Chapter 2. Tuners and radio receivers

58



Tuners and Radio Receivers



9 clarity - which requires freedom from unwanted interference and

noise, whether this originates within the receiver circuit or from external

sources;

9 linearity - which implies an absence of any distortion of the signal

during the transmission/reception process.

These requirements for receiver performance will be discussed later under

the heading of receiver design. However, the quality of the signal heard

by the listener depends very largely on the nature of the signal present at

the receiver. This depefids, in the first place, on the transmitter and the

transmission techniques employed.

In normal public service broadcasting - where it is required that the

signal shall be received, more or less uniformly, throughout the entire

service area - the transmitter aerial is designed so that it has a uniform,

360 ~, dispersal pattern. Also the horizontal shape of the transmission

'lobe' (the conventional pictorial representation of relative signal strength,

as a function of the angle) is as shown in Fig. 2.1.

The influence of ground attenuation, and the curvature of the earth's

surface, mean that in this type of transmission pattern the signal strength,

gets progressively weaker as the height above ground level of the receiving

aerial gets less, except in the immediate neighbourhood of the transmitter.

There are a few exceptions to this rule, as will be shown later, but it is

generally true, and implies that the higher the receiver aerial can be

placed, in general the better.



THE INFLUENCE OF THE IONOSPHERE

The biggest modifying influence on the way the signal reaches the receiver

is the presence of a reflecting - or, more strictly, refracting - ionised



" - ' - - - " - -



Fig. 2.1



Typical transmitter aerial lobe pattern.



Tuners and Radio Receivers



59



band of gases in the outer regions of the earth's atmosphere. This is

called the ionosphere and is due to the incidence of a massive bombardment

of energetic particles on the outer layers of the atmosphere, together with

ultra-violet and other electromagnetic radiation, mainly from the sun.

This has the general pattern shown in Fig. 2.2 if plotted as a measure

of electron density against height from the surface. Because it is dependent

on radiation from the sun, its strength and height will depend on whether

the earth is exposed to the sun's radiation (daytime) or protected by its

shadow (night).

As the predominant gases in the earth's atmosphere are oxygen and

nitrogen, with hydrogen in the upper reaches, and as these gases tend to

separate somewhat according to their relative densities, there are three

effective layers in the ionosphere. These are the 'D' (lowest) layer, which

contains ionised oxygen/ozone; the 'E' layer, richer in ionised nitrogen

and nitrogen compounds; and the 'F' layer (highest), which largely consists

of ionised hydrogen.

Since the density of the gases in the lower layers is greater, there is a

much greater probability that the ions will recombine and disappear, in

the absence of any sustaining radiation. This occurs as the result of

normal collisions of the particles within the gas, so both the 'D' and the

'E' layers tend to fade away as night falls, leaving only the more rarified

'F' layer. Because of the lower gas pressure, molecular collisions will

occur more rarely in the 'F' layer, but here the region of maximum

electron density tends to vary in mean height above ground level.



H e ~ t (Kin)



00



m



00



""



'F' myer (H-)



2OO

100-



'D' layer



0 -



Fig. 2.2



The electron density in the ionosphere.



E~'~n ~



(m~)



Tuners and Radio Receivers



60

Critical frequency



The way in which radio waves are refracted by the ionosphere, shown

schematically in Fig. 2.3, is strongly dependent on their frequency, with

a 'critical frequency' ('Fc') dependent on electron density, per cubic

metre, according to the equation

Fc



=



9V'Nmax



where Nmax is the maximum density of electrons/cubic metre within the

layer. Also, the penetration of the ionosphere by radio waves increases as

the frequency is increased. So certain frequency bands will tend to be

refracted back towards the earth's surface at different heights, giving

different transmitter to receiver distances for optimum reception, as shown

in Fig. 2.4, while some will not be refracted enough, and will continue

on into outer space.

The dependence of radio transmission on ionosphere conditions, which,

in turn depends on time of day, time of year, geographical latitude, and

'sun spot' activity, has led to the term 'MUF' or maximum usable frequency,

for such transmissions.

Also, because of the way in which different parts of the radio frequency

spectrum are affected differently by the possibility of ionospheric refraction,

the frequency spectrum is classified as shown in Table 2.1. In this VLF

and LF signals are strongly refracted by the 'D' layer, when present, MF

signals by the 'E' and 'F' layers, and HF signals only by the 'F' layer, or

not at all.

