ozQRP MDT Instructions for use

MDT Construction Manual –Issue 2 Page 1
MDT
DSB TRANSCEIVER
CONSTRUCTION MANUAL

MDT Construction Manual –Issue 2 Page 2
CONTENTS
1Introduction..............................................................................................................................................................4
2DSB vs SSB .................................................................................................................................................................5
3DSB transmitter.......................................................................................................................................................6
4Direct Conversion receiver................................................................................................................................. 7
5MDT Block Diagram...............................................................................................................................................8
6Circuit Description .................................................................................................................................................9
6.1 Variable Frequency Oscillator (VFO)........................................................................................................................9
6.2 Mixer .......................................................................................................................................................................................9
6.3 Microphone amplifier ................................................................................................................................................... 10
6.4 Transmit amplifier......................................................................................................................................................... 10
6.5 Power Supply and RX/TX switching ...................................................................................................................... 11
6.6 Receive audio.................................................................................................................................................................... 11
7Kit Supplied Parts ................................................................................................................................................ 17
8Individual Parts List............................................................................................................................................ 19
9Construction........................................................................................................................................................... 21
9.1 General ................................................................................................................................................................................ 21
9.2 Building the PCB.............................................................................................................................................................. 22
10 Final Assembly...................................................................................................................................................... 33
10.1 Microphone socket......................................................................................................................................................... 33
10.2 Terminating plugs .......................................................................................................................................................... 34
10.3 Front Panel ........................................................................................................................................................................ 35
10.4 Mounting the PCB........................................................................................................................................................... 35
11 Testing and alignment ....................................................................................................................................... 36
11.1 General ................................................................................................................................................................................ 36
11.2 Power on............................................................................................................................................................................. 36
11.3 Receive ................................................................................................................................................................................ 36
11.4 Transmit ............................................................................................................................................................................. 37
11.5 Marking the VFO scale.................................................................................................................................................. 38
12 Modifications......................................................................................................................................................... 40
12.1 Setting the VFO range ................................................................................................................................................... 40
12.2 Receiver alignment ........................................................................................................................................................ 40
12.3 Crystal operation ............................................................................................................................................................ 40
12.4 Using a loudspeaker ...................................................................................................................................................... 40
13 Operation................................................................................................................................................................. 41
13.1 Setting up ........................................................................................................................................................................... 41
13.2 Receiving ............................................................................................................................................................................ 41
13.2.1 SSB.......................................................................................................................................................... 41
13.2.2 CW........................................................................................................................................................... 41
13.3 Transmitting ..................................................................................................................................................................... 42

MDT Construction Manual –Issue 2 Page 3
List of Figures
Figure 1 DSB generation................................................................................................................................................6
Figure 2 Direct Conversion receiver.........................................................................................................................7
Figure 3 MDT Block diagram.......................................................................................................................................8
Figure 4 Carrier oscillator.......................................................................................................................................... 12
Figure 5 Mixer................................................................................................................................................................. 13
Figure 6 Microphone Amplifier ............................................................................................................................... 14
Figure 7 Transmit.......................................................................................................................................................... 15
Figure 8 Receive Audio................................................................................................................................................ 16
Figure 9 Diode identification.................................................................................................................................... 22
Figure 10 DIP ICs ........................................................................................................................................................... 23
Figure 11 Relay install................................................................................................................................................. 23
Figure 12 Non-polarised capacitors ...................................................................................................................... 24
Figure 13 LPF capacitor marking............................................................................................................................ 24
Figure 14 Trimcap install........................................................................................................................................... 25
Figure 15 BD139 location .......................................................................................................................................... 25
Figure 16 BD139 lead identification...................................................................................................................... 26
Figure 17 LED lead bending...................................................................................................................................... 27
Figure 18 LED install.................................................................................................................................................... 27
Figure 19 Pot with tab removed.............................................................................................................................. 31
Figure 20 Component overlay.................................................................................................................................. 32
Figure 21 Mic connector wiring .............................................................................................................................. 33
Figure 22 Typical circuit voltages........................................................................................................................... 39
Figure 23 DSB transmit waveform......................................................................................................................... 42
Change History
Date
Issue
Comments
12-6-15
1
First release
20-6-15
2
Corrected values for R43 and C13, and other minor typos.

