TB8100 Service Manual Reciter Circuit Description 47© Tait Electronics Limited September 20062.5 Exciter RF CircuitryRefer to Figure 2.10 on page 49 and Figure 2.11 on page 50.2.5.1 Frequency Control LoopAudio modulation of the exciter synthesizer is implemented in thefrequency control loop (FCL). It uses a three-point modulation schemeinvolving the FCL_VCXO and VCO signals.The FCL consists of reference oscillators, clock buffers, twisted ring counterphase detectors, low pass filters (LPFs), ADCs, the FCL processor anddigital-to-analogue converters (DACs).ReferenceOscillators Modulation to the FCL_VCXO reference oscillator requires the use of theFCL_VCXO_CTRL and SYN_VCO_MOD signals to apply:■ a constant DC offset to the FCL_VCXO signal until it achievesfrequency lock to the internal referenced TCXO;■ frequency modulation to the FCL_VCXO and VCO simultaneouslyfrom the transmit audio signal; the transmit audio signal has a range of 0to 3kHz.The modulated signal from the VCXO is attenuated by the bandwidth ofthe loop filter in the low pass filter (i.e. 150Hz). To obtain flat modulationacross the audio band, the VCO is also modulated simultaneously to obtaina composite high pass filter response. Figure 2.9 shows the relationshipsbetween the frequency modulation gain characteristics of the VCXO andVCO.Clock Buffers The TCXO and VCXO signals are squared up and buffered as digitalsignals using hex inverters.Twisted RingCounter The VCXO and TCXO signals are phase shifted and multiplied by XOR(exclusive_or) logic. This is achieved using a twisted ring counter, whichalso divides both signals by four.Low Pass Filter There are two output signals from the twