Chapter 3 - R/C Electronics
The
Superheterodyne
Did your ever wonder why, or
how, modern radio systems work?
Specially when you think that just a few years’ ago there were only a
handful of channels, and now there are so many more? And they’re much “closer” together.
As you probably know, radios
work by having a transmitter create an electro-magnetic signal that alternates
from a positive direction to a negative direct many million times per
second. A bit like reversing the direction/polarity
of a battery -- first one way, then the other -- but faster. Of course, in a transmitter, the
alternations are smooth progressive changes from one polarity to the other. The “Frequency” of that alternation, for
aircraft radios, is in the ballpark of 72,000,000 times per second (Usually
referred to as 72 mega hertz).
If we could make a receiver
that receives/responds to a signal of only one frequency we could arrange for
various transmitters on slightly different frequencies with receivers that
respond only to one particular transmitter.
Well, this is exactly what happens.
Each slightly different frequency is called a “Channel”
The greater the number of
channels that are required within a specific frequency range requires that the
“Channels” are closer together; the frequency difference between channels gets
smaller. The two main tricks are to get
a transmitter that accurately transmits on a specific frequency (“Channel”) and
for it not to drift away from that frequency, and, to make a receiver respond
to that frequency and no others. That’s
where modern day Superheteradyne receivers come in.
Let’s draw another
comparison. I think we all understand
that any machine, such as a reciprocating engine, works better at one
particular RPM. The RPM is actually the
“Frequency” of the machine. Think about
why we have tuned pipes or how/why a constant speed prop. works. Keep this thought.
Since a receiver is
manufactured to respond to any frequency (that it is tuned to) in the 72
megahertz range, its “Front end” must accommodate all valid frequencies
(Channel 11 to 60). However, as we
discussed, any machine is more efficient if its frequency is constant. Therefore, if an electronic circuit is
designed to respond to a fixed frequency, it’s efficiency is much
improved. In a Superheteradyne, the
incoming signal (which can be any frequency from channel 11 to channel 60) from the transmitter is electronically mixed
(in a Mixer) with a signal generated within the receiver (Local
Oscillator). The tuning crystal we
install into receiver actually creates this internal signal. It is arranged that the difference between
the transmitter frequency and the local oscillator is always the same
regardless of the channel tuned to.
When you electrically mixes two different frequencies, the result is a
new signal at a frequency equal to the difference. Therefore, the output from the mixer is ALWAYS the same frequency
(Intermediate frequency). The following
electronics (Intermediate Frequency, IF, amplifier). Because the IF is ALWAYS
constant frequency, the IF amplifier can be very selective to that
frequency. This is what allows the very
sharp, specific and selective accuracy that we expect from our RC equipment. This intermediate frequency is 10.7
megahertz (10,700,000 cycles per second).
This is still a very high
frequency. Even the best accuracy in
this range is not good enough for the selectivity we require in a modern RC
radio system. To get the closeness of
frequencies that we now enjoy, we must have an intermediate frequency much
lower than 10.7 MHz.
What happens now is the 10.7
MHz is applied to yet another mixer.
There is also another local oscillator.
The result is a much lower frequency IF -- 465 KHz (465,000 cycles per second). Such a frequency can be tuned and made
selective to within 20 Khz, which is today’s accepted channel separation. These dual mixer and dual intermediate frequency
superheteradyne receivers are usually referred to as “Dual Conversion”
receivers.
As a side issue. When two transmitters are operating that are
23 channels apart, the two signals “Mix” in the air. Since each channel is separated by 20 KHz, the resulting frequency
is 23 times 20 KHz which equals 460 KHz.
This is real close to the Dual Converted second intermediate frequency
of 465 KHz. This can interfere with any
third and subsequent receivers that are switched on at the same time. That’s why, channel 42 and 19, for example,
shouldn’t fly together....
Remember, none of this
matters if you haven’t charged your batteries.
AM
vs. FM
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Did you ever wonder at what
was the difference between AM and FM?
Did you every wonder at why FM is preferred over AM? And what about PPM and PCM? Did you ever wonder?
