ozQRP MST400 Instructions for use

MST Construction Manual –Issue 1 Page 1
MST
CONSTRUCTION MANUAL

MST Construction Manual –Issue 1 Page 2
CONTENTS
1Introduction..............................................................................................................................................................4
2Block Diagram..........................................................................................................................................................5
3Circuit Description .................................................................................................................................................6
3.1 SSB generator..................................................................................................................................................6
3.2 Transmit mixer...............................................................................................................................................6
3.3 Power amplifier .............................................................................................................................................7
3.4 Receive mixer..................................................................................................................................................7
3.5 Receive audio..................................................................................................................................................8
4Parts List.................................................................................................................................................................. 14
4.1 Main Board Parts........................................................................................................................................ 14
4.2 Band Specific Parts .................................................................................................................................... 16
4.3 Off Board Parts............................................................................................................................................ 16
5Construction........................................................................................................................................................... 17
5.1 General............................................................................................................................................................ 17
5.2 Construction Steps..................................................................................................................................... 17
6Enclosure................................................................................................................................................................. 23
6.1 Rear Panel ..................................................................................................................................................... 23
6.2 Front Panel.................................................................................................................................................... 23
6.3 Mounting the PCB....................................................................................................................................... 23
6.4 Front panel label......................................................................................................................................... 24
6.5 Fitting the DDS VFO................................................................................................................................... 26
7Wiring up................................................................................................................................................................. 27
8Testing and alignment ....................................................................................................................................... 29
8.1 General............................................................................................................................................................ 29
8.2 Power on........................................................................................................................................................ 29
8.3 Receive............................................................................................................................................................ 30
8.4 Transmit......................................................................................................................................................... 30
8.5 Carrier frequency adjustment............................................................................................................... 32
9Operation................................................................................................................................................................. 33

MST Construction Manual –Issue 1 Page 3
List of Figures
Figure 1 MST Block diagram........................................................................................................................................5
Figure 2 SSB generator...................................................................................................................................................9
Figure 3 Transmit mixer............................................................................................................................................. 10
Figure 4 Power amplifier............................................................................................................................................ 11
Figure 5 Receive mixer................................................................................................................................................ 12
Figure 6 Receive audio................................................................................................................................................ 13
Figure 7 Component overlay .................................................................................................................................... 22
Figure 8 Front panel drilling guide........................................................................................................................ 25
Figure 9 DDS VFO front panel mounting ............................................................................................................. 26
Figure 10 Wiring diagram.......................................................................................................................................... 28
Figure 11 Typical circuit voltages........................................................................................................................... 31
Figure 12 Crystal filter response............................................................................................................................. 32

MST Construction Manual –Issue 1 Page 4
1INTRODUCTION
The MST (Minimalist Sideband Transceiver) is an easy to build and fun to use single sideband
QRP transceiver for the 40M or 80M amateur bands.
The MST transceiver follows a minimalist design making it inexpensive and simple to build yet
still delivering excellent performance and a pleasure to operate.
The MST PCB contains a complete SSB transceiver except for a VFO. You can add a VFO of your
own, as long as it has the correct frequency range and drive level, but to obtain best results it
should be used with the companion DDS VFO. The DDS VFO offers drift free performance and
features a clear backlit LCD display, selectable frequency steps and programmable IF offset.
MST Features:
1. Complete SSB transceiver on a single PCB (just add a VFO).
2. Superhet receiver using a 4 pole 10MHz crystal filter.
3. 5W PEP minimum power output using a rugged power MOSFET output stage.
4. Unwanted sideband suppression is typically 40dB.
5. All spurious transmit outputs below -45dBc.
6. AF and microphone gain controls.
7. Easy to adjust and set up.
8. Front panel LED transmit power and modulation indicator.
9. Plenty of audio output to drive a loudspeaker.
10. High quality double sided PCB with groundplane, solder mask and silk screen.
11. Simple and easy to build using all through hole components.
12. No complicated coil winding required. Uses inexpensive commercial coil assemblies for
tuned circuits.
A range of kits containing PCBs and hard to get parts for the MST and the DDS VFO are available
from www.ozQRP.com.

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2BLOCK DIAGRAM
Figure 1 MST Block diagram
Receive Path
Transmit Path

