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REMOTE CONTROL
BY BOB YOUNG
Building a complete remote
control system for models; Pt.2
This month, we present the circuit description
of the Silvertone Mk.22 24-channel AM receiver.
Although designed primarily for the radio
control of models, it also lends itself to a myriad
of non-modelling applications.
The receiver is a “three-PCB” arrangement, with PCB1 for the receiver,
PCB2 for the first eight channels in
the decoder and PCB3 for the last 16
channels. This month, we are describing the circuit operation, with the
construction to follow next month.
The design of any electronic device
represents a series of compromises
which eventually lead to a completed
unit. In fact, many of the requirements
imposed on the designer are conflicting in nature and we will discuss these
conflicts as we go along.
Basically, the design requirements
for a receiver intended for use in the
radio control of models are: small
physical size, low cost, out-of-sight
range on a low power transmitter
(200-600mW), good noise rejection,
ability to operate in close physi
cal
proximity to other transmitters (some
of which may be only 10-20kHz away),
temperature stability, and the ability
to operate with one cell in the battery
pack short circuited.
Quite a number of prototypes were
produced during the development of
the Mk.22. For those who are curious
about the Mk.22 designation, the last
production Silvertone receiver was
the Mk.14. Mk.15 - Mk.21 were proThis larger-than-life
size photo shows
the completed
receiver assembly.
Note the socket
for the plugin crystal. The
resistors, capacitors
& transistors are
surface-mounted on
the copper side of
the board.
duced during the development of this
unit. The main problems encountered
were PCB layout problems causing
front end instability, excessive noise,
oscillator stability and local oscillator
injection levels and coil phasing. You
will note that all of these are essentially RF problems. The IF stages were
no problem.
The resulting receiver is a very useful little unit which gives surprisingly
good results considering its simplicity.
As my mate Klaus (who provided
valuable assistance with this project,
including the test flying) pointed out,
there is not a lot that can be done with
a couple of transistors and IF cans.
Sensitivity
Receiver sensitivity is approximately 2µV with about 1µV thrown
away in the audio slicer. This results
in a receiver of approximately 3µV
sensitivity. Translated into practical
terms, the result is about 600 metres
ground range (depending on condi
tions) and about 1.5km in the air or
over water.
In R/C modelling, it is important
that the transmitter and receiver do
not provide excessive performance.
This is because many modelling sites
are in close proximity to each other
and excessive transmitter power
or receiver sensitivity can result in
interfield interference. The trick is to
provide just enough performance to
do the job reliably.
The band spacing on this receiver
is 20kHz and this spacing can be used
with complete safety. In addition,
the receiver layout has a very small
cross-section and this allows the board
to be mounted at right angles to the
February 1995 77
C13
2.2
R9
180k
R10
2.2k
C16
47
B
E
L5, L6 : TOKO M113CN 2K218 DC
L4 : LMC 4100A
L2 : LMC 4101A
L1 : LMC4102A
XTAL1 : 30MHz SERIES MODE
3RD OVERTONE
R11
470
E
C
C
VIEWED FROM
ABOVE
B
R2
2.2k
S
Q3
BFT25
R5
1k
E
C10
4.7pF
4.7PF
C
B
30MHz
XTAL1
C11
22pF
V+
C7
15pF
S
R3
100k
78 Silicon Chip
C9
.01
C12
.01
L3
D1
BFR92A
S
F
F
L5
ANTENNA
1
E
B
ANTENNA
2
C4
10pF
10pF
C5
3.3pF
C2
10pF
10pF
C1
.0047
S
F
L6
F
C3
.001
B
R1
680
E
E
C
L4
Q1
BFT25
R4
2.2k
C8
.001
B
R6
1.5k
E
R7
2.2k
C6
2.2
R8
1M
CF1
BFB455
B
Q4
BFT25
Q2
BFT25
C
Circuit details
SILVERTONE MK22 RECEIVER
B
D2
BAS16
R13
10k
L1
L2
direction of travel, even in the most
slender of models.
A plug-in crystal facility is also
provided to allow the crystal to be
quickly changed on the field. The
machine-wound RF coils suggested
are only suitable for 29MHz but with
handwound coils, this receiver will
tune over the range 27-40MHz.
