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National Company Advertisement: Noise Limiters
September 1950 QST Article

September 1950 QST

September 1950 QST  Cover - RF CafeTable of Contents

Wax nostalgic about and learn from the history of early electronics. See articles from ARRL's QST, published December 1915 - present. All copyrights hereby acknowledged.

In classic National Company style, this full-page advertisement took the form of a mini tutorial / anecdote about using equipment and devices they sold - both for Hams and for design engineers. September 1950's topic, which appeared in QST magazine, was the use of noise limiters in CW (Morse code) transmitter / receiver sets. The noise limiters functioned as squelch controls and were adjustable for both positive and negative excursions. According to John J. Nagle (K4KJ), writing on the QSL.net website, "HRO" stood for "Helluva Rush Order," which was a rearrangement of the originally proposed "Hell Of a Rush" that came about because of a hurried modification to an existing product.

National Company Advertisement

{Number one hundred ninety-seven of a series}

National Company Advertisement, September 1950 QST - RF CafeIt is interesting to note the increasing interest lately in the use of noise limiters for C. W. use. Of course, the value of these simple little devices for phone operation has been well established for some time. The usefulness of of such a device was brought home quite forcibly to the writer while operating 80 meter C. W. (believe it or not!) at W1WB/1 during the recent field day. The transmitter was a surplus job that left the receiver running while the transmitter was keyed. The head-phones clamped over our ears with huge rubber earflaps that must have been designed for ears much larger. Every time the key was operated, a terrific clatter rattled through our head like a rivet hammer in the subway. We tried removing the phones during transmission but with one hand holding the key on a knee and the other hand operating it, there was too much delay getting rid of the key and getting hold of the phones again. And you know how much time a C. W. man will allow you for such things during Field Day operation! In this instance, a noise limiter would have been worth while just to save wear and tear on the ear drums.

While a single-ended noise limiter with a fixed adjustment is satisfactory for phone operation, it is far from adequate for good noise-limiting with C. W. signals. At National, we recognized this fact years ago and that is the reason that the noise limiter used in the HRO-7 and HRO-50 has for many years been of the double-ended type with an adjustable rather than a fixed threshold.

Phone limiters usually are adjusted to clip the positive modulation swing at about forty per cent with one diode. Further, this adjustment can be made automatic so that this clipping level can be maintained as the signal strength varies. A second diode is not needed to limit the negative modulation swing as this is taken care of by the detector diode itself.

When receiving C. W. signals, conditions are considerably different. The operating level is now usually set not by the signal but by the strength of the injected BFO carrier! If a signal is being received that is many times weaker than the BFO injection, noise peaks can now go to several hundred per cent of the signal voltage in both positive and negative directions. The limiter which worked so well on phone is now almost useless. It is now necessary to clip both sides of the audio signal delivered from the second detector and to adjust the clipping level to match the amount of audio recovered. Only then can good C. W. limiting be obtained. The above is just what the HRO limiter was designed to do years ago. It also helps to keep key clicks from wrecking your ear drums and can be used as a C. W. AVC as mentioned in a recent QST article. If you C. W. men have not been using your limiter for these purposes, you haven't been getting the most out of your HRO.

Cal Hadlock, W1CTW

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