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APHY 103: COMMUNICATION CIRCUITS Baquiran, Teresa Lean Z.

Bino, Charmine B.

Borlagdan, Paolo P.

Communication systems are widely used in our everyday life even in the past when we still

dont have improved and high technology systems for communicating. These systems are

composed of a wide range of systems including analog and digital systems. Communication

systems are systems that send information from one point to another over relatively long distances.

Some of the categories of communication systems are radio, television, telephony, radar,

navigation, satellite, data (digital), and telemetry. In order to send information, many

communication systems use either amplitude modulation (AM) or frequency modulation (FM).

Other modulation methods are phase modulation, pulse modulation, and frequency shift keying

(FSK).

BASIC RECEIVERS

The aim of this section of the study is to first describe basic heterodyne receivers; to define

AM and FM; and finally, to discuss the major functional blocks of an AM receiver and an FM

receiver.

Receivers that are based on the superheterodyne principle are standard in one form or

another in most types of communication systems and are found in systems such as standard

broadcast radio, stereo, and television. Superheterodyne receiver uses frequency mixing to convert

a received signal to a fixed IF (intermediate frequency) which is more convenient to process than

the original radio carrier frequency.

AMPLITUDE MODULATION

A method of sending audible information such as voice or music, by electromagnetic waves

that are broadcast through the atmosphere is called amplitude modulation or AM. In AM, the

carrier, a signal with frequency fc, the amplitude is varied according to modulating signal which

can be audio signal. Figure 17-1 found in Floyds Electronic Devices 7th edition book shows an

example of an amplitude modulated signal. The carrier frequency allows the receiver to be turned

to a specific known frequency. The resulting AM waveform contains the carrier frequency, the

upper-side frequency which is just equal to the carrier frequency plus the modulating frequency

(fc + fm), and a lower-side frequency which is equal to the carrier frequency minus the modulating

frequency (fc fm).

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Superheterodyne AM Receiver

The illustration shown in Figure 17-3 shows the different parts of a superheterodyne

AM receiver and the signal flow through it.

The frequency band for AM broadcast receivers is 540 kHz to 1640 kHz. As we can see

from the figure above, the different parts of the superheterodyne AM receiver are illustrated and

how the signal is processed on each part. The antenna picks up all radiated signals and feeds them

into the RF amplifier. These picked up signals are very small. The RF amplifier can be adjusted or

tuned to select and amplify any carrier frequency within the AM broadcast band. Only the selected

frequency and its two side bands pass through the amplifier. Local oscillator is the circuit that

generates a steady sine wave at a frequency 455 kHz above the selected RF frequency. The mixer

has two inputs that comes from the local oscillator and the RF amplifier. The signals from these

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two are mixed by a nonlinear process called heterodyning to produce a sum and difference

frequencies. For the IF or the intermediate amplifier, the input is the 455 kHz AM signal, a copy

of the original AM carrier signal except that the frequency has been lowered to 455 kHz. This also

significantly increase the level of the signal. The detector recovers the modulating signal or the

audio signal from the 455 kHz intermediate frequency. The output of the detector is just the audio

signal since at this point the IF is no longer needed. The audio and power amplifier amplifies the

detected audio signal and drives the speaker to produce sound. Finally, the automatic gain control

or the AGC provides a dc level out of the detector that is proportional to the strength of the received

signal. This level is fed back to the IF amplifier and sometimes to the mixer and RF amplifier to

adjust the gains so as to maintain constant signal levels throughout the system over the wide range

of incoming carrier signal strengths.

FREQUENCY MODULATION

In frequency modulation (FM), the modulating signal (audio) varies the frequency of a

carrier as opposed to the amplitude, as in the case of AM. Figure 17-4 shows the basic frequency

modulation.

The standard FM broadcast band consists of carrier frequencies from the 88 MHz to 108

MHz, which is significantly higher that AM.

Superheterodyne FM receiver

The FM receiver is similar to the AM receiver in many ways, but there are

significant differences. Figure 17-6 shows the different parts of an FM receiver and how

the signal flow through it.

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RF amplifier for this receiver is capable of amplifying any frequency between 88

MHz and 108 MHz. It is highly selective so that it passes only the selected carrier frequency and

significant side-band frequencies that contain audio. The local oscillator produces a sine wave at

a frequency 10.7 MHz above the selected RF. The mixer in this figure is the same with the AM

receiver except that its output is 10.7 MHz FM signal regardless of the RF carrier frequency. IF

amplifier amplifies the 10.7 MHz FM signal. The limiter removes the unwanted variations in the

amplitude of the FM signal as it comes out of the IF amplifier and produces a constant amplitude

FM output at the 10.7 MHz intermediate frequency. The discriminator has the same function with

the detector in AM receiver which recovers the audio from the FM signal. For certain reasons, the

higher modulating frequencies are amplified more than the lower frequency at the transmitting end

of an FM system by a process called preemphasis. The de-emphasis circuit in the FM circuit bring

the high-frequency audio signals back to the proper amplitude relationship with the lower

frequencies. The audio and power amplifiers have same function as in AM system and therefore

can be shared in a dual AM/FM configuration.

