1
00:00:04,310 --> 00:00:09,610
Lot's of things in our lives transmit signals.
From your cell phone when it's making a call,

2
00:00:09,610 --> 00:00:15,660
to your computer when it's sending an email,
to your local radio station when it's broadcasting.

3
00:00:15,660 --> 00:00:20,369
Here you see two objects that receive signals
- they are radio receivers. Have you ever

4
00:00:20,369 --> 00:00:24,580
wondered how they pick out the signal they
want out of all of the radio waves around

5
00:00:24,580 --> 00:00:29,029
them? In this video, we're going to find out.

6
00:00:29,029 --> 00:00:35,180
This video is part of the information flow
video series. A system is shaped and changed

7
00:00:35,180 --> 00:00:40,310
by the nature and flow of information into,
within, and out of the system.

8
00:00:40,310 --> 00:00:45,360
Hi, my name is Elena Glassman, and I am a
graduate student in the Electrical Engineering

9
00:00:45,360 --> 00:00:49,440
and Computer Science Department at MIT.

10
00:00:49,440 --> 00:00:54,490
Before watching this video, you should be
familiar with basic electrical circuits, and

11
00:00:54,490 --> 00:00:59,540
how inductors, capacitors, and AM radio transmitters
work.

12
00:00:59,540 --> 00:01:03,770
After watching this video, you will be able
to explain how a basic radio receiver circuit

13
00:01:03,770 --> 00:01:10,770
functions to select particular radio frequencies.

14
00:01:13,049 --> 00:01:19,049
This Hammarlund HQ120 communications receiver
was introduced in 1938. It tunes in radio

15
00:01:19,049 --> 00:01:25,590
waves ranging from 540 thousand cycles per
second to 30 million cycles per second. It

16
00:01:25,590 --> 00:01:31,439
can decode signals encoded with Amplitude
Modulation, abbreviated as AM, or "continuous

17
00:01:31,439 --> 00:01:35,408
wave" modulation, abbreviated as CW.

18
00:01:35,408 --> 00:01:41,749
This Drake R-4 was introduced later, in 1964,
and is optimized for amateur radio bands within

19
00:01:41,749 --> 00:01:47,819
the same frequency range covered by the HQ120.
Both of these radios were popular with amateur

20
00:01:47,819 --> 00:01:52,530
radio operators.
Amateur radio operators are radio enthusiasts

21
00:01:52,530 --> 00:01:57,399
who pass U.S. Federal Communications Commission
tests to receive official government call

22
00:01:57,399 --> 00:02:01,170
signs and the right to broadcast on select
bands.

23
00:02:01,170 --> 00:02:06,009
Amateur radio operators are anything but amateur,
serving as part of an important worldwide

24
00:02:06,009 --> 00:02:11,890
network. When cell phones, the internet, or
other communication networks are down, amateur

25
00:02:11,890 --> 00:02:17,810
radio operators can help pass along important
information. Amateur radio operators have

26
00:02:17,810 --> 00:02:22,650
historically experimented with and advanced
cutting-edge radio technologies. (viewers

27
00:02:22,650 --> 00:02:25,640
hear radio making some noise)

28
00:02:25,640 --> 00:02:30,099
Let's delve deeper into how these radios receive
information.

29
00:02:30,099 --> 00:02:35,599
Recall that radio waves are electromagnetic
waves whose frequencies fall in a certain

30
00:02:35,599 --> 00:02:42,500
range. A variety of data types can be transmitted
over radio waves by systematically modulating

31
00:02:42,500 --> 00:02:44,730
some property of the wave.

32
00:02:44,730 --> 00:02:50,739
The information we want to transmit--say speech
or music--has a much lower frequency than

33
00:02:50,739 --> 00:02:56,709
the carrier radio wave. We can encode this
lower frequency by modulating the amplitude

34
00:02:56,709 --> 00:03:03,140
of the carrier wave. This is called Amplitude
Modulation, which is used in AM radio.

35
00:03:03,140 --> 00:03:09,629
CW radio stands for continuous wave modulation
and is traditionally used to transmit Morse

36
00:03:09,629 --> 00:03:12,870
code by switching the carrier signal on and
off.

37
00:03:12,870 --> 00:03:19,870
All of the signals being transmitted around
the globe are superimposed on to each other.

38
00:03:20,360 --> 00:03:26,299
Our goal is to understand how a basic radio
circuit, called the regenerative circuit,

39
00:03:26,299 --> 00:03:32,150
works to pick out one frequency. This circuit
was a breakthrough in radio technology both

40
00:03:32,150 --> 00:03:35,930
in terms of amplification and selectivity.

41
00:03:35,930 --> 00:03:42,329
It was invented in 1914 by American electrical
engineer Edwin Armstrong when he was an undergraduate

42
00:03:42,329 --> 00:03:44,700
at Columbia University.

43
00:03:44,700 --> 00:03:51,379
This circuit was widely used in radio receivers,
called regenerative receivers, between 1920

44
00:03:51,379 --> 00:03:58,379
and World War II. They are still used in low-cost
electronic equipment such as garage door openers.

