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.
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 October 27, 2016
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