TB8100 Service Manual Reciter Circuit Description 51© Tait Electronics Limited September 2006The FCL_VCXO reference oscillator is modulated by the FCL. The FCLitself is locked to an internal reference 12.8MHz TCXO. When the TCXOis active, the exciter synthesizer is locked to an “internal reference mode”(by default). Alternatively, the FCL is locked to a phase locked 12.8MHzexternal reference oscillator. When this external oscillator is active, theexciter synthesizer is locked to an “external reference mode”. In operationonly one reference oscillator is active at any given time. Refer to“Reference Switch” on page 35 for details on the phase-locked 12.8MHzexternal reference oscillator.The FCL_VCXO reference oscillator is buffered and divided down to theappropriate 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 feedbacksignal using the prescaler and programmable dividers of the synthesizer IC.Phase lock is achieved when both divider references have same phase andfrequency content (i.e. their difference is zero or DC). This is achieved bythe phase detector (part of the synthesizer IC), which compares both dividerreferences and delivers an error signal. A ±4mA charge pump circuit (alsopart of the synthesizer IC) and the active loop filter circuit convert this errorsignal to a DC voltage (0 to 22V 1 ) to tune the VCO for correction. Theloop 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 ReciterThe exciter VCO consists of a high Q VCO, modulation based on varicapdiodes, low noise amplifier, harmonic filter, fixed slope attenuator, finaldriver and a fixed attenuator.High Q VCO The VCO BJT transistor operates in a common emitter, Colpitts oscillatorconfiguration (virtual ground at the base). The LC resonator is coupledbetween the collector and base. A high Q trimmer is coupled to the baseend, and varactor diodes are coupled to the collector end. This forms a highQ resonator which is both mechanically and electronically tunable.Mechanical tuning is possible by adjusting the trimmer. Changes in thecontrol voltage from the loop filter are applied to the varactors to facilitateelectronic tuning.1. The normal lock range is between 3V and 16V.
