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Items relevant to "80-Metre DSB Amateur Transmitter":
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|
80 metre DSB
amateur
g
Ri
If you are studying for your novice licence,
you will want to get on the air as cheaply
& easily as possible. This little 80-metre
transceiver is the way to do it. It uses no
integrated circuits & all the parts are cheap
& readily available.
By LEON WILLIAMS, VK2DOB
There’s a certain fascination and
challenge in extracting the most
performance from the least number
of components and this project is
the outcome of a desire to do just
that. The 3.5MHz to 3.7MHz, or
80-metre Amateur band, is ideal for
the experimenter. The relatively low
frequencies allow the use of semiconductors generally meant for audio
applications. As well, construction
techniques are not as critical as for
VHF or UHF circuits.
At night, signals can be quite strong
and therefore receivers do not need
November 1994 53
54 Silicon Chip
100pF
220pF
470
Q1
470
E
.001
0.1
Q7
BC549
B
MIC GAIN
VR2
500
2.2M
10k
.0056
.012
10k
10
16VW
150
220
B
Q8
BC549
E .01
Q2
BC337 C
B
1
E 16VW
C
100
10
16VW
1M
0.1
.01
.01
D2
1N4148
CARRIER
6T
NULL
VR1
200
56
T1
D1
1N4148
470
+RX
.022
220W
16VW
1
1M
100
16VW
1k
B
4.7k
4T
VOLUME 10
16VW
E
C
6T
Q6
BC549
4T
T2
100
1
Q9
BC549 C 16VW
B
VR3
E 50k
LOG
5.6k
0.1
56
0.1
80 METRE DSB TRANSCEIVER
10
16VW
100pF
Q1
BC549 C 68k
B
OPTIONAL OSCILLATOR
330pF
100pF
330pF
33k
F1
3.58MHz
ZD1
15V
1W
MICROPHONE
TUNE
VR4
20k
LIN 0.1
100
F1
3.58MHz
10k
0.1
56
470
E
Q10
BC549 C
B
5.6k
100
0.1
220
1k
0.1
0.1
0.1
22
10
D5
68pF
10
16VW
D6
1N4148
+TX
E
L1
6T
D4
PLASTIC
SIDE
4.7k B
470
25VW
E
2. 2W
330
0.5W
E
C
C
L4
2.2uH
100pF
Q5
BD139
0.1
0.1
C
B
E
E
TX
RX
560pF
L2
2.2uH
220pF
820pF
13.8V
+V
820pF
L3
2.2uH
100pF
B
C
HEADPHONES
S1
VIEWED FROM BELOW
B
C
+RX
560pF
100
E 16VW
C Q12
BC549
220
E
1k B
Q11
BC337
820pF
2. 2
E B
Q4
BD139 C
B
2x1N4148
0.1
Q3
D3
BD139 C
1N4004
B
6T
T3
820
0.5W
10
+TX
+V
ANTENNA
to be extremely sensitive and it is
possible to work long distances (DX)
with low power. From my location
near Canberra I have easily worked
New Zealand with just a few watts of
output power when conditions were
favourable.
This transceiver is about as simple
as can be. The transmitter and receiver
share a balanced mixer; in transmit
mode it operates as a balanced modulator and in receive mode as a product
detector. The carrier oscillator is also
common to both transmit and receive
modes and for simplicity, can be crystal controlled. Also delightfully simple is the method of using a ceramic
resonator to form a simple variable
oscillator. The main feature of this
transceiver is the use of cheap and
common components. Other features
include a power output of about 1.5
watts PEP and easy single PC board
construction.
There are no expensive and hard to
get integrated circuits and no difficult
alignment procedures to undertake.
The transmissions are Double Side
band or DSB. This means that the
carrier is nulled out and only the two
sidebands (upper and lower) are transmitted. This is much more efficient
than conventional AM (amplitude
modulation) because there is no RF
output when there is no modulation.
This means the output stage is not
wasting power and heating up while
you are not talking.