All in all, it’s a very useful little
receiver which will satisfy all but the
most demanding modellers.
Q6
BC848
R12
1k
C14
2.2
C C15
.047
V+
Q5
BC848
E
C
+4.8V
TB1
Fig.1 (left): the receiver follows
conventional superhet principles &
features a crystal controlled local
oscillator (Q3 & Xtal1), a double tuned
front end feeding a conventional
transistor mixer (Q1), two IF stages
working at 455kHz (Q2 & Q4), & the
transistorised equivalent of an anode
bend detector (Q6).
The receiver follows conventional
superhet principles and features a
crystal controlled oscillator, a double
tuned front end feeding a conventional
transistor mixer, two IF stages work
ing at 455kHz, and the transistorised
equivalent of an anode bend detector.
Fig.1 shows the details.
The transmitted signal arrives at
the antenna and is fed into either
the primary or the secondary of coil
L5, depending upon the application.
Antenna 1 is intended for coax-feed
remote antennas, while Antenna 2
is the normal model aircraft antenna
(usually one metre of flexible hook-up
wire). If signal-to-noise ratio is more
important than range in your application, then use Antenna 1, even for the
flexible wire antenna. This will result
in a much cleaner signal at very low
signal strengths but will cost about
6-8dB in gain.
Diode D1 acts as a clamp to prevent
mixer overload when the transmitter
antenna is very close to the receiver
antenna. This is a serious problem
in model applications, as modellers
often need to stand over their models
in order to operate them unassisted.
A common trick is to stand astride a
model aircraft, for example, with the
tailplane hooked behind the ankles
whilst the motor is run up to clear
the plug and check the mixture. This
will result in a very high signal level
at the receiver mixer if precautions
are not built into the front end to
compensate.
earth/antenna and the transmitter
antenna, these two signals (which are
opposite in phase) can cancel each
other out, the nett result being a com
plete loss of signal and what is known
as a glitch. This is a momentary loss
of signal which clears itself almost
immediately after it occurs.
This problem can and does occur in
most model receivers and accounts for
some of the mysterious little hiccups
which occur from time to time.
Local oscillator
This views shows the completed receiver (right) together with a companion
8-channel decoder unit (to be described next month). The two units can be fitted
together inside a small metal case.
Thus, D1 clamps the signal to 0.6V
maximum. The downside to D1 is that
it can introduce intermodulation effects at the mixer. For this reason, D1’s
physical characteristics are extremely
important, if another transmitter is
operated close by and on an adjacent
channel.
From experience, I know that a
1N4148 diode works well in this
application. However, the Mk.22 receiver uses the base-emitter junction
of a VHF transistor (BFR92A) for this
diode and this also works extremely
well. Coil L6 provides additional fre
quency selectivity and also matches
the 1-metre wire antenna into the base
of the mixer.
Before leaving the antenna coils,
there is one very important point to
bring to light regarding the earth/antenna relationship. Ideally, the signal
appears in its strongest form across
the antenna and is balanced against
a very strong ground connection. In
model work and particularly model
aircraft work, howev
er, there is no
ground connection and the battery
and interwiring have to work as a solid
earth. The problem is, this wiring varies from model to model, depending
on the size of the model, number of
channels, servos and the neatness of
the installation.
In some cases, signal inversion can
take place across coil L5, where the
antenna is acting as a counterpoise
(earth) and the earth wiring is acting as
the antenna. In freak cases, depending
on the polarisation of the receiver
Fig.2: this scope
photograph shows
the output signal
on the collector of
detector stage Q6.
Transistor Q3 functions as a local
oscillator and runs at the carrier frequency plus 455kHz. In Australia,
local oscillators run on the high side
of the carrier in the 29MHz band, due
to possible image problems from the
30MHz band. The opposite is the case
on the 36MHz band where the local
oscillator runs on the low side of the
carrier.
Coil L3 forms the tank coil for the
local oscillator, while its secondary
provides low impedance matching
for injecting the oscillator signal into
the emitter of Q1 via C12. C7 provides
the fine tuning for the crystal frequency. The crystal can be pulled about
1-1.5kHz by adjusting C7 and C10. The
values presented on the circuit are for
Showa brand crystals and may need
some adjustment if different brands of
crystals are used.