Glossary:
AM -- Amplitude Modulation
FM -- Frequency Modulation
Modulation -- The method by
which information is attached to a radio signal for transmission
In “The Superheterodyne” we
saw that radio signals are transmitted at different frequencies, called
channels. This is to keep the
information I’m sending to my plane separate from the information you are
sending to your plane. It’s a bit like
a multi-lane highway where each lane is a channel and the cars are the
information which are separated by the lane markings. The road itself is the medium -- the air. OK, no worries so
far. But let us look at what we mean by
“Sending information”.
Let’s take a simple radio
transmitter (TX) and receiver (RX).
Let’s also assume that in this example the receiver is connected to a
light bulb so that when the TX is transmitting, the light bulb comes on. Let’s take it one step further and say that
the stronger the signal being received by the RX, the brighter the bulb will
glow. OK so far. In the system just described we get some
knowledge on how strong the TX signal is by looking at the brightness of the
bulb -- right. Well we have just
transmitted “Information” over a radio link.
Let us now add a control to
the TX (like a volume control on a TV) that allows us to change the output
power of the TX up and down. As we move
the control, the light bulb at the receiver will brighten and dim corresponding
to the control movements. If we
replaced the control with a microphone, and the bulb with a loudspeaker, we
could transmit voice, music, etc. Just
like the Broadcast boys do.
So, what have we done? We’ve developed a system that allows us to
transmit information by varying the output power of a transmitter. We “Modulate” the output power -- we MODULATE
the AMPLITUDE of the power -- thus we have an AM system.
Notice, in the diagram
above, how the signal from the microphone (Modulation) alters the height
(Amplitude) of the radio signal.
Notice, also, that the radio frequency (The number of ups and downs per
inch) stays the same.
OK, let’s go back to the
bulb. Just suppose that we change the
receiver so the bulb doesn’t respond to signal strength, but does respond to
how close the transmitted signal’s frequency is to the frequency set at the
receiver. That means, if the RX
receives a signal from a TX that is exactly the same frequency, the bulb will
glow brightly. But if the TX frequency
is off, the bulb will glow dimmer.
To repeat what we did to the
AM system, let’s now consider controlling (Altering) the TX frequency with the
electrical impulses from a microphone and replacing the RX bulb with a
loudspeaker. Now we have a FREQUENCY
MODULATED (FM) broadcast system.
Notice here, that the signal
from the microphone (Modulation) changes the frequency of the radio signal, not
its amplitude. Of course the amount of
change in the radio signal’s frequency is substantial smaller that the
frequency difference between adjacent radio channels.
Well, of course, in R/C we
don’t transmit voice or music from a microphone. What we do want to do is to transmit some information that
represents the position of a control stick -- the elevator stick, or the rudder
stick (the sticks on an R/C TX are essentially the same as the ‘volume control’
we used in the above examples). And, of
course we don’t use loudspeakers on our receivers either. Rather, we have servos that change their
position relative to the position of the TX control stick. More about this in part 3.
So, I guess the next
question is what are the other differences between AM and FM, and why is FM
more desirable than AM. Good
question. I’ll try to answer it for
you.
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If you have a radio in your
car that can select AM and FM stations I’m going to ask you to try our a little
experiment. This is how it goes. Next time you’re in your car, tune to a
local FM station. What you will here is
a clear, noise free transmission. Now
tune to an AM station. What you will
hear now is a lot more “Static or background” noise. Some or most of this noise will alter as you rev. your engine.
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What’s happening is
interference - Electro-magnetic interference.
The noise that alters with your engine’s RPM is the easiest to
explain. In a four cylinder auto
engine, there will be two high voltage sparks generated. So, at 3,000 RPM there will be 6,000 sparks
generated every minute, or 100 sparks
every second. Each spark actually
transmits, into the air, a very short, but high intensity, electro-magnetic
power spike. This power spike, because
of its nature, contains millions of different radio frequencies. Any receiver close by, like the one in your
car, will accept this power spike as a sudden increase in its received energy
level. So to the RX, it looks like the
transmitter has momentarily increased its power.
You’ve probably guessed what
comes next. If the RX is AM, this
sudden apparent increase in energy/power is considered a valid amplitude
modulation. The RX’s output device responds. In your car it’s the loudspeaker that “Pops”
with a background “Buzz” that varies with the engine speed. In R/C, it’s the servo that moves to an
erroneous position that could cause the plane to crash.
In an FM system, the RX
still receives this spike as increased energy/power, but, if you remember, it
only responds to frequency changes, not amplitude changes. The FM system is much less
interference/noise prone than AM.
What else causes this
electro-magnetic interference? Metal to
metal joints that move, mechanical engine parts moving fast, movement in the
servo itself. All these can and do cause
amplitude related interference.
So, the rule is: In most
cases, an AM system is OK, but it’s well worth the extra $20 - $30 it will cost
for a comparable FM system. Your plane
is worth much more than the difference in cost.
Don’t forget when buying a
new radio, what ever type it is. Steer
clear of channels 20 and 21. On these
channels you’ll get interference from a different source, namely the local TV
transmitter station.
Your homework assignment!
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Picture this. You are 10 years old and so is your buddy
who lives across the street from you.
You can see each other from you second floor bedroom windows, but the
distance is too far for you to hear each other. Neither of your houses has a telephone. You see each other at school every day and during the days at
weekends. Your parents are very strict
and you both have a curfew of 8:30 P.M.
You both decide to set up a
communications system so that you “talk” to each other after curfew. The materials that each of you has includes
a light bulb, switch and battery. You
know how to wire it up so that you can pulse the bulb on and off with the
switch.
What plan might you and your
buddy devise to use this equipment as communications devices. The answer in the next section - “PPM vs. PCM”
PPM
vs. PCM
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Did you ever wonder at what
was the difference between AM and FM?
Did you every wonder at why FM is preferred over AM? And what about PPM and PCM? Did you ever wonder?
In the last section we
discussed the AM and FM technologies.
In this third part of the R/C System discussion we will look at the
difference approaches that are employed to actually tell each servo what to do.
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You will have heard the
terms “Digital Proportional” and “Pulse Position”, etc. So let’s first take a look at the word “Pulse”.
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To “pulse” something means
to cause something to happen for a relatively short time then to stop. The turn indicators on your car pulses. Your heart pulses. In an R/C system, the transmitter is pulsed too. In an AM (Amplitude Modulation) TX (See Part
2), the amplitude, or TX power, is pulsed from low power to high power. In an FM (Frequency Modulation) TX, the
frequency of the TX is pulsed away from nominal momentarily. By the way, there are several answers to the
homework assignment in Part 2. But
probably the most obvious is that you may have chosen to learn the Morse
Code. The Morse Code describes a unique
series of long and short pulses for each letter and symbol in the
alphabet. By pulsing the signal light
on and off for the right amount of time and in the right sequence, one person
can correspond with another by pulsing their light. So, in the Morse code, we must be able to differentiate between
long and short pulses.
This leads us to a couple
more questions. One, how does that
control the position of a servo, and two, how can we operate multiple control
channels on the same system. We’ll address
question one first.
The servo electronics is
designed so that it receives a pulse of electric current from the receiver that
corresponds to the pulse transmitted by the TX. It is also configured so that the position of the servo motor
will correspond to the length of that pulse.
For example, if the pulse were to be short, say 1 mSec
(one thousandth of a second) the servo motor will move to its most extreme
counterclockwise position and stay there.
If the pulse were to be long, say 2 mSecs, the servo motor will go to
its extreme clockwise position and stay there.
For pulse widthorizontal stabiliser between 1 and 2 mSecs, the
servo motor will go to and stay at a proportionally corresponding
position. This system is technically
called “Pulse Width Modulation” (PWM).
In R/C we are more apt to call it “Pulse Position Modulation” (PPM).
To create pulses of
differing widthorizontal stabiliser, the transmitter uses a circuit called a
encoder that modulates the TX with a pulse width corresponding to position of
the TX control stick.
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In the diagram above, you
can see what could be the control signals for the rudder stick. When the stick is pushed to the left, the
transmitter pulses are short. Since the
TX transmits about a hundred pulses per second, all the pulses will be short
while the rudder stick is held to the left.
