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TB8100 Service Manual Reciter Circuit Description 51

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 for details on the phase-locked 12.8MHz

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

) to tune the VCO for correction. The

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.

2.5.3 VCO - VHF Reciter

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.

High Q VCO 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.

Tait TB8100 Service Manual

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