scope:

Introduction

In many countries an acute problem has arisen resulting from interference to television receivers, radio receivers and associated equipment caused by the increased use of radio transmitters in the frequency range 1.5 MHz to 30 MHz.

Once course of action which can be taken to help alleviate the problem is to ensure that the equipment liable to be affected has an adequate degree of immunity from these r.f. fields.

Investigations into interference to television receivers, radio receivers and associated equipment have shown that, in the frequency range considered, the asymmetric currents induce by the interfering r.f. field on the antenna cable shield, on the mains cable and on other leads are the major causes of interference. In addition to the induced asymmetric currents, interference may also result from the fundamental or harmonics of the transmitted signal appearing at the antenna input either by direct pick-up of the signal by the antenna or because the antenna cable has a high transfer impedance.

In this report, a method of measurement of immunity from the asymmetric interference currents is described. It is known as the current injection method. In this method, interference r.f. currents are injected onto the leads and cables associated with the equipment under test by a suitable coupling network, thus simulating the effects of an ambient r.f. field.

The immunity of an equipment is expressed in terms of the e.m.f. of an interference source with 150 Ω internal impedance and which generates just perceptible interference.

Immunity values are given which, if met, will ensure a reasonable degree of protection in practical situation in the frequency range 26 MHz to 30 MHz.

Note - The immunity figures indicated in this report are not C.I.S.P.R. recommended limits, but only suggested values.

Besides the interfering signal (the unwanted signal), a standard r.f. test signal (the wanted signal) is applied to the equipment under test. The purpose of this wanted input signal firstly is to set the receiver under test to the proper operating conditions, such as a synchronized picture and the normal a.g.c point in r.f. and i.f. amplifiers. The wanted signal also enables the detection of some interference effects which would not be evident in its absence, such as picture synchronization degradation, detuning, colour impairment and audio distortion.

For audio equipment, an audio wanted signal is used only for calibration purpose; the immunity test is made without any wanted signal.

This report, which is not a C.I.S.P.R. Recommendation since insufficient experimental evidence has yet been gained, has been issued to provide guidance to those who need to take action to reduce the number of interference cases caused by the use of radio transmitters in the frequency range 1.5 MHz to 30 MHz.

A C.I.S.P.R. publication containing methods of measurement for several immunity aspects, including the current injection method, and for various frequency ranges is under preparation. As soon as it has been published, this report will be withdrawn.