitude

also called second detector• mixer/converter 1st detector

Kinds of Detector:1. Peak Detector - noncoherent

RC filter output difference frequenciesof LSF, Carrier and USF

Ex. Vout = 300 – 298 kHz= 2kHz

•Difference between AM modulator & AM demodulator:- AM modulator output is tuned to the sum frequencies (up-converter)- AM demodulator output is tuned to the difference freq.

(down-converter)

Percent of Modulation: a. no modulation peak detector is a filtered halfwave rectifier

output voltage = peak input voltage – 0.3Ab. With modulation increase in the variations in the output voltage

follows the shape of the AM envelope

Detector Distortion: based on the RC value short RC time constant is neededa. Rectifier distortion short RCb. Diagonal clipping long RC

RCmfm 2

112

(max)

For 70% modulation:

RCfm 2

1(max)

Automatic Gain Control Circuits adjusts the voltage gain of received signal toincrease weak signals and decrease strong RF signal that may overdrivethe receiver

Types of AGC:1. Simple AGC – monitors the received signal level and sends a signal back to the

RF and IF amplifiers to adjust their gain automatically

2. Delayed AGC – prevents the AGC feedback voltage from reaching the RF and IF amplifiers until the RF level exceeds a predetermined magnitude

3. Forward AGC – receive signal is monitored closer to the front end of the receiverand correction voltage is fed forward to the IF amplifiers

Squelch Circuit – to quiet a receiver in the absence of a received signal- keeps the audio section of the receiver turned off or muted RF signal the absence of a received signal

disadvantage – weak RF signals will not produce an audio output.

Noise Limiters & Blankers Noise limiters - use diode limiters or clippers in the audio section

- limiting or clipping threshold level is normally established just above the max. peak level of the audio signal.

Blanking ckt - detects the occurrence of a high-amplitude, short duration noise spike, then mutes the receiver by shutting off a portion of the receiver for the duration of the pulse

Alternate Signal – to – Noise Measurements

To measure sensitivity, measure the

A. Signal Plus Noise-to-noise reading (S+N)/NSteps:1. An RF carrier modulated 30% by a 1-kHz tone is applied to the input of the receiver2. Measure the total audio power at the receiver’s output S+N3. Remove the modulation from the RF signal4. Measure the total audio power again N5. Get the (S+N)/N value

B. Signal-to-notched noise ratioSteps:6. An RF carrier modulated 30% by a 1-kHz tone is applied to the input of the receiver7. Measure the total audio power at the receiver’s output S+N8. Insert 1-kHz notch filter between receiver output and the power meter9. Measure power (with noise)

meaningful only if the notch filter has extremely narrow bandwidth and introduces 40 dB or more of attenuation to the signal

Linear IC AM receivers:a. LM 1820b. LM 386 – LIC audio amplifier

Double-Conversion AM Receivers- for good image-frequency rejection use high IF leads to unstable IF- use two IF to solve the problem

First IF relatively high frequency for good image-frequency rejectionsecond IF low frequency for easy amplification

First IF = 10.625 MHz - pushes the image-freq. 21.25 MHz away from the desired RFSecond IF = 455 kHz

Net Receiver Gain – ratio between the modulated signal level at the output of the receiver (audio) to the RF signal level at the input of the receiver- difference between the audio signal level in dBm and the RF signal level in dBm

dBdBdB lossesgainsG

Wheregains = RF amplifier gain + IF amplifier gain + audio amplifier gainlosses = preselector loss + mixer loss + detector loss

Example: For an AM receiver with a -8dBm RF input signal level and the followinggains and losses, determine the net receiver gain and the audio signal level

Gains: RF amplifier=33 dB, IF amplifier = 47 dB, audio amplifier =25 dBLosses: preselector loss =3 dB, mixer loss = 6 dB, detector loss=8 dB

Solution:the sum of the gains is = 33 + 47 + 25 = 105 dBthe sum of the losses is = 3 + 6 + 8 = 17 dBnet receiver gain, G = 105 – 17 = 88 dBaudio signal level = -80 dBm + 88 dB = 8 dBm

Net receiver gain = includes only the components within the receiverSystem gain = includes all the gains and losses incurred by a signal as it propagates from

the transmitter output stage to the output of the detector in the receiverand includes antenna gains, transmission line and propagation losses