Communication TheoryUnit 3Lecture-1AM Transmitters Block schematicAM broadcast transmitter

AM TransmittersDefinition: Any set-up that transmits AM signal is called as AM transmitter.Two types 1) Low level AM transmitter: If the amplification process takes place after the modulation, then it is called low level transmitter.

2) High level AM transmitter: If the amplification process takes place before the modulation, then it is called low level transmitter.

AM Transmitters Block SchematicLow level transmitter:

High level transmitter: ModulatorInput message signalInput carrier signalClass B amplifierInput message signalInput carrier signalClass B AmplifierModulatorAntennaAntenna

AM Broadcast TransmitterIt is an example for high level transmitterUsed for transmission of speech, talk, entertainment programmes, etc.Power output is in the range of 1KW to 100KWBlock Diagram:Master oscillatorBuffer AmplifierFrequency MultiplierDriver AmplifierModulatorAF voltage amplifierClass B Push Pull AmplifierMicrophone

AM Broadcast Transmitter

Master Oscillator (MO):The circuit must provide a carrier of specific frequency.Frequency should be drift freeThe generated frequency must be easily adjustableMust withstand changes in supply voltageCommonly used master oscillators are Clapp, Colpitt, Hartley, etc.Buffer Amplifier:To provide isolation between the MO and the subsequent stagesBuffer amplifier is normally a storage circuitIf the subsequent stages suddenly draw huge current from MO, then the frequency stability will be affected. Buffer amplifier is used to prevent this situationA tuned class A voltage amplifier is used as buffer amplifierHence, even if there is a change in MO, it will not affect the rest of the circuits.

AM Broadcast TransmitterFrequency Multiplier:MO cannot generate very high frequency since this will affect the frequency stability .Hence, MO will generate only sub-multiple of the frequency which will be further increased with frequency multipliers.Also called as harmonic generators.Normally, it is a class-C tuned amplifierDriver Amplifier:Also called as Intermediate Power Amplifier (IPA)To increase the power of the output signalMore than one stage of amplification is also used.It is a class C amplifierModulator:Any one of the modulators can be used. All the above mentioned five blocks are called as RF Chain

AM Broadcast Transmitter The second part of the AM transmitter is called as AF chainIt consist of a microphone (message signal) and a voltage amplifierFinally, the amplified signals are supplied to the modulator along with the carrier.The modulated AM signal is transmitted through the transmitting antennaFor safety purpose (to avoid over-heating), the following cooling systems are employed (a) Forced air cooling (b) Water cooling

Communication TheoryLecture -41) TRF receiver

TRF receiverTRF = Tuned Radio FrequencyFirst framed receiverBlock Diagram:

1st RF amplifier2nd RF amplifierDetectorAudioamplifierPoweramplifierLoudSpeaker

TRF receiver1st RF amplifier:Normally, a tuned amplifierIt is used to select only the desired frequency rangeAmplifies the voltage of the desired received signal2nd RF amplifier:A tuned amplifierIt will reject the unwanted frequencies escaped from the 1st RF amplifier

TRF receiverDetector:Any one of the demodulator (detector) can be used. a) Square law detector b) Envelope detector c) Synchronous detectorAudio amplifier:Increases the voltage of the received signalPower amplifier:Increases the power level of the received signal

Advantages: Simple to designDisadvantages:Instability in gainWide variation in bandwidthQUESTIONS??Why RF amplifier?

Communication TheoryLecture -51) Superhetrodyne receiver

Superhetrodyne receiverSuper Supersonic signals (very high frequency)Hetro differentDyne Beating (or) mixingThus, in superhetrodyne receiver, the incoming signal is modified before given to the user.Single superhetrodyne : one time frequency changesDouble superhetrodyne : two time frequency changesBlock Diagram:

RF amplifierMixerDetectorAudioamplifierPoweramplifierLoudSpeakerIF amplifierLocal oscillatorGanged tuning

fo

fs

fo- fs

Superhetrodyne receiverRF amplifier:Normally, a tuned amplifierIt is used to select only the desired frequency rangeAmplifies the voltage of the desired received signalMixer:Generates both the sum and the different of input frequencies.One input is signal frequency (fs) and the other input is local oscillator frequency (fo).But, only the difference is chosen and it is called as Intermediate Frequency (IF). fi = fo fsThe RF amplifier, mixer and local oscillator are joined together by ganged tuning and hence the difference will be always (fi) Even if the incoming signal changes, the local oscillator frequency also changes correspondingly to maintain constant IF.

