Gibilisco S. Teach Yourself Electricity and Electronics

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The Twin T Oscillator

An audio oscillator circuit that is popular for general-purpose use is the twin T oscillator (Fig. 24-18).

The frequency is determined by the values of the resistors R and capacitors C. The output is a near-

perfect sine wave. The small amount of distortion helps to alleviate the irritation produced by an ab-

solutely pure sinusoid. The circuit shown in this example uses two PNP bipolar transistors. They are

biased for class A amplification.

The Multivibrator

Another popular AF oscillator circuit makes use of two identical common emitter or common

source amplifier circuits, hooked up so that the signal goes around and around between them. This

is sometimes called a multivibrator circuit, although that is technically a misnomer, the term being

more appropriate to various digital signal-generating circuits.

In the example of Fig. 24-19, two N-channel JFETs are connected to form a multivibrator for

use at AF. Each stage amplifies the signal in class A, and reverses the phase by 180°. Therefore, the

signal goes through a 360° phase shift each time it gets back to any particular point. A 360° phase

shift is equivalent to no phase shift at all, so it results in positive feedback.

Audio Oscillators 401

24-18 A twin T audio oscillator using two PNP bipolar

transistors. The frequency is determined by the

values of the resistors R and the capacitors C.

The frequency of the circuit shown in Fig. 24-19 is set by means of an LC circuit. The coil uses a

ferromagnetic core, because stability is not of great concern and because such a core is necessary to ob-

tain the large inductance needed for resonance at AF. Toroidal cores or pot cores are excellent in this

application. The value of L can range from about 10 mH to as much as 1 H. The capacitance is cho-

sen according to the formula for resonant circuits, to obtain an audio tone at the frequency desired.

IC Oscillators

In recent years, solid-state technology has advanced to the point that whole circuits can be etched

onto silicon chips. Such devices are called integrated circuits (ICs). The operational amplifier, also

called an op amp, is a type of IC that is especially useful as an audio oscillator because it has high

gain, and it can easily be connected to produce positive feedback.

Quiz

Refer to the text in this chapter if necessary. A good score is at least 18 correct. Answers are in the

back of the book.

1. The decibel is a unit of

(a) relative signal strength.

(b) voltage.

402 Amplifiers and Oscillators

24-19 A multivibrator audio oscillator using two N-channel JFETs. The

frequency is determined by the value of the inductor L and the

capacitor C.

(d) current.

2. An oscillator at RF requires the use of

(a) a common drain or common collector circuit.

(b) a stage with gain.

(d) a quartz crystal.

3. Suppose a circuit is found to have a gain figure of −15 dB. Which of the following statements

is true?

(a) The output signal is stronger than the input signal.

(b) The input signal is stronger than the output signal.

(d) The output signal is 15 times as strong as the input signal.

4. In an oscillator circuit, the feedback should be

(a) as great as possible.

(b) kept to a minimum.

(d) done through a transformer whose wires can be switched easily.

5. A power gain of 44 dB is equivalent to which of the following output/input power ratios?

(a) 44:1

(b) 160:1

(d) 25,000:1

6. An RF choke

(a) passes RF signals but blocks dc.

(b) passes both RF signals and dc.

(d) blocks both dc and RF signals.

7. The optimum capacitance values in an amplifier circuit depend on

(a) the power-supply voltage.

(b) the power-supply polarity.

(d) the input signal frequency.

8. An oscillator might fail to start for any of the following reasons, except

(a) low battery voltage.

(b) low stage gain.

Quiz 403

(d) a high-impedance load.

9. In which of the following FET amplifier types does drain current flow for exactly 50 percent

of the signal cycle?

(a) Class A

(b) Class AB

(d) Class B

10. The frequency at which a quartz crystal oscillator produces energy is largely dependent on

(a) the load impedance.

(b) the physical thickness of the quartz wafer.

(d) the power-supply voltage.

11. Which bipolar amplifier type has some distortion in the signal wave, with collector current

during most, but not all, of the cycle?

(a) Class A

(b) Class AB

(d) Class B

12. An RF oscillator usually

(a) produces an output signal with an irregular waveshape.

(b) has most or all of its energy at a single frequency.

(d) employs an RC circuit to determine the output amplitude.