Transistor Q1 functions as the mixer
and the resulting 455kHz IF signal is
derived from the composite signal by
L4. C8 damps L4 to prevent ringing if
it occurs. It is not fitted with the 4000
series coils provided in the kit but
may be required if different brands of
coils are used.
Q2, L2, Q4 & L1 provide the IF amplification, with R6 acting as the main
gain control. Increasing its value will
reduce the gain (the value shown on
Fig.1 provides near maximum gain).
Ceramic resonator CF1 across Q4’s
emitter resistor (R11) sharpens the
bandpass characteristic of the IF stage
by approximately 3dB and is a useful
addition.
Detection & AGC
Q6 acts as the transistorised equivalent of an anode bend detector and
provides the recovered audio signal
as well as the AGC control voltage.
Diode D2 and capacitor C15 rectify and
filter out the 455kHz component. The
recovered audio will be approximately
February 1995 79
Frequency Control At Flying Fields
The receiver presented in this
article is intended for use on the
29MHz band and, in fact, the machine-wound coils recom
mended
will only tune from 27-29MHz.
Hand-wound coils will allow the unit
to be tuned through the full range of
frequencies available to modellers
from 27-40MHz. However, its use on
modelling frequencies outside the
29MHz band is not recommended for
several reasons, as set out below. In
addition, non-modelling applications
will need to take into account the
relevant Department of Transport
and Communications regulations.
27MHz Citizens Band (26.95727.282MHz): the original garbage
band, cluttered with cosmic noise
and thus given over to experimenters
from the early days. It was heavily
used by modellers for many years
until CB traffic made it too dangerous. This band is very busy with CB
traffic and now frowned upon by the
authorities for modelling use. Two frequencies are given over to children’s
toys and “toy” walkie talkies.
29MHz Band (29.72-30.00MHz): a
specific modelling band allocated
when the CB band became unusable (c. 1975) and the recommended
band for this receiver. The frequencies recommended for use in this
band are set out in Table 1
Crystals on these frequencies
are available from most good hobby
shops. This band is used extensively
3V p-p at high signal levels. The slicer
in the decoder (to follow) rejects the
bottom 1V of the audio output and
passes only the clean, high level signal
to the audio amplifier.
As the signal strength increases, the
80 Silicon Chip
Silvertone Keyboards are the recommended method of frequency control
for all national events sanctioned by the Model Aeronautical Association
of Australia (MAAA). Illustrated are the 29MHz board (standing) and the
new expanded 36MHz two-board set. The expanded 36MHz band, soon to
be released, now features 59 frequencies at 10kHz spacing.
Table 1
Channel
TX
RX
10
29.725
30.18
12
29.745
30.20
14
29.765
30.22
16
29.785
30.24
18
29.805
30.26
20
29.825
30.28
22
29.845
30.30
24
29.865
30.32
26
29.885
30.34
28
29.905
30.36
30
29.925
30.38
32
29.945
30.40
34
29.965
30.42
36
29.985
30.44
by modellers favouring 2-channel
equipment (cars and boats) but almost deserted now on flying fields
due to the rush to 36MHz. This is a
wise choice if you just want to go
to the field and fly, free of channel
clutter and waiting time.
36MHz band (36.00-36.60MHz):
soon to be expanded and opened
up for use with a 10kHz frequency
voltage at the collector of Q6 falls towards ground and the bias supplied to
Q1, Q2 & Q4 via R9, R2, R4 & R7 falls,
thus reducing the gain of these stages.
Capacitor C6 filters out any audio on
the AGC line, while R9 & C6 together
spacing. The Mk.22 is not recommended for 10kHz spacing and is
thus not recommended for use on
the 36MHz band.
40MHz band (40.66-40.70MHz):
another of the original modelling allocations but now not recommended
due to heavy traffic from hospital
pagers and the like on 40.680MHz.
Channel 50 (40.665) and Channel
53 (40.695) are still OK for 10kHz or
wider bandwidth receivers in areas
free of this traffic.
The Silvertone Keyboard pictured
above was designed in 1969 to allow
the mixing of equipment with various
bandwidth characteris
tics at busy
flying fields. It is basically a graphic
representa
tion of the frequency
allocation laid out on a 1-inch =
10kHz grid.