LINEAR MULTIPLIER

The linear multiplier is a key circuit in many types of communication systems. In this

section of communication circuits our aim is to discuss the function of a linear multiplier, to

describe the multiplier quadrants and transfer characteristics, and show some application of

multipliers.

For a four-quadrant multiplier can accept any of the four possible input polarity

combinations and produce an output with corresponding polarity. The four-quadrant multiplier is

shown in Figure 17-7.

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For the multiplier transfer characteristics for a typical IC linear multiplier of two input

voltages Vx and Vy . Values of Vx run along the horizontal axis and the values of Vy are the sloped

lines. To find the ouput voltage from the transfer characteristics graph, find the intersection of the

two input voltages Vx and Vy. Then find the output by projecting the point of intersection over the

vertical axis. The scale factor, K is set to 1/10 which is basically the attenuation that reduces the

output by a fixed amount. This is adjustable but the typical value of 0.1 or 1/10.

Some basic applications of linear multiplier are multiplier, squaring circuit, divide circuit,

square root circuit, and mean square circuit. Figure 17-13 shows the illustration of a multiplier. It

basically multiply two voltages.

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AMPLITUDE MODULATION

Amplitude modulation is basically a multiplication process. From the Figure 17-18, we can

see that the carrier signal and the modulation signal are the inputs to the amplitude modulator

which basically a multiplier. Therefore, the output voltage is the input voltage multiplied by the

voltage gain.

For example, if the gain of an amplifier is made to vary sinusoidally at a certain frequency,

and an input signal is applied at a higher frequency, the output signal will have higher frequency.

However the amplitude will vary according to the variation in gain as shown in the figure.

Therefore, amplitude modulation is basically a multiplication process. This is shown in Figure 17-

19.

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Mixer

For this part of the study our aim is to describe and discuss the function of a mixer, AM

demodulator, IF amplifiers, and Audio amplifiers

The basic property of the mixer is that it is actually a linear multiplier whose output is not

directly proportional to its input but to the product of its inputs. The mixer changes the frequency

of a signal to another value. It takes the incoming modulated RF signal along with the signal from

the local oscillator and produces a modulated signal containing not only the original frequency but

also the frequencies equal to the difference and sum of its two input frequencies.

Figure 1. Operation of a mixer

AM Demodulation

Demodulation is the process of separating or extracting the modulation from a signal. A

demodulator is an electronic circuit used to recover the information content from a modulated

carrier wave. An AM signal consists of a carrier which acts as a reference.

When demodulating a signal, there are two steps to be considered. First is to create a

baseband signal and second is to use a filer. A basic AM demodulator is shown in figure 1. It is

composed of a linear multiplier to create the baseband signal and a low pass filter to remove any

unwanted high frequency elements from the demodulation process. The critical frequency of the

filter is the highest audio frequency that is required for a given application. For figure 1, the critical

frequency is set to 15 kHz.

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Figure 2. Basic AM demodulator

IF and Audio amplifiers

IF amplifiers are used to raise the signal level to a level that can be used by a given circuit

to properly utilize the received information. It provides amplification of the modulated IF signal

out of the mixer before it is applied to the demodulator. In a receiver, it is a tuned amplifier with a

specified bandwidth operating at a center frequency of 455 kHz for AM and 10.7 MHz for FM set

to operate at a single resonant frequency that remains the same over the entire band of carrier

frequencies that can be received. Audio amplifiers on the other hand are used in a receiver system

to provide amplification of the recovered audio signal. As seen in figure 3, it is connected following

the detector to drive the speaker.

Figure 3. Audio amplifier in a receiver system

Audio amplifiers typically have bandwidths of 3 kHz to 15 kHz depending on the

requirement of the system. An example of this device is the LM386 audio amplifier which

operates from any dc supply voltage in the 4V to 12V range, making it a good choice for battery

operation.

Problem Solving

Determine the output voltage for a four-quadrant linear multiplier whose transfer

characteristics is given in Figure 17-8. The input voltages are Vx = -2 V and Vy = +10 V.

If a 455 kHz IF modulated by a 1 kHz audio frequency is demodulated, what frequency

or frequencies appear on the output of the demodulator?

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FREQUENCY MODULATION

Modulation: A carrier (high frequency signal; usually sinusoidal) is modified by a modulating

signal (voice, video/digital signal, audio, etc.) This can be done using either amplitude modulation

or frequency modulation.

AM versus FM

Amplitude Modulation: Variations in the modulating signal changes the value of the carrier signal

amplitude, with the frequency (of the carrier) being held constant.

