# Review Notes in Modulation for ECE Board Exam

Compiled review notes in Modulation as one topic in ECE Board Exam taken from different sources including but not limited to Communications books, past Board Exams Questions, Journals and the Internet.

This is the Review Notes in Modulation as one topic in ECE Board Exam taken from various sources including but not limited to past Board Examination Questions in Electronic System and Technologies (EST), Communications Books, Journals and other Communications References. This particular Coaching Notes in Communications Engineering has random Questions and Answers in random topics. Make sure to familiarize this review notes to increase the chance of passing the ECE Board Exam.

### Summary of Important Modulation Review Notes

50 dB is the maximum sideband suppression value using filter system

Modulation index determines the number of sideband components in FM

H3E transmit only one sideband

Baseband frequency produces sidebands on FM

Spectrum analyzer displays the carrier and the sidebands amplitude with frequency to frequency

Mixer is also known as converter

A3H emission transmit the lower sideband and half of the upper sideband

An FM receive signal vary in frequency with modulation

The process of impressing intelligence on the carrier is called modulation

Spectrum analyzer is an electronic instrument used to show both the carrier and the sidebands of a modulated signal in the frequency domain

Amplitude is varied at the carrier by the intelligence during modulation in an AM system

The difference between the RF carrier and the modulating signal frequencies is called the LSB

Buffer stage in a radio transmitter isolates the oscillator from the load

The frequency of the unmodulated carrier in FM system is center frequency

The ratio of maximum deviation to the maximum modulating frequency is called deviation ratio

A carrier signal has a frequency of 20 kHz and above

In a FM system, if modulation index is doubled by halving the modulating frequency, there will be no effect on the maximum deviation

Armstrong system is considered as an indirect method of generating FM

To generate an SSB or DSB signal one must use a circuit known as balanced modulator

An interfering signal with a frequency equal to the received signal plus twice the IF is called image frequency

A3E – double sideband full carrier emission type

R3E – single sideband reduced carrier emission type

J3E – a single sideband suppressed carrier emission type

B8E – independent sideband emission type

C3F – vestigial sideband emission type

H3E – single sideband full carrier emission type

G3E – phase modulation emission type

Better fidelity is not an advantage of SSB over AM

The advantage of a high level modulated AM transmitter is higher value of operating power

The advantage of a low-level modulated AM transmitter is less audio power required

Interference to other radio services is the bad effect caused by overmodulation in AM transmission

Selectivity of a radio receiver refers to its ability to reject an unwanted signal

F3E emission is frequency modulation

AM transmission power increase with modulation

Capture effect locks the FM receiver to a stronger signal

The highest percentage of modulation for AM is 100%

In FM, the Carson’s Rule states that the bandwidth is equal to twice the sum of the modulating frequency and frequency deviation

The carrier swing of an FM transmitter when modulated by 75% is 112.5 kHz

Frequency modulation – the modulation system inherently more resistant to noise

Subcarriers that are arranged so that the channels occupying adjacent frequency bands with some frequency space between them is known as guard bands

Modulation of an RF carrier results in multiple channels, smaller antennas, and directional propagation

Modulation is a process which occurs in the transmitter

Demodulation is a process which occurs in the receiver

Buffer amplifier part of the transmitter that protects the crystal oscillator from “pulling”

The amplitude of a sine wave which is modulated by a musical program will be complex, contain fundamental frequencies, and contain harmonic frequencies

The result of the gain level being too high for signals entering the modulator is distortion and splatter

Amplitude modulation causes the amount of transmitter power to increase

When the amplitude of the modulating voltage is increased for AM, the antenna current will increase

A second modulating tone having the same amplitude but a different frequency is added to the first at the input to the modulator. The modulation index will be increased by a factor of sqr(2)

Unwanted sidebands in SSB equipment can be suppressed by phasing method and filter method

Envelope detection is concerned with the process of rectification

Diagonal clipping in envelope detection will result in distortion

Product detection requires the process of heterodyning

A sine wave which is coherent with carrier has identical frequency and phase angle

Frequency modulation and phase modulation are collectively referred to as angle modulation

In FM the change in carrier frequency is proportional to amplitude of the modulating signal

A louder sound, when generating the modulating waveform for FM, will cause a greater frequency deviation

Varactor diode – a device whose capacitance is deliberately made to be a function of the applied voltage