While single sideband (SSB) is
the most used mode on the Amateur
bands, an SSB transceiver is a lot more
complex than a DSB type. In any case,
a DSB signal is compatible with an
SSB receiver and has the advantage
that the receiving station can choose
either USB or LSB mode. The receiver
is a direct conversion type where the
incoming signal is mixed directly with
the carrier frequency to produce an
Fig.1 (left): this transceiver is about
as simple as can be. The transmitter
and receiver share a balanced mixer
(T1); in transmit mode it operates as
a balanced modulator and in receive
mode as a product detector. The
carrier oscillator is also common
to both transmit and receive modes
and, as an option, it can be crystal
controlled.
PARTS LIST
1 PC board, code 06110941,
143 x 71mm
1 Jiffy box, 196 x 112 x 60mm
1 black binding post
1 red binding post
1 3.58MHz ceramic resonator (F1)
1 SPDT toggle switch (S1)
1 4-pin microphone panel socket
1 square mount S0239 panel
socket
1 6.5mm stereo jack socket
1 200Ω horizontal trimpot (VR1)
1 500Ω horizontal trimpot (VR2)
1 50kΩ log potentiometer (VR3)
1 20kΩ linear potentiometer (VR4)
2 knobs
15 PC stakes
4 F14 balun formers (L1,T1,T2,T3)
3 2.2µH RF inductors (L2,L3,L4)
Semiconductors
7 BC549 NPN transistors
(Q1,Q6-Q10,Q12)
2 BC337 NPN transistors (Q2,Q11)
3 BD139 NPN transistors (Q3-Q5)
5 1N4148 diodes
(D1,D2,D4,D5,D6)
1 1N4004 diode (D3)
1 15V 1W zener diode (ZD1)
Capacitors
1 470µF 25VW electrolytic
audio signal. Once again, an SSB signal
is compatible, the only disadvantage
being that there is equal response to
both the lower and upper sidebands.
Now let’s have a look at the circuit
diagram of Fig.1.
Carrier oscillator
The carrier oscillator is formed
around Q1. It is configured as a Colpitts
oscillator with feedback provided by
the capacitors connected to the base
and emitter. The oscillator frequency is set by F1, a 3.58MHz ceramic
resonator. This is used in preference
to a crystal because it can be pulled
in frequency quite easily by altering
the circuit capacitance around it.
This is achieved by using a variable
capacitance diode, which is in fact a
15V 1W zener diode, ZD1. These are
cheaper and easier to get than a dedicated Varicap. A variable resistor and
a 10kΩ series resistor provide a means
of varying the frequency.
3 100µF 16VW electrolytic
5 10µF 16VW electrolytic
3 1µF 16VW electrolytic
13 0.1µF monolithic
1 .022µF MKT polyester or
greencap
1 .012µF greencap
3 .01µF ceramic
1 .0056µF greencap
1 .001µF ceramic
3 820pF ceramic
2 560pF ceramic
1 330pF ceramic
1 220pF ceramic
4 100pF ceramic
1 68pF ceramic
Resistors (0.25W, 1%)
1 2.2MΩ
3 470Ω
2 1MΩ
1 330Ω 0.5W 5%
1 68kΩ
4 220Ω
1 33kΩ
1 150Ω
3 10kΩ
3 100Ω
2 5.6kΩ
3 56Ω
2 4.7kΩ
1 22Ω
3 1kΩ
2 10Ω
1 820Ω 0.5W 5% 2 2.2Ω
Miscellaneous
Screws, nuts, spacers, medium-duty
hook-up wire, shielded cable, scrap
aluminium.
A 0.1µF capacitor is included as
protection against noise on the supply
rail modulating the oscillator. The
prototype tuned from 3.568MHz to
3.583MHz and while this is not a big
range, it allows greater flexibility than
when a crystal is used. Note that the
oscillator does not have any voltage
regulation and it is important to use
a regulated power supply to stop the
oscillator changing frequency while
transmitting.
The small value capacitors around
the oscillator are specified as ceramics
in the parts list. This was satisfactory
in the prototype, however if excessive frequency drift is experienced,
polystyrene capacitors may need to
be substituted.
Q2 operates as a buffer stage and
provides a low impedance drive for
the balanced mixer.