MDT Construction Manual –Issue 2 Page 4
1INTRODUCTION
The MDT (Minimalist Double Sideband Transceiver) is an inexpensive and easy to build kit for
the 40M band. It is ideal for the first time builder as all parts except the microphone socket are
mounted on a single PCB and all the components are through hole. There is a SMD tuning diode
but this comes pre-installed. The PCB is a high quality double sided type with ground plane,
solder mask and silk screen.
The plastic case is small and lightweight and can easily be held in one hand. The front and rear
panels, which are made of PCB material, come pre-cut and feature silkscreened labeling.
The low receive current of the MDT means it is ideal for battery operation. The transmitter
outputs between 1.5 and 2 watts DSB which is ample for short distance contacts through the
day and long distance contacts under good conditions. DSB transmissions are entirely
compatible with SSB transceivers, and in fact most operators won’t be aware unless you tell
them.
Building the MDT is quick and easy. The receiver doesn’t require any alignment and the only
setup required for the transmitter is setting the microphone gain and balancing the mixer to
null out the carrier.
A kit of parts for the MDT including everything you need, such as enclosure and front and rear
panels is available from www.ozQRP.com.
MDT Specifications and features:
1. Size 130mm x 100mm x 50mm.
2. Direct Conversion receiver. Sensitivity 0.4uV for 10dB S+N/N.
3. Double Sideband transmitter. Nominal 1.5W output. Up to 2W depending on power
supply voltage.
4. Selectable frequency range. 7.090MHz - 7.130MHz or 7.050MHz - 7.110MHz.
5. Microphone amplifier accepts standard low impedance dynamic or Electret microphone
with selectable on-board bias resistor.
6. LED transmit power and modulation indicator.
7. 3.5mm stereo headphone connector. Can power external loudspeaker.
8. Carrier suppression up to 50dB.
9. All spurious transmit outputs better than -46dBC.
10. Receive current approximately 50mA.
11. Transmit current approximately 250mA at maximum power output.
12. Reverse polarity protection using a series-diode.

MDT Construction Manual –Issue 2 Page 5
2DSB VS SSB
Why DSB? The answer is simple. A DSB transceiver is less expensive, less complicated and
easier to build and align than a SSB transceiver. This is due mainly to a DSB transceiver not
having a crystal filter, IF amplifier and multiple mixers that are required in a SSB design. Note
that in a DSB transceiver the receiver is more often referred to as a Direct Conversion (DC)
receiver.
For the first time builder or for a small and cheap rig, DSB is ideal. Over the years countless
amateurs have started out this way.
While a DSB rig has many advantages for the home builder, there are some things to consider.
Firstly, a DSB transmitter occupies twice the bandwidth of a SSB transmitter. On a quiet band
this does not cause any problems but on a crowded band it may not be as easy to find a free spot
to operate without interfering with nearby stations. Secondly, the Direct Conversion receiver
has equal response to both sidebands. This means you hear signals on both upper and lower
sidebands simultaneously. This results in a slightly higher noise level and the possibility of
hearing two separate stations at the same time.
There are, however, a couple of nice advantages when a Direct Conversion receiver is used with
a DSB transmitter. Firstly, you can operate with SSB stations using Upper Sideband (USB) or
Lower Sideband (LSB) without having to change controls or move frequency. Secondly, being
able to hear both sidebands means that you can check for other stations on both sides of your
frequency before transmitting and avoid interfering with them.