A reactance modulator is one method of obtaining direct FM

VCO – a device, now available in IC form, is useful for direct FM and as one element in the phase-locked loop

Multiplication is a frequency change process, whereby the phase deviation and frequency deviation are multiplied by some fixed constant

Foster-Seeley discriminator – a circuit that has the function of demodulating the frequency-modulated signal

The ratio detector is superior to the slope detector because it is less sensitive to noise spikes and interference causing AM

One implementation of a pulse-averaging discriminator is a triggered multivibrator

Two different signals can be coherent if they have the same frequency

A quadrature detector requires that the inputs are coherent

In a phase-locked loop, the VCO is the abbreviation for Voltage-controlled oscillator

LSB and USB – the output of a balanced modulator

If the modulation index of an AM wave is changed from 0 to 1, the transmitted power is increased by 50%

RF carrier is not a baseband signal of modulation

If the unmodulated level peak carrier amplitude is doubled in an AM signal, the percent modulation is 100

Balanced modulator circuit when inserted in the equipment suppressed the carrier

The carrier of a 100% modulated AM wave is suppressed, the percentage power saving is 66.66%

If the modulation index of an AM wave is doubled, the antenna current is also doubled, the AM system being used is J3E

100% modulation in AM means a corresponding increase in total power by 50%

A single-tone amplitude modulated wave has 3 components

A carrier signal has constant peak amplitude

The modulating system is frequency modulation if the modulating frequency is doubled, the modulation index is halved, and the modulating voltage remains constant

The modulation index of an FM signal if its modulating frequency is doubled is one-half the original index

A3E – standard way of designating AM

Discriminator is the circuit used to detect frequency modulated signal

Baseband is an information signal that is sent directly without modulating any carrier

Both frequency and phase modulation utilize angle modulation

Bandwidth – it is the width of frequencies within the spectrum occupied by signal and used by the signal for conveying information

H3E transmit only one sideband

Continuous modulation is a kind of modulation in which the modulated wave is always present.

Pulse modulation – a type of modulation in which no signal is present between pulses

Coefficient of modulation is the amount of amplitude change present in an AM waveform

Carrier shift is a form of amplitude distortion introduced when the positive and negative alternations in the AM modulated signals are not equal

The advantage of phase modulation over direct FM frequency modulation is that the oscillator is crystal-controlled

If the spectrum is shifted in frequency with no other changes, this is known as frequency translation

Balanced modulator – a device which is capable of causing frequency translation

If the frequency of each component in a signal spectrum is increased by the same fixed amount, this is known as frequency translation and up conversion

Any device to be used as a frequency multiplier must be nonlinear

Push-push – a particular amplifier circuit used for frequency doubling

Frequency division is useful in the implementation of a frequency synthesizer

Balanced modulator can be used as a phase detector

A particular frequency synthesizer contains only a single crystal. This synthesizer is known as indirect

A recognizable feature of a CW transmitter is keyed transmitter, power amplification, and frequency generation

The term “pulling” refers to the change of the crystal oscillator frequency by loading

When frequency modulation is achieved by initial phase modulation, this is called indirect FM

A disadvantage of direct FM is the need for AFC

Direct FM can be achieved by a reactance tube modulator and a varactor diode

A receiver in which all RF amplifier stages require manual tuning to the desired RF is called TRF

It is often necessary to precede the demodulator by amplifier stages in a receiver because of weak antenna signals

A serious disadvantage of the TRF receiver is the bandwidth variations over the tuning range

Modulator is not part of a superheterodyne receiver

R-F amplifier element will not be found in every superheterodyne receiver

Mixer element of a superheterodyne receiver must be nonlinear

The change of the modulated carrier frequency from the original RF to the I-F of the superheterodyne receiver is known as frequency translation

The key to achieving receiver sensitivity is the reduction of internal noise

In comparing the S/N ratio for the input to the receiver with the S/N ratio for the output, the latter is smaller

Noise figure – the characteristic of a receiver that specifies the self-generated noise

The ratio of the superheterodyne receiver response at the desired carrier frequency to that at the image frequency is called the image rejection ratio

The core of an IF transformer usually contains powered iron

Shape factor is a measure of skirt steepness

AGC is the function which tends to maintain the sound volume level of a voice receiver nearly constant for a large signal strength range

