The October 1930 issue of Radio News featured a revolutionary new radio design which was to completely alter the future of radio. This was the Stenode Radiostat which was designed by Dr. James Robinson, radio engineer of the Receptite Company of London, England.

Briefly, this design was a typical superheterodyne with a quartz crystal filter circuit just ahead of the second detector. The schematic diagrams illustrate the design. The text of the article had this to say: "

The last stage of i.f. amplification is obtained by means of a neutralized triode in preference to a screen-grid tube, owing to the superior power-handling capacity of the former. This stage of amplification immediately precedes the quartz crystal and its associated circuit, which may be termed the heart of the apparatus; this portion of the circuit is shown in greater detail in the accompanying diagram. The quartz crystal is used instead of a number of sharply-tuned cascade resonant circuits in order to obtain high selectivity; and that used in the Radiostat has a decrement of the order of 0.00004. It is obvious that the selectivity given by such an arrangement is much higher than could be easily obtained by a more normal method of cascade tuned circuits. The band width passed by this crystal circuit is only about 50 cycles; consequently any interference on a frequency different from the frequency of the desired station by more than 50 cycles should be completely eliminated."

The final IF stage of the Stenode Radiostat

The article continues with further descriptions of the design and a discourse on the previously presumed need for a wider pass-band of receive the sidebands created by the carrier modulating audio signal. The theory here was that sidebands were not needed for the reception of the audio signal and that by using this design stations could be separated by just one kilocycle on the broadcast band, effectively increasing the number of stations which could be on the air by a factor of ten.

Circuit of the Stenode Radiostat

Radio News featured this revolutionary design in following issues, each one elaborating on the theory and design of the Radiostat. Along the way they conceded that there was some loss in higher frequencies of audio but that this was compensated for by designing the audio amplifier to have more gain at the higher frequencies. The December issue featured comments by members of the profession, such as McMurdo Silver, James Millen, and others. Needless to say, there was some skepticism in their comments.

The "excitement" died down after the February issue and we presume the idea died soon after. The crystal filter, of course, was used (later) in communications receivers for reception of c.w. signals. Anyone who has tried to used the crystal filter on AM signals knows all-too-well what happens to the audio.

Cost will Limit Application
By McMurdo Silver

The Stenode Radiostat is a very interesting receiver design from a scientific viewpoint in affording an idea of what can be done by use of an extremely selective i.f. amplifier. Through its use, a present balance between image frequency and adjacent channel selectivity is upset in favor of a higher i.f. frequency which will require less selection ahead of the first detector to give the image frequency suppression which present broadcast conditions require.

There appears to be nothing in Mr. W. T. Cocking's article, (October Radio News) nor in what has appeared in the British press, however, which vitiates the present theory of side-band transmission advanced by J. R. Carson and others. The fact that a great deal of audio-frequency compensation is necessary seems to corroborate this. Fortunately, a great deal of frequency in discrimination can be tolerated by the average listener, and this probably accounts for the arrangement sounding "very good."

Its high cost, with other shortcomings which are recognized by the designer will probably limit its application to experimental and special purpose receivers for some time to come, since extremely good adjacent channel selectivity may be secured by conventional circuit designs and at a much lower cost. It can also be shown mathematically that the fidelity of a series of broadly tuned circuits in cascade is better than that of a single circuit giving equivalent selectivity at 10 or 20 kc from resonance. This practice is followed in the better current receiver designs and limits the amount of audio compensation required to a very reasonable value for excellent fidelity.