03_AM Transmission and Reception

7/21/2019 03_AM Transmission and Reception

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Jan Doris C. Latada , ECE

Holy Cross of Davao College, Inc.



AM Transmission

Low-Level DSBFC AM Tx1

2

3

High-Level DSBFC AM Tx

Carrier Shift



Block Diagram of Low-Level AM

DSBFC Tx



MODULATING SIGNAL: acoustical transducers

(microphone, magnetic tape, cd or phonograph

record)

PRE-AMP: a sensitive class A linear voltage amplifier

with high input impedance. Use to raise the

amplitude of the source signal to a usable level whileproducing minimum nonlinear distortion.

MODULATING SIGNAL DRIVER: a linear amplifier

used to amplify the information signal to an

adequate level to sufficiently drive the modulator.

RF Carrier OSCILLATOR: common used is the crystal-

controlled oscillator for accuracy and stability.



BUFFER AMPLIFIER: a low-gain, high-input impedance

linear amplifier used to isolate the oscillator from the

high-power amplifier. Also provides a relatively constantload to the oscillator that reduces the occurrence and

magnitude of short-term frequency variations. (usually

emitter follower of IC op-amp)

MODULATOR: modulation takes place

INTERMEDIATE AND FINAL POWER AMPLIFIERS: either

class A or B push-pull modulators to maintain symmetry

in the AM envelope. COUPLING NETWORK: used for impedance matching of

the final power amp and the transmission line and

antenna.



Block Diagram of High-Level

AM DSBFC Tx



MODULATING SIGNAL & CARRIER: processed in the

same manner as in the low-level Tx in addition of

power amplifier.

HIGH LEVEL TX: The modulating signal and carrier

undergoes additional power amplification prior to

modulator stage to obtain 100% modulation. MODULATOR: serves as the final power amplifier,

usually a drain-, plate, or collector-modulated class C

amplifier. Function, (1) modulation circuit (2) class C

power amplifier for efficiency, (3) frequency up-

converter (translates the LF signal to RF signals to be

efficiently radiated from an antenna.)



Carrier Shift

Sometimes called upward or downward

modulation.

A form of amplitude distortion introduced

when positive and negative alternations in theAM signals are not equal (i.e. non-symmetrical

modulation).

May be either positive or negative.

It is an indication of the average voltage of

an AM modulated signals.



Carrier Shift

Positive Carrier

Shift

Negative CarrierShift



AM Reception

Block Diagram of Simple Rx1

2

3

4

Receiver Parameters

DSBFC Receivers

DSBFC Receiver Operations



AM Reception

A receiver must able to:

receive, amplify and demodulate an AM wave.

It must also be capable of “bandlimiting” the

total RF spectrum to a specific desired band of

frequencies. The selection process calledTUNING the receiver.



Block Diagram of Simple AM Rx



AM Demodulation

RF SECTION: the first stage of the receiver. Aka

receiver front end. Its function are detecting,

bandlimiting, and amplifying the received RF signals.

MIXER/CONVERTER: down converts the received RFfrequencies to intermediate frequencies (IFs)

IF SECTION: amplification and selectivity

AM DETECTOR: demodulates the AM wave andconverts it to the original information.

AUDIO SECTION: amplifies the recovered information



Receiver Parameters

Selectivity

Bandwidth Improvement

Sensitivity

Dynamic Range

Shape Factor

Fidelity

Insertion Loss



Receiver Parameters

SelectivityUsed to measure the ability of the receiver to

accept a given band of frequencies and reject

all others.

It is the degree of distinction made by the

receiver between the desired signal and

unwanted signals.Example:

commercial AM, BW = 10kHz; the receiver must limit

its BW to 10kHz.



Receiver Parameters

Selectivity

EXAMPLE:

A TRF receiver is to be designed with a single tuned

circuit using a 10µH inductor. Calculate the capacitance

range of the variable capacitor required to cover the

entire AM band (535-1605 kHz) and also calculate the

bandwidth at 540 kHz and 1600 kHz (assume Q = 110)

=

Where: fr = resonant frequency in Hz

=

2



Receiver Parameters

Shape Factor

EXAMPLE:

If a typical AM broadcast-band radio receiver has a -

3dB bandwidth of 10 kHz and a -60-dB bandwidth of 20

kHz, compute for the shape factor.

=(− )

(− )

Where: SF = shape factor (unitless)

B(-60 dB) = bandwidth 60 dB

below maximum signal level

B(-3 dB) = bandwidth 3 dB below

maximum signal level



Receiver Parameters

Bandwidth ImprovementDecreasing the BW reduces the noise by the

same proportion, thus increasing the S/N ratio

improving the system performance.

