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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 TC3 and capacitor C26
are used to equalize the capacitance on the mixer sides, while trimpot VR1 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.
Diode D5 is used as an RF switch. With no DC current flowing through the diode it is high
impedance to RF. In TX state 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 D8 performs a similar function.
3.3 C
F
RYSTAL
ILTER
The crystal filter is a 10MHz 5 pole ladder type using closely matched crystals on the same
frequency. Capacitors C28 to C33 are selected to provide an approximate 2.7KHz bandwidth.
Resistors R26 and R27 terminate the crystal filter in the correct resistance to give low ripple in
the pass band.
3.4 T
RANSMIT MIXER
The transmit mixer is based around U6 a SA612 balanced mixer. The 10MHz LSB signal from the
crystal filter is fed single ended into pin 1 while the other input on pin 2 is grounded to RF by
C59. The VFO signal of 300 to 600mV pk-pk is fed into pin 6. Transistors Q10 and Q11 act as
emitter follower buffers providing a balanced low impedance feed for the transmit band pass
filter. The output of the transmit band pass filter is capacitively coupled to the pre-driver built
around transistor Q11 which has both series and shunt feedback. The collector load is a
broadband transformer (T3) with a 10 to 3 turn ratio. R52 determines the DC collector current,
while R53 and C66 set the AC gain.
3.5 P
OWER AMPLIFIER
Transmit signal from the pre-driver is applied to the driver stage built around transistor Q13.
The circuit is identical to the pre-driver except for the use of a BD139 transistor and a larger
quiescent current of around 60mA. This is necessary to give linear operation with the larger
output power. The design is well proven using both shunt and series feedback to provide low
input and output impedance and good stable gain on the HF bands. The collector load is a
broadband transformer (T4) with a 10 to 3 turn ratio.
The power amplifier (Q14) is an IRF510 MOSFET and has been used in many designs. It is a
good candidate for the HF bands and can provide in excess of 5 Watts PEP from a 13.8 V drain
supply. The output from the driver is applied across resistor R60 and becomes the AC drive
component for Q14 gate. Zener diode ZD4 and trimpot VR3 provides a stable and variable DC
gate voltage to place Q14 just into conduction for linear service. There is a short ramp up of the
gate voltage when switching to TX state as capacitor C71 charges and is included to provide a
smooth gate voltage transition.
The drain load for Q14 is a broadband bi-filar wound transformer (T5) and was found to
provide maximum output into a 50 ohm load. The waveform from Q14 can be high in harmonics
and so a 7 pole low pass filter is included to reduce the level of harmonic and other spurious
energy to an acceptable level.
MST3 Construction and Operation Manual – Issue 1
Page 9
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