Each modeller is supplied with
a key, the width of which is proportional to the bandwidth of his
equipment. Thus, a 10kHz system
uses a 1-inch key, while a 20kHz
system uses a 2-inch key. To reserve a frequency block in order to
fly safely, the correct width key is
simply inserted into the appropriate
slot in the board, thus reserving the
frequencies required.
provide the AGC time constant to
filter out any flutter caused by rapid
variations in signal strength. These
can occur due to high speed aircraft
flying by the transmitter or through
weak signal areas.
Finally, Q5 and C16 provide the
power supply filtering. In operation,
the capacitance of C16 is multiplied
by the gain of Q5, thus resulting in a
very simple and effective filter.
Unusual Use For A Speed Control Unit
Substitute at your peril
Now a few general notes on the
overall design of the receiver. First,
substitute values at your own peril.
And to those who wish to do their
own through-hole layout, the best
of luck. Half of the prototypes were
rejected because of layout problems.
RF circuits are very sensitive to
board layout and consequently the
layout forms a major component in
the design.
Capacitor C14 is a layout compensation filter and must be mounted
in the physical location shown on
the component overlay. C13 is there
to provide spike suppression on the
power rail input.
For those still determined to press
on, use 2N3646 or BF494 transistors in
the RF and IF stages. These will give
the best noise and AGC characteristics.
The surface mount BFT25 transistors
used in the unit described here were
chosen for the same reason and were
selected after trying many types. Let
me tell you, these are an expensive
transistor but are well worth the
money in this application. Also, use
a BC847 in the DC and audio stages.
Try not to substitute for the IF coils
as they are the heart of the system
and a change here can create all sorts
of havoc.
RF coils
The only other components which
are critical are the RF coils. These may
be hand-wound and Neosid make a
neat little 4mm coil former which will
fit the PC board with only a slight joggle of the mounting pins. Use 12 turns
of 28 B&S wire with a 33pF capacitor.
The secondary consists of three turns
of the same wire. Be sure to follow
the start and finish instruction on the
schematic.
Reduce the capacitor to increase
the frequency –there is no need to
change the turns. They should tune
to 40MHz with about 22pF of capacitance.
You can use a 1N4148 diode for D1
but do not substitute anything else. In
addition, make sure that you use NPO
capacitors on all of the values up to
.001µF. The rest of the components
One of our readers, Peter Barsden of WA, has sent along some interesting
photos of his gyrocopter (no details provided) which is fitted with a pre-rotator. This unit consists of an electric motor (located at the top of the mast)
and this spins up the rotor before takeoff to reduce the takeoff distance. The
electric motor is controlled by a Speed1B speed control unit fitted with a
self- contained pulse generator, as published in Silicon Chip in November
& December 1992, January 1993 and April 1993. Peter has purchased six
of these units and appears to have convinced his friends that the Speed1B
is the way to go.
are not that critical. The resistors can
all be 1/8W types.
Surface mount components
Finally, I failed to stress one important point last month on the hand assembly of surface mount components.
The manufactures do not recommend
surface mount components for hand
assembly due to the risk of thermal
shock cracking the substrate of some
of the components.
In practise, this can be minimised
by heating the pad first and letting the
solder flow from the tip of the iron to
the component (ie, apply the solder
to the tip of the iron and not to the
component).
Remember also that the iron and
the solder (with flux) must be applied
simultaneously to the joint. Do not try
to transfer solder from the iron to the
joint. Also, try to avoid touching the
component with the tip of the iron. As
you will recall, I suggested soldering
one pad of each component first by
sliding their ends into molten solder.
This minimises the thermal shock.
Looked at in this light, it is probably
a good idea to immediately solder
the second pad of a component after
the first (ie, while it is still warm),
rather than after all components have
been mounted. In practise, I have
hand-mounted thousands of these
components with no signs of visible
damage but do try to be as careful as
possible.
To recap my previous advice, use
a low wattage iron (20W), keep the
iron temperature as low as practical
and avoid touching the component
with the tip of the iron. Next month,
we shall continue with details of the
SC
board assembly and alignment.
Acknowledgement
I would like to thank everyone at
Borundi Electronics for the assistance and cooperation given to
me throughout this project. Without the use of their prototyping
PCB facilities, I would have faced
great difficulties in completing this
design.
February 1995 81
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