Squelch – the function which tends to silence the receiver in the absence of transmitter carrier

Noise blanker device is incorporated in a communications receiver to reduce impulse noise

If the input to a detector stage is an amplitude-modulated (A3E) IF signal then the output from the stage is the audio voice information

In a capacitive type, reactance-tube modulator connected across an oscillator tune circuit, a more negative voltage on the grid of the reactance tube will cause an increase of the oscillator frequency

The limiting condition for sensitivity in a communications receiver is the noise floor of the receiver

When a communications receiver is tuned to a strong signal, the AGC bias is measured and found to be zero. The fault cannot be caused by an open circuit in the AGC’s filter capacitor

Cross-modulation interference – the term used to refer to the condition where the signals from a very strong station are superimposed on other signals being received

The limiter stage of an FM receiver limits the amplitude of the IF signal to the required level

Motorboating (low-frequency oscillations) in an amplifier can be stopped by connecting a capacitor between the B+ and lead ground

Crossmodulation – an effect in which, the modulation of an unwanted signal is transferred to the desired carrier

Leads should be kept as short as possible in radio circuit so that stray coupling is minimized

4 voice transmissions can be packed into a given frequency band for amplitude-compandored single-sideband systems over conventional FM-phone systems

Neutralization of an RF amplifier stage can be necessary in order to prevent the generation of spurious oscillations

The ability of a communications receiver to perform well in the presence of strong signals outside the band of interest is indicated by blocking dynamic range

RF amplifier, mixer, IF amplifier, and AF amplifier are stages that are common to both AM and FM receivers

Filter ringing occurs during CW reception if too narrow a filter bandwidth is used in the IF stage of a receiver

IF amplifier stage mainly determines a communication receiver’s sensitivity

The main advantage of FM over AM is better signal-to-noise-ratio

Low-level modulation – an amplitude modulation created in an amplifier before the final RF stage

Receiver desensitizing can be reduced by ensuring good RF shielding between the transmitter

In a narrow-band FM system, the deviation ratio is commonly one and the highest audio frequency is generally limited to 3,000 Hz

A3C – a type of emission is produced when an amplitude modulated transmitter is modulated by a facsimile signal

The noise generated which primarily determines the signal to noise ratio in a VHF (150 MHz) marine band receiver is in the receiver front end

Cross-modulation in a receiver can be reduced by installing a filter at the receiver

F3E is the emission designation for FM telephony

The cause of receiver desensitizing is the presence of a strong signal on a nearby frequency

In a phase-modulated signal (indirect FM), the frequency deviation is directly proportional to the carrier amplitude only

An RF stage precedes the mixer stage in a superheat receiver. One advantage of including this RF stage is letter rejection ratio

Bandwidth and noise figure are two factors that determine the sensitivity of a receiver

An undesirable effect of using too-wide a filter bandwidth in the IF section of a receiver is that the undesired signals will reach the audio stage

FM receiver – a system containing a limiter stage, a discriminator, and a de-emphasis circuit

The limiter stage of an FM receiver prevents any amplitude modulation of the IF signal

High selectivity occurs when the degree of coupling between a receiver’s RF stage is loose

A carrier is phase modulated by a test tone. If the amplitude and the frequency of the tone are both doubled, the amount of deviation is multiplied by four

2.4 kHz is the degree of selectivity desirable in the IF circuitry of a single-sideband receiver

The component most apt to break down in the radio circuit is the resistor

The base in an RF amplifier is grounded in order to avoid the requirement of neutralizing the stage

The AM detector performs two basic functions in the receiver. It rectifies and filters

A varactor diode can be used in direct FM modulator circuit, AFC circuit in a direct FM transmitter and in phase-modulator circuit

Receiver interference is not reduced by including an insulating enclosure around the receiver

Television is the emission C3F

Limiter stage in an FM receiver is responsible for drastically reducing the effect of static noise during the reception of a signal

The letter “SSSC” stand for single sideband, suppressed carrier

For many types of voices, the ratio of PEP-to-average power during a modulation peak in a single-sideband phone signal is approximately 2.5 to 1

In most mixers, the oscillator frequency is higher than the carrier frequency of the input signal.