Where: BI = bandwidth improvementBRF = RF bandwidth (Hz)

BIF = IF bandwidth (Hz)

=



Receiver Parameters

Noise Figure Improvementthe corresponding reduction in noise figure

due to the reduction in bandwidth

EXAMPLE:

Determine the improvement in the noise figure for a

receiver with an RF bandwidth equal to 200 kHz and an IF

bandwidth equal to 10 kHz.

=



Receiver Parameters

SensitivityThe minimum RF signal that can be detected

receiver input and still produce a usable

demodulated information signal.

It is the ability of a receiver to reproduce

weak signals expressed in µV. Also called receiver

threshold.Typical Values (commercial AM):

S/N ratio – 10dB

Power output – 1/2W (27dBm)



Receiver Parameters

SensitivityTypical sensitivity for commercial AM is 50μV.

Factors:

– Noise power at the input – Noise Figure

– Sensitivity of the AM detector

– Bandwidth improvement factorNOTE:

The best way to improve the sensitivity of a Rx is

to reduce the noise level.



Receiver Parameters

Dynamic Range

The input power range over which the

receiver is useful.

The difference in dB between the minimum

input level necessary to discern a signal and the

input level that will overdrive the receiver and

produce distortion.

= ( )

()

= ( )

()



Receiver Parameters

Dynamic Range

EXAMPLE:

A receiver has a dynamic range of 90 dB. It

has 1.5 nW sensitivity (threshold). Determine the

maximum allowable input signal.



Receiver Parameters

Fidelity

The ability of a communications system to

produce an exact replica of the original source

information.

Any frequency, phase, or amplitude

variations that are present in the demodulated

waveform that were not in the originalinformation signal are considered distortion.



Receiver Parameters

Distortions: Phase distortion

– is not particularly important for voice transmission but can be

devastating to data transmissions.

Amplitude distortion

– occurs when the Amplitude-vs-Frequency characteristics of a

signal at the output of a receiver differ from those of the

original information signal.

– Result of non-uniform gain in amplifiers and filters.

Frequency distortion

– result of harmonic and intermodulation distortion caused by

nonlinear amplification



Receiver Parameters

Insertion Loss

Associated with the frequencies that fall

within the passband of a filter.

ratio of the power transferred to the load

with filter (Pout) to the power transferred to

the load w/o filter (Pin).

IL dB = 10log



DSBFC Receivers

Tuned Radio-Frequency (TRF) Receiver

Superheterodyne Receiver



DSBFC Receivers

Tuned Radio-Frequency (TRF) RxThe earliest and simplest design radio receiver

used in single-channel LF applications.

Ganged capacitors



DSBFC Receivers

Tuned Radio-Frequency (TRF) Rx

RF STAGE: 2 or 3 RF amplifier are required to

filter an amplify the received signals to a level

sufficient to drive the detector. Integrated with

transformer-coupled tank circuits

DETECTOR: converts RF signals directly to

information.

AUDIO STAGE: amplifies the information signals

to a usable level



DSBFC Receivers

Tuned Radio-Frequency (TRF) Rx

Advantages

Simple

High sensitivity

NOTE:

TRF Rx are seldom used, except for special-purpose single

station Rx.



DSBFC Receivers

Tuned Radio-Frequency (TRF) RxDisadvantages

Tracking errors – capacitances in the cascaded RF

amplifiers will always have differences in value no matter

how small, and this will affect the resonant frequency.

Inconsistent Bandwidth, resulting to poor Selectivity -

Caused by Skin Effect.

– @ high Frequencies, current flow is limited to the surface of theconductor.

– This reduces the effective area (A), and increases resistance (R),

and decreases Quality Factor (Q), thus affecting Selectivity.



DSBFC Receivers

Superheterodyne Receiver Widely used because of its gain, selectivity and

sensitivity characteristics.

invented by Major Edward Armstrong down converts the incoming RF signal to IF

signal before processing and before the

extraction of the information signal

NOTE:

“ Heterodyning” means mixing two frequencies in a

nonlinear device or translate one frequency to

another using nonlinear mixing.



DSBFC Receivers




DSBFC Receivers

Superheterodyne ReceiverPreselector:

a broad-tuned BPF withan adjustable centerfrequency

Provide enough initial bandlimiting to prevent aspecific unwanted RF called the image frequencyfrom entering the receiver.

