Gibilisco S. Teach Yourself Electricity and Electronics

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Detectors 431

25-14 At D, a product detector using diodes. At E, a product

detector using an NPN bipolar transistor biased as a class B

amplifier. Both of these circuits can also be used as signal

mixers.

432 Wireless Transmitters and Receivers

be set so certain standard tone pitches result (such as 2125 Hz and 2295 Hz in the case of 170-Hz

shift). Unlike the situation with CW reception, there is little tolerance for BFO adjustment varia-

tion or error.

Detection of FM

Frequency-modulated (FM) signals can be detected in various ways. These methods also work for

phase modulation.

Slope detection: An AM receiver can detect FM in a crude manner by setting the receiver fre-

quency near, but not on, the FM unmodulated-carrier frequency. An AM receiver has a filter with

a passband of a few kilohertz, having a selectivity curve such as that shown in Fig. 25-14B. If the

FM unmodulated-carrier frequency is near either edge, or skirt, of the filter response, frequency

variations in the incoming signal cause it to swing in and out of the receiver passband. This causes

the instantaneous receiver output to vary. The relationship between the instantaneous FM devia-

tion and the instantaneous output amplitude is not linear, however, because the skirt of the pass-

band is not a straight line, as is apparent in the figure. The result is an unnatural-sounding received

signal.

PLL: If an FM signal is injected into a PLL, the loop produces an error voltage that is a dupli-

cate of the modulating waveform. A limiter, which keeps the signal amplitude from varying, can be

placed ahead of the PLL so the receiver does not respond to AM. Weak signals tend to abruptly ap-

pear and disappear, rather than fading, in an FM receiver that employs limiting.

Discriminator: This type of FM detector produces an output voltage that depends on the in-

stantaneous signal frequency. When the signal is at the center of the passband, the output voltage is

zero. If the frequency falls below center, the output voltage becomes positive. If the frequency rises

above center, the output becomes negative. The relationship between the instantaneous FM devia-

tion and the instantaneous output amplitude is linear, so the output is a faithful reproduction of the

incoming signal data. A discriminator is sensitive to amplitude variations, but this can be overcome

by a limiter.

Ratio detector: This type of FM detector is a discriminator with a built-in limiter. The original

design was developed by RCA (Radio Corporation of America), and is used in high-fidelity receivers

and in the audio portions of TV receivers. A simple ratio detector circuit is shown in Fig. 25-14C.

The balance potentiometer should be adjusted for the best received signal quality.

Detection of SSB

For reception of SSB signals, a product detector is preferred, although a direct-conversion receiver can

also do the job. A product detector also works well for the reception of CW and FSK. The incom-

ing signal combines with the output of an unmodulated LO, producing audio or video. Product de-

tection is done at a single frequency, rather than at a variable frequency as in direct-conversion

reception. The single, constant frequency is obtained by mixing the incoming signal with the out-

put of the LO.

Two product-detector circuits, which are also representative of the mixers used in superhet re-

ceivers, are shown in Fig. 25-14D and E. At D, diodes are used; there is no amplification. At E, a

bipolar transistor is employed; this circuit provides some gain. The essential characteristic of either

circuit is the nonlinearity of the semiconductor devices. This generates the sum and difference fre-

quency signals that result in audio or video output.

Audio Stages

Enhanced selectivity can be obtained by tailoring the frequency response in the AF amplifier stages

following the detector, in addition to the RF selectivity provided in the IF amplifier stages preced-

ing the detector.

Filtering

A voice signal requires a band of frequencies ranging from about 300 Hz to 3000 Hz. An audio

bandpass filter, with a passband of 300 Hz to 3000 Hz, can improve the quality of reception with

some voice receivers. An ideal voice audio filter has little or no attenuation within the passband

range, and high attenuation outside the range, with a near-rectangular response.