As an alternative to a ceramic
resonator, a 3.579MHz crystal can be
used for the oscillator, to give fixed
November 1994 55
10uF
470
100pF
Q11
560pF
560pF
820pF
0.1
100uF
10uF
330
0.1
ANTENNA
SOCKET
220pF
L2
10
820
1k
Q10
10uF
470uF
820pF
100pF
L3
Q9
2. 2
2. 2
L1
1k
220
4.7k
100
D6
1uF
1uF
D4
13.8V
B C E
0.1
L4
220
470
10uF
220
0.01
0.1
D5
.001
VR2
68pF
100
0.1
Q9
10
22
820PF
1uF
2
B
C
E
220
.022
0.1
1
0.1
100uF
Q5
Q3
5.6k
0.1
Q4
T3
4.7k
.012
D3
0.1
1k
.01
100
.0056
Q8
Q6
T1
1M
10k
10k
Q7
2.2M
0.1
100pF
D1
5.6k
10uF
D2
.01
150
470
10k
Q2
100pF
ZD1
0.1
Q1
VR1
0.1
1M
F1
56
330pF
T2
56W
56W
68k
33k
0.1
100uF
CONNECTIONS MADE TO GROUND PLANE
1
HEADPHONES
2
S1
MICROPHONE
VR3
VR4
frequency operation. This alternative
is shown on the circuit diagram of
Fig.1.
Microphone input
Transistor Q7 is the microphone
amplifier and its gain is variable by
adjusting the emitter degeneration
with potentiometer VR2. The circuit
should provide enough gain for most
microphones, however low output microphones may need an extra external
amplifier. A .001µF capacitor is wired
across the input of the amplifier to filter out any RF that may make its way
in via the microphone lead.
Q8 performs the dual role of buffer
stage and low pass filter. The buffer
stage provides a high impedance
load to Q7 and provides a low output
impedance drive for the balanced
modulator. The low pass filter has
a cutoff frequency of about 2kHz. A
56 Silicon Chip
DSB transmitter occupies twice the
bandwidth of an SSB signal and we
must limit the audio response to avoid
interference to adjacent stations.
Balanced modulator
The balanced modulator components are transformer T1, two 1N4148
diodes (D1 & D2) and a 200Ω trimpot
(VR1). T1 is a trifilar wound transformer, where three lengths of wire
are twisted together and wound on
a former as one. This provides close
coupling between the windings and
also aids in the balance or nulling of
the carrier.
Let’s look at how it works in transmit
mode, firstly with no audio input from
the microphone amplifier stages. The
high level RF from Q2 causes current
to flow in the secondary winding of T1.
A .01µF capacitor effectively grounds
the centre of the winding to RF. Due
Fig.2: this component overlay diagram
shows all the components which
must have their leads soldered to the
top & bottom of the PC board; all the
relevant component leads are marked
with a black star dot.
to the phasing of the windings (shown
by dots on the circuit), the two diodes
conduct during the negative half of
the RF cycle. Thus, equal currents
will flow through the diodes and the
resulting voltage at the wiper of VR1
will be zero.
In the next (positive) half cycle, the
diodes will be turned off and again
no voltage will appear at the wiper of
VR1. When an audio signal appears at
the centre of the winding, depending
on the instantaneous voltages, one of
the diodes will conduct more than
the other.
The result is that the modulator is
unbalanced and a voltage will appear
at the wiper of VR1. This voltage follows the envelope of the original audio signal and is a suppressed carrier
double sideband (DSB) signal. A 56Ω
resistor provides a broadband resistive
termination. Ideally D1 and D2 should
be a matched pair, however we can
get good results by adjusting VR1 to
obtain the deepest carrier null (we’ll
talk more about this aspect later).
The output of the balanced modulator is coupled by transformer T2
to the RF driver stage Q3. It is biased
in class A, with a collector current of
50mA. The collector load for Q3 is
transformer T3 with the secondary
winding driving the output stage Q4
and Q5 which are two BD139 transistors in parallel except for their separate
emitter resistors.
These resistors stabilise the AC
and DC gain, ensure that the current
is shared more or less equally between the two transistors and prevent
thermal runaway. The transistors are
biased in class AB which means that
the transistors are just conducting
when there is no input signal. D3 and
an 820Ω resistor provide a stabilised
base voltage and the final stage draws
30mA under no-signal conditions (ie,
with no speech into the microphone).
