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The principle of frequency modulation (FM) is that the earner wave of a transmitter receives a constant level of modulation, but that its frequency deviates by a predetermined amount above and below the nominal figure. The standard deviation has been for many years 75 kHz. The applied modulation decides the number of deviations that occur per second, e.g. an applied modulation of 400 Hz produces 400 deviations every second, and so on. The system is capable of carrying the full AF range with ease. This method of modulation had been tested as long ago as just before the First World War, when it seemed likely to prove easier to achieve with the primitive equipment then available, but improvements to tubes (valves) and other technical advances reversed this situation and FM fell into long disuse.
In 1922 a mathematician called J.R Carson ‘proved’ to his own satisfaction that FM would never be a practical possibility (despite the 1913 tests) since, he averred, the bandwidth requirements would be colossal, or even ‘infinite’. Shortly after this E.H. Armstrong commenced experimental FM transmissions. He had concluded that the manmade interference and atmospherics which plagued radio communications were largely in effect amplitude modulated, and he saw in FM the means of combating them. Influenced by Carson’s gloomy predictions Armstrong at first tried very narrow deviation bandwidths, and found that there was no discernible improvement in signal to noise ratio over AM. Undeterred, he tried wider bandwidths with very encouraging results. He was soon able to show that an FM transmitter on VHF uses less bandwidth, expressed as a percentage of the carrier frequency, than does an AM station on MW!
Armstrong’s work was for some years carried out in Marcellus Hart Research Laboratory at New York’s Columbia University, followed by a short period in 1934 when he operated a 44 MHz, 2 kW transmitter situated on the top of the Empire State Building, then the world’s tallest structure. In 1937 he set about building a high power transmitter at Alpine, New Jersey. Transmissions from this location proved conclusively that FM was able to offer substantial advantages over AM. Since the effective power of the transmitter is not affected by the depth of the modulation, nor its frequency, it is able to operate continuously at or near its maximum. Not only are the signals to a large extent immune from interference as mentioned above, but there is considerable resistance to the other kind of interference due to another station working on the same frequency. This would have to have a power of over 50% of the desired station at the receiver before it could be heard. By comparison, even a very weak AM station can spoil the reception of another in certain conditions. This co-channel interference, as it is called, is particularly trouble some on MW during the hours of darkness, when the effective range of transmitters is greatly increased. Thus FM is able to provide excellent sound quality unimpaired by fading or the several forms of interference.
By the 1940s FM had been adopted generally in the US and magnificent receivers were being manufactured to take advantage of its potentialities. Given that the war and its aftermath were bound to delay the introduction of FM to the UK to a certain extent, progress here was still pitifully slow. The BBC installed a 25 kW FM experimental transmitter at Wrotham alongside another for a system called HIFAM (HIgh Fidelity AM Broadcasting) which seems to have been rather a superfluous exercise since HIFAM had already been tried and rejected in the USA. In the event a full service was not established by the BBC until 1955, and it started with the crippling disadvantage that it merely duplicated the existing Home, Light and Third programs, thus providing little or no incentive for the public to buy FM receivers. Listeners on the whole remained indifferent to the advantages that FM offered in the way of reduced interference and better sound quality since this could be fully realised only if an expensive new aerial system was installed — and why do that to get the same programs? It is fair to say that the absence of any competition in FM broadcasting held back its popularity for twenty years or more.
Some aspects of FM receiver design
International standards having been set down many years ago, receiver design must to a great extent follow a pattern. The most favored band for FM broadcasting (although others are in use) is between about 87 and 108 MHz. Note that until comparatively recently this band was occupied above 100 MHz in the UK by mobile radio services for the police and fire services, and it was not until these had been banished to much higher frequencies that commercial broadcasting on FM could take place. Channel spacing is nominally 220 kHz, which is ample to allow for the standard deviation of 75kHz (i.e. a total of 150 Hz per channel). In practice, however, some stations are slightly offset from the frequencies allocated to them; and the actual spacing may differ. The standard IF for FM receivers is 10.7 MHz, and for once this seems to be strictly adhered to. The bandwidth of the IF transformers is likely to be rather more than 150 kHz, to allow for ‘drift’ in the RF tuning during the warming-up period, the usual figure being 200 kHz.
A different type of detector has to be used in FM receivers which will respond to its different form of modulation, and one which enjoyed early popularity in the US was that same Foster-Seeley that had been developed originally for use in AFC systems. However this detector also would respond to any stray AM that may have found its way into a receiver and it had to be preceded by a ‘limiter’ tubes (valves) which passed FM but not AM. A later development, called the ratio detector, is largely self-limiting and was used in all UK FM receivers.
It was discovered in the US that the signal to noise ratio could be further improved if the higher audio frequencies were artificially boosted prior to trans mission, this process being called pre-emphasis. At the receiver de-emphasis is applied by a simple resistance-capacity network in the AF circuitry following the detector.
Design aspects of British FM receivers
The lack of competition in broadcasting already mentioned dictated to a large extent how FM receivers should be designed. For instance, at first a number of firms tried the experiment of making small FM only sets, but their restriction only to BBC transmissions prevented them from becoming really popular. Combined AM/FM receivers then appeared, the usual coverage being M LW and VHF, short waves being dumped. As regards AM the usual four- tubes (valves) plus rectifier circuitry was favored, with a separate tuner for FM. Dual frequency IF transformers were used capable of working at either the usual 465 kHz or thereabouts for AM and 10.7 MHz for FM. To boost the sensitivity on FM the AM frequency changer was switched to act as an FM IF amplifier.
By the time these AM/FM sets appeared the small B9G all-glass tubes (valves) were almost universally used in UK receivers and the vast majority of AC- only models had a line-up consisting of an ECH81 triode-heptode frequency changer, an EF89 RF pentode as IF amplifier, an EABC8O or — 81 triple diode-triode as AM detector, FM ratio detector and AF amplifier, and an EL84 output pentode. The rectifier would be an EZ8O or perhaps a metal contact cooled type. Some early VHF tuner units used a pair of EF8O high slope RF pentodes as RF amplifier and self-oscillating frequency changer but most makers preferred a special double triode, the ECC85, which did both jobs. In AC/DC receivers the tube (valve) used were, respectively, the UCH81, UF89, UABC80/81, UL84, UY85 and UCC85.