Radio Broadcast (May 1928-Apr 1929)

Laboratory Technique for Making MEASUREMENTS ON BROADCAST RECEIVERS By L. M. HULL Formerly, Engineering Department, General Radio Co. f ■ iHE general subject of overall measureI merits on any kind of a radio set is one about which engineers do a great deal of talking but not much writing. Thus, the literature of the subject is in a rather unsatisfactory state, although the technique of providing small measured radio-frequency voltages at definitely localized points is quite generally understood, and is now being employed extensively for the standardization of broadcast receivers, even in laboratories which do not profess to be centers of research. Most laboratory measurements on a radio receiver are based upon the assumption that the action of a wave field upon an exposed antenna, in building up a high-frequency voltage across a receiver input impedance at the base, may be simulated so far as the receiver is concerned by a locally generated signal which is fed into the receiver through a local impedance having reactive constants equivalent to those of the antenna. For example, the effective height of an antenna is any length in meters which makes a conventional formula give the right numerical result. But on many occasions in this cruel competitive age the design engineer is forced into the embarrassing responsibility of deciding whether receiver A will give more service per dollar than receiver B, without having the leisure or the facilities for operating both A and B in a hundred different localities on a hundred different antennas. The problem of making a laboratory measurement from which valid generalizations can be derived is then of vital importance which cannot be resolved by definition. The practice of measuring a receiving set by assuming an "equivalent" local or dummy Audio Oscillator ■|IJljl|IJ|— 1 135 V. Standard Signal Generator } Local 5 Antenna Twisted Leads Receiver 1 Output Meter -s J 6 V. = Ground Fig. 2 — Method of testing receiver. antenna and impressing a voltage in series with it would seem to be justified by theory. This This article by Dr. Hull, formerly of the Engineering Staff of the General Radio Company, treats of a subject that has been hotly discussed pro and con by engineers ever since there were any broadcast receivers. How to measure accurately the overall gain of radio sets has been a serious problem and the device which Dr. Hull describes here is, as far as we know, the first of its type to be generally available. Some of the material in this article was presented by Dr Hull in a paper delivered before the Radio Club of America. — The Editor. theoretical argument, as outlined, is not new. It was recently summarized by Colebrook JOS 7o.5 18 178< 18 . SL :2n. ' >2si HK >HH 05 0.5 w Radio Current Meter Fig. 1 Modulation Voltmeter -Schematic diagram of standard signal generator. (Experimental Wireless and the Wireless Engineer, p. 567, Nov., 1927) and objections to it may be answered by two kinds of experimental evidence as follows: Experimental Evidence FIBST, if an antenna excited by a wave field is series-tuned to some frequency lower than its fundamental, and various resistances are inserted at the base (enough to vary the current at the base over a wide range) a linear relation will be obtained between current and resistance, indicating that the voltage due to the wave field acts like a constant voltage in series with some impedance, which is substantially independent of the current, at least over certain ranges. Second, if a tuned receiver input circuit is compared with a pure resistance on an antenna excited by a wave at frequencies below the fundamental, and then compared at the same frequency excited by a local generator through an impedance equal to the antenna impedance as measured at the base, the relative factors of merit will be the same for each form of measurement. Thus it may be concluded that the use of a local signal in measuring antenna-operated receivers is partially justified by the theory I have outlined, and is better justified by experience. A primary necessity for such measurements is a local signal generator of such a form that a known minute radio-frequency voltage may be produced between two particular terminals and nowhere else. With this available we can forsake the pernicious practice of measuring the individual amplifier stages, detectors and what not, independently, and multiplying the results together to arrive at the performance of the set. I do not question the value of the piecemeal measurements; they constitute essential steps in the design. But what we are now concerned with is an appraisal of the final result. There are two schools of thought with regard to the design of refined local sources. One advocates the inductive-coupler method, in which a measured current is passed through an exposed coil and the small test voltage is picked up on a second coil having a small calculated mutual inductance with the first. A • february, 1929 page 230 •