L1 is the collector load and the
output signal is fed from it through a
low-pass filter before connection to the
antenna. The 330Ω resistor in parallel
with the collector coil is included to
suppress a spurious signal that was
noticed during develop
ment. It is
important that the low-pass filter is
used because quite large harmonics
can be produced in the RF amplifier.
The filter is basically a double Pi filter
with notch frequencies at 7MHz, set by
L2 and a 220pF capacitor, and 10MHz,
set by L3 and a 100pF capacitor.
When the output signal was viewed
on a spectrum analyser, all harmonics
were at least 45dB below the signal
fundamental.
Receive circuit
The signals from the antenna flow
through the just mentioned low-pass
filter and this helps attenuate strong
out-of-band signals. The signal then
passes through a bandpass filter form
ed by L4 and an 820pF capacitor. A
100pF and a 68pF capacitor match
this bandpass filter to the impedances
of the low-pass filter and receive preamplifier. Diodes D4 and D5 protect
tran
sistor Q6 from damage during
transmit mode.
Transistor Q6 is the receive preamplifier. The collector load of Q6 is
one winding of transformer T2 and
the output is coupled to the product
detector via the 4-turn winding (of T2)
All of the circuitry is on a double-sided PC board with a ground plane on the
top. Note the two BD139 transistors which are bolted together with heatsink
flags. These function as the RF output transistors.
and potentiometer VR1. The product
detector uses the same components
as used for the balanced modulator in
transmit, except that the signal path
is now reversed. When there are no
signals coming from the antenna, the
balance is maintained and no audio
signals are produced at the centre tap
of T1. When a signal is tuned in, the
balance is upset and a voltage representing the audio signal is produced
and passed to the first audio stage Q9.
A 56Ω resistor and two .01µF capacitors filter out any RF that may be on
the audio signal.
The signal level at this point is quite
small and so Q9 is configured for high
gain. The collector load is essentially a
5.6kΩ resistor in parallel with a .022µF
capacitor. This combination acts as a
low-pass filter, where the gain of the
stage is greatest at low frequencies and
drops off rapidly at higher frequencies. This is necessary to filter out
adjacent signal interference and it is
in these audio stages that the adjacent
channel selectivity of the receiver is
determined.
The output of Q9 is passed to the
volume control VR3. This is the only
gain control for the receiver and needs
to be adjust
ed for differing signal
strengths as there is no automatic
gain control. The second audio stage
is Q10 and again low-pass filtering is
accomplished by the collector combination of a 5.6kΩ resistor in parallel
with a 0.1µF capacitor. The audio
output stage is Q11 and provides
enough power to drive a pair of low
impedance headphones. Power supply decoupling is included to ensure
amplifier stability.
Transmit/receive switching
Normally, a relay is used in a
transceiver to switch the antenna and
power supply between the transmit
and receive circuits. Relays are both
bulky and expensive, so this design
avoids them by using some novel
techniques.
The antenna and low-pass filter are
permanently connected to both the
transmit output stage (Q4) and the receive bandpass filter. During transmit,
the receiver (ie, the input of Q6) is protected by a pair of back-to-back diodes
(D4 & D5) which limit the voltage to
about 1.2V peak-to-peak. The 100pF
capacitor feeding the receive bandpass
filter is small enough in value to avoid
affecting the operation of the low-pass
filter. During receive, Q4 and Q5 are
turned off and the collectors exhibit
a high enough impedance to avoid
November 1994 57
Fig.3: here are the full size etching patterns for the double-sided PC board.
attenuating the signal on its way to
the receive section.
Power supply switching has been
simplified by using the transmit/
receive switch, S1. Power is permanently connected to the audio section,
the carrier oscillator section and the
RF output stage collectors. When the
switch is in transmit, power is applied
to the microphone amplifier, the RF
driver and the RF output base bias
circuit. Power is also applied to diode
D6 which turns on Q12 and mutes the
receive audio sections.