A CW or FSK signal requires only a few hundred hertz of bandwidth to be clearly read. Audio

CW filters can narrow the response bandwidth to 100 Hz or less, but passbands narrower than

about 100 Hz produce ringing, degrading the quality of reception. With FSK, the bandwidth of the

filter must be at least as large as the difference (shift) between mark and space, but it need not, and

should not, be much greater.

An audio notch filter is a band-rejection filter with a sharp, narrow response. An interfering car-

rier that produces a tone of constant frequency in the receiver output can be greatly attenuated with

this type of filter. Audio notch filters are tunable from at least 300 Hz to 3000 Hz. Some sophisti-

cated AF notch filters can tune themselves automatically. When an interfering carrier appears and

remains for a few tenths of a second, the notch is activated and centers itself on the audio frequency

of the detected offending signal.

Squelching

A squelch silences a receiver when no signal is present, allowing reception of signals when they ap-

pear. Most FM communications receivers use squelching systems. The squelch is normally closed,

allowing no audio output, when no signal is present. The squelch opens, allowing everything to be

heard, if the signal amplitude exceeds the squelch threshold, which can be adjusted by the operator.

In some systems, the squelch does not open unless the signal has certain characteristics. This is

known as selective squelching. The most common way to achieve this is the use of subaudible (below

300 Hz) tone generators, or AF tone-burst generators. The squelch opens only when an incoming

signal is modulated by a tone, or sequence of tones, having the proper characteristics. This can pre-

vent unauthorized transmissions from accessing repeaters or being picked up by receivers.

Television Reception

A television (TV) receiver has a tunable front end, an oscillator and mixer, a set of IF amplifiers, a

video demodulator, an audio demodulator and amplifier chain, a picture CRT or display with asso-

ciated peripheral circuitry, and a loudspeaker.

Fast-Scan TV

Figure 25-15 is a block diagram of a receiver for conventional analog FSTV. In the United States,

conventional FSTV broadcasts are made on 68 different channels numbered from 2 through 69.

Each channel is 6 MHz wide, including video and audio information. Channels 2 through 13 com-

prise the VHF TV broadcast channels. Channels 14 through 69 are the UHF TV broadcast channels.

Television Reception 433

In digital cable television, there are more channels. The number of possible channels is virtually un-

limited, because signals are not transmitted over the air and do not consume EM spectrum space.

Slow-Scan TV

A slow-scan television (SSTV) communications station needs a transceiver with SSB capability, a stan-

dard TV set or personal computer, a video camera, and a scan converter that translates between the

SSTV signal and either FSTV imagery or computer video data. The scan converter consists of two

data converters (one for receiving and the other for transmitting), some digital memory, a tone gener-

ator, and a TV detector. Scan converters are commercially available. Computers can be programmed

to perform this function. Some amateur radio operators build their own scan converters.

Specialized Wireless Modes

Some less common wireless communications techniques are effective under certain circumstances.

Dual-Diversity Reception

A dual-diversity receiver can reduce the fading that occurs in radio reception at high frequencies (ap-

proximately 3 to 30 MHz) when signals are propagated by means of the ionosphere. Two receivers

are used. Both are tuned to the same signal, but they employ separate antennas, spaced several wave-

lengths apart. The outputs of the receiver detectors are fed into a common audio amplifier, as shown

in Fig. 25-16.

Dual-diversity tuning is critical, and the equipment is expensive. In some installations, three or

more antennas and receivers are employed. This provides superior immunity to fading, but it com-

pounds the tuning difficulty and further increases the expense.

434 Wireless Transmitters and Receivers

25-15 Block diagram of a conventional FSTV receiver.

Synchronized Communications

Digital signals require less bandwidth than analog signals to convey a given amount of information

per unit time. Synchronized communications refers to a specialized digital mode, in which the trans-

mitter and receiver operate from a common time standard to optimize the amount of data that can

be sent in a communications channel or band.