Reduces the noise BW to the minimum level.



DSBFC Receivers

Superheterodyne ReceiverRF Amplifier:

Determines the sensitivity of the receiver.

Is the primary contributor of noiseOffers several advantages:greater gain, thus better selectivity

improved image-frequency rejection

better S/N ratio

better selectivity.



DSBFC Receivers




DSBFC ReceiversSuperheterodyne Receiver

Local Oscillator:

Linked to the preselector so that it

varies with the carrier frequency.Mixer/Converter:

A nonlinear device in which

heterodyning takes place

converts RF to IF frequencies.

Note:

Common IF in AM broadcast-band Rx

is 455kHz.




During the demodulation process in theSuperheterodyne receiver:

– The RF is converted to IF (frequency conversion)

– IF is converted to the source information

Intermediate Frequency (IF)

– a frequency to which a carrier frequency is shifted as

an intermediate step in transmission or reception

– created by mixing the carrier signal with a local

oscillator signal in a process called heterodyning,

resulting in a signal at the difference or beat

frequency.




Reasons for using IF

1. At very high (gigahertz) frequencies, signal processing

circuitry performs poorly. Active devices such as

transistors cannot deliver much amplification (gain). Soa high frequency signal is converted to a lower IF for

more convenient processing.

2. Without using an IF, all the complicated filters and

detectors in a radio or television would have to be

tuned in unison each time the frequency was changed,

as was necessary in the early tuned radio frequency

receivers.




Reasons for using IF

3. To improve frequency selectivity.

Selectivity is achieved through filtering. But at high

frequencies, the filter’s bandwidth increases

proportionately.

The narrower the bandwidth of the filter, the more

selectivity can be achieved.Thus, RF is converted to lower frequencies, IF, and

filtering is done at that frequency.




Frequency Conversion- In the Mixer/Converter, RF signals are

combined with the LO frequency in a nonlinear

device.- At the incoming RF signal is mixed with the LO

frequency, and the Mixer/Converter produces the

sum and/or difference of the two signals, which isthe IF.

- During this process, shape of the envelope

remains the same , and BW is unchanged.




Frequency ConversionThe adjustment for the center frequency of the

preselector and LO are gang tuned (mechanically

tied together).

High-side Injection or High-beat Injection

When LO is tuned above the RF

Low-side Injection or Low-beat Injection When LO is tuned below the RF

IF RF LO f f f

IF RF LO f f f

Where: fLO = local frequency (Hz) fRF= radio frequency (Hz)

fIF = intermediate frequency (Hz)





DSBFC Receivers Operations







IF Section:

Consists of a series of IF amplifiers and BPF often calledthe IF strip.

IF is always lower in frequency than the RF because it iseasier and less expensive to construct high-gain, stableamplifiers for the LF signals.



DSBFC Receivers

Superheterodyne ReceiverIF Section:

It provides most of the receiver gain andselectivity

The IF center frequency and BW are constant for

all stations and are chosen so that their frequencyis less than any of the RF signals to be received.

Low frequency IF amplifiers are less likely to

oscillate than their RF counterparts.



DSBFC Receivers

Superheterodyne ReceiverCommon Intermediate Frequencies (IF)

System IF

AM Broadcast 455 kHz

AM broadcast (automobiles) 262.5 kHz

FM broadcast 10.7 MHz

FM Two-way radios 21.4 MHz

Picture IF 41.25 MHz

Sound IF 45.75 MHz

Radar Tx/Rx 30 or 60 MHz

Satellite Rx 70 MHz



DSBFC Receivers OperationsImage-Frequency

Any frequency other than the selected RF

carrier that, if allowed to enter a receiver and mix

with the LO, will produce a cross-product frequency

equal to intermediate frequency.



DSBFC Receivers OperationsImage-Frequency

For high-side injection: For low-side injection: = +

= +

=

=



DSBFC Receivers Operations

Image-Frequency Rejection RatioMeasure of the ability of a preselector to reject

the image frequency.

221 Q IFRR

Where:

im

RF

RF

im

f

f

f

f

NOTE: If more than one tuned circuit is used the

total IFRR is the product of two ratios.







Detector Section:

Converts the IF signal back to

the original source information.called an audio detector or the

second detector

May be a single diode, phase-locked loop or balanced

modulator.






DSBFC Receivers

Superheterodyne ReceiverAudio Amplifier Section:

Comprises several cascaded

audio amplifiers thatamplifies the demodulated

information to a level that

will drive one or morespeakers.

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