Frequency Modulation: Carrier frequency varies according to the state of the modulating signal.

Figure 4. Frequency Modulation of a signal.

Figure 1 shows the basic frequency modulation of a signal. The frequency of the carrier signal

increases/decreases from its normal value, with the deviation controlled by the amplitude of the

modulating signal. In terms of amplitude, an increasing modulating signal raises the carrier

frequency above the normal value. A decreasing modulating signal likewise decreases the carrier

frequency. These relationships does not always hold true, as the reverse relationships can also be

applied. The amount of variation to normal value of the carrier frequency is referred to as the

frequency deviation. Maximum deviation is then observed during the maximum amplitude of the

modulating signal.

On the other hand, the frequency of the modulating signal determines the rate at which frequency

deviations occur, or how many times (per second) the carrier frequency increases/decreases from

its normal value. So for a modulating signal of a 1 kHz sine wave, the carrier frequency varies

1000 times per second.

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Basic Frequency Modulator

Figure 5. VCO as a frequency modulator

A frequency modulated (FM) signal can be produced from a carrier signal by using a voltage-

controlled oscillator. Recall that the frequency generated by a VCO is determined by the input

voltage. For an input voltage that varies over time (e.g. sine wave), the resulting signal is then

frequency modulated, as shown above.

Advantages and Disadvantages of FM (as compared to AM)

Advantages:

Noise Immunity the presence of limiter circuits in an FM receiver, majority of noise

levels are attenuated, resulting to a stable output.

Capture Effect another benefit due to the limiters, interfering signals possessing the same

frequency as the FM signal are rejected. Out of two or more FM signals that are present

simultaneously, only the strongest signal persists.

Transmitter Efficiency linear amplifiers are not needed, since FM signals exhibit constant

amplitude (as oppose to AM signals).

Disadvantages:

Excessive Spectrum Use the wider bandwidth of FM signals occupies more spectrum

space, giving way to unnecessary bandwidths.

Complex Circuitry circuits use in frequency modulation and demodulation are more

complex than the basic circuits needed in AM. FM transmission ICs has so far solved this

problem.

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FREQUENCY DEMODULATION

Demodulation: The process of extracting the original modulating signal from a modulated signal

(also known as detection). Demodulator circuits accepts the modulated signal, and outputs the

modulating signal used for modulation.

Some Frequency Demodulation methods:

Slope Detection

Phase-Shift Discrimination

Radio Detection

Quadrature Detection

Phase-locked Loop Demodulation

Demodulation using Phase-locked Loops (PLLs) is one of the widespread methods used for such

purposes. In addition to demodulation, PLLs are also used in other communication applications

such as TV receivers, tone decoder, telemetric receiver, modems, data synchronizers, etc.

Basic PLL Concepts

The phase-locked loop is a frequency-sensitive feedback circuit consisting of three basic

components: the phase detector, a low-pass filter, and a voltage-controlled oscillator (VCO).

Figure 6. Block diagram of a Phase-locked loop.

The phase detector is used to compare the input (incoming signal Vin), with the VCO signal (Vc).

The output from the phase detector is then filtered (low pass), becoming proportional to the phase

difference between Vin and Vc. Filtering is done to rid the signal of high-frequency noise. The

filtered signal is then fed to the VCO, where it forces the VCO output to have the same frequency

with the input. The PLL finishes synchronization when the input and the VCO output attains the

same frequency. Any changes in the input frequency modifies the VCO output. To use the PLL as

a frequency demodulator, we only need to provide an FM signal as the PLL input. In this case, the

PLL will produce an output (VCO output) having the same frequency as the FM input, which is

basically the modulating signal of the FM input.

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The LM565 phase-locked loop

The LM565 is a general-purpose PLL IC consisting of a phase detector, a low pass filter (which is

formed by and internal resistor and an external capacitor), a VCO, and an amplifier. These are

indeed the basic PLL components, with an added amplifier to allow operation for small input

signals. The standard operating frequency ranges from 0.001 Hz up to 500 kHz.

Figure 7. PLL LM565 internal diagram.

As mentioned, the LM565 can be used as an FM demodulator by introducing an FM input to the

phase detector, and the VCO output must be fed back to the phase detector. The integrated circuit

outputs the demodulated signal after the VCO output matches the FM input frequency.

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REFERENCES

1. Floyd, T. L., Electronic Devices 7th Edition (International). 2005

2. Frenzel, L. E., Principles of Electronic Communication Systems 3rd Edition. 2008

3. Amplitude modulation AM demodulation. Retrieved from http://www.radio-

electronics.com/info/rf-technology-design/am-reception/amplitude-modulation-detection-

demodulation.php

Images Retrieved from:

Electronic Devices 7th Edition (Floyd). Chapter 17 Communication Circuits.

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http://zone.ni.com/reference/en-XX/help/370592E-01/digitizers/reference_clock/

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