REMOTE CONTROL
BY BOB YOUNG
Radio control receivers
using amplitude modulation
This month we will look at amplitude
modulated receivers for use in R/C models.
They have the advantage of being simple, easy
to understand, and above all, reliable.
Why use amplitude modulation
(AM) when all the movers and shakers in the R/C movement talk endlessly about FM (frequency modulation)? There are several very sound
reasons for my preference for AM. To
begin, this column is written for beginners and sports flyers, and they do
not face the demanding situations that
the experts face in competition work.
Secondly, beginners are never quite
sure of whether they will be good at
R/C work or even if they will enjoy it,
to the level that they wish to stay in
the hobby. So expensive equipment
is a waste if it is just going to be
resold or sit on a shelf.
Thirdly, and most importantly, AM
still gives the best value for the dollar.
FM sets are expensive to purchase,
much more expensive to repair and
most important of all, much more
expensive and sometimes more difficult to change crystals in.
This is a very important practical
consideration. Many modellers end
up with lots of pairs of crystals, as the
ability to change frequencies on a
crowded field or on a race day is
important. All of these disadvantages
come for a very slender (if any) performance increase.
Narrow band FSK
The point to keep in mind here is
that FM as used in R/C model equipment is not really true FM. It is more
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SILICON CHIP
correctly defined as Narrow Band Frequency Shift Keying (NBFSK) and
believe me, the emphasis is on narrow band. The typical frequency shift
range is from 1.5-2.SkHz.
Now everybody "knows" that FM
is better than AM and this may very
well be true. But for it to be true, we
are speaking of frequency shifts in
excess of ±S0kHz. At this figure, there
is a marked improvement in the signal to noise ratios over AM.
With frequency shifts down around
1-3kHz, the signal to noise ratios are
similiar to, if not worse, than AM and
any talk of FM (NBFSK) R/C systems
being better than AM systems in this
regard is nonsense.
NBSK advantages
NBFSK systems do have two distinct advantages over AM. The AGC
problem in AM receivers can be tricky
and the time constants must be carefully set. A model moving very
quickly past a Tx can create a lump in
the AGC line which will show up as a
glitch in the controls. This is particularly true of high speed model aircraft but can apply to boats and cars.
Secondly, the frequency spacing can
be moved a little closer with NBFSK.
Typically, AM receivers operate on
20kHz spacing and are hard pressed
to come down to lOkHz. It can be
done but it is expensive. As many
clubs only allow 20kHz spacing on
their fields, this is really not a serious
disadvantage.
NBFSK disadvantage
NBFSK receivers do have one very
serious disadvantage. They suffer
from "capture", in which an interfering Tx can override the master Tx.
Control will thus pass to the interfering Tx if it is strong enough. On the
other hand, AM receivers will often
fight their way through interference.
Even brief snatches of recovered control are sometimes sufficient to keep
the model flying until the interference has passed.
From a design point of view, the
NBFSK system places much more
stringent demands on the designer
and the components he uses. To begin with, in an AM system, the crystal locks the transmitter oscillator onto
frequency. Thus, the problems associated with the design of the supply
rails, for example, are minimal. In
other words, the crystal stabilises the
electronics.
The situation in a11 NBFSK transmitter is much more complex. Here
the electronics stabilises the crystal; a
"cart before the horse" situation if
ever I saw one. To achieve the frequency shift required for modulation,
the electronics must be able to pull
the crystal "off frequency" and herein
lies the danger in the NBFSK system.
Supply rails to the transmitter oscillator must be well stabilised with
regards to voltage and must be heavily decoupled. Also, the temperature
stability of the components must be
excellent and the oscillator design
very precise.
The crystals used in AM systems
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Fig.1: this is the circuit of a simple AM R/C receiver which has been thoroughly
proved over many years. The CMOS ICs provide audio gain and squaring (ICt)
and serial to parallel conversion for the 8 channels (IC2).
are simple third overtone series mode
types which are very cheap to produce at 27-40MHz. However, they
cannot be pulled off frequency very
easily. This makes it sound like a bad
thing when in fact it is a good thing;
such is the nature of the AM/FM
conflict. Typically, series mode crystals can only be pulled about 1-1.5kHz
in our bands.
As a result, fundamental crystals
are required for NBFSK systems and
these can be pulled up to 5kHz off
frequency easily. They are, however,
much more expensive to produce. In
addition, they are even more expensive if cut to 30MHz, which is about
the limit of fundamental crystal technology at the moment. Thus, we have
an additional problem in that the crystals used in R/C sets are cut to one
half of the Tx output frequency and
doubled in one of the Tx stages.
All of this amounts to much greater
complexity and a much higher price.
Now the important point here is that
90% of all modellers only require one
thing of their R/C systems and that is
that the integrity of the radio link
must be perfect. In other words, the
commands sent must equal the commands received. AM will do this at a
much lower cost/complexity factor
than FM.
The reliability of AM sets is also
better than FM sets, mainly due to
the simplicity involved. This is not to
say that FM is unreliable; far from it.
In fact, it is a tribute to the modern
component industry that this system
works as well as it does. But AM will
do the job with less fuss and a much
greater cost effectiveness.
