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CHAPTER 3:CHAPTER 3:AM RECEIVERSAM RECEIVERS

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TopicsTopics

• AM DemodulatorsAM Demodulators• Tuned Radio Frequency ReceiversTuned Radio Frequency Receivers• Superheterodyne ReceiversSuperheterodyne Receivers• RF Section and CharacteristicsRF Section and Characteristics• Path and Frequency ChangingPath and Frequency Changing• Intermediate Frequency (IF) & IF Intermediate Frequency (IF) & IF

AmplifierAmplifier• Detector and Automatic Gain Detector and Automatic Gain

Control (AGC)Control (AGC)

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AM DemodulatorAM Demodulator

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DemodulatorDemodulator

• Definition:Definition:– A demodulator is an electronic circuit used to recover the

information content from the carrier wave of a signal. The term is usually used in connection with radio receivers, but there are many kinds of demodulators used in many other systems. Another common one is in a modem, which is a contraction of the terms modulator/demodulator.

– For AM, the most popular demodulator used are the Envelop Detector and Product Detector.

RF Section

IF Section Demodulator

AF Stage

Figure 3.1 Receiver Block Diagram

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AMAM DEMODULATORDEMODULATOR

• Demodulation of DSBFC AMDemodulation of DSBFC AM– Simplest demodulator for DSBFC is Simplest demodulator for DSBFC is envelop detectorenvelop detector..– The recovery of the baseband signal undergoes the The recovery of the baseband signal undergoes the

process of rectifying the incoming signal, remove half of process of rectifying the incoming signal, remove half of the envelop, then use low pass filter to remove the high the envelop, then use low pass filter to remove the high frequency component of the signal.frequency component of the signal.

– Major advantage of AM = ease of the demod process.Major advantage of AM = ease of the demod process.– No need for synchronous demodulator.No need for synchronous demodulator.

Figure 3.2 Envelope detection of a conventional AM signal

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AM Demodulation AM Demodulation

• Demodulation of SSBSC AMDemodulation of SSBSC AM– For SSBSC, For SSBSC, product detectorproduct detector is used to recover the signal. is used to recover the signal.– The product detector multiplies the incoming signal by the

signal of a local oscillator with the same frequency and phase as the carrier of the incoming signal.

AM orSSB

ProductDetector

BeatFrequencyOscillator

(BFO)

Low PassFilter(LPF)

AudioOut

– After filtering, the original audio signal will result.

– This method will decode both AM and SSB, although if the phase cannot be determined a more complex setup is required.

Figure 3.3 Product Detector for AM and SSB

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Demodulator CircuitDemodulator CircuitDiode Demodulator• D1 rectifies the signal producing only positive

result.• The rectified signal will quickly charge C1.

• RC time constant of R1 and C1 is made long enough so that C1 does not have to discharge before the next pulse is received.

• Voltage across C1 follows the amplitude variation of carrier signal, not the carrier signal itself.

• Finally DC component is removed by C2.

Figure 3.4: Diode Demodulator

Figure 3.5: Transistor Demodulator

Transistor DemodulatorTransistor Demodulator• AM input is applied to AM input is applied to QQ11 base (common emitter). base (common emitter).• CC11 is the coupling capacitor is the coupling capacitor block DC from the block DC from the

input source.input source.• RR11 and and RR22 establish base bias and establish base bias and RR33 establish establish

collector bias.collector bias.• Transistor is biased-for-class B operation that Transistor is biased-for-class B operation that

allows positive pulses on the output.allows positive pulses on the output.• CC22 filter out carrier frequency. filter out carrier frequency.• CC33 removes DC component. removes DC component.

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AM RECEIVERSAM RECEIVERS

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Process in a ReceiverProcess in a Receiver

1.1. Signal received at antenna is very low, need Signal received at antenna is very low, need to amplify (LNA) and tuned to desired freq. to to amplify (LNA) and tuned to desired freq. to avoid interference.avoid interference.

2.2. Detector finds the info signal from the rf Detector finds the info signal from the rf signal.signal.

3.3. Further amplification needed to give it Further amplification needed to give it enough power to drive a loudspeaker.enough power to drive a loudspeaker.