Features of a transmitter’s buffer stage include improvement in frequency stability of the oscillator

A3F – type of emission produced when an amplitude modulated transmitter is modulated by a television signal

A pi network is a network consisting of one inductor and two capacitors

A G3E FM-phone signals is produced with a reactance modulator on the oscillator

Installing resistive spark plugs is a way of eliminating auto interference to radio reception

The carrier in an AM transmitter is the transmitter’s output signal when the modulation is present

Final IF amplifier, which also acts as a limiter stage, feeds the discriminator of an FM receiver

In an FM receiver, the discriminator stage has the IF signal as input and the audio signal as output

Capture effect - the loudest signal received is the only demodulated signal

A double-sideband phone signal can be generated by modulating the plate voltage of a class-C amplifier

Pre-emphasis is used in FM transmitters to improve the signal-to-noise ratio of high modulating frequencies

The result of cross-modulation is that the modulation of an unwanted signal is heard on the desired signal

Television is emission F3F

F3C emission is produced when a frequency modulated transmitter is modulated by a facsimile signal

Two AM transmitting antennas are close tighter. As a result the two modulated signals are mixed in the final RF stage of both transmitters. The resultant effect on the other station is intermodulation interference

Desensitizing – the term used to refer to the reduction of receiver gain caused by the signal of a nearby station transmitter in the same frequency band

Bandwidth of emission and occupied bandwidth is the bandwidth occupied by the carrier, both sidebands and harmonics

A class-C RF amplifier is collector amplitude modulated and its average dc level collector current does not change. This means a normal condition

Amplitude of the modulating signal determines the percentage modulation of an FM transmitter

Deviation ratio of an FM transmitter is the ratio of the maximum frequency swing to the highest modulating frequency

The main purpose of the beat frequency oscillator (BFO) is to generate an output, whose frequency differs from the IF by 1 kHz

Normally, a linear class B RF power amplifier operates with a bias approximately equal to projected cut-off

The purpose why an RF amplifier is operated under linear class-B conditions (as opposed to class-C) is to amplify an AM signal

Cross-modulation interference – the term used to refer to the condition where the signal from a very strong station are superimposed on other signal being received

Peak negative voltage is the amplitude of the maximum negative excursion of a signal as viewed on an oscilloscope

FM and double sideband AM – type of emission that suffer most from selective fading

In an FM-phone signal, percentage of modulation is the ratio between the actual frequency deviation to the maximum frequency deviation

Capture effect is used to refer to the reception blockage of one FM-phone signal by another FM-phone signal

A receiver selectivity of 10 kHz in the IF circuitry is optimum for double-sideband AM type of signal

If the envelope of modulation is constant in amplitude this means zero-modulation

Amplitude modulation is the same as linear mixing

The negative half of the AM wave is supplied by the tuned circuit in a diode modulator

Having the carrier vary a resistance can produce AM

Amplitude modulators that vary the carrier amplitude with the modulating signal by passing it through an attenuator network is the principle of variable resistance

PIN diode is used to produce AM at very high frequencies

Demodulator circuit recovers the original modulating information from an AM signal

Envelope detector is the most commonly used amplitude demodulator

Balanced modulator circuit generates the upper and lower sidebands and suppresses the carrier

Lattice modulator is a widely used balanced modulator

In a diode ring modulator, the diode act like switches

The output of a balanced modulator is DSB

The principal circuit in the popular 1496/1596 IC balanced modulator is a differential amplifier

The most commonly used filter in SSB generators uses crystals

In the phasing method of SSB generation, one sideband is canceled out due to phase shifting

A balanced modulator used to demodulate a SSB signal is called a product detector

Frequency translation is done with a circuit called a mixer

Mixing for frequency conversion is the same as linear summing

Lower complexity and cost is not a major disadvantage of FM over AM

The primary disadvantage of FM is its excessive use of spectrum space

Noise is primarily high-frequency spikes

The receiver circuit that rids FM of noise is the limiter

The Am signals generated at a low level may only be amplified by Class C amplifier

SSB means Single sideband with suppressed carrier

Filter – a circuit used to select the desired output from a mixer

DSB is the output of a balanced modulator

The acronym SSSC refer to Single sideband, suppressed carrier

Demodulation process occurs in the receiver

BFO is usually used to demodulate SSB or CW signal

Diode detector is the most widely used amplitude modulator

Lattice modulator is the most widely used balanced modulator

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