In synchronized digital communications, also called coherent communications, the receiver and

transmitter operate in lock-step. The receiver evaluates each transmitted binary digit, or bit, for a

block of time lasting for the specified duration of a single bit. This makes it possible to use a receiv-

ing filter having extremely narrow bandwidth. The synchronization requires the use of an external

frequency/time standard. The broadcasts of standard time-and-frequency radio stations such as

WWV or WWVH can be used for this purpose. Frequency dividers are employed to obtain the nec-

essary synchronizing frequencies. A tone or pulse is generated in the receiver output for a particular

bit if, but only if, the average signal voltage exceeds a certain value over the duration of that bit. False

signals, such as can be caused by filter ringing, sferics, or ignition noise, are generally ignored, be-

cause they rarely produce sufficient average bit voltage.

Experiments with synchronized communications have shown that the improvement in S/N

ratio, compared with nonsynchronized systems, is several decibels at low to moderate data speeds.

Further improvement can be obtained by the use of DSP.

How DSP Can Improve Reception

In DSP with analog modes such as SSB or SSTV, the signals are first changed into digital form by

A/D conversion. Then the digital data is cleaned up so the pulse timing and amplitude adhere

Specialized Wireless Modes 435

25-16 Block diagram of a dual-diversity radio receiver system.

strictly to the protocol (standards) for the type of digital data being used. Finally, the digital signal

is changed back to the original voice or video by D/A conversion.

Digital signal processing can extend the range of a wireless communications circuit, because it

allows reception under worse conditions than would be possible without it. Digital signal process-

ing also improves the quality of marginal signals, so that the receiving equipment or operator makes

fewer errors. In circuits that use only digital modes, A/D and D/A conversion are irrelevant, but

DSP can still be used to clean up the signal. This improves the accuracy of the system, and also

makes it possible to copy data over and over many times (that is, to produce multigeneration dupli-

cates).

The DSP circuit minimizes noise and interference in a received digital signal as shown in Fig.

25-17. A hypothetical signal before DSP is shown at the top; the signal after processing is shown at

the bottom. If the incoming signal is above a certain level for an interval of time, the DSP output is

high (also called logic 1). If the level is below the critical point for a time interval, then the output is

low (also called logic 0).

Multiplexing

Signals in a communications channel or band can be intertwined, or multiplexed, in various ways.

The most common methods are frequency division multiplexing (FDM) and time division multiplex-

ing (TDM). In FDM, the channel is broken down into subchannels. The carrier frequencies of the

signals are spaced so they do not overlap. Each signal is independent of the others. In TDM, signals

are broken into segments by time, and then the segments are transferred in a rotating sequence. The

receiver must be synchronized with the transmitter by means of a time standard such as WWV.

Multiplexing requires an encoder that combines or intertwines the signals in the transmitter, and a

decoder that separates or untangles the signals in the receiver.

436 Wireless Transmitters and Receivers

25-17 Digital signal

processing can clean

up a signal,

improving reception.

Spread Spectrum

In spread-spectrum communications, the main carrier frequency is rapidly varied independently of

signal modulation, and the receiver is programmed to follow. As a result, the probability of cata-

strophic interference, in which one strong interfering signal can obliterate the desired signal, is near

zero. It is difficult for unauthorized people to eavesdrop on a spread-spectrum communications link

unless they gain access to the sequencing code, also known as the frequency spreading function. Such a

function can be complex, and can be kept secret. If the transmitting and receiving operator do not

divulge the function to anyone, and if they do not tell anyone about the existence of their contact,

then no one else on the band will know the contact is taking place.

During a spread-spectrum contact between a given transmitter and receiver, the operating fre-

quency can fluctuate over a range of kilohertz, megahertz, or tens of megahertz. As a band becomes

occupied with an increasing number of spread-spectrum signals, the overall noise level in the band

appears to increase. Therefore, there is a practical limit to the number of spread-spectrum contacts

that a band can handle. This limit is roughly the same as it would be if all the signals were constant

in frequency, and had their own discrete channels.