RFAmplification

DetectorAudio

Amplification

Antenna

Speaker

Fig. 3.6 Simple block diagram of a receiver

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RECEIVER PARAMETERSRECEIVER PARAMETERS

• Parameters used to evaluate the ability Parameters used to evaluate the ability of a receiver to successfully demodulate of a receiver to successfully demodulate radio signal :-radio signal :-– SelectivitySelectivity– SensitivitySensitivity– Bandwidth Improvement FactorBandwidth Improvement Factor– Dynamic RangeDynamic Range– FidelityFidelity– Insertion LossInsertion Loss

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SelectivitySelectivity

• Ability of a receiver Ability of a receiver to accept a given to accept a given band of frequency band of frequency and reject all others.and reject all others.

• Obtained using Obtained using tuned circuits.tuned circuits.

• Selectivity Q, is Selectivity Q, is given by:given by:

• The bandwidth curve The bandwidth curve from the tuned from the tuned circuit is:circuit is:

• Higher Q the narrower Higher Q the narrower the BW and the better the BW and the better the selectivity. the selectivity.

• i.e. using the bandwidth i.e. using the bandwidth of the receiver at the – of the receiver at the – 3dB points 3dB points not not necessarily show necessarily show rejection characteristicrejection characteristic

• Most common used two Most common used two points; another at -60dB points; another at -60dB ratio of the two called ratio of the two called shape factor:shape factor:R

XQ L

( 60 )

( 3 )

dB

dB

BSF

B

Q

fBW r

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Example 3.1Example 3.1

1.1. High-Q tuned cct are used to keep the High-Q tuned cct are used to keep the BW narrow to ensure that only desired BW narrow to ensure that only desired signal is passed. Assumed that 10signal is passed. Assumed that 10H coil H coil with resistance of 20with resistance of 20 is connected in is connected in parallel with 101.4pF variable capacitor. parallel with 101.4pF variable capacitor. The circuit resonates at what freq.?The circuit resonates at what freq.?

2.2. What is the inductive reactance?What is the inductive reactance?3.3. What is the selectivity of the cct?What is the selectivity of the cct?4.4. The bandwidth of the tuned cct?The bandwidth of the tuned cct?5.5. Find the upper and lower cutoff Find the upper and lower cutoff

frequencies?frequencies?

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Answer Eg. 3.1Answer Eg. 3.1

1. 1.

2.2.

3.3.

4.4.

5. One half on each 5. One half on each side of center freq. side of center freq. of 5MHz is of 5MHz is 318.47/2 = 0.159 318.47/2 = 0.159 MHz.MHz.

MHzLC

fr 52

1

3142 LfX rL

7.15R

XQ L

kHzQ

fBW r 47.318

MHzf

MHzf

841.4159.05 Lower,

159.5159.05 Upper,

1

2

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• The minimum RF signal that can be detected at The minimum RF signal that can be detected at the input of a receiver and still produce a usable the input of a receiver and still produce a usable demodulated info signal.demodulated info signal.

• Also called receiver Also called receiver threshold.threshold.• Depends on the noise power present at the input Depends on the noise power present at the input

of the receiver, the receiver’s noise figure, of the receiver, the receiver’s noise figure, sensitivity of the AM detector and the bandwidth sensitivity of the AM detector and the bandwidth improvement factor of the receiver.improvement factor of the receiver.

• The best way to improve sensitivity is by reducing The best way to improve sensitivity is by reducing the noise level the noise level reduce temperature, reduce reduce temperature, reduce bandwidth of the receiver, or improving receiving bandwidth of the receiver, or improving receiving noise figure.noise figure.

SensitivitySensitivity

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• One way of reducing the noise level is by reducing the One way of reducing the noise level is by reducing the bandwidth of the signalbandwidth of the signal

• There is limitation for reducing the bandwidth to make sure There is limitation for reducing the bandwidth to make sure information is not lostinformation is not lost

• As RF bandwidth at the input of the receiver is higher than As RF bandwidth at the input of the receiver is higher than the IF bandwidth at the output of the receiver, reducing the the IF bandwidth at the output of the receiver, reducing the RF bandwidth to IF bandwidth ratio effectively reducing the RF bandwidth to IF bandwidth ratio effectively reducing the noise figure of the receiver, thus reducing the noisenoise figure of the receiver, thus reducing the noise

• Bandwidth improvement expressed mathematically asBandwidth improvement expressed mathematically as

• Noise figure improvement expressed asNoise figure improvement expressed as

NFNFimprovement improvement = 10 log BI= 10 log BI

RF

IF

BBI

B

Bandwidth Improvement Bandwidth Improvement FactorFactor

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Dynamic RangeDynamic Range

• The minimum input level necessary to The minimum input level necessary to discern a signal and the input that will discern a signal and the input that will overdrive the receiver and produce overdrive the receiver and produce distortion.distortion.