Additionally, the associated wavelengths of the transmissions (from

100000-1000 m in the case of the VLF and LF signals, are so long that

the earth's surface appears to be smooth, and there is a substantial



F


\



///

Earth's surface



\



Tx



Fig. 2.3



The refraction of radio waves by the ionosphere.



61



Tuners and Radio Receivers

~J



'V



Esrlh's surface



Fig. 2.4



The influence of frequency on the optimum transmitter to receiver

distance- the 'skip distance'.



Classification of radio frequency spectrum



Table 2.1



i



3-30

30-300

300-3000

3-30

30-300

300-3000

3-30



VLF

LF

MF

HF

VHF

UHF

SHF

i



,,,,,



,



,



kHz

kHz

kHz

MHz

MHz

MHz

GHz



,



reflected 'ground wave' which combines with the direct and reflected

radiation to form what is known as a 'space wave'. This space wave is

capable of propagation over very long distances, especially during daylight

hours when the 'D' and 'E' layers are strong.



VHF/SHF effects

For the VHF to SHF regions, different effects come into play, with very

heavy attenuation of the transmitted signals, beyond direct line-of-sight

paths, due to the intrusions of various things which will absorb the signal,

such as trees, houses, and undulations in the terrain. However, temperature inversion layers in the earth's atmosphere, and horizontal striations

in atmospheric humidity, also provide mechanisms, especially at the higher

end of the frequency spectrum, where the line of sight paths may be

extended somewhat to follow the curvature of the earth.



Tuners and Radio Receivers



62



Only certain parts of the available radio frequency (RF) spectrum are

allocated for existing or future commercial broadcast use. These have

been subclassified as shown in Table 2.2, with the terms 'Band 1' to

'Band 6' being employed to refer to those regions used for TV and FM

radio.

The internationally agreed FM channel allocations ranged, originally,

from Channel 1 at 87.2-87.4 MHz, to Channel 60, at 104.9-105.1 MHz,

based on 300 kHz centre-channel frequency spacings. However, this allocation did not take into account the enormous future proliferation of local

transmitting stations, and the centre-channel spacings are now located at

100 kHz intervals.

Depending on transmitted power, transmitters will usually only be

operated at the same frequency where they are located at sites which are

remote from each other. Although this range of operating frequencies

somewhat overlaps the lower end of 'Band 2', all of the UK operating

frequencies stay within this band.

Radio broadcast band allocations



Table 2.2

ii



l



ii



Wavelength

LW

MW

SW



i



Allocation

150-285 kHz

525-1605 kHz

5.95-6.2 MHz

7.1-7.3 MHz

9.5-9.775 MHz

11.7-11.975 MHz

15.1-15.45 MHz

17.7-17.9 MHz

21.45-21.75 MHz

25.5-26.1 MHz



Band



49

40

30

25

19

16

13

11



M

M

M

M

M

M

M

M



Note National broadcasting authorities may often overspill these frequency

limits.

,.



Band



,=.



.



Wavelength allocation

4 1 - 6 8 MHz

87.5-108 MHz

174-223 MHz

470-585 MHz

610-960 MHz

11.7-12.5 GHz



I



II

III

IV

V

V|

i



Tuners and Radio Receivers



63



WHY Vl-~ TRANSMISSIONS?

In the early days of broadcasting, when the only reliable long to medium

distance transmissions were thought to be those in the LF-MF regions of

the radio spectrum, (and, indeed, the HF bands were handed over to

amateur use because they were thought to be of only limited usefulness),

broadcast transmitters were few and far between. Also the broadcasting

authorities did not aspire to a universal coverage of their national audiences. Under these circumstances, individual transmitters could broadcast

a high quality, full frequency range signal, without problems due to

adjacent channel interference.

However, with the growth of the aspirations of the broadcasting authorities, and the expectations of the listening public, there has been an

enormous proliferation of radio transmitters. There are now some 440 of

these in the UK alone, on LW, MW and Band 2 VHF allocations, and

this ignores the additional 2400 odd separate national and local TV

transmissions.

If all the radio broadcast services were to be accommodated within the

UK's wavelength allocations on the LW and MW bands, the congestion

would be intolerable. Large numbers would have to share the same

frequencies, with chaotic mutual interference under any reception conditions

in which any one became receivable in an adjacent reception area.



Frequency choice

The decision was therefore forced that the choice of frequencies for all

major broadcast services, with the exception of pre-existing stations, must

be such that the area covered was largely line-of-sight, and unaffected by

whether it was day or night.

