Chapter 2: Modulation

MODULATION

It is a process of impressing low-frequency information signals into a high frequency carrier signal.

MODULATION

It is also defined as the process of changing one or more properties of the analog carrier in proportion with the information signal.

DEMODULATION

It is the reverse process of modulation where the received signals are transformed back to their original form.

CARRIER

It is a high-frequency signal (the one that is modulated) used to carry information from source to destination.

INFORMATION

It is also known as intelligence, modulating signal or baseband signal, which is an understandable information (one that is modulating) such as voice, audio, video, documents, etc.

ADVANTAGES OF MODULATION

It is extremely difficult to radiate low-frequency signals from an antenna in the form of electromagnetic energy: With modulation, the size of antennas is more practical (shorter) since the wavelengths of frequencies used as carriers are shorter (high frequencies).

Information signals often occupy the same frequency band, and if signals from two or more sources are transmitted at the same time, they would interfere with each other. With modulation, there will be no interference with the signals, since every transmitter can use different frequencies.

LOW-FREQUENCY SIGNALS

Have longer wavelengths, thus, require longer antennas for transmission.

HIGH-FREQUENCY SIGNALS

Have shorter wavelengths, thus, require shorter antennas for transmission.

ANALOG MODULATION

It is a modulation technique wherein both the carrier and information are in analog forms.

AMPLITUDE MODULATION (AM)

It is a modulation technique, invented by Reginald Fessenden, wherein the information signal is analog, and the amplitude of the carrier is varied proportional to the information signal.

535-1605 kHz.

Standard AM range

455 kHz.

Standard AM Intermediate Frequency

AM Modulation Index

Also known as coefficient of modulation or depth of modulation.

AM Modulation Index

It is the term used to describe the amount of amplitude change (modulation) present in an AM waveform signal.

AM Percent Modulation

It is the coefficient of modulation expressed as a percentage

ENVELOPE

The curve produced by joining the tips of the individual RF cycles of a modulated wave.

ENVELOPE

Contains the information.

1

Ideal value for m

OVERMODULATION

The condition where m is greater than 1
(should be avoided as it introduces serious distortion of the modulating signal)

SPLATTER

Colloquial term used to describe additional side frequencies produced by overmodulation or distortion in an AM system.

SIDEBAND POWER

The useful power in an AM signal

SIDEBAND POWER

Increases with modulation, becoming one-third the total signal power for 100% modulation. The higher the percentage modulation, the greater the sideband power and the stronger and more intelligible the transmitted and received signal.

TRAPEZOIDAL PATTERNS

Used for observing the modulation characteristics of AM transmitters.

CRYSTAL OSCILLATOR

It provides a stable carrier frequency at low power.

CRYSTAL OSCILLATOR

It is an RF oscillator which can be a Hartley, Colpitts, Clapp, etc.

BUFFER AMPLIFIER

It is a class A RF amplifier that isolates the crystal oscillator to improve its stability. It is connected after the oscillator to prevent it from loading (changing value of load voltage).

BUFFER AMPLIFIER

It has a high input impedance and low output impedance (common- collector or emitter-follower configuration) to couple the high output impedance of the oscillator and the low input impedance of the IF power amplifier.

INTERMEDIATE POWER AMPLIFIER

It is a class C RF amplifier that raises the output of the buffer to a level sufficient to drive the modulated RF amplifier.

INTERMEDIATE POWER AMPLIFIER

It amplifies the signal from the oscillator.

MODULATED POWER AMPLIFIER

It is a class C RF amplifier that supplies the energy which is required to drive the antenna system at the rated RF power for high-level modulation.

MODULATED POWER AMPLIFIER

It multiplies the signals from the IF power amplifier and push-pull modulator.

SPEECH AMPLIFIER

It is a class A AF pre-amplifier that raises the level of the input AF after being subjected to processing and filtering.

SPEECH AMPLIFIER

It amplifies the weak audio frequency so that it can be detected by the driver amplifier.

SPEECH AMPLIFIER

It improves the signal-to-noise ratio before mixing.

DRIVER AMPLIFIER

It is a class A/B/AB that supplies the necessary audio power to drive the class B modulator.

DRIVER AMPLIFIER

It amplifies the output of the speech amplifier so that it can be detected by the push-pull modulator.

PUSH-PULL MODULATOR

It is a class B AF output amplifier that varies the plate voltage of the class C RF amplifier in accordance with the frequency and the amplitude of the AF signal.

LINEAR POWER AMPLIFIER

It is a class A/B/AB amplifier which provides linear power amplification of the amplitude-modulated output signal from the class C modulated power amplifier.

LINEAR POWER AMPLIFIER

It is used for low-level modulation.

LOW-LEVEL MODULATION

The modulation takes place prior to the output element of the final stage of the transmitter.

HIGH-LEVEL MODULATION

The modulation takes place in the fine element of the final stage where the carrier signal is at its maximum amplitude.

