Tuners and radio receivers

BACKGROUND

The nineteenth century was a time of great technical interest and experiment in both Europe and the USA. The newly evolved disciplines of scientific analysis were being applied to the discoveries which surrounded the experimenters and which, in turn, led to further discoveries.

One such happening occurred in 1864, when James Clerk Maxwell, an Edinburgh mathematician, sought to express Michael Faraday’s earlier law of magnetic induction in proper mathematical form. It became apparent from Maxwell’s equations that an alternating electromagnetic field would give rise to the radiation of electromagnetic energy.

This possibility was put to the test in 1888 by Heinrich Hertz, a German physicist. He established that this did indeed happen, and radio transmission became a fact. By 1901, Marconi, using primitive spark oscillator equipment, had transmitted a radio signal in Morse code across the Atlantic. By 1922 the first commercial public broadcasts had begun for news and entertainment.

By this time, de Forrest’s introduction of a control grid into Fleming’s thermionic diode had made the design of high power radio transmitters a sensible engineering proposition. It had also made possible the design of sensitive and robust receivers. However, the problems of the system remain the same, and the improvements in contemporary equipment are merely the result of better solutions to these, in terms of components or circuit design.

BASIC REQUIREMENTS

These are

• selectivity – to be able to select a preferred signal from a jumble of competing programmes;

• sensitivity – to be sure of being able to receive it reliably;

• stability – to be able to retain the chosen signal during the required reception period;

• predictability – to be able to identify and locate the required reception channel;

• clarity – which requires freedom from unwanted interference and noise, whether this originates within the receiver circuit or from external sources;

• 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 depends, 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 – ionized 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.

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

c=9√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 1 00 000–1000 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

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.

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...