• Minimum receive level is a function of Minimum receive level is a function of front-end noise, noise figure and the front-end noise, noise figure and the desired signal quality.desired signal quality.

• Input that produce distortion is a function Input that produce distortion is a function of the net gain of the receiver.of the net gain of the receiver.

• 1 dB compression point is used for the 1 dB compression point is used for the upper limit for usefulness.upper limit for usefulness.

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FIGURE 3.7 Linear gain, 1-dB compression point, and third-order intercept distortion for a typical amplifier

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FidelityFidelity

• A measure of the ability of the A measure of the ability of the receiver to produce, at the output of receiver to produce, at the output of the receiver, an exact replica of the the receiver, an exact replica of the original source information.original source information.

• Any amplitude, frequency or phase Any amplitude, frequency or phase variations present in the variations present in the demodulated waveform that are not demodulated waveform that are not included in the original signal are included in the original signal are consider as distortion.consider as distortion.

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Insertion LossInsertion Loss• Loss occur when a signal enter the Loss occur when a signal enter the

input of the receiver.input of the receiver.• Parameters associated with the Parameters associated with the

frequencies that fall within the frequencies that fall within the passband of a filter.passband of a filter.

• Defined as the ratio of the power Defined as the ratio of the power transferred to the load with a filter in transferred to the load with a filter in the circuit to the power transferred the circuit to the power transferred to the load without a filter.to the load without a filter.

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Tuned Radio Frequency Tuned Radio Frequency ReceiverReceiver

•Tuned RF Receiver (TRF)Tuned RF Receiver (TRF)– It is the earliest and simplest It is the earliest and simplest

receiver design (Fig. 3.8).receiver design (Fig. 3.8).– TRF consist of RF amplifiers TRF consist of RF amplifiers

stages, detector and audio stages, detector and audio amplifier stages (Fig. 3.9)amplifier stages (Fig. 3.9)

– The received signal is tuned by LC The received signal is tuned by LC circuit to a passband centered at circuit to a passband centered at carrier frequency.carrier frequency.

– Selectivity pass only the desired Selectivity pass only the desired signal, others are rejected.signal, others are rejected.

– The tuned signal is boost up by an The tuned signal is boost up by an amplifier for better info detection.amplifier for better info detection.

– Signal info detection is made at Signal info detection is made at the demodulator and further the demodulator and further amplified for the speaker output.amplified for the speaker output.

DEMODULATOR

LC

Antenna

RecoveredOutput

Figure 3.8 Basic TRF receiver block diagram, showing simple structure.

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FIGURE 3.9 Noncoherent tuned radio frequency receiver block diagram

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TRF cont…TRF cont…

– TRF has highTRF has high sensitivity sensitivity – ability to drive the speaker – ability to drive the speaker to an acceptable level (to amplify).to an acceptable level (to amplify).

– Disadvantages :-Disadvantages :-• BW is inconsistent and varies with center BW is inconsistent and varies with center

frequency when tuned over a wide range of input frequency when tuned over a wide range of input frequencies frequencies selectivityselectivity changes, (means the changes, (means the extent to which a rx can differentiate between the extent to which a rx can differentiate between the desired signal and other signal).desired signal and other signal).

• Instability due to the large number of RF amplifier Instability due to the large number of RF amplifier all tuned to the same center frequency all tuned to the same center frequency oscillation.oscillation.

• Gain is not uniform over a wide range of frequency.Gain is not uniform over a wide range of frequency.

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Superheterodyne ReceiverSuperheterodyne Receiver

• Superhets was designed to overcome the Superhets was designed to overcome the problems in TRF.problems in TRF.

• Complex circuitry compared to TRF but excellent Complex circuitry compared to TRF but excellent performance under many conditions (Fig. 3.10).performance under many conditions (Fig. 3.10).

• Heterodyne mean:Heterodyne mean:– to mix 2 frequencies together in a nonlinear device or to mix 2 frequencies together in a nonlinear device or – to translate one frequency to another using nonlinear to translate one frequency to another using nonlinear

device.device.

• Superhets concept:Superhets concept:– Rx tunes to desired signal and converts the signal to Rx tunes to desired signal and converts the signal to

intermediate frequency via a signal mixing circuit.intermediate frequency via a signal mixing circuit.– Then IF signal is optimized to fully recovered the Then IF signal is optimized to fully recovered the

modulated info signal. modulated info signal.