In receive, power is switched to the
receive RF preamp and the audio mute
transistor Q12 is turned off. There is a
small turn-off delay as the 10µF capacitor discharges via the 4.7kΩ resistor
and the base of Q12. This is done to
58 Silicon Chip
avoid any signal from the microphone
feeding through to the audio amplifier
stages while the microphone amplifier
is turning off.
Construction
All parts except for the controls
and sockets are mounted on a PC
board coded 06110941. The PC board
is double-sided, with the top side
being a continuous groundplane with
clearances for the component leads.
Components which require a ground
plane connection are soldered to the
top and these points are marked with
a black star symbol on the component
overlay. The electrolytic capacitors
get their earth connection through the
earth leads of adjacent components
which are themselves soldered on the
bottom and top of the board. This can
be seen on the wiring diagram of Fig.2.
As with any RF project, keep the
component leads as short as possible.
The overlay diagram shows the variable frequency oscillator components
installed.
After you have checked the board for
any defects, commence by soldering
in the resistors, then install PC stakes
for the external connections. Continue
with the capacitors, diodes, ceramic
resonator and the prewound 2.2µH RF
chokes. Install the transistors, taking
particular care with the orientation of
the BD139s.
The output pair need to be installed
about 5mm above the board. Place a
3mm screw through the mounting
holes of the two transistors while they
dsb
Fig.4: this full-size artwork can be used as a drilling template for the front panel.
Rx
phones
Tx
Ri
80mg
allel to each other. While holding
one end of the set of wires secure in
a vice, twist the other end until there
is about five twists per centimetre. A
hand drill or a battery operated drill
with variable speed control would be
handy for this job.
Wind the wires on the former as
discussed before. Cut off the excess
and untwist the ends for identification
with a multimeter. The start of one and
the finish of another winding need to
mic
All coils are wound on 2-hole F14
ferrite balun formers, using enamelled
copper wire:
• L1: 6 turns 22 B&S enamelled copper wire
• T1: 6 trifilar turns 26 B&S enamelled
copper wire
• T2: primary 4 turns; secondary 4
turns; Q6 collector winding 6 turns 26
B&S enamelled copper wire
• T3: primary 6 turns; secondary 4
turns 26 B&S enamelled copper wire
• L2, L3 & L4 are prewound 2.2µH
RF chokes.
L1 is straightforward, as is T3 except
that it has two windings. T2 has three
windings. The winding ends can be
identified by scraping the enamel off
the ends of the wires and checking for
continuity with a multimeter. Ideally
each winding would use a different
colour wire or you could use a spot
of paint; some form of identification
needs to be used so that the winding
polarities are as specified.
When pulling the wire through
the balun formers, try not to damage
the enamel. This can happen as the
wire passes over the sharp edges of
the holes and could ultimately cause
shorted turns.
Note that T1 is wound using the
trifilar method: take three 400mm
lengths of wire and place them par-
audio
Coil winding details
tune
are being soldered in. The holes need
to be in line, so that a small heatsink
can be attached. This can be simply
constructed from two pieces of scrap
aluminium 16mm wide by 28mm long.
These are formed into two “L” shapes
with a bend at 8mm. A hole is drilled
in the centre of the short leg of each
piece. One is placed in between Q4
and Q5 and the other is placed against
the metal surface of Q5. A screw is
then passed through the assembly and
tightened with a nut.
Next comes the coil winding. Normally this involves cans and formers
with slugs and can be an quite involved. This project makes it simple
by requiring just a few turns of wire
wound on 2-hole ferrite balun formers.
These are sold in two sizes, the one
required measures about 12 x 12 x
7mm. A turn is defined as passing a
wire up through one hole, out the other
end and feeding it back again down the
other hole, so that both ends (start and
finish) of the wire are at the same end
of the former.
be joined to form the centre tap of the
secondary winding. The remaining
winding becomes the primary.
Final assembly
The PC board is housed in a plastic jiffy box measuring 196 x 112 x
60mm. On the front panel are knobs
for tuning (VR4) and audio volume
(VR3), the transmit/receive switch, the
microphone socket and the headphone
socket. The PC board is mounted on
November 1994 59
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The rear panel of the transmitter carries the SO239 antenna socket & binding
posts for the power supply connections (13.8V DC).
the bottom of the box with four spacers
secured with 3mm screws and nuts.
