exam for Electronic Engineering
Jocelyn yang 
EE1301Session17-TelegraphTubes-Class.pdf
2/28/2018
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EE 1301: MODERN ELECTRONIC TECHNOLOGY
SESSION #17: RADIO TELEGRAPHY TUBES, 2/26/2018
Instructor: Joseph Cleveland, Ph.D. Email: [email protected]
Thought for the Day
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The Relay and Telegraphy
Wireless Messaging
• 1894: Guglielmo Marconi becomes fascinated with Hertzian waves generated by electromagnetic interactions. – Marconi built wave-generating
equipment at his family’s estate and began sending signals to locations a mile away.
• 1896: Marconi moves to England and receives support from interested backers, including the British Post Office.
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The Start “Instant” Radio Messaging …
• 1897: Marconi was broadcasting up to 12 miles and had applied for his first patents.
• 1898: He set up a wireless station on the Isle of Wight that allowed Queen Victoria to send messages to her son Prince Edward aboard the royal yacht.
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The Start “Instant” Radio Messaging …
• 1901: Marconi transmits Morse code for the letter “s” from Poldhu, Cornwall, to Signal Hill, St. John’s, Newfoundland.
• In 1909 Marconi receives the Nobel Prize in Physics for the radio telegraph
• Shipping companies realize the usefulness of the radio telegraph for passenger communication, navigation reports and distress signals
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The Start “Instant” Radio Messaging …
• 1913: Marconi Company radio telegraph operator on the Titanic sends the distress signal received by RMS Carpathia
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Type of telegraph key used on the Titanic
The Start “Instant” Radio Messaging …
• Radio telegraphy used a resonant transformer with spark gap (Nikola Tesla, 1891) to generate high voltage (>25,000 V), high frequency AC signals
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Titanic Wireless Telegraph
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The Titanic SOS
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https://www.youtube.com/watch?v=snkwsU98QlQ
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The Vacuum Tube
A switch that improved telegraphy
Vacuum Tube Diode
• 1904: John Fleming invents the vacuum tube diode
• A heated wire boils off electrons in vacuum
• Electrons attracted to positive voltage on the anode
• If the plate is made negative the electrons are repelled: no current!
• We have a “switch”
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e- e- e- e-
Glass Envelope
Plate (anode)
Filament (cathode)
+
+20-200 V 5-12 V
Vacuum
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Vacuum Tube Diode
• If the telegraph signal ++, fewer electrons reach the anode, so diode is “OFF”
• If the telegraph signal 0, more electrons reach the anode, so diode is “ON”
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Heater
Cathode Anode
200-300 VDC
R
e-
I C
Vacuum Tube Diode …
• The vacuum tube diode allowed improved detection of radio telegraph signals because it would “saturate”
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+
-
The capacitor C stores charge between the RF pulses and reproduces the Morse code.
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Diode Operation
• Symbol
Biased “ON”
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Anode Cathode
+
-
I
+
- I=0
Biased “OFF”
I
Vacuum Tube Triode
1906: Lee de Forest invented the triode, an improvement on the Fleming vacuum tube diode.
• It enabled amplification of a signal voltage applied to the grid
• How does this work?
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Electrons
Anode
Cathode
+V
Heater
Control Grid
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Vacuum Tube Triode
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vout = Vpp – ipR = Vpp – vgR
Vpp
ip = vg
vgrid vout
If the grid voltage becomes more positive, the electrons move faster: more current.
If the grid voltage becomes less positive, the electrons move slower: less current.
“gain” = R
+
Vacuum Tube Triode …
• Triodes became useful a switches for use in digital computers
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Each tube needs heater current
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Technology Problems
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By the 1940s, some computational devices reached the level at which the losses from failures and downtime outweighed the economic benefits.
ENIAC
19,000 tubes 200 kW of power ~5000 instructions/sec
Technology Problems …
• Tubes still used but platforms are very heavy and consume much power.
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Big transformers for filament and plate current
Drove the need to reduce the weight and power used by vacuum tube devices with a different technology
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End of Session
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