45
00:03:58,890 --> 00:04:05,890
Our Hammerlund and Drake radios use more sophisticated
circuitry, also invented by Armstrong. However,

46
00:04:05,890 --> 00:04:12,890
we're going focus on the more basic and still
powerful regenerative circuit.

47
00:04:15,939 --> 00:04:22,939
The regenerative circuit seen here is a classical,
elegant electrical circuit that amplifies

48
00:04:23,229 --> 00:04:27,250
while selecting a particular radio wave frequency.

49
00:04:27,250 --> 00:04:33,800
Let's start by identifying the elements from
this diagram in our actual radio. This is

50
00:04:33,800 --> 00:04:38,340
the antenna for the radio.
And this is the symbol for that antenna in

51
00:04:38,340 --> 00:04:43,220
our diagram.
This dial allows us to control the frequency

52
00:04:43,220 --> 00:04:48,470
we're selecting. And this is the variable
capacitance element that determines which

53
00:04:48,470 --> 00:04:53,000
frequency this basic regenerative receiver
circuit will select for.

54
00:04:53,000 --> 00:04:55,490
This is a vacuum tube in the radio.

55
00:04:55,490 --> 00:05:01,680
And in the diagram. It is going to act as
an amplifier.

56
00:05:01,680 --> 00:05:06,400
Together the volume control and the speaker
enable us to hear the information decoded

57
00:05:06,400 --> 00:05:08,360
from the radio signal.

58
00:05:08,360 --> 00:05:14,020
These are represented by a variable resistor
and the headphone cartoon in our diagram.

59
00:05:14,020 --> 00:05:19,569
Now let's understand how this circuit works!
Radio waves induce waves of alternating current

60
00:05:19,569 --> 00:05:24,720
through the antenna. By the phenomenon of
inductive coupling, the radio wave energy

61
00:05:24,720 --> 00:05:30,300
is picked up by this tuned circuit, which
is just an inductor and variable capacitor

62
00:05:30,300 --> 00:05:36,300
in parallel. This circuit acts like an echo
chamber for radio waves.

63
00:05:36,300 --> 00:05:41,110
To get a better sense of how this parallel
inductor and capacitor work, let's look at

64
00:05:41,110 --> 00:05:44,580
a system with which you may be more familiar--brass
instruments.

65
00:05:44,580 --> 00:05:50,500
Brass players create lots of sound frequencies
when they blow through their mouthpieces.

66
00:05:50,500 --> 00:05:54,729
[sound of buzzing]
The brass tube acts as a resonant cavity or

67
00:05:54,729 --> 00:06:01,139
echo chamber. Sound waves bounce back and
forth between each end of the tube.

68
00:06:01,139 --> 00:06:06,099
Particular frequencies constructively interfere
or reinforce each other, while other frequencies

69
00:06:06,099 --> 00:06:11,370
destructively interfere, and therefore have
their amplitude diminished.

70
00:06:11,370 --> 00:06:17,259
As the slide is moved, the length of the tube
changes, and that changes which frequencies

71
00:06:17,259 --> 00:06:23,680
are reinforced and which frequencies are damped.
This parallel inductor and capacitor act the

72
00:06:23,680 --> 00:06:30,680
same way for radio waves. Changing the capacitance
of the capacitor is equivalent to changing

73
00:06:30,780 --> 00:06:34,430
the length of the trombone's tube by moving
the slide.

74
00:06:34,430 --> 00:06:40,050
The noise produced by buzzing into the mouthpiece
and filtering it with the brass tube is sufficient

75
00:06:40,050 --> 00:06:42,199
to produce audible sound.

76
00:06:42,199 --> 00:06:48,990
But radio waves may be coming from transmitters
very far away, and may have very small amplitudes

77
00:06:48,990 --> 00:06:55,909
by the time they reach us. It's not uncommon
for these receivers to pick up radio stations

78
00:06:55,909 --> 00:07:02,810
part way around the globe. How are we going
to deal with this? We need amplification!

79
00:07:02,810 --> 00:07:08,099
The radio waves echoing in the tuned circuit
here are applied to the input of this vacuum

80
00:07:08,099 --> 00:07:14,810
tube, which acts as an amplifier. An amplifier
reproduces the signal applied to its input,

81
00:07:14,810 --> 00:07:21,639
but with greater magnitude. The added energy
comes from a local power source, like a battery.

82
00:07:21,639 --> 00:07:26,409
That might be enough amplification, but we
can do even better using positive feedback!

83
00:07:26,409 --> 00:07:33,409
The tickler, located here, takes the amplified
radio signal from the vacuum tube and feeds

84
00:07:34,110 --> 00:07:40,009
it back into the echo chamber, to reinforce
new radio waves coming in from the antenna

85
00:07:40,009 --> 00:07:42,050
at that same frequency!

