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3 C
D
IRCUIT
ESCRIPTION
3.1 C
O
ARRIER
SCILLATOR
Transistor Q2 is configured as a Colpitts oscillator and acts as the 10MHz carrier oscillator in
transmit and beat frequency oscillator (BFO) in receive. The frequency is determined by crystal
X1 and inductor L1 in conjunction with trimmer capacitors TC1 and TC2. When transistor Q1 is
turned off only TC2 is in circuit and this is adjusted to set the upper frequency of the oscillator.
When transistor Q1 is turned on by applying 12V to the U/L connector, TC1 is connected in
parallel with TC2. TC1 is then adjusted to set the lower frequency. Note that the trimmers are
not labeled USB and LSB because this depends on whether the VFO is above or below the IF
frequency.
The power supply to the oscillator is regulated with an 8V regulator REG1.
The oscillator feeds buffer stage Q3 via a small capacitor to provide minimal loading on the
oscillator. The buffer stage gives a low impedance drive for the balanced modulator. A lower
level output of around 500mV pk-pk is obtained at the junction of Q2 emitter resistors to feed
the receive product detector.
3.2 SSB
GENERATOR
Transistor Q4 is the microphone pre-amplifier with a 20K ohm input impedance and gain of
around 10. C9 is included to prevent RF feeding into the amplifier. The output of Q4 is fed to the
Mic gain control via C15.
If an Electret microphone is used, R8 provides a DC bias current and is enabled by shorting LK1.
If a dynamic microphone is used LK1 is left open. Transistor Q5 is the second microphone
amplifier and has a gain of about 10. It is biased for higher current and has a low value collector
resistor to enable it to drive the balanced modulator. R16 and C19 form a low pass filter to keep
RF out of the amplifier.
The balanced modulator is a diode switching type and doubly balanced. The carrier signal,
which is much larger in amplitude than the audio signal, is applied simultaneously to both sides
of the mixer through trimpot VR1. The carrier signal alternately turns on the diode pairs to give
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 D1 and D4 causing them to conduct and become a low
resistance. When the carrier signal goes negative diodes D2 and D3 conduct. Note that capacitor
C25 holds the junction of D1 and D2 at ground for RF. As the currents are equal through each of
the conducting diodes the differential voltage across the primary winding of T1 does not change
and as a result no RF is present at the secondary winding. If an audio signal is injected into the
bridge at the junction of D1 and D2 the mixer balance is upset because the audio changes state
much less frequently than the carrier signal and the resultant instantaneous diode currents are
not equal. As a result a signal is now output on the secondary of T1, which is a double sideband
suppressed carrier waveform.
MST3 Construction and Operation Manual – Issue 1
Page 8
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