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Audio Frequency Interference (AFI) {Reprinted from Radio Communication

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Audio Frequency Interference (AFI) {Reprinted from Radio Communication Tony Tregale, VK3QQ FEDERAL EMC CO-ORDINATOR 38 Wattle Drive. Watsonia. Vic 3087 This month we have reprinted an article which appeared in AR in September 1974. Although the monetary values have increased since this time the basic problems are still the same and the solutions may help many of our younger amateurs and members. Audio frequency interference (AFI) {Reprinted from Radio Communication. April 1973) P W Waters, G30JV (Reprinted from Amateur Radio. September 1974) 8 Gay Bowers. Hockley. Essex. UK THE PROBLEM THE CAUSE The current boom in hi-fi sales has led to an Before discussing the various ways in increase in the number of cases of interference which this kind of interference can be caused by radio transmitters operating in prevented, it is necessary to understand how close proximity to audio equipment. Almost the RF signal reaches the amplifier, is rectified, all audio equipment now being produced for and emerges at the speaker as an unwanted the domestic market is enti rely solid state and signal. Fig 1 shows typical audio amplifier low this changeover from valves to transistors has signal stages. In the case of the transistor coincided with a hi-fi boom, making it difficult version notice the base/emitter junction. This to assess to what extent transistors are forms a fairly effective junction diode and any responsible for the increase in the number of RF signal that reaches this stage will be cases of interference. Certainly transistorised rectified and passed on as an audio signal to equipment appears to be far more susceptible the following stages. Similar comments apply Fig 1. Typical first stages in audio amplifiers than the older valve equipment. Also of to the valve stage. RF energy reaching the significance is the now widespread usage of which are open to RF interference, (a) grid of the valve is likely to be rectified by transistor, and (b) valve. magnetic cartridges which require amplifier non-linear action and the resultant demodu- sensitivities of the order of 3 or 4 mV. This lated signal passed on through the following THE TREATMENT usually necessitates one or two additional stages as an audio signal. With the modern There are two basic ways of tackling stages of amplification, whereas the older hi-fi amplifier, having a high overall gain and interference in audio equipment. Either the type of crystal and ceramic cartridges having an output rating of 10 to 15 W/channel or even circuit can be modified to prevent the far higher outputs require far less gain from more, RF breakthrough can be dramatic. rectification occurring, or the RF signals can the amplifier. be prevented from reaching the amplifier Unlike television interference, there is There are a number of paths the radio circuitry by fitting filters to the various usually very little that can be done at the signal can take to reach the circuitry of an connecting leads. transmitter end to prevent the trouble. Apart audio amplifier. In high RF fields even direct Dealing firstly with the rectification prob- from reducing power, moving aerials or pickup by the circuit board is possible. lem, it has already been shown how, in a switching off altogether, the cure must be at Normally, however, the signal is fed to the transistor amplifier, trouble usually arises the complainant's end. Like all kinds of amplifier via the various connecting cables, when RF reaches the base/emitter junction of interference this poses a social problem. The which make very good "aerials". Two of the a transistor. Similarly, in a valve amplifier, too average cost of a stereo radiogram is around most common sources of trouble seem to be much RF energy on the grid can also result in £80-£100. and for a hi-fi installation com- the mains cable and the speaker leads. The rectification. Clearly, if the RF signal can be prising separate amplifier, speakers, turntable mains connection, because of the modern bypassed to earth without degrading the and possibly VHF tuner the price rises to the ring mains circuits, results in the entire house wanted audio signal then the problem will be region of £150 to £200. Any person having power wiring being connected to the amplifier solved. The most obvious solution that comes spent this amount of money is not going to and acting as a long-wire "aerial". to mind is to fit a capacitor between input and take kindly to hearing a burst of CW or earth of the amplifier of such a value that "distorted" SSB coming through in the middle As for the speaker leads, stereo reproduc- while it looks like a near short circuit at RF it of his or her favourite record. Unfortunately, tion requires a pair of speakers to be offers a high impedance to audio frequencies. telling your neighbour that the interference is separated from the amplifier and turntable Unfortunately, between the input and the not the fault of the transmitter, but his own unit in order to obtain the stereo effect. In transistor base or valve grid may be several equipment, is not going to ease the matter practice this means that the speakers are very inches of wire or circuit board, switch even though it is probably true. often positioned several yards away from the contacts with their associated connecting Of course, each case has to be dealt with on amplifier, the length of twin flex usually used leads and other components. Bypassing at its merits and no hard and fast rules can be for this purpose making a good "aerial", the remote input socket of the amplifier may laid down. It is of prime importance to use possibly resonant on or near one of the HF therefore not be sufficient. tact, patience and common sense. A special amateur bands. This, of course, ignores the A far more effective method of preventing mention should be made here of the case of other connecting cables from record turn- rectification is to solder a capacitor directly interference from an AM transmitter. A table. tape unit. VHF tuner etc. It is not across the base/emitter junction or between sensitive hi-fi system may well be picking up surprising, therefore, that a substantial RF control grid and cathode to prevent an RF such a signal and relaying it in "full frequency signal can find its way into an amplifier potential difference between the electrodes. stereo sound" — a situation which calls for several hundred feet distant from the Fig 2 illustrates the required circuit special tact transmitter. modification. The capacitance value C can !>.,„,. 4(| _ AMATEUR RADIO. Xmvml.cr I MCI .
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