86
00:07:42,050 --> 00:07:48,229
A frequency is strengthened by the echo chamber
alone, so it becomes even larger in amplitude

87
00:07:48,229 --> 00:07:54,569
after amplification and positive feedback
into the same echo chamber! Likewise, any

88
00:07:54,569 --> 00:07:59,889
frequencies that did not resonate well in
the echo chamber will be diminished further

89
00:07:59,889 --> 00:08:06,590
with each circulation back through the loop.
This echo chamber with its output amplified

90
00:08:06,590 --> 00:08:12,800
and fed back into itself is what is responsible
for achieving our stated goal of selecting

91
00:08:12,800 --> 00:08:19,800
only one frequency to tune in to, in the presence
of all the other frequencies carrying information.

92
00:08:20,289 --> 00:08:25,590
Now that we have selected and amplified the
carrier frequency, we still need to extract

93
00:08:25,590 --> 00:08:31,129
the original information.

94
00:08:31,129 --> 00:08:37,940
You might expect our signal after amplification
to look like this. But the information encoded

95
00:08:37,940 --> 00:08:44,940
in the amplitude would be hard to decode because
the average amplitude is constant everywhere.

96
00:08:45,010 --> 00:08:50,860
Luckily for us, the vacuum tube is a nonlinear
amplifier.

97
00:08:50,860 --> 00:08:57,760
It accentuates the lower half of the signal
and diminishes the top half, giving us a signal

98
00:08:57,760 --> 00:09:04,760
that looks more like this.
Since headphones are natural smoothing filters

99
00:09:05,350 --> 00:09:11,510
that convert variations in current to air
pressure waves, the user hears the original

100
00:09:11,510 --> 00:09:18,510
information encoded in the AM signal. If we
hadn't accentuated the lower half of the signal

101
00:09:18,700 --> 00:09:21,149
before smoothing, you wouldn't hear anything.
There's one last aspect of this simple regenerative

102
00:09:21,149 --> 00:09:21,250
receiver we'd like to point out.

103
00:09:21,250 --> 00:09:21,480
After all this selective amplification of
the carrier frequency, we must still extract

104
00:09:21,480 --> 00:09:21,630
the information carried by the envelope of
its amplitude.

105
00:09:21,630 --> 00:09:21,890
We take advantage of the imperfect amplification
of the vacuum tube to accentuate the lower

106
00:09:21,890 --> 00:09:21,959
half of the signal.

107
00:09:21,959 --> 00:09:22,180
Since headphones are natural smoothing filters
that convert variations in current to air

108
00:09:22,180 --> 00:09:22,410
pressure waves, the user hears the original
information encoded in the AM signal.

109
00:09:22,410 --> 00:09:29,410
It was extremely clever to use a single tube
to amplify, provide selectivity, and demodulate

110
00:09:30,760 --> 00:09:37,470
an AM signal at a time when tubes were very
expensive and considered cutting edge.

111
00:09:37,470 --> 00:09:43,120
This Drake R-4 radio uses more sophisticated
circuitry but even modern radio receivers

112
00:09:43,120 --> 00:09:50,120
are built with digital circuits that operate
on the same basic principles!

113
00:09:54,180 --> 00:10:00,800
Radio is a cool, handy way to disseminate
information, so humans have created lots and

114
00:10:00,800 --> 00:10:07,290
lots of electromagnetic waves and transmitted
them into the air at various frequencies.

115
00:10:07,290 --> 00:10:11,320
We've even standardized who can transmit and
at what frequency.

116
00:10:11,320 --> 00:10:18,320
Here's the radio spectrum that is available.
You see AM and FM broadcast within this spectrum.

117
00:10:19,120 --> 00:10:25,779
This radio spectrum has been divided up even
further for different uses: mobile phones,

118
00:10:25,779 --> 00:10:31,750
amateur astronomy, satellites, space research,
amateur radio, and Earth research.

119
00:10:31,750 --> 00:10:38,060
Zooming out further we see that lots of different
communities have their own spot allocated.

120
00:10:38,060 --> 00:10:43,470
Out of all of that radio activity, we can
now pull out what we personally want. Maybe

121
00:10:43,470 --> 00:10:49,220
it's finding a friend or fellow amateur radio
operator transmitting from another continent,

122
00:10:49,220 --> 00:10:56,220
or maybe it's the BBC, broadcasting international
news.

123
00:10:58,339 --> 00:11:04,970
To summarize, in this video, we have explained
how the components in the regenerative receiver

124
00:11:04,970 --> 00:11:11,970
circuit work together to dampen unwanted frequencies,
while selectively amplifying and demodulating

125
00:11:12,540 --> 00:11:19,540
a desired AM signal. This is no small feat,
given the amount of radio waves humans broadcast!

126
00:11:19,990 --> 00:11:25,660
We've also demonstrated the tuning process
with a Drake R-4 radio receiver built for

127
00:11:25,660 --> 00:11:32,459
amateur radio operators in the 1960s. Learning
about these radio receivers with my dad when

128
00:11:32,459 --> 00:11:38,649
I was a kid was instrumental in developing
my own interest in electrical engineering!

129
00:11:38,649 --> 00:11:43,160
It was really fun to share some of the circuitry
with you. If you want to try something a little

130
00:11:43,160 --> 00:11:49,320
more hands-on, I recommend checking out the
local amateur radio community or signing up

131
00:11:49,320 --> 00:11:55,740
for a course in electrical engineering!