It is true that there are rare occasions when, even on Band 2 VHF,

there are unexpected long-distance intrusions of transmissions. This is

due to the occasional formation of an intense ionisation band in the lower

regions of the ionosphere, known as 'sporadic E'. Its occurrence is unpredictable and the reasons for its occurrences are unknown, although it has

been attributed to excess 'sun spot' activity, or to local thermal ionisation,

as a result of the shearing action of high velocity upper atmosphere

winds.

In the case of the LW and MW broadcasts, international agreements

aimed at reducing the extent of adjacent channel interference have continually restricted the bandwidth allocations available to the broadcasting

authorities. For MW at present, and for LW broadcasts as from 1 February

1988- ironically the centenary of Hertz's original experiment- the channel

separation is 9 kHz and the consequent maximum transmitted audio

bandwidth is 4.5 kHz.



64



Tuners and Radio Receivers



In practice, not all broadcasting authorities conform to this restraint,

and even those that do interpret it as 'fiat from 30 Hz to 4.5 kHz and - 5 0

dB at 5.5 kHz' or more leniently, as 'fiat to 5 kHz, with a more gentle

roU-off to 9 kHz'. However, by comparison with the earlier accepted

standards for MW AM broadcasting, of 30 H z - 12 kHz, • 1 dB, and with

less than 1% THD at 1 kHz, 80% modulation, the current AM standards

are poor.

One also may suspect that the relatively low Standards of transmission

quality practicable with LF/MF AM broadcasting encourages some broadcasting authorities to relax their standards in this field, and engage in

other, quality degrading, practices aimed at lessening their electricity

bills.

AM or FM?

Having seen that there is little scope for high quality AM transmissions

on the existing LW and MW broadcast bands, and that VHF line-of-sight

transmissions are the only ones offering adequate bandwidth and freedom

from adjacent channel interference, the question remains as to what style

of modulation should be adopted to combine the programme signal with

the RF carrier.



Modulation systems

Two basic choices exist, that of modulating the amplitude of the RF

carrier, (AM), or of modulating its frequency, (FM), as shown in Fig.

2.5. The technical advantages of FM are substantial, and these were

confirmed in a practical field trial in the early 1950s carried out by the

BBC, in which the majority of the experimental listening panel expressed

a clear preference for the FM system.

The relative qualities of the two systems can be summarised as follows.

AM is:

9 the simplest type of receiver to construct

9 not usually subject to low distortion in the recovered signal

9 prone to impulse-type (e.g. car ignition) interference and to

'atmospherics'

9 possibly subject to 'fading'

9 prone to adjacent channel or co-channel interference

9 affected by tuning and by tuned circuit characteristics in its frequency

response.

FM:

9 requires more complex and sophisticated receiver circuitry



65



Tuners and Radio Receivers



s



Fig. 2.5



Carrier modulation systems.



9

s

9

9



can give very low signal distortion

ensures all received signals will be at the same apparent strength

is immune to fading

is immune to adjacent channel and co-channel interference, provided

that the intruding signals are less strong.

9 has, potentially, a flat frequency response, unaffected by tuning or

tuned circuit characteristics

9 will reject AM and impulse-type interference

9 makes more efficient use of available transmitter power, and gives, in

consequence, a larger usable reception area.



On the debit side, the transmission of an FM signal requires a much

greater bandwidth than the equivalent AM one, in a ratio of about 6:1.

The bandwidth requirements may be calculated approximately from the

formula

B. = 2M + 2 D K



where B. is the necessary bandwidth, M is the maximum modulation

frequency in Hz, D is the peak deviation in Hz, and K is an operational

constant (= 1 for a mono signal).

However, lack of space within the band is not a significant problem at

VHF, owing to the relatively restricted geographical area covered by any

transmitter. So Band 2 VHF/FM has become a worldwide choice for high

quality transmissions, where the limitations are imposed more by the



66



Tuners and Radio Receivers



distribution method employed to feed programme signals to the transmitter,

and by the requirements of the stereo encoding/decoding process than by

the transmitter or receiver.



FM BROADCAST STANDARDS

It is internationally agreed that the FM frequency deviation will be 75

kHz, for 100% modulation. The stereo signal will be encoded, where

present, by the Zenith-GE 38 kHz sub-carrier system, using a 19 kHz + 2

Hz pilot tone, whose phase stability is better than 3~ with reference to the

38 kHz sub-carrier. Any residual 38 kHz sub-carrier signal present in the

composite stereo output shall be less than 1%.

Local agreements, in Europe, specify a 50 its transmission pre-emphasis.

In the USA and Japan, the agreed pre-emphasis time constant is 75 tts.