13. A class C amplifier can be made linear by

(a) reducing the bias.

(b) increasing the drive.

(d) no means; a class C amplifier is always nonlinear.

14. A frequency synthesizer has

(a) high power output.

(b) high frequency drift rate.

(d) an adjustable waveshape.

15. A graphic equalizer is a form of

(a) bias control for an NPN bipolar transistor.

404 Amplifiers and Oscillators

(b) gain control for an RF oscillator.

(d) circuit for adjusting the waveform of an RF oscillator.

16. If the impedance of the load connected to the output of an oscillator is extremely high,

(a) the frequency will drift excessively.

(b) the power output will be reduced.

(d) it is no cause for concern; in fact, it is a good thing.

17. Suppose a certain bipolar-transistor PA is 66 percent efficient. The output power is 33 W.

The dc collector power input is

(a) 22 W.

(b) 50 W.

(d) impossible to determine without more information.

18. The arrangement in the block diagram of Fig. 24-20 represents

(a) a waveform analyzer.

(b) an audio oscillator.

(d) a sine wave generator.

19. A tuned RF PA must always be

(a) set to work over a wide range of frequencies.

(b) adjusted for maximum power output.

(d) operated in class C.

Quiz 405

24-20 Illustration for Quiz Question 18.

20. Class B amplification can be used to obtain low distortion for audio applications

(a) by connecting two amplifiers in cascade, thereby maximizing the gain and generating a

pure sine wave output.

(b) by biasing the bipolar transistor or FET beyond cutoff or pinchoff, thereby ensuring that

the output is in phase with the input.

pinchoff, in a push-pull configuration.

(d) by biasing the bipolar transistor or FET in the middle of the straight-line portion of the

characteristic curve.

406 Amplifiers and Oscillators

IN RADIO OR WIRELESS COMMUNICATIONS, A TRANSMITTER CONVERTS DATA INTO ELECTROMAGNETIC

(EM) waves intended for recovery by one or more receivers. In this chapter, we’ll look at how data is

converted to EM waves and transmitted, and then examine how the resulting EM fields can be in-

tercepted and received.

Oscillation and Amplification

A radio transmitter employs one or more oscillators to generate an RF signal, and amplifiers to gen-

erate the required power output. You just learned how these circuits work. Most transmitters have

mixers in addition to the oscillating and amplifying stages. Signal mixing is commonly done with

diodes, and was discussed in Chap. 20.

Modulation

Modulation is the process of writing data onto an electric current or EM wave. The process can be

done by varying the amplitude, the frequency, or the phase of the current or wave. Another method

is to transmit a series of pulses, whose duration, amplitude, or spacing is made to vary.

The Carrier

The heart of a wireless signal is a sine wave known as the carrier. The lowest carrier frequency used

for radio communications is a few kilohertz (kHz). The highest frequency is in the hundreds of giga-

hertz (GHz). For efficient data transfer, the carrier frequency must be at least 10 times the highest

frequency of the modulating signal.

On/Off Keying

The simplest form of modulation is on/off keying. This can be done in the oscillator of a radio

transmitter to send Morse code, which is a binary digital mode. The duration of a Morse-code dot

is one bit (binary digit). A dash is 3 bits long. The space between dots and dashes within a charac-

407

CHAPTER

Wireless Transmitters

and Receivers

ter is 1 bit. The space between characters in a word is 3 bits. The space between words is

7 bits. The key-down (full-carrier) condition is called mark, and the key-up (no-signal) condition

is called space.

Morse code is slow. Human operators use speeds ranging from about 5 words per minute

(wpm) to 40 or 50 wpm. A few human operators can work at 60 to 70 wpm. These people usually

copy the signals in their heads.

Frequency-Shift Keying

Digital data can be sent over wireless links by means of frequency-shift keying (FSK). In some FSK

systems, the carrier frequency is shifted between mark and space conditions, usually by a few hun-

dred hertz or less. In other systems, a two-tone audio-frequency (AF) sine wave modulates the car-

rier. This is known as audio-frequency-shift keying (AFSK).

The two most common codes used with FSK and AFSK are Baudot (pronounced “baw-DOE”)

and ASCII (pronounced “ASK-ee”). The acronym ASCII stands for American Standard Code for In-

formation Interchange.

