Schematic of the regenerative circuit
invented by Armstrong
As aforementioned either crystal
as well as one-valve receivers were unable to detect long distance
radio signals. In this way soon appeared in the market the
first multiple stage receivers using several amplifying circuits
with tuning radio frequency as known as TRF.
In spite of its better sensitivity they still had some disadvantages
such as the production of annoying hisses during the tuning process.
In 1912, Edwin H. Armstrong invented a new type of circuit known
as regenerative circuit in which the AUDION behave like an amplifyer
as well as a generator of electromagnetic waves. Either the
TRF as well as the regenerative circuits were big steps in radio
receivers' development when operating for tuning in low frequencies.
However, with the growing of the radio broadcasting stations now
operating in frequencies higher than 10 MHz those types of receivers
had difficulties to detect them.
In the early twenties Armstrong again invented a new type of circuit
now known as superetherodyne circuit. Such kind of circuit
was more selective and stable and so became the base for the manufacturing
of all types of radio receivers.
In 1930, started radio boom and later in 1933 appeared in the market
a new circuit known as frequency modulation or FM. Invented
by the so famous Armstrong in this new system it could eliminate the
phenomenon of static found in the conventional amplitude modulation.
Schematic of the superheterodyne
circuit invented by Armstrong circa 1919 where:
a - signal
b - oscillator
c - mixer
d - IF amplifier
e - detector
f - audio amplifier
Schematic of the FM circuit invented
by Armstrong in 1933 where:
d - IF amplifier
e - limitter