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An Example of Modern Receiver Design Trends
NEW FEATURES IN THE SILVER RADIO
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What's new in radio?" is the question that is being asked nowadays. Quite a few things are new in broadcast receiver design this year. One good way of answering the question is to review briefly the trends evident at the R. M. A. trade show, and to describe a typical example of a modern radio receiver employing a majority, at least, of these features. Such is the purpose of this paper; the new design trends which should be looked for in the 1929-30 season being listed below, while their application to a practical receiver is well exemplified in the Silver Radio chassis to be described in some detail.
(a) The use of 224-type screen-grid a.c. tubes as both r.f. amplifiers and power detectors, to the probable almost complete exclusion of the 226 and 227-type tubes heretofore popular.
(b) The general use of "band-selector" tuning as an aid in attaining the desirable ideal of a rectangular response curve, with its freedom from sideband cutting (attenuation of high musical frequencies).
(c) The widespread use of 245-type tubes in push pull, feeding a high-grade dynamic loud speaker unit of improved response over the musical range of 50 to 5000 cycles, the amplifier having an undistorted power output of about three watts.
(d) Automatic regulation of fluctuations in a.c. power line voltages.
(e) A tendency to use smaller, or even selfcontained, antennas for convenience, when receiver sensitivity permits.
In addition to these general trends in the design of all-electric broadcast receivers, and in addition to the fact that each season prices come down, or actual values given increase (which amounts to the same thing), certain other trends will be evident among a few manufacturers, as follows:
(f) Use of but one a.f. stage following a power detector in sets where the attenuation of high frequencies resulting from the necessary high-ratio a.f. coupling transformer is not considered of as great importance as the cost saving resulting.
(g) A tendency toward condenser loud speakers, not at all general, because of present poor efficiency, high polarizing voltages needed, excessive bulk, and fragility of existing types.
(h) Use of so-called automatic tuning; as, for example, devices for selection of a small pre-selected group of stations by means of a multiplicity of buttons instead of the usual single selector knob.
(i) Extension of the reproduced audio tone range up to 4000 or 5000 cycles to give naturalness and brilliancy, with the addition of an overtone cutout switch to diminish high-pitched atmospheric noises in bad weather.
By McMURDO SILVER
President, Silver-Marshall, Ine.
Features of Silver Radio
The receiver here described embodies all of the above features except f, g, and h, which will not be found in wide general application this year.
An article dealing briefly with the one audio stage trend, (f), will be found on page 15 of the May, 1929, Radio Broadcast. Another article, upon condenser loud speakers, (g), appears upon page 369 of the April, 1929, Radio Broadcast. Automatic tuning, (h), usually accomplished by mechanical selectors, is so simple as to require little special description.
At this stage of the radio art it is unfortunately difficult to make evident the merits of a receiver simply and concisely
Fig. 1 — Rear view of the new Silver Radio showing chassis and dynamic loud speaker.
Fig. 2
-The chassis of the Silver Radio with shield cover removed.
(as should be the case) by means of quantitative measurements, as few overall receiver measurement curves have been published (due in a large .measure to the difficulty of making such measurements, even on older type sets). In presenting Figs. 3, 4, and 5 there is little data to compare them with and the curves must, therefore, be interpreted to be rendered readily comprehensible. Incidentally, it is to be hoped that, as overall measurements are now possible in any well-equipped radio laboratory, manufacturers will have more recourse to facts and less to fanciful "sales talks" for radio receivers in the future.
Fig. 3 illustrates the overall sensitivity of the set of Figs. 1, 2, and 8 under two different antenna conditions. Curve a is for the small self-contained screen antenna (8"x 24") attached to the bottom of the cabinet, while curve b is for a typical broadcast antenna of characteristics stated on the curve. The order of sensitivity shown is thought to be greater than that of any other receiver offered today. Measurements on typical six-tube, onedial a.c. sets having three tuned circuits show an average sensitivity of forty to sixty microvolts per meter, and measurements on other sets of the same general type, having four tuned circuits (4 tuned r.f. stages), show about 15 to 20 microvolts per meter sensitivity. Considering curve b of Fig. 3, indicating an average sensitivity of 2 microvolts under standard conditions, the sensitivity of this set is seen to be about seven to ten times that of the typical t.r.f. set of four tuned circuits, and about twenty to thirty times that of the average three-tuned-circuit set. Suffice it to say that a sensitivity of 1.2 to 3.5. microvolts per meter, (the antenna input required to produce 50 milliwatts output, a purely arbitrary measurement standard) is sufficient to bring in almost any station audible above average prevailing noise levels. Comparative tests have furthered the belief that no more sensitive receiver may be used in the average home to-day.
Overall Fidelity
In fig. 4 appear two overall fidelity curves for the receiver, taken at the middle of the broadcast band (1000 kc, 300 meters).
If the reader unthinkingly compares curve a with typical curves of audio amplifiers such as are often furnished to indicate the audio response characteristic of a radio receiver, this fidelity does not appear to be particularly startling. Here again, the dearth of overall fidelity measurements of existing receivers prevents the true excellence indicated by curve a from being appreciated at first glance. Referring again to a previous article in Radio Broadcast as one of the few sources of overall measurements that tell any real story of receiver merit, the reader's attention is called to the fidelity curve of Fig. 5, page 16 of the May
• JULY -1929 •
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