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Contents
13.1 Transmitter Modulation Types and Methods
13.1.1 Data, Morse Code or Pulse Transmitters
13.1.2
Project
: Simple CW Transmitters
13.1.3 Amplitude-Modulated Full-Carrier Voice
Transmission
13.1.4 Single-Sideband Suppressed-Carrier
Transmission
13.1.5 Angle-Modulated Transmitters
13.1.6 The Superhet SSB/CW Transmitter
13.1.7 CW Operation
13.1.8 Wideband Noise
13.1.9 Automatic Level Control (ALC)
13.1.10
Project:
The MicroT2 — A Compact
Single-Band SSB Transmitter
13.1.11
Project:
The MkII — An Updated Universal
QRP Transmitter
13.2 VHF Signal Sources
13.2.1 Overview
13.2.2 Oscillator-Buffer
13.2.3 Frequency Multiplier
13.2.4 Output Bandpass Amplifier
13.2.5 Tuning Up the Circuit
13.2.6 Modulating the VHF Sources
13.2.7 Creating a Direct Conversion Receiver
13.3 Increasing Transmitter Power
13.3.1 Types of Power Amplifiers
13.3.2 Linear Amplifiers
13.3.3 Nonlinear Amplifiers
13.3.4 Hybrid Amplifiers
13.4 Transceiver Construction and Control
13.4.1 Upconverting Architecture
13.4.2 Break-In CW Operation
13.4.3 Push-To-Talk for Voice
13.4.4 Voice-Operated Transmit-Receive
Switching (VOX)
13.4.5 TR Switching
13.4.6 TR Switching With a Linear Amplifier
13.5 Transceiver Projects
13.5.1 Transceiver Kits
13.5.2 Project: The TAK-40 SSB CW Transceiver
13.5.3
Project:
A Homebrew High Performance
HF Transceiver — the HBR-2000
13.6 References and Bibliography
Chapter 13 —
CD-ROM Content
Supplemental Articles and Projects
•
“Designing and Building Transistor Linear Power Ampliiers” Parts 1 and 2 by
Rick Campbell, KK7B
•
“A Fast TR Switch” by Jack Kuecken, KE2QJ
•
“A Homebrew High Performance HF Transceiver — the HBR-2000” by
Markus Hansen, VE7CA
•
“The MicroT2 — A Compact Single-Band SSB Transmitter” by Rick Campbell,
KK7B
•
“The MkII — An Updated Universal QRP Transmitter” by Wes Hayward,
W7ZOI
•
“The Norcal Sierra: An 80-15 M CW Transceiver” by Wayne Burdick, N6KR
(plus supporting iles)
•
“The Rockmite — A Simple Single-Band CW Transceiver” by Dave Benson,
K1SWL (plus supporting iles)
•
“The TAK-40 SSB/CW Transceiver” by Jim Veatch, WA2EUJ
•
“A Transmitter for Fox Hunting” by Mark Spencer, WA8SME
•
“The Tuna Tin 2 Today” by Ed Hare, W1RFI
•
“VHF Open Sources” by Rick Campbell, KK7B (plus parts placement guides)
Chapter
13
Transmitters
and Transceivers
Transmitter technology has advanced in a parallel process similar to that of the technology
of receivers. While transmitters are composed of many of the same named blocks as those
used in receivers, it’s important to keep in mind that there are significant differences. An RF
amplifier in a receiver may deal with amplifying picowatts while one in a transmitter may
output up to kilowatts. While the circuits may even look similar, the size of the components,
especially cooling systems and power supplies, may differ significantly in scale. Still, many
of the same principles apply.
Transceivers — the combination of a receiver and transmitter in a single physical piece of
equipment — are the norm in Amateur Radio today. Separate receivers and transmitters are
no longer offered by the major manufacturers, although many amateurs find this an easier
approach when constructing homebuilt equipment. With receivers covered in their own
chapter, this edition of the book combines the previously separate chapters on the closely
related technology of transmitters and transceivers.
Transceivers achieve many economies by sharing receiver and transmitter elements such as
high-performance components and circuits, power supplies and antenna switching circuits, as
well as the physical enclosures and operating controls themselves. The sharing is facilitated
by control and switching circuitry as discussed in the transceiver sections of this chapter.
Transmitters (and transceivers) may contain hazardous voltages, and at higher power levels
RF exposure issues must be considered — review the
Safety
chapter for more information.
Techniques for transmitter measurement are covered in the
Test Equipment and Measure-
ments
chapter.
Transmitters are the companion
to the receivers discussed in the
previous chapter. As with receiver
design, the basic elements of
transmitters such as oscillators
and modulators are described
in other chapters of this book.
Transceivers—the combination of
a transmitter and receiver in the
same package — add switching
and signal control circuitry.