MDT Construction Manual –Issue 2 Page 6
3DSB TRANSMITTER
Figure 1 shows how a Double Sideband signal is generated. The mixer used here is not to be
confused with an audio mixer that combines, for example, microphones. The mixer here is more
correctly called a multiplier, where the inputs are multiplied in the same way as in a
mathematical equation. When multiplying sine waves there are two main outputs and these are
the sum and difference of the frequencies of the input signals.
The first input to our mixer is from the VFO or carrier oscillator. The second input is audio from
the microphone amplifier.
The dominant outputs of the mixer are the sum and difference frequencies, that is, the sum and
difference of the carrier and audio frequencies. In this case, 7.101MHz (7.100MHz + 1KHz)
upper sideband, and 7.099MHz (7.100MHz –1KHz) lower sideband.
The important thing to note is that only the sidebands are present at the output of the mixer as
the carrier and audio signals have been suppressed by the action of the balanced mixer.
The diagram in Figure 1 at top right shows the DSB output signal in the time domain, or how it
would be seen on an oscilloscope. Note the overlapping envelope shape that follows the audio
waveform. The diagram at bottom right shows the DSB output signal in the frequency domain
and how it would be seen on a spectrum analyser. The horizontal axis is frequency and the
vertical axis is amplitude. The dotted vertical line in the middle indicates the suppressed carrier
frequency.
By contrast, if this was a SSB transmitter, there would be a crystal filter placed after the mixer
and one of the sidebands would be filtered out. However, it would then be necessary to add
another mixer to move the SSB signal onto the wanted transmit frequency.
Figure 1 DSB generation

MDT Construction Manual –Issue 2 Page 7
4DIRECT CONVERSION RECEIVER
Figure 2 shows the simplified diagram of a Direct Conversion receiver. As with a DSB generator
the mixer has two inputs and an output, but this time the signal directions are reversed.
Signals from the antenna are presented to the mixer, and mixed with the VFO signal. The output
again contains sum and difference signals. The sum frequency of 7.101MHz + 7.100MHz
(14.201MHz) is easily filtered out by a low pass audio filter. However the difference frequency
of 7.101MHz –7.100MHz (1KHz) can pass through the filter and be heard in the headphones.
This is the upper sideband response as the antenna signal frequency of 7.101MHz is above the
7.100MHz VFO frequency.
Note that there is also another antenna signal that can be heard. This is the lower sideband
signal at 7.099MHz. This would also produce a 1KHz tone in the headphones.
This ability to simultaneously detect both upper and lower sidebands is an important
characteristic of a Direct Conversion receiver.
Both Figure 1 and Figure 2 show a single 1KHz tone for the audio signal. This is done to make it
easier to understand the process involved. In practice there would be a range of voice band
frequencies present, but the same mixing conversion principle applies.
Figure 2 Direct Conversion receiver

MDT Construction Manual –Issue 2 Page 8
5MDT BLOCK DIAGRAM
Figure 3 MDT Block diagram

MDT Construction Manual –Issue 2 Page 9
6CIRCUIT DESCRIPTION
6.1 VARIABLE FREQUENCY OSCILLATOR (VFO)
Transistor Q1 is configured as a Colpitts oscillator and acts as the carrier oscillator in transmit
and beat frequency oscillator (BFO) in receive. The frequency is set predominantly by a ceramic
resonator (X1) and a dual variable capacitance (varicap) diode D1. The capacitance of D1 is
altered by a varying the DC voltage applied to the Cathodes through the Tune control VR1 and
R3. To help stabilize the oscillator and minimize frequency drift the power supply to the
oscillator and the Tune control is regulated with a 9.1V Zener diode ZD1.
The VFO has effectively two ranges through the Range link. If the link is closed both varicap
diodes are in parallel circuit and the tuning range is approximately 60KHz. With the link open,
only one varicap diode is in use and the range change is reduced to around 40KHz. Capacitor C3
is not normally used.
The oscillator signal is fed to the emitter follower buffer stage Q2 via a small capacitor (C6). The
buffer stage provides light loading of the oscillator and a low impedance drive for the mixer.
6.2 MIXER
The balanced mixer is a diode switching type and doubly balanced. It performs two functions. In
TX mode it mixes the VFO carrier signal with the microphone audio signal to produce DSB while
in RX mode it mixes the antenna signal with the VFO signal to produce received audio.
The carrier signal, which is much larger in amplitude than the audio signal, is applied
simultaneously to both sides of the mixer through trimpot VR2. The carrier signal turns on the
diodes to form a low resistance and is why it is referred to as a switching mixer. As the carrier is
capacitively coupled it swings both positive and negative around ground potential. When the
carrier is positive, current flows through diodes D2 and D5 causing them to conduct and
become a low resistance. When the carrier goes negative diodes D3 and D4 conduct. Note that
capacitor C10 holds the junction of D2 and D4 at ground for RF. As the currents are equal
through each of the conducting diodes the differential voltage across winding one of T1 does not
change and no RF is present at winding three, the output of T1. If an audio signal is injected into
the bridge at the junction of D2 and D4 the mixer balance is upset because the audio changes
state much less frequently than the carrier signal and the instantaneous diode currents are not
equal. As a result a signal is now output on winding three of T1, which is a double sideband
suppressed carrier waveform.
Due to variations in component parameters the mixer balance is not exact and if not
compensated for the carrier balance would be poor. Trimmer capacitor TC1 and C9 are used to
equalize the capacitance on the mixer sides, while trimpot VR2 is used to balance the diode
currents in each side. They are adjusted together to bring the modulator into balance. In
practice up to 50dB of carrier suppression can be achieved.