Superhetrodyne receiverMixer:IF is constant for specific applications. E.g: for audio signals, IF is 455 KHz.IF amplifier:It is a tuned voltage amplifier.It increases the voltage of the input signal (amplifies) only if the frequency is equal to IF.Detector:Any one of the demodulator (detector) can be used. a) Square law detector; b) Envelope detector; c) Synchronous detectorAudio amplifier:Increases the voltage of the received signalPower amplifier:Increases the power level of the received signal

Superhetrodyne receiverAdvantages:Uniform gain and uniform bandwidthImproved stabilityImproved selectivityQuestions??Why the frequency is fo fs and not fs - fo?Why IF is fo fs and not fo + fs ?

Communication TheoryLecture -61) Image frequency signals and their rejection

Image frequency signalsCase (1): fi = fo-fsFor an audio signal, if fs = 545KHz, then fo adjust itself to 1000KHz to make sure fi=455 KHz.Case (2): fi =fs-foSuppose, if noise affects the input signal and fs increases to 645KHz, then fo can also adjust itself to 200KHz to make sure fi=455 KHz.You do not know that noise has affected the signal. But on seeing 455KHz at the output of mixer, you may think that fs=645KHz which is actually wrong

Image frequency signalsDefinition: Any unwanted signals which enter the receiver by generating the required IF is called as image frequency signals(fs) = fs+2fiWays of rejecting image frequency signals:Increasing the number of tuned circuitsHigh IFIncreasing the quality factor (Q)

Communication TheoryUnit 3Lecture-7Frequency conversionIF amplifier

Frequency ConversionDefinition: A frequency changer is a device that mixes the incoming signal and generates the sum and difference of the input signals. It is also called as mixer.Two types 1) Additive Mixer:If both the input signals are given to the same electrode, then it is called as additive mixer. 2) Multiplicative mixerIf the two input signals are given to different electrodes, then it is called as multiplicative mixer.

Schematic of mixersAdditive Mixer:

MixerOutputmessageCarrier

Operation of Additive MixerAny non-linear device can be called as mixer.It is same as that of square law demodulator.The input expression is:

LetThe output expression is

From the output, we can see that both sumand difference frequencies are generated.

IF AMPLIFIER

Schematic of mixersMultiplicative Mixer:

MixerOutputmessageCarrier

IF AMPLIFIERIF is a tuned voltage amplifierIt amplifies only if the incoming frequency is IFCircuit diagram: Refer the lab observationChoice of IF:If IF is too high, adjacent channel rejection will be poorIf IF is too high, tracking becomes difficultIf IF is too high, selectivity becomes poorIf IF is too low, image signal rejection will be poorIf IF is too low, gain will be low.Values of IF:TV receivers = 26 MHz to 45 MHzFM receivers = 10 MHzMicrowave receivers = 30 MHzAudio signals = 455 KHz

Communication TheoryLecture -8Characteristics of receiverMerits and demerits of receivers

Characteristics of receiverSensitivity:Measure of the receivers ability to receive the weak signal.Usually in the range of few microvoltsSelectivity:Capability to pick up a desired signal while rejecting other signals.Adjacent channel (or) second channel selectivity:The ratio of the adjacent (or) second channel input to the desired input is called as adjacent channel selectivity

Characteristics of receiverFidelity:The ability of the receiver to exactly reproduce the original signal is called as fidelity.Signal to noise ratio:Signal to noise ratio (SNR) is the ratio of the signal power to the noise power at the receiver output.Usually measured in dB.MERITS AND DEMERITS OF RECIVERS:Refer TRF receiver and superhetrodyne receiver

Communication TheoryLecture -9Tracking and alignment

Tracking and AlignmentThe local oscillator frequency (fo) track the input signal frequency (fs) to ensure constant IF in superhetrodyne receiver.The input signal (fs) is always a frequency range. (e.g audio signal is 20Hz to 20KHz).To track the lower value of (fs), the formula is:

To track the upper value of (fs), the formula is:

Tracking and AlignmentSelection of Cmax and Cmin are crucial for proper tracking.The ratio of Cmax : Cmin should be close to each other for practical feasibility.But, Cmax and Cmin are calculated based on the formula:

Cp = Padder capacitor (series capacitor)CT = Trimmer capacitor (shunt capacitor)Thus, proper selection of Cp and CT which leads to proper selection of Cmax and Cmin for proper tuning is called as tracking.Types:Two point tracking: If the minimum and maximum ranges are tracked, then it is called as two-point tracking.b) Three point tracking: If the minimum, maximum and mid ranges are tracked, then it is called as three-point trackingTracking error: The error occurred during tracking (non-constant IF) is called as tracking error