EFFICIENCY OF A TRANSMITTER

An important specification, particularly in view of the large power levels often used.

EFFICIENCY OF A TRANSMITTER

Calculated by dividing the output power by the input power of a transmitter.

COMMON-BASE CONFIGURATION

Used in AM amplifiers since it can operate in high frequencies.

COMMON-EMITTER CONFIGURATION

Is not used due to its high miller effect at high frequency of operation.

one-half

For a 100% modulation, the power of the modulating amplifier is ______of the supply power.

DIODE ENVELOPE DETECTOR

Also known as peak detector. It consists of a diode in series with an RC low pass filter. The diode acts as a rectifier that allows only one-half of the modulated AM signal to pass (the other half being clipped). The rectified signal is fed to a low pass filter, where the low-frequency signal (modulating signal) is recovered while the high-frequency carrier is filtered out.

CRYSTAL DETECTOR

It is the first rectifying detector.

CAT WHISKERS

A thin, pointed wire pressed against the surface of the crystal, when a sensitive spot is found, more current flows in one direction than in the opposite thus rectification occurs.

POWER DETECTOR

It is a type of detector using amplifying devices like transistors in place of a diode to provide rectification and amplification at the same time.

GRID LEAK DETECTOR

It consists of a tuned circuit, a rectifier and an RC low pass filter for the recovery of the modulating signal. It has better sensitivity than diode detector.

REGENERATIVE & AUTODYNE DETECTOR

It employs a tickler coil which generates energy from the plate circuit into the grid circuit of the triode. When the regenerative detector oscillates it becomes an autodyne detector (self-heterodyning) or product detector and can be used to detect CW, SSB, or FSK emission.

DETECTION

Means the extraction of the information signal from the AM

signal; is only part of the demodulator system.

DISTORTION

Any frequency, phase, or amplitude variations that are present in the demodulated waveform that were not in the original information signal. When noise is added to the AM signal, the output at the detector is always distorted.

FIDELITY

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

TRF (TUNED RADIO FREQUENCY) RECEIVER

It is one of the earliest types of AM receivers, invented by Reginald Fessenden, and is probably the simplest designed radio receivers available today.

Its components include: antenna, RF amplifier, detector, AF amplifier, and speaker.

DISADVANTAGES OF TRF

Its bandwidth is inconsistent and varies with center frequency when tuned over a wide range of frequencies (i.e., as the frequency is increased, bandwidth also increases).

It is unstable due to the large number of amplifiers all tuned to the center frequency.

Its gains are not uniform over a wide range of frequencies.

Components of TRF: RF AMPLIFIER

it amplifies the weak RF signal. It has a variable resistor that controls the RF gain and sensitivity. It also contains the pre-selector, which is used to tune to the desired frequency.

Components of TRF: DETECTOR

It makes use of regenerative detector that provides rectification and detection for modulated signals. It extracts the information.

Components of TRF: AF AMPLIFIER

It amplifies the recovered information (weak AF signal). It is a volume-controlled amplifier that raises the power level of the AF (audio) signal to a value sufficient to drive the loudspeaker of the receiver.

HETERODYNE

To mix two frequencies together in a nonlinear device or to translate one frequency to another using nonlinear mixing.

SUPERHETERODYNE RECEIVER

Was invented by Edwin Armstrong, its gain, selectivity and sensitivity are far superior to the other

receivers.

Its components include: antenna, RF amplifier, mixer & local (crystal) oscillator, IF section, detector (second detector), AF amplifier, and speaker.

Components of the Superhet: LOCAL OSCILLATOR

It is a stable crystal oscillator whose frequency beats with the incoming signal to produce the correct Intermediate frequency.

Components of the Superhet: MIXER

It down-converts the received RF frequencies to IFs (intermediate frequencies).

Components of the Superhet: MIXER

It is the first detector which operates in a nonlinear, fashion and provides the action which produces the desired intermediate frequency. Its output includes the original, sum and difference of the local oscillator frequency and the RF Input frequency.

Components of the Superhet: IF SECTION

Its primary functions are amplification and selectivity.

Components of the Superhet: IF SECTION

It consists of a series of IF amplifiers and bandpass filters and is often known as IF strip. It is tuned in most cases to 455 kHz. It amplifies the 455 kHz output of the mixer and rejects the remaining output frequencies. Its selectivity characteristics determine the selectivity of the receiver.

Components of the Superhet: SECOND DETECTOR

It demodulates the 455 kHz IF signals and recovers the original modulating signal or audio signal.

SELECTIVITY

It is a receiver parameter that is used to measure the ability of the receiver to accept a given band of frequencies and reject all others.

SENSITIVITY

Also known as receiver threshold. It is the minimum RF signal level that can be detected at the input to the receiver and still produce a usable demodulated information signal.