MDT Construction Manual –Issue 2 Page 10
Received signals from the antenna are fed to a broadly tuned band pass filter formed with L1,
C14 and C15. The values of C14 and C15 are selected to form a tuned circuit with L1 at 7MHz.
The ratio of C14 and C15 provide impedance matching between the tuned circuit and the 50
ohm antenna source. It also sets the loaded ‘Q’ of the tuned circuit and as a result the overall
filter bandwidth. The filter is connected to the receive pre-amplifier via a small 100pF coupling
capacitor (C13). The pre-amplifier stage is formed around transistor Q3 in a common emitter
configuration and provides around 10 times amplification. The collector load is winding two of
the mixer transformer T1, and here the signal is mixed with the carrier. The resultant audio
appears at the junction of D2 and D4.
6.3 MICROPHONE AMPLIFIER
Transistor Q7 is the microphone pre-amplifier with an input impedance of around 10K ohm and
a gain of around 40 set mainly by C34 and R28. C32 is included across the input to prevent RF
feeding into the amplifier. The amplified output appears across the 5K ohm trimpot (VR3) in the
collector. This becomes a pre-set microphone gain control, and the signal from the wiper is fed
to the microphone amplifier stage Q8. This stage only has a gain of around 3, but it’s biased for
higher current and a low value collector resistor so it can drive the balanced modulator. C33 and
C36 provide heavy low pass filtering to limit the transmitted bandwidth.
If an Electret microphone is used, R25 provides a DC bias current and is enabled by shorting the
EL link. If a dynamic microphone is used the link is left open.
6.4 TRANSMIT AMPLIFIER
Transmit signal from the mixer is applied to the driver stage built around transistor Q4. A
BD139 works well here when biased with about 50mA of collector current. The design is well
proven using both shunt and series feedback to provide low input and output impedance and
good stable gain on the low HF bands.
The power amplifier stage is formed from two BD139 transistors (Q5 and Q6) in parallel. They
operate in class B and provide up to 2 Watts PEP of power from a 13.8 V supply. The bases of
the transistors are held at around 0.6 volts DC by the voltage reference formed by R18 and
diode D6. This holds the transistors at or just below the point of conduction and so draw very
little current with no RF drive. The 1.5 ohm resistors in the emitters force the transistors to
share the load more equally, and provide a small amount of negative feedback which improves
stability and prevents thermal runaway.
The collector load for Q5 and Q6 is a toroidal inductor L2. The specified inductance was found to
provide maximum output into the low pass filter. The waveform from Q5 and Q6 can be high in
harmonics and so a 5 pole low pass filter is included to reduce the level of harmonic and other
spurious energy to an acceptable level. L4 and a 150pF capacitor form a parallel tuned circuit to
give sharp attenuation of the second harmonic.
As a visual indication of power output and modulation, the transmit signal is sampled by
capacitor C28 and ground referenced by R21. The signal is rectified by D9 and filtered by C29.
This drives the front panel LED via current limiting resistor R22.