This gives a slightly better receiver S/N ratio, but a somewhat greater

proneness to overload, with necessary clipping, at high audio frequencies.



STEREO ENCODING/DECODING

One of the major attractions of the FM broadcasting system is that it

allows the transmission of a high quality stereo signal, without significant

degradation of audio q u a l i t y - although there is some worsening of S/N

ratio. For this purpose the permitted transmitter bandwidth is 240 kHz,

which allows an audio bandwidth, on a stereo signal, of 30 Hz-15 kHz,

at 90% modulation levels. Lower modulation levels would allow a more

extended high-frequency audio bandwidth, up to the 'zero transmission

above 18.5 kHz' limit imposed by the stereo encoding system.

It is not known that any FM broadcasting systems significantly exceed

the 30 H z - 1 5 kHz audio bandwidth levels.

Because the 19 kHz stereo pilot tone is limited to 10% peak modulation,

it does not cause these agreed bandwidth requirements to be exceeded.



GE/ZENITH 'PILOT TONE' SYSTEM

This operates in a manner which produces a high-quality 'mono' signal in

a receiver not adapted to decode the stereo information, by the transmission

of a composite signal of the kind represented in Fig. 2.6. In this the

combined left-hand channel and right-hand channel (L+R) - mono signal is transmitted normally in the 30 H z - 1 5 kHz part of the spectrum,

with a maximum modulation of 90% of the permitted 75 kHz deviation.

An additional signal, composed of the difference signal between these



67



Tuners and Radio Receivers

%

10090-



i!



R



(mono)

'L+R'

30 Hz-15 kHz



'L-R'



lO

15



Fig. 2.6



19



23



38



53



v



kHz



The Zenith-GE 'pilot tone' stereophonic system.



channels, ( L - R ) , is then added as a modulation on a suppressed 38 kHz

sub-carrier. So that the total modulation energy will be the same, after

demodulation, the modulation depth of the combined (L-R) signal is held

to 45% of the permitted maximum excursion. This gives a peak deviation

for the transmitted carrier which is the same as that for the 'mono'

channel.



Deemling

This stereo signal can be decoded in two separate ways, as shown in Figs

2.7 and 2.8. In essence, both of these operate by the addition of the

two ( L + R ) and ( L - R ) signals to give LH channel information only, and

the subtraction of these to give the RH channel information only.

In the circuit of Fig. 2.7, this process is carried out by recovering the

separate signals, and then using a matrix circuit to add or subtract them.

In the circuit of Fig. 2.8, an equivalent process is done by sequentially

sampling the composite signal, using the regenerated 38 kHz sub-carrier

to operate a switching mechanism.

Advocates of the matrix addition method of Fig. 2.7 have claimed

that this allows a better decoder signal-to-noise (S/N) ratio than that of

the sampling system. This is only true if the input bandwidth of the

sampling system is sufficiently large to allow noise signals centred on the

harmonics of the switching frequency also to be commutated down into

the audio spectrum.

Provided that adequate input filtration is employed in both cases there

is no operational advantage to either method. Because the system shown

in Fig. 2.8 is more easily incorporated within an integrated circuit, it is

very much the preferred method in contemporary receivers.

In both cases it is essential that the relative phase of the regenerated



Tuners and Radio Receivers



68



Low-pass finer

' mono



'



i



Corr~

~Xx~~an~



.....



" 1



(L,+,R!+(L- R)=,~.



J M'~ I (L+R)-(L:R)=,R

L ..... j-



~~n~.



.



.



.



-~,~--



:



Stotoo output



.



v



~,o,,~. ,,,.,. ,:r~~,,:~

L

Fig. 2.7



,~kHz._ [



i



__ _!



Matrix addition type of stereo decoder.

Fmqmm~ ~

~ ~ ~ e t



~kX~f~r



19 kHz I



x2



J o 8

.



.



.



.



.



J



38kHz I



,,



T



~



II



I 38kHz

I



i,..~__~~_



I



'



L



-



60 kHz



'--



Fig. 2.8



-



. . . .



R

~



-O



Synchronous switching stereo decoder.



38 kHz sub-carrier is accurately related to the composite incoming signal.

Errors in this respect will degrade the 35-40 dB (maximum) channel

separation expected with this system.

Because the line bandwidth or sampling frequency of the studio to

transmitter programme link may not be compatible with the stereo encoded

signal, this is normally encoded on site, at the transmitter, from the

received LH and RH channel information. This is done by the method

shown in Fig. 2.9.