I always feel sad when I see a beginner stagger out of a hobby shop
loaded down with expensive equipment he does not really need, or for
that matter, know how to operate to
its full potential. Nor is he ever likely
to reach this level within the lifetime
...
of his first radio. Such is modern
merchandising.
AM receiver circuit
Let us turn now to Fig.1 , which is a
typical AM single conversion Rx of
the type used throughout the . R/C
industry for many, many years. It gave
excellent results and a lot of fun to
untold thousands of modellers. This
type of Rx, incidentally, is still used
in the current generation of two channel systems. Even here though, the
relentless demand for increased complexity in all things (the "Gingerbread
Syndrome", I call it) is forcing the
pace on the development of much
more complex voltage regulation circuits, for example.
In Fig.1 we see a simple superhet
Rx using a local oscillator (Q6), mixer
(Ql), two IF (intermediate frequency)
stages (QZ, Q3) and an active detector
(Q4). Audio amplification after the
detector is provided by a transistor
(Q5) and this is followed by several
stages of squaring using a CMOS
74C04 inverting buffer (ICl). The seDECEMBER 1990
111
The circuit of the 8-channel receiver shown in Fig.1 can be built into a very
small box as this original Silvertone unit shows. The circuitry was on two small
PC boards and there was provision for crystal changing. The slightly larger
Futaba receiver at left is a 3-channel unit.
rial to parallel conversion and decoding is performed by the 74C164 shift
register (ICZ), which gives eight channels of decoded information out.
(In R/C work, a control output is
referred to as a channel, hence eight
channels can control eight separate
controls, each giving left and right,
up and down, etc).
The designer of a receiver for R/C
use faces several problems which are
relatively unique in Rx design. The
overriding factor is that the final unit
must be small, light in weight and
cheap to produce. In addition to this,
it must have good sensitivity and be
capable of sustaining crash after crash.
Some of these crashes can provide
"G" forces that can wrench components completely out of the PC hoard.
At all times, the unit is subjected to
high levels of engine vibration and
high levels of in-flight "G" forces.
Thus, all tall components or components with thin leads must be bonded
to their neighbours at the top end
with contact cement or similar. I have
had components fall completely out
of the PC board under extreme engine
vibration.
The salt water hazard
The receivers in model cars and
boats are regularly immersed in water (salt, brackish or fresh). Salt water
can electroplate the copper from the
PC board to the plastic Rx case in 10
minutes if the board is not correctly
coated.
The moral here is get that model
out of the water quickly, get the power
off pronto, wash it out with fresh
water from the bottle you carry espe112
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cially for this purpose and then flush
with methylated spirits to absorb all
residual water. The metho comes from
the other bottle you carry just for this
purpose. The reason you carry these
bottles, of course, is that you are operating near water regularly.
You do carry these bottles don't
you?
Another serious problem is that of
the Rx coming into extremely close
proximity of anything up to 15 or 20
transmitters, all on very close frequencies. The typical frequency spacing
on most club fields is 20kHz.
The Rx design shown in Fig.1 was
one of the first in this country designed specifically for close band
spacing. Prior to Silvertone introducing this system in 1969, the band
spacing on club fields was 50kHz.
While the design is very basic and
cheap to build, it gave good performance on 15kHz spacing and revolutionised frequency utilisation in this
country.
We had to devise a new method of
frequency control to handle the number of transmitters on the air at any
one time, and this system is now the
Australian standard.
We also had to pull a few devious
little tricks to achieve this spacing
from such a simple design and here
are a few of them.
One of the big problems faced in
model transmitters is the "hole" or
weak signal area off the tip of the Tx
antenna. This can be overcome with
more Tx power, greater Rx sensitivity
or more effective antennas. This Rx
used a bifilar wound antenna coil (11)
which was intended for use with a
centre-fed antenna. We envisioned
self-adhesive burglar alarm tape on
the wing leading edges. The.se would
have doubled as turbulators to enhance lift. As it turned out, this Rx
was sensitive enough not to require
such an antenna system but it did
work well in practice.
Diode D1 across 11 clamps the
input signal to 0.6V and helps prevent front end overload. AGC (automatic gain control) was applied to
the mixer (Q1) and the two IF stages
(Q2, Q3). This was unorthodox but it
definitely helped when others stuck
their Tx antennas through the covering on your wings.
The active detector Q4 gives good
AGC and a high level of audio output. This audio is then "sliced" at
approximately 1V above ground, thus
eliminating the low level noise and
adjacent channel interference present in the audio output. There are, of
course, more modern and elegant
ways to achieve this slicing and op
amp slicers are great.
The output of Q5 is a straight line
with no noise with no carrier present.
Filtered audio is applied to pin 9 of
IC2 (enable). Thus, with no carrier,
IC2 was switched off and this further
ensured that there were no spurious
servo output signals.
The rest of the circuit was fairly
straightforward and followed conventional superhet practise. This was a
nice little Rx and it stayed in production for 15 years. I can still recommend it to anyone who just wants to
fly and fly and fly and not get bogged
down in endless discussions about
the latest and greatest in the gingerbread line.
For those who prefer to roll their
own, I have included circuit values.
Go and have some fun!