In radioteletype (RTTY) FSK and AFSK systems, a terminal unit (TU) converts the digital sig-

nals into electrical impulses that operate a teleprinter or display characters on a computer screen.

The TU also generates the signals necessary to send RTTY as an operator types on a keyboard. A de-

vice that sends and receives AFSK is sometimes called a modem. This acronym stands for modulator/

demodulator. A modem is basically the same as a TU. Figure 25-1 is a block diagram of an AFSK

transmitter.

The main advantage of FSK or AFSK over on/off keying is the fact that there are fewer errors

or misprints, because the space part of the signal is identified as such, rather than existing as a gap

or pause in the data. A sudden noise burst in an on/off keyed signal can confuse a receiver into read-

ing the space as a mark signal, but when the space is positively represented by its own signal, this is

less likely to happen.

408 Wireless Transmitters and Receivers

25-1 Simplified block diagram of an AFSK transmitter.

Amplitude Modulation

An AF voice signal has frequencies mostly in the range between 300 Hz and 3 kHz. Some charac-

teristic of an RF carrier can be varied, or modulated, by these waveforms, thereby transmitting voice

information. Figure 25-2 shows a simple circuit for obtaining amplitude modulation (AM). This cir-

cuit can be imagined as an RF amplifier for the carrier, with the instantaneous gain dependent on

the instantaneous audio input amplitude. Another way to think of this circuit is as a mixer that

combines the RF carrier and audio signals to produce sum and difference signals at frequencies just

above and below that of the carrier.

The circuit shown in Fig. 25-2 works well, provided the AF input amplitude is not too great. If

the AF input is excessive, then distortion occurs, intelligibility is degraded, system efficiency is re-

duced, and the bandwidth of the signal is increased unnecessarily.

The extent of AM is expressed as a percentage, from 0 percent (an unmodulated carrier) to

100 percent (full modulation). Increasing the modulation past 100 percent causes the same prob-

lems as excessive AF input. In an AM signal modulated 100 percent,

⁄3 of the power is used to convey

the data, and the other

⁄3 is consumed by the carrier wave.

Figure 25-3 shows a spectral display of an AM voice radio signal. The horizontal scale is cali-

brated in increments of 1 kHz per division. Each vertical division represents 3 dB of change in sig-

nal strength. The maximum (reference) amplitude is 0 dB relative to 1 mW (abbreviated as 0 dBm).

The data exists in sidebands above and below the carrier frequency. These sidebands resemble the

sum and difference signals produced by a mixer. In this case the mixing occurs between the AF input

signal and the RF carrier. The RF between −3 kHz and the carrier frequency constitutes the lower

Modulation 409

25-2 An amplitude modulator using an NPN bipolar transistor.

sideband (LSB); the RF from the carrier frequency to +3 kHz represents the upper sideband (USB).

The bandwidth is the difference between the maximum and minimum sideband frequencies, in this

case 6 kHz.

In an AM signal, the bandwidth is twice the highest audio modulating frequency. In the exam-

ple of Fig. 25-3, all the voice energy is at or below 3 kHz, so the signal bandwidth is 6 kHz. This is

typical of a communications signal. In standard AM broadcasting, the AF energy is spread over a

wider bandwidth, nominally 10 kHz.

Single Sideband

In AM, most of the RF signal power is consumed by the carrier alone; the two sidebands are mirror-

image duplicates. This is inefficient, and is also unnecessarily redundant! If the carrier and one of

the sidebands is eliminated, these shortcomings can be overcome. That makes the signal stronger for

a given amount of RF power, or allows the use of lower RF power in a given communications sce-

nario. Another bonus is the fact that the bandwidth is reduced to less than half that of an AM sig-

nal modulated with the same data, so more than twice as many signals can fit into a specific range,

or band, of frequencies.

When the carrier is removed from an AM signal along with one of the sidebands, the remain-

ing RF energy has a spectral display resembling Fig. 25-4. This is single sideband (SSB) transmission.

Either the LSB or the USB alone can be used, with equally good results.

410 Wireless Transmitters and Receivers

25-3 Spectral display of a typical AM voice

communications signal.