Ampliiers for power levels
above 100 W (at HF) are covered
in the
RF Power Ampliiers
chapter. The
DSP and Software
Radio Design
chapter has more
information on digital techniques
and architectures.
This chapter includes a trio
of QRP transmitter projects,
two transceiver projects, and
supporting information and articles
that can be found on the
Handbook
CD-ROM. Rick Campbell, KK7B,
contributed a new section on
design and construction of VHF
signal sources.
13.1 Transmitter Modulation Types
and Methods
Current Transceiver Overview
A supplemental article on
this book’s CD-ROM describes
a range of commercial HF and
VHF/UHF transceivers. With each
subsequent edition, the overview is
updated and the previous version
moved to the ARRL website for
future reference.
13.1.1 Data, Morse Code or Pulse Transmitters
The simplest transmitter consists of an oscillator generating a signal at the frequency we
want to transmit. If the oscillator is connected to an antenna, the signal will propagate outward
and be picked up by any receivers within range. Such a transmitter will carry little informa-
tion, except perhaps for its location — it could serve as a rudimentary beacon for direction
finding or radiolocation, although real beacons generally transmit identification data. It also
indicates whether or not it is turned on, perhaps useful as part of an alarm system.
To actually transmit information, we must
modulate
the transmitter. The modulation
process, covered in detail in the
Modulation
chapter, involves changing one or more of the
signal parameters to apply the information content. This must be done in such a way that the
information can be extracted at the receiver. As previously noted, the parameters available
for modulation are:
Frequency
— this is the number of cycles the signal makes per second.
Amplitude
— although the amplitude, or strength, of a sinusoid is constantly changing
with time, we can express the amplitude by the maximum value that it reaches.
Phase
— the phase of a sinusoid is a measure of when a sinusoid starts compared to another
Transmitters and Transceivers
13.1
sinusoid of the same frequency.
We could use any of the above parameters
to modulate a simple transmitter with pulse
type information, but the easiest to visualize
is probably amplitude modulation. If we were
to just turn the transmitter on and off, with it
on for binary “one” and off for a “zero,” we
could surely send Morse code or other types
of pulse-coded data. This type of modula-
tion is called “On-Off-Keying” or “OOK”
for short.
Some care is needed in how we implement
such a function. Note that if we performed the
obvious step of just removing and turning on
the power supply, we might be surprised to
find that it takes too much time for the voltage
to rise sufficiently at the oscillator to actually
turn it on at the time we make the connection.
Similarly, we might be surprised to find that
when we turn off the power we would still be
transmitting for some time after the switch is
turned. These finite intervals are referred to
as
rise
and
fall times
and generally depend
on the time constants of filter and switching
circuits in the transmitter.
reviews of commercial multimode 100 W HF
transceivers.
Fig 13.1
shows the CW keying
waveform of a transmitter with good spec-
trum control. The top trace is the key closure,
with the start of the first contact closure on
the left edge at 60 WPM using full break-in.
Below it is the nicely rounded RF envelope.
Fig 13.2
shows the resultant signal spectrum.
Note that the signal amplitude is about 80 dB
down at a spacing of ±1 kHz, with a floor of
–90 dB over the 10 kHz shown.
Figs 13.3
and
13.4
are similar data taken from a different
manufacturer’s transceiver. Note the sharp
corners of the RF envelope, as well as the time
it takes for the first “dit” to be developed. The
resulting spectrum is not even down 40 dB at
±1 kHz and shows a floor that doesn’t quite
make –60 dB over the 10 kHz range. It’s easy
to see the problems that the latter transmit-
ter will cause to receivers trying to listen to
a weak signal near its operating frequency.
The unwanted components of the signal are
heard on adjacent channels as sharp clicks
when the signal is turned on and off, called
key clicks
. Note that even the best-shaped
keying waveform in a linear transmitter will
become sharp with a wide spectrum if it is
used to drive a stage such as an external power
amplifier beyond its linear range. This gener-
ally results in clipping or limiting with sub-
sequent removal of the rounded corners on
the envelope. Trying to get the last few dB
of power out of a transmitter can often result
in this sort of unintended signal impairment.
HBK0497
I
F
I
F
I
F
N+]
I
F
I
F
Fig 13.2 — The resultant signal spectrum
from the keying shown in Fig 13.1. Note
that the signal amplitude is about 80 dB
down at a spacing of ±1 kHz, with a loor
of –90 dB over the 10 kHz shown.
REAL WORLD CW KEYING
In the
Modulation
chapter, the impor-
tance of shaping the time envelope of the
keying pulse of an on-off keyed transmitter
is discussed. There are serious ramifications
of not paying close attention to this design
parameter. The optimum shape of a transmit-
ter envelope should approach the form of a
sinusoid raised to a power with a tradeoff
between occupied bandwidth and overlap
between the successive pulses. This can be
accomplished either through filtering of the
pulse waveform before modulation in a linear
transmitter, or through direct generation of
the pulse shape using DSP.