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FIGURE 3.10 AM superheterodyne receiver block diagram

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Stages in SuperhetsStages in Superhets

• RF Stage:RF Stage:– Which takes the signal from the antenna and amplifies it Which takes the signal from the antenna and amplifies it

to a level large enough to be used in the following stages.to a level large enough to be used in the following stages.

• Mixer and Local Oscillator:Mixer and Local Oscillator:– Converts the RF signal to IF signal.Converts the RF signal to IF signal.

• IF Stage:IF Stage:– Further amplifies the signal and has bandwidth and Further amplifies the signal and has bandwidth and

passband shaping appropriate for the received signal.passband shaping appropriate for the received signal.

• Detector Stage:Detector Stage:– Recovers (demodulates) the info signal from the carrier.Recovers (demodulates) the info signal from the carrier.

• AF Stage:AF Stage:– The received signal is amplified for loudspeaker or The received signal is amplified for loudspeaker or

interconnections to comm systems.interconnections to comm systems.

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RF Stage and RF Stage and CharacteristicsCharacteristics

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RF Stage and RF Stage and CharacteristicsCharacteristics

• The RF section is a tunable circuit connected to the The RF section is a tunable circuit connected to the antenna.antenna.

• It is where the wanted signal is selected and the It is where the wanted signal is selected and the unwanted signal is rejected.unwanted signal is rejected.

• Some basic receiver does not have amplifier but for rx Some basic receiver does not have amplifier but for rx that has one is much more superior in performance.that has one is much more superior in performance.

• The main advantage having RF amplifiers are:The main advantage having RF amplifiers are:– Greater gain – better sensitivityGreater gain – better sensitivity– Improved image frequencyImproved image frequency– Improve SNRImprove SNR

• Two main characteristic of RF stage are:Two main characteristic of RF stage are:– Sensitivity – ability to amplify weak signalsSensitivity – ability to amplify weak signals– Selectivity – ability to reject unwanted signalsSelectivity – ability to reject unwanted signals

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Path and Path and Frequency Frequency ChangingChanging

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Path & Frequency Path & Frequency ChangingChanging

• Converter / Mixer (Fig. 3.11)Converter / Mixer (Fig. 3.11)– RF is down converted to IF, but shape of the envelope RF is down converted to IF, but shape of the envelope

remains the same remains the same info is conserved, bandwidth is info is conserved, bandwidth is unchanged.unchanged.

– Output of the mixer : infinite no. of harmonic and cross Output of the mixer : infinite no. of harmonic and cross product including product including ffRFRF, f, fLOLO, f, fRFRF + f + fLOLO , f , fRFRF – f – fLO.LO.

– LO is designed so that its frequency is always above or LO is designed so that its frequency is always above or below the desired RF carrier by an amount equal to IF below the desired RF carrier by an amount equal to IF center frequency.center frequency.

– ffLOLO is usually higher than is usually higher than ffRFRF because up conversion leads to a because up conversion leads to a smaller tuning range (smaller ratio of the maximum to smaller tuning range (smaller ratio of the maximum to minimum tuning frequency) minimum tuning frequency) much easier to design an much easier to design an oscillator that is tunable over a smaller frequency ratio.oscillator that is tunable over a smaller frequency ratio.

– If mixer and LO are in a single stage, it is called converter.If mixer and LO are in a single stage, it is called converter.– Common IF : 455 kHz.Common IF : 455 kHz.– Adequate selectivity because it is difficult to design sharp Adequate selectivity because it is difficult to design sharp

band bass filter if the center frequency is very high.band bass filter if the center frequency is very high.– Center frequency is fixed and factory-tuned Center frequency is fixed and factory-tuned effectively effectively

suppressed because of its high selectivity.suppressed because of its high selectivity.

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fi,fo

fo + fi

fo – fi or fi - fo

Tuned circuit orfilter

Mixer

fo

fi

Figure 3.11: Mixer input - output

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Intermediate Intermediate Frequency & Frequency & IF AmplifiersIF Amplifiers

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IF & IF AmplifiersIF & IF Amplifiers

• Intermediate FrequencyIntermediate Frequency– Sum or difference in the output of a mixer that Sum or difference in the output of a mixer that

enters the IF stage.enters the IF stage.– A down-converted frequency that carries the A down-converted frequency that carries the

information.information.

• IF amplifiersIF amplifiers– One or more stage(s).One or more stage(s).– Provide most gain and selectivity.Provide most gain and selectivity.– IF is much lower than RF IF is much lower than RF easier to design easier to design

and good sensitivity is easier to obtain with and good sensitivity is easier to obtain with tuned circuit.tuned circuit.