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3CIRCUIT DESCRIPTION
3.1 SSB GENERATOR
Transistor Q1 is configured as a Colpitts oscillator and acts as the 10MHz carrier oscillator in
transmit and beat frequency oscillator (BFO) in receive. The frequency of crystal X1 is set by
trimmer capacitor TC1 to a frequency slightly above the upper crystal frequency response. This
results in LSB being generated. Capacitor C1 is included in the PCB pattern but not used in this
application. The power supply to the oscillator is regulated with a 9.1V Zener diode ZD1.
The oscillator feeds buffer stage Q2 to provide a low impedance drive for the balanced
modulator. A lower level output of around 500mV pk-pk is obtained at the junction of Q1
emitter resistors to feed the receive product detector.
Transistors Q3 and Q4 are configured as a shunt feedback audio amplifier which has stable gain
and low output impedance. The low input impedance makes it ideally suited for low impedance
microphones. C16 is included to prevent RF feeding into the amplifier while C19 provides high
frequency roll off.
The balanced modulator is a diode switching type and doubly balanced. When the carrier signal
is positive diodes D1 and D4 conduct and when it is negative diodes D2 and D3 conduct. The
result is that no RF is present at the output transformer T1. If an audio signal is injected into the
bridge the balance is upset and a double sideband suppressed carrier signal is produced at the
output of T1. Note that capacitor C8 holds the junction of D1 and D2 at ground for RF.
Due to variations in component parameters the balance is not exact and so trimcap TC2 and
trimpot VR1 are adjusted to bring the modulator into balance. In practice up to 50dB of carrier
suppression can be achieved.
Diode D5 is used as an RF switch. With no DC current flowing the diode is a high impedance to
RF. In transmit around 6mA of DC current flows through D5 and it becomes a low impedance
path for RF. This feeds the output of the balanced modulator into the crystal filter. In receive D6
performs a similar function.
The crystal filter is a 10MHz 4 pole ladder type using closely matched crystals on the same
frequency. Capacitors C11 to C15 are selected to provide a usable 2.5KHz bandwidth. Resistors
R9 and R10 terminate the crystal filter in the correct resistance to give low ripple in the pass
band.
3.2 TRANSMIT MIXER
The transmit mixer is based around U5 a SA612 balanced mixer. The 10MHz LSB signal from the
crystal filter is fed single ended into pin1 while the other input on pin 2 is grounded to RF by
C65. The VFO signal of around 300mV pk-pk is fed into pin 6. The balanced output of the mixer
is fed to transformer T7 which is tuned to the difference of the VFO and carrier frequencies. The
transformer used here is actually a 10.7MHz IF transformer with an integral 47pF capacitor. An
external capacitor is added to lower the resonate frequency to match the transmit frequency.

MST Construction Manual –Issue 1 Page 7
A link coupled winding provides a low impedance output for the pre-driver built around
transistor Q9. The collector load is another IF transformer (T8) and resonated to the transmit
frequency in the same way as T7. In practice the two transformers form a 2 pole band pass filter
and adjusted to give the desired passband.
3.3 POWER AMPLIFIER
Transmit signal from the pre-driver is applied to the driver stage built around transistor Q6. A
BD139 works well here when biased with about 60mA 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 is an IRF510 MOSFET and has been used in many designs. It is a good
candidate for the HF bands and easily provides over 5 Watts PEP of power from a 13.8 V drain
supply. The output from the driver is applied across resistor R38 and becomes the AC drive
component for Q7 gate. Zener diode ZD4 and trimpot VR2 provides a stable and variable DC
gate voltage to place Q7 just into conduction for linear service. There is a short ramp up of the
gate voltage when switching to transmit as capacitor C54 charges and is included to provide a
smooth gate voltage transition.
The drain load for Q7 is a broadband transformer T6. The turns ratio is chosen for best match
and to provide maximum output into a 50 ohm load. The waveform from Q7 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. The values of the low pass filter components are band
dependant.
When the PTT is operated the transmit/receive relay is energized and the filtered transmit
signal is passed to the antenna. When the PTT is not operated the relay switches the antenna
through to the receive circuits. The relay also switches power to the transmit and receive
sections as required.
As a visual indication of power output and modulation, the transmit signal is sampled, rectified
and filtered by R39, D11 and C63. The resultant voltage is buffered by transistor Q8 to drive a
front panel LED via current limiting resistor R41.
The power supply is made available at an auxiliary connector and is intended to power a
companion VFO.
3.4 RECEIVE MIXER
Signals from the antenna are applied to a bandpass filter formed with two transformers T2, T3
and capacitors C21, C22 and C23. The antenna is link coupled to T2 while the output is fed from
a tap on the tuned primary winding of T3 to provide proper impedance matching.
The mixer U1 is another SA612. The input is protected with a pair of back to back diodes and fed
single ended into pin 1. Pin 2 is grounded to RF by C25. Zener diode ZD2 provides a stabilized
6.8 volt supply. VFO signal is injected into pin 6 at about 300mV pk-pk. The balanced output
which is a difference signal at approximately 10MHz is fed to broadband transformer T4. The
output of T4 is passed to the crystal filter when DC current flows through R19 and into D6.

MST Construction Manual –Issue 1 Page 8
3.5 RECEIVE AUDIO
The 10MHz SSB intermediate frequency (IF) signal from the crystal filter is applied to the
product detector U2. The product detector is formed with another SA612 and mixes the IF
signal with the 10MHz BFO signal to produce an audio output. The BFO is adjusted slightly
above the crystal filter upper response so that Lower Side Band signals are detected correctly.
A low noise balanced input audio amplifier is formed with U3 a NE5534. A reference supply for
the non-inverting input is obtained from the Zener diode stabilized supply for U2. The high
frequency response of U3 is limited by C35 and C42, while capacitors C37 and C36 reduce the
low frequency response.
The output of U3 is fed to a 1uF coupling capacitor and 1K resistor (R28) to the AF gain
potentiometer. When switching to transmit mode MOSFET Q5 is turned on rapidly through D9,
charging C45, and shorting the audio line to ground. When returning to receive mode Q5 slowly
turns off as C45 discharges via resistor R29. This circuit produces a very nice action eliminating
unwanted audio clicks and pops when switching between receive and transmit modes.
Audio from the AF gain control is amplified by U4, an LM386, to drive a loudspeaker. U4 is
configured for extra gain by including C41 and R26. C40 guards against RF entering the
amplifier input, while R30 and C47 help stabilize the output.

MST Construction Manual –Issue 1 Page 9
Figure 2 SSB generator

MST Construction Manual –Issue 1 Page 10
Figure 3 Transmit mixer