The differences between well-designed
and poor pulse shaping can perhaps be best
described by looking at some results. The fol-
lowing figures are from recent
QST
product
HBK0494
13.1.2
Project:
Simple CW
Transmitters
The schematic for a very basic low power
HF CW oscillator transmitter is shown in
Fig 13.5.
Using a crystal-controlled oscillator
gains frequency accuracy and stability but
gives up frequency agility.
In this design, the key essentially just turns
power on and off. The value of the keying
line 0.1 µF bypass capacitor was chosen
so it would not create an excessive rise or
fall time at reasonable keying speeds. This
transmitter will generate a few mW of RF
power at the crystal frequency, but it has a
number of limitations in common with early
vacuum tube transmitters that were such an
improvement over the spark transmitters that
preceded them.
First, the oscillator is dependent on the
environment. Changes in the antenna from
wind, for example, change the load and can
cause the frequency to shift. Second, the
oscillator generates signals at harmonics of
the fundamental frequency. Third, every time
the key is closed the oscillator must start up,
taking a short but audible time for the output
frequency to stabilize. This creates a distinc-
tive change in output frequency referred to as
“chirp,” since the signal sounds like a bird’s
chirp in the receiver.
By adding an amplifier stage to the oscilla-
tor transmitter, the oscillator can be isolated
from changes in the environment to improve
0 .010.020.030.040.050.060.070.08
Seconds
Fig 13.3 — The CW keying waveform of a
transmitter with poor spectrum control.
The top trace is the key closure, with the
start of the irst contact closure on the left
edge at 60 WPM using full break-in. Note
the sharp corners of the RF envelope that
result in excessive bandwidth products.
HBK0496
HBK0495
0
-20
-40
-60
-80
0.0
0.01 0.02 0.03 0.04 0.05 0.06 0.07
0.08
-100
14015 14017 14019 14021 1402314025
Seconds
Fig 13.1 — The CW keying waveform of a
transmitter with good spectrum control.
The top trace is the key closure, with the
start of the irst contact closure on the left
edge at 60 WPM using full break-in. Below
that is the nicely rounded RF envelope.
Fig 13.4 — The resultant signal spectrum
from the keying shown in Fig 13.3. The
resulting spectrum is not even down
40 dB at ±1 kHz and shows a loor that
doesn’t quite make 60 dB below the car-
rier across the 10 kHz.
13.2
Chapter 13
(
www.pwpublishing.ltd.uk
). An article by
Ed Hare, W1RFI, describing the Tuna-Tin
2 transmitter that was included in previous
editions of the
ARRL
Handbook
is available
on this book’s CD-ROM and listed in the
References section of this chapter.
Fig 13.6
is the schematic of a much bet-
ter, but still simple, CW transmitter. It is an
update to the “Pebble Crusher” originally
designed by Doug DeMaw, W1FB (SK), and
described in his
QRP Notebook
(out of print).
The transmitter uses an oscillator followed
by a stage of amplification to about 1/2 W of
output. Note that the output circuit includes
a filter to reduce any harmonic output below
the levels required by FCC rules.
The circuit is a two transistor transmitter
using 2N2222A devices. These are small-
signal devices so the transmitter output is
only about
1
⁄
2
W. What might surprise the
reader is the number of inductors (coils) in
the design, but the circuit has good perfor-
mance in impedance matching and harmonic
reduction. A careful design ensures a clean
output waveform, low in harmonic and other
spurious content.
The oscillator is a variable frequency crys-
tal oscillator (VXO) in which the 100 pF
variable capacitor between the crystal and
ground enables the oscillator frequency to be
shifted. Depending upon individual crystals,
that movement can be on the order of 5 kHz.
Increasing the variable capacitance too much
may cause the transistor to cease oscillating.
The 150 pF capacitor in the emitter of the os-
cillator controls the oscillator feedback. This
value is a compromise that appears to work
well — but if the transistor fails to oscillate,
try increasing this value a little.
The 4.7 µF electrolytic capacitor across
the
KEY
connection minimizes key clicks by
softening the keying waveform. Lowering
this value gives a harder keying waveform and
increasing it will further soften the waveform.
A 10 W resistor is added to the base of the
transistor to reduce the risk of high frequency
parasitic oscillation.
A 22 µH RF choke (RFC1) provides the
RF collector dc supply for the oscillator. A
molded inductor was used in the original de-
sign. If this isn’t available, about six turns of
thin enameled wire wound through a small
ferrite bead (type 43 mix) would give roughly
the same inductance value.
The output from the collector goes to a
single-element harmonic filter in a pi configu-
ration. A 100 pF variable capacitor tunes the
filter to resonance at the oscillator frequency.
