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September 1935 QST
 Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
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When designing a receiver or transmitter
using discrete components rather than connectorized components or packaged integrated
circuits, where the interfaces are at or near 50 + j0 Ω, adding frequency
selectivity beyond that provided by the generic response requires inserting separate
filters. If you are designing the entire signal path, including the biasing, feedback
(if any), and stage interfaces from scratch, you can include features that increase
frequency selectivity. In the "old days" with vacuum tubes and interstage coupling
transformers being commonplace, the addition of a few capacitors made response peaking
a simple advantage to implement. The National Company frequently advertised in
QST magazine to appeal to Hams with their extensive line of radio
wares.
National Company Advertisement
Number nineteen of a series
 Last march we mentioned that much interest is
being shown in I. F. transformers having variable coupling as a means of controlling
selectivity. This idea is particularly interesting to us, because as far back as
May 1933 we advertised variable coupling as a feature of our earliest model air-dielectric-tuned
I.F. transformer. In this unit, the adjustment was made by a screw which moved the
coils closer or farther apart.
This scheme was abandoned by us in later models, because we felt that it was
not particularly desirable. Its only virtue is its ability to act as a selectivity
control and this we feel can be better accomplished by other means. Its disadvantages
are numerous, among them being the mechanical complication of changing coupling
by a panel control, and the pronounced effect of the adjustment on gain and other
circuit constants.
Variable selectivity presents a problem quite different in broadcast receivers
than in communication receivers. In the former, the aim is "High Fidelity," which
requires a sub-normal selectivity. In amateur receivers, the object is to eliminate
interfering signals which necessitates abnormally high selectivity. Because of this
difference, two entirely different types of control are desirable.
 The most advanced thought on variable selectivity
for broadcast receivers inclines toward some form of non-mechanical semi-automatic
device. A number of schemes have been suggested for doing this, and it is probable
that these will be incorporated in some of the newer receivers for the coming season.
In one system, the coupling is controlled by the strength of the received signal,
on the theory that strong local stations can override interference by mere volume,
and consequently do not require complete elimination of unwanted signals by high
selectivity. The controlling circuit is similar to a conventional AVC circuit, but
is operated by the audio signal rather than the carrier. Of the many schemes suggested
for obtaining the actual selectivity change without moving parts, only one will
be mentioned; namely, connecting a variable load resistance across one of the I.F.
transformers. This is quite practical if a specially designed I.F. transformer is
used. Since the plate resistance of the 58 varies with the suppressor voltage applied,
this tube may be used as the variable load resistor.
To come back to amateur receivers, however, the control should be in the direction
of increased selectivity, not decreased. We think that the only practical answer
to this is the Single Signal Filter, such as used in our communication receivers
and discussed in detail on this page last March. In this filter, a knob on the front
panel gives a variable admittance to the I.F. amplifier of from several kilocycles
down to a few cycles.
We realize keenly that it is unwise to say that any development is valueless,
and we do not wish to be dogmatic about it. However, we believe our comments above
pretty well sum up the situation as matters stand. So for the present anyway, we
will stick to fixed coupling. It seems the best way, particularly as it gives us
the assurance that after they leave the factory, our I.F. units will stay at the
optimum coupling value for best gain and selectivity.
James Millen
Here are all the National Company advertisements I have:
- National's Company:
Merry Christmas and Happy New Year, January 1941
- National Company:
Noise Limiters, September 1950
- National Company: Modulators, April
1942
- National Company: Switches,
May 1939
- National Company: TMS Condenser, March 1939
- National Company: TVI, October
1953
- National Company:
Amateur Radio Activity, December 1952
- National Company: HRO-50
Receiver, May 1951
- National Company:
Noise Limiters, September 1950
- National
Company: Modulation, October 1947
- National Company:
Merry Christmas and Happy New Year, January 1942
- National Company:
Merry Christmas and Happy New Year, January 1941
- National Company: Switches,
May 1939
- National Company: AC Power Strip,
April 1939
- National Company: TMS Condenser,
March 1939
- National Company Catalog: NC80X
/ NC81X Receiver Advertisement, September 1937
- National Company:
IF Circuits, September 1935
Posted July 26, 2022
(updated from original post on 10/27/2016)
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