0.16 Lecture 17:modulation, broadcast radio  (Page 3/4)

4/ Synchronous detection

Synchronous detection is effectively used when the oscillators in the modulator and detector are the same. For example in the chopper-stabilized DC amplifier shown below.

In DC amplifiers, the signal component at DC interacts with the biasing of the transistors in the amplifier complicating the design. In a chopper stabilized amplifier, the DC signal is modulated, amplified by an AC amplifier, and then detected. Thus, amplification at DC is achieved with an AC amplifier.

III. BROADCAST AM RADIO

1/ Brief history

1864 James Clerk Maxwell published his equations of electromagnetism.

1887 Heinrich Hertz proved that waves travel through the “ether” by creating a spark in a gap between two wires and picking up a voltage in a loop of wire — the first transmitter and receiver of electromagnetic waves.

Brief history, cont’d

1896 Guglielmo Marconi took out patents on a system of wireless telegraphy.

1906 Lee de Forrest invented the triode vacuum tube for sensitive detection of telegraphy signals. Despite the fact that De Forrest did not understand how the “audion” worked this invention began the modern electronic era.

1906 Reginald Aubrey Fessenden transmitted voice and music over radio waves using a 100,000 Hz alternator designed by Charles Steinmetz at General Electric Company. This was the beginning of broadcasting of audio over the airways.

1912 Edwin Howard Armstrong analyzed the operation of the audion tube and made the first vacuum tube amplifier as part of a sensitive receiver of wireless telegraphy. He used regeneration now called feedback. Noting that the vacuum tube circuit could be made to oscillate, he used this to make the first electronic transmitter. Armstrong’s work ushered in the modern era of radio transmission and reception.

1916 David Sarnoff, working for the Marconi Company, envisaged “music boxes” (radios) as consumer products and the system of radio broadcast as we have it today.

1918 Armstrong enlisted in the army during World War I and worked for the Signal Corps in Paris. He developed the superheterodyne receiver which became, and is to this day, the basis of AM radio receivers.

1919 The Radio Corporation of America was formed out of General Electric Company and the American Marconi Company with David Sarnoffas commercial manager. Between 1918 and 1923 radio broadcasting became pervasive as inexpensive radios became widely available. Sarnoffb ecame president and perhaps the most powerful person in the burgeoning communications industry.

2/ The conception of broadcast radio

The important conception was to develop a radio broadcast system that consisted of relatively small number of transmitters each transmitting at different (carrier) frequencies and a large numbers of inexpensive receivers that could be tuned to different transmission frequencies. In order to be sufficiently inexpensive so that everyone could own one, the receivers had to be simple to manufacture. Thus, the system consisted of a small number of expensive transmitters and a large number of inexpensive receivers.