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• Image Frequency & Image Frequency & RejectionRejection – It is formed after the mixer It is formed after the mixer

circuitry.circuitry.– It is an image of the input It is an image of the input

frequency that enters the frequency that enters the mixer.mixer.

– Represented in two form: high Represented in two form: high side injection and low side side injection and low side injection.injection.

– The image is an equal distance The image is an equal distance from the LO frequency on the from the LO frequency on the other side of it from the signal.other side of it from the signal.

– An image must be rejected An image must be rejected prior to mixing, because it’s prior to mixing, because it’s indistinguishable and indistinguishable and impossible to filter out. impossible to filter out.

f i f LO f image

IF IF

f if LOf image

IFIF

Fig. 3.12 High-side Injection

Fig. 3.13 Low-side Injection

IFiimage

IFiimage

fff

fff

2

:side lowFor

2

:sidehigh For

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• Image Frequency Rejection Ratio Image Frequency Rejection Ratio – Is defined as the ratio of voltage gain at Is defined as the ratio of voltage gain at

the input frequency to which the receiver the input frequency to which the receiver is tuned to gain the image frequency.is tuned to gain the image frequency.

– Numerical measure of the preselector Numerical measure of the preselector ability to reject the image frequency.ability to reject the image frequency.

log20)(

1,, 22

dBIR

bandwidthBwhereBf

R

X

circuittunedoffactorQualityQ

fi

fs

fs

fsi

f

f

f

fratiorejectionThewhere

QIRRejectionImageThe

L

im

RF

RF

im

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Determine the image frequency for a standard Determine the image frequency for a standard broadcast band receiver using 455-kHz IF and tuned broadcast band receiver using 455-kHz IF and tuned to station at 620 kHz.to station at 620 kHz.

The first is determine the frequency of the LOThe first is determine the frequency of the LOThe LO frequency minus the desired station’s frequency of 620 The LO frequency minus the desired station’s frequency of 620 kHz should equal the IF of 455 KHzkHz should equal the IF of 455 KHzHence,Hence,

fLO – 620 kHz = 455 kHzfLO – 620 kHz = 455 kHzfLO = 620 KHz + 455 kHzfLO = 620 KHz + 455 kHzfLO = 1075 kHzfLO = 1075 kHz

Now determine what other frequency, when mixed with 1075 Now determine what other frequency, when mixed with 1075 kHz, yields an output component at 455 kHzkHz, yields an output component at 455 kHz

X – 1075 kHz = 455 kHzX – 1075 kHz = 455 kHzX = 1075 kHz + 455 kHzX = 1075 kHz + 455 kHzX = 1530 kHzX = 1530 kHz

Thus, 1530 is the Thus, 1530 is the image frequencyimage frequency in this situation. To solve in this situation. To solve the problem associated with image frequency, sometimes a the problem associated with image frequency, sometimes a technique known as double conversion is employed.technique known as double conversion is employed.

Example 3.2Example 3.2

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Detector And Detector And Automatic Gain Automatic Gain

ControllerController

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• Detector/DemodulatorDetector/Demodulator– to recover the original signalto recover the original signal– eg : diode detector eg : diode detector

• Audio amplifierAudio amplifier– to amplify the detected audio signal to be to amplify the detected audio signal to be

passed to the userpassed to the user• Automatic Gain Control Automatic Gain Control

– A dc level proportional to the received signal’s A dc level proportional to the received signal’s strength is extracted from the detector stage strength is extracted from the detector stage and fed back to the IF and sometimes to the and fed back to the IF and sometimes to the mixer and/or the RF amplifier.mixer and/or the RF amplifier.

– This is the automatic gain control (AGC) level, This is the automatic gain control (AGC) level, which allows relatively constant receiver which allows relatively constant receiver output for widely variable received signals.output for widely variable received signals.

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– The AGC help to maintain a constant output The AGC help to maintain a constant output voltage level over a wide range of RF input signal voltage level over a wide range of RF input signal levelslevels

– Without AGC, to not miss a weak station, you Without AGC, to not miss a weak station, you would probably blow out your speaker while a would probably blow out your speaker while a weak station may not be audible.weak station may not be audible.

– The received signal from the tuned station is The received signal from the tuned station is constantly changing as a result of changing constantly changing as a result of changing weather and atmospheric conditions.weather and atmospheric conditions.

– The AGC allows you to listen to a station without The AGC allows you to listen to a station without constantly monitoring the volume control.constantly monitoring the volume control.

Cont..Cont..

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Figure 3.14 Automatic Gain Control