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MDT Construction Manual – Issue 2 Page 9
6 C
IRCUIT
D
ESCRIPTION
6.1 V
ARIABLE
F
REQUENCY
O
SCILLATOR
(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 M
IXER
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.