Vco - Vhf Reciter - Tait TB8100 Service Manual

2.5.3

VCO - VHF Reciter

High Q VCO
TB8100 Service Manual
© Tait Electronics Limited September 2006
The FCL_VCXO reference oscillator is modulated by the FCL. The FCL
itself is locked to an internal reference 12.8MHz TCXO. When the TCXO
is active, the exciter synthesizer is locked to an "internal reference mode"
(by default). Alternatively, the FCL is locked to a phase locked 12.8MHz
external reference oscillator. When this external oscillator is active, the
exciter synthesizer is locked to an "external reference mode". In operation
only one reference oscillator is active at any given time. Refer to
"Reference Switch" on page 35
external reference oscillator.
The FCL_VCXO reference oscillator is buffered and divided down to the
appropriate divider reference within the synthesizer IC:
VHF - 3.125kHz (default) or 2.5kHz
UHF - 6.25kHz (default) or 5kHz.
The same divider reference is maintained by dividing the VCO feedback
signal using the prescaler and programmable dividers of the synthesizer IC.
Phase lock is achieved when both divider references have same phase and
frequency content (i.e. their difference is zero or DC). This is achieved by
the phase detector (part of the synthesizer IC), which compares both divider
references and delivers an error signal. A ±4mA charge pump circuit (also
part of the synthesizer IC) and the active loop filter circuit convert this error
signal to a DC voltage (0 to 22V
loop filter has a bandwidth of 150Hz and filters the VCO control lines,
reference side bands and spurious signals.
Note
The VCO frequency increases as the control line voltage increases.
The exciter VCO consists of a high Q VCO, modulation based on varicap
diodes, low noise amplifier, harmonic filter, fixed slope attenuator, final
driver and a fixed attenuator.
The VCO BJT transistor operates in a common emitter, Colpitts oscillator
configuration (virtual ground at the base). The LC resonator is coupled
between the collector and base. A high Q trimmer is coupled to the base
end, and varactor diodes are coupled to the collector end. This forms a high
Q resonator which is both mechanically and electronically tunable.
Mechanical tuning is possible by adjusting the trimmer. Changes in the
control voltage from the loop filter are applied to the varactors to facilitate
electronic tuning.
1. The normal lock range is between 3V and 16V.
for details on the phase-locked 12.8MHz
1
) to tune the VCO for correction. The
Reciter Circuit Description 51