Q

Stands for quality (merit), for which this factor measures just how close to perfect a tuner or tuner component can be, and it affects the bandwidth and the selectivity of a certain receiver.

The higher the Q, the better the circuit, the lower the losses, the closer the circuit is to being perfect.

SHAPE FACTOR

It is the ratio of the bandwidth 60 dB below maximum signal level and bandwidth 3 dB below maximum signal level.

BANDWIDTH IMPROVEMENT

It is a noise reduction ratio achieved by reducing the bandwidth.

NOISE FIGURE IMPROVEMENT

It is the corresponding reduction in the noise figure due to the reduction in bandwidth expressed mathematically in decibels.

HIGH SIDE INJECTION

Also known as high-beat injection, wherein the local oscillator frequency is tuned to a frequency higher than the input RF frequency.

LOW SIDE INJECTION

Also known as low-beat injection, wherein the local oscillator frequency is tuned to a frequency lower than the input RF frequency.

IMAGE FREQUENCY

It is any frequency other than the selected radio frequency carrier that (if allowed to enter a receiver and mix with local oscillator) will produce a cross-product frequency that is equal to intermediate frequency.

IMAGE FREQUENCY REJECTION RATIO (IFRR)

It is the numerical measure of the ability of a pre-selector to reject the image frequency; the higher the IFRR, the better.

TUNED CIRCUITS

Virtually all communication equipment contains; circuits made up of inductors and capacitors that resonate at specific frequencies.

IFs (intermediate frequencies)

Refer to frequencies that are used within a transmitter or receiver that fall somewhere between the radio frequencies and the original source information frequencies.

NARROWBAND AMPLIFIERS

RF amplifiers designed for a narrow range of frequencies to avoid problems with noise and spurious signals. Wideband amplifiers are also used.

CLASS A (RF AMPLIFIER)

Operation of RF amplifier that has the least distortion but is very inefficient.

CLASS C (RF AMPLIFIER)

Operation of RF amplifier, which is the most efficient, but its extreme nonlinearity makes it unsuitable for

many signals.

LOCAL OSCILLATOR

The stability of a superheterodyne receiver depends directly on this.

SQUELCH CIRCUIT

Also called a muting circuit, the squelch is used to keep the receiver audio turned off until an RF signal appears at the receiver input.

SQUELCH CIRCUIT

It is used to quiet a receiver in the absence of a received signal.

DOUBLE SIDEBAND SUPRESSED CARRIER (DSBSC)

It is a form of amplitude modulation in which the carrier is totally suppressed.

BALANCED MODULATOR

it is a circuit that generates a DSB signal suppressing the carrier and leaving only the sum and difference frequencies at the output. The output of a balanced modulator can be further processed by filters or phase-shifting circuitry to

eliminate one of the sidebands, resulting in an SSB signal.

SINGLE SIDEBAND SUPRESSED CARRIER (SSBSC)

It is a form of amplitude modulation in which the carrier is totally suppressed and one of the sidebands removed.

PEAK ENVELOPE POWER (PEP)

It is the RMS power developed at the crest of the modulation envelop. The power of an SSBSC signal is usually expressed in PEP.

FILTER METHOD

It is the simplest method of generating SSB. It uses LC, crystal, ceramic or mechanical filters to eliminate one of the sidebands of the SSB.

It is easy to construct, however, filter response is not ideal, so the sideband may be cut or there will be still a small portion of the other sideband.

It consists of one balanced modulator.

PHASE SHIFT METHOD

It makes use of two balanced modulators and two phase shifters. It reduces the bulkiness of the filters and its inherent disadvantages.

THIRD (WEAVER) METHOD

It retains the advantages of the phase shift method without the disadvantage of the AF phase shift method. It consists of four balanced modulators.

SINGLE SIDEBAND FULL CARRIER (SSBFC)

It is a form of amplitude modulation in which the carrier is transmitted at full power but only one of the sidebands is transmitted.

SINGLE SIDEBAND REDUCED CARRIER (SSBRC)

It is also known as single-sideband reinserted carrier, and it is a form of amplitude modulation in which one sideband is totally removed and the carrier voltage is reduced to approximately 10% of its unmodulated amplitude.

SSB TRANSMITTERS

Usually work by filtering a DSBSC signal to remove the unwanted sideband, then using a mixer to move the signal to the operating frequency.

SSB SIGNALS

Require linear amplification.

SSB RECEIVERS

Require a beat frequency oscillator (BFO) to reinsert the

carrier, and generally use product detectors.

INDEPENDENT SIDEBAND (ISB)

It is a form of AM in which a single carrier frequency Is independently modulated by two different modulating signals (i.e., it contains different information for each sideband).

VESTIGIAL SIDEBAND (VSB)

It is a form of amplitude modulation in which the carrier and one complete sideband are transmitted, but only part of the second sideband is transmitted.

This VSB system is used for the picture portion of a commercial television broadcasting signal.