The square mount SO239 antenna
socket is mounted in the bottom right
corner of the base of the box with a
solder lug under one of the retaining
nuts for the earth connection.
The power supply binding posts
are mounted on the base above the
antenna socket. All wiring from the
PC board is done with hook-up wire
except the microphone lead which
should be via shielded audio cable.
Twist the wires to the antenna socket, the tune control and the volume
control. Keep the wiring to the front
panel as short as possible but long
enough so that the PC board can be
accessed when the front panel is lent
forward.
Note that the headphone socket is a
stereo type wired for mono operation.
Testing
Once construction is complete,
check all the wiring one more time.
Place the transmit/receive switch in
receive and connect a power supply
to the binding posts. The transceiver
is designed to be run off 13.8V DC
regulated and draws about 70mA,
however there should be no troubles
with a voltage between 12V and 15V.
A supply of 15V should be considered a maximum and 12V will give a
reduced power output, compared to
the nominal setting of 13.8V
Plug headphones into the phones
socket and advance the volume control. A hiss should be heard indicat-
ing that the audio stages are working
correctly. Check that the oscillator is
working by measuring the frequency
with a frequency counter at the emitter
of Q2. Failing this, listen on a receiver
placed nearby which is tuned to the
oscillator frequency. Inject a signal
into the antenna socket at 3.58MHz
and check that you can hear a tone of
about 1kHz – rotate the tune control
until the tone is heard.
You should only need very light coupling to the antenna socket for a good,
clean tone. If you fail to hear a hiss, the
fault will be later in the audio sections
and if you hear a hiss but no tone then
look for trouble in the RF sections or
around the early audio stages. Unless
you have a second transmitter or a
friend nearby, you will probably have
to wait till late afternoon to receive
off-air voice signals.
Before testing the transmitter, plug a
50Ω dummy load or wattmeter into the
antenna socket and place a multimeter
set to the 1A range in the supply positive lead. Place the modulator balance
trimpot (VR1) at halfway. Switch to
transmit and without a microphone
connected, check the current; it should
read about 180mA. A reading far from
this indicates a fault and should be
looked into.
The next step is to balance the modulator. This can be done by using a low
power wattmeter, a dummy load and
an oscilloscope or a second receiver. A
dummy load can be simply two 100Ω
1W resistors in parallel wired across
the antenna socket. In all the methods,
the aim is to rotate balance control
VR1 until minimum output power is
obtained. This should be at half way,
but it may need adjusting a little either
way to obtain balance.
If you are using a receiver be careful
to avoid picking up the direct signal
from the oscillator which can cause
misleading S-meter readings. With
the carrier nulled, plug in a microphone and either listen to yourself
on a second receiver or have someone
else listen while whistling into the
microphone. Advance the mic gain
control VR2 until the signal starts
to distort and just back it off a little.
Driving the transmitter too hard will
cause a distorted signal and should be
avoided at all times. The transmitter
draws about 400mA on voice peaks.
Operating
Before you can transmit with this
project you must hold a current amateur radio licence. To obtain the best
results with any radio it is important to
use an effective antenna. With a QRP or
low power transmitter it is even more
important, because we want as much
signal radiated as possible. The most
popular antenna for the 80-metre band
is the half wave dipole, which is about
40 metres long and generally fed at the
centre with 50Ω coax cable.
While the antenna is very important,
the band conditions can also play a
large part in getting good contacts.
Sometimes the band can be noisy or
propagation poor, so do not expect to
work long distances every time.
When making a CQ call, it is
helpful to say that you are operating
QRP. This stirs the curiosity of those
listening and also explains the possibility of your low signal strength.
Most stations on the air will be using
commercial transceivers with much
greater output powers than your 1.5W,
so a little patience and skill is needed
to get contacts. You will, however, be
pleasantly surprised with the signal
reports you get.
If you intend to contact other DSB
stations, it will be necessary to adjust
the tune control very accurately. In
fact the two carrier frequencies should
be exactly the same frequency and in
phase to recover the audio properly.
This will generally not be possible
but, with a little knob twiddling a
success
ful contact will be possible.
This problem does not occur with
SC
SSB signals.
November 1994 61
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