A common method of generating spread spectrum is frequency hopping. The transmitter has a

list of channels that it follows in a certain order. The receiver must be programmed with this same

list, in the same order, and must be synchronized with the transmitter. The dwell time is the inter-

val at which the frequency changes occur, which is the same as the length of time that the signal re-

mains on any given frequency. The dwell time should be short enough so that a signal will not be

noticed, and not cause interference, on any frequency. There are numerous dwell frequencies, so the

signal energy is diluted to the extent that, if someone tunes to any particular frequency in the se-

quence, the signal is not noticeable.

Another way to get spread spectrum, called frequency sweeping, is to frequency-modulate the

main transmitted carrier with a waveform that guides it up and down over the assigned band. This

FM is independent of signal intelligence. A receiver can intercept the signal if, but only if, its tun-

ing varies according to the same waveform, over the same band, at the same frequency, and in the

same phase as that of the transmitter.

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. A radio wave has a frequency of 1.55 MHz. The highest modulating frequency that can be

used effectively is about

(a) 1.55 kHz.

(b) 15.5 kHz.

(d) 1.55 MHz.

2. The reflected wave

(a) arrives in phase with the direct wave.

(b) arrives out of phase with the direct wave.

(d) is always horizontally polarized.

Quiz 437

3. An advantage of FSK over on/off keying is the fact that FSK

(a) offers better frequency stability.

(b) can provide faster data rates.

(d) Forget it! On-off keying is just as good as FSK.

4. The highest layer of the ionosphere is

(a) the D layer.

(b) the E layer.

(d) none of the above.

5. If an AM signal is modulated with audio having frequencies up to 5 kHz, then what is the

complete signal bandwidth?

(a) 10 kHz

(b) 20 kHz

(d) 500 kHz

6. An LSB, suppressed-carrier signal can be demodulated by

(a) an envelope detector.

(b) a diode.

(d) a product detector.

7. Which of the following modes is used to send image data over telephone lines?

(a) On/off keying

(b) Fax

(d) Product detection

8. The S/N ratio is a measure of

(a) the sensitivity of a receiver.

(b) the selectivity of a receiver.

(d) the efficiency of a transmitter.

9. Suppose an SSB suppressed carrier is at 14.335 MHz, and audio data is contained in a band

from 14.335 to 14.338 MHz. What is this mode?

(a) DSB

(b) LSB

(d) AFSK

438 Wireless Transmitters and Receivers

10. A receiver that responds to a desired signal, but not to another signal very close by in

frequency, has good

(a) sensitivity.

(b) noise figure.

(d) selectivity.

11. Fill in the blank in the following sentence to make it true: “The deviation of a narrowband

voice FM signal normally extends up to

either side of the unmodulated-carrier frequency.”

(a) 3 kHz

(b) 5 kHz

(d) 3 MHz

12. An FM detector with built-in limiting is

(a) a ratio detector.

(b) a discriminator.

(d) a product detector.

13. In which mode of PM does the peak power level of the pulses vary?

(a) PAM

(b) PDM

(d) PFM

14. A continuously variable signal (such as music audio) can be recovered from a signal having

only a few discrete levels or states by means of

(a) a ratio detector.

(b) a D/A converter.

(d) an envelope detector.

15. In which mode are signals intertwined in the time domain at the transmitter, and then

separated again at the receiver?

(a) FDM

(b) AFSK

(d) None of the above

16. Which of the following modes can be demodulated with an envelope detector?

(a) AM

(b) CW

Quiz 439

(d) USB

17. The bandwidth of a fax signal is kept narrow by

(a) sending the data at a slow rate of speed.

(b) maximizing the number of digital states.

(d) using pulse modulation.

18. The dynamic range in a superhet is largely influenced by the performance of the

(a) local oscillator.

(b) product detector.

(d) selectivity in the IF chain.

19. Frequency sweeping can be used to get a transmitter to produce

(a) spread-spectrum signals.

(b) time division multiplexed signals.

(d) double sideband, suppressed carrier signals.

20. Fill in the blank to make the following sentence true: “The reception of

can be

improved by the use of DSP.”

(a) SSB signals

(b) SSTV signals

(d) any of the above

440 Wireless Transmitters and Receivers