During this time, of course, the servo will be holding its travel
extreme counterclockwise. As the rudder
stick is moved to its center, the TX pulses will lengthen, causing the servo to
move to its center position. Continuing
the stick movement to the right, the TX pulses will lengthen even more and will
eventually reach its maximum length.
The servo will respond to this by moving to its extreme clockwise
position. The actual details of this
vary between different manufacturers.
But the concept remains the same.
So there you have it. Pulse Position Modulation (PPM) at work.
Before we take a look at
PCM, let’s first discuss the second question -- How can we control multiple
channels from one TX.
First, don’t get confused by
the term “Channels” When we are talking
about transmitter frequencies, channel refers to the actual frequency. There are 50 channels (Channel 11 to 60) and
each channel is a different radio frequency.
When we talk about controls and servos, etc. the word channel means the
actual number of individual control outputs from the receiver. The most common radio system in R/C aviation
is a 4 channel system (Rudder, Elevator, Ailerons, Throttle). If you want to use retractable wheels, bomb
drop, flaps etc., you will need to use a radio that has more than 4 channels.
I will assume a 4 channel
radio for the purposes of this discussion.
The PPM discussed above really looked at only a single channel (We used
the rudder as the control of choice).
So, in a 4 channel system we need some extra stuff.
As mentioned before, the TX
transmits about a hundred pulses per second.
It varies between manufactures, but for discussion purposes we’ll say
the TX transmits a pulse every 16 mSecs (actually, that’s about 80 pulses per
second). In a 4 channel system we will
identify this train of pulses in a repeating sequence of 5 pulses. We’ll call them pulses 1 through 5.
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As we know, the signal
pulses vary in width from their minimum to there maximum (1 mSec to 2 mSec in
our example). The first pulse in each
sequence of 5 pulses is shorter than the minimum that a control pulse can go
(less that 1mSec). This is the
synchronizing pulse. When the RX
recognizes this synchro pulse, it resets its counters. The very next pulse after the synchro pulse
is the control pulse for channel 1. The
one after that is channel 2, and so on.
If you think about it, this means that the control pulse for any given
channel occurs once in every 5 pulses.
That is once in every 5 X 16 mSec or every 80 mSecs.
That is the PPM multi
-channel R/C system in a nutshell. Now for PCM.
Although Futaba calls their
PPM system “Digital Proportional” in the ‘Conquest’ 4 channel system, it truly
isn’t digital. Let me explain.
The term “Digital” refers to
the simplest form of recognition. In
any digital system, there are two conditions used - usually referred to as “ON”
and “OFF”, or “Zero” and “One” A pulse
system that does not rely on the shape or duration/length of the pulse - just
whether it’s there or not, is digital.
In the PPM system described above, pulses were used to control the
position of a servo, but not by their presence; rather by measuring their pulse
widthorizontal stabiliser.
In a true digital system
pulses are still used, but they are all the same width. The message is sent by whether or not a
pulse is present. But let’s give
Futaba’s marketing department some poetic license. The direction indicators on your car is a digital system. The information as to whether the car in
front of you will turn or not is indicated by whether or not you see the light
pulsing.
Digital and Binary Counting
To use digital pulses to
convey more information, we can set up a system where pulses are still sent,
but different information is indicated by varying the quantity of pulses sent;
-- Digital counting. More commonly
known as “Binary counting”.
Decimal System
How do you count? (Can you count?). If you are like most people on this planet,
you use the decimal system. That means
you are limited to ten different counting symbols, namely, zero through nine (0
-- 9). That’s it, there are no others
to use. So, to count beyond nine, we
must be creative. Here’s how. First, count from 0 to 9. now what?
Well the rule says go back to zero and increment the number to the left
by one. This makes 10 (ten). Now count as normal by taking the zero up to
nine again to get 19. Now apply the
rule. The 9 goes back to zero and we
increment the number to the left by one
to get 20. Simple, right? When we get to 99, we apply the same
rule. The right hand nine does to zero
and we increment the next number. But
in this case that, too, is a nine. So
it goes to zero as well and we increment the next number to get 100. Notice, in this counting system we assume an
infinite number of zeros to the left exist, we just don’t show them.
000
001
:
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009
010
011
:
:
019
020
:
:
029
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100
101
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:
199
200