It should be adjusted for maximum output
consistent with a clean CW signal.
If the intent is to use the oscillator stage as a
transmitter without additional amplification,
the filter is designed to have a 50 W output
impedance for a matched 7 MHz antenna.
The antenna can be connected in place of T1
in the schematic.
A 4:1 impedance ratio transformer (T1)
matches the output of the oscillator to the
Fig 13.5 — A simple solid-state low-power
HF crystal-controlled oscillator-transmit-
ter. The unspeciied tuned circuit values
are resonant at the crystal frequency.
stability. Adding filtering at the output ad-
dresses the problem of harmonics.
The following project by Rev George
Dobbs, G3RJV, was published in the Spring
2012 issue of
QRP Quarterly
and is reprinted
courtesy of the QRP Amateur Radio Club
(
www.qrparci.org
) and
Practical Wireless
Fig 13.6 — The circuit of the Pebble Crusher transmitter. The oscillator section at left can be used as a standalone transmitter by
replacing T1 with an antenna. The output of the ampliier circuit is approximately
1
⁄
2
W.
RFC1, RFC2 — 22 µH or 6t #30 AWG on
ferrite bead (type 43 mix)
T1 — 12t #24 AWG (primary) and 6t #24
L1 — 32t #30 AWG on T50-6 toroid core
L2 — 28t #24 AWG on T50-6 toroid core
AWG (secondary) on FT37-43 toroid
core (see text)
FB — ferrite bead (type 43 mix)
Transmitters and Transceivers
13.3
transmitter would actually be implemented.
The upper portion is the RF channel, and you
can think of the previously described Tuna
Tin Two transmitter as a transmitter that we
could use, after shifting the frequency into
the voice portion of the band. The lower por-
tion is the audio frequency or AF channel,
usually called the
modulator
, and is nothing
more than an audio amplifier designed to be
fed from a microphone and with an output
designed to match the anode or collector
impedance of the final RF amplifier stage.
The output power of the modulator is ap-
plied in series with the dc supply of the output
stage (only) of the RF channel of the trans-
mitter. The level of the voice peaks needs to
Fig 13.7 — G3RJV’s “Anglicised” version of the Pebble Crusher is built on 0.1 inch
spacing perfboard. The heat sink for the output transistor at lower right can be a com-
mercial unit or created from metal strip or tubing (see text).
core and L2 has 28 turns of #24 AWG enam-
eled copper wire also on a T50-6 core. The
transformer (T1) is wound on a ferrite FT37-
43 core. Wind the primary first with 12 turns
of #24 AWG spread out over three-quarters of
the core circumference. Then add the second-
ary winding which is made up of 6 turns of
#24 AWG wire between turns of the primary
winding at the grounded end.
Fig 13.7
shows 2N2222A transistors in
metal TO-18 cases, but the plastic TO-92
version would also work. An advantage of
using the metal TO-18 is that a small heat
sink can be attached to the output transistor
to dissipate surplus heat. If a commercial
heat sink is not available, a small piece of
aluminum or copper strip can be formed into
a heat sink. The strip is wrapped around a drill
bit the same diameter as the transistor case,
one side overlapping where the ends of the
metal meet. This is then squeezed to make a
tight fit on the TO-18 case. A small piece of
1
⁄
4
inch OD brass tubing slightly flattened and
cemented to the transistor casing with epoxy
will work as well.
applications using data applied to our “pulse”
transmitter described earlier. Here we will
talk about the more direct application of the
analog voice signal to a radio signal.
A popular form of voice amplitude modu-
lation is called
high-level amplitude modula-
tion.
It is generated by mixing (or modulating)
an RF carrier with an audio signal.
Fig 13.8
shows
the conceptual view of this
.
Fig 13.9
is a more detailed view of how such a voice
Fig 13.8 — Block diagram of a conceptual
AM transmitter.
13.1.3 Amplitude-Modulated
Full-Carrier Voice Transmis-
sion
While the telegraph key in the transmitters
of the previous section can be considered a
modulator
of sorts, we usually reserve that
term for a somewhat more sophisticated sys-
tem that adds information to the transmitted
signal. As noted earlier, there are three signal
parameters that can be used to modulate a
radio signal and they all can be used in vari-
ous ways to add voice (or other information)
to a transmitted signal.
One way to add voice to a radio signal is to
first convert the analog signal to digital data
and then transmit it as “ones” and “zeros.”
This can be done even using the simple tele-
graph transmitters of the last section. This is
a technique frequently employed for some
Fig 13.9 — Block diagram of a 600 kHz AM broadcast transmitter.
Fig 13.10 — The range of of spectrum used by a 600 kHz AM broadcast signal showing
sidebands above and below a carrier at 600 kHz.
13.4
Chapter 13
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