Tait TB9100 - Page 96

96 RF Circuitry TB9100 Reciter Service Manual
© Tait Electronics Limited January 2006
The synthesizer uses a phase-locked loop to lock the exciter VCO to a
modulatable frequency reference. The synthesizer IC receives the divider
and control information from the RISC processor via a 3-wire serial bus
(clock, data and enable). When the data bits are latched in, the synthesizer
processes the incoming signals from the VCO feedback signal (f
vcofb
) and
the reference oscillator (f
ref
).
The VCO feedback attenuator is a resistive divider that terminates the VCO
feedback signal in a fixed low impedance (50Ω). This attenuates the VCO
RF level down to a level suitable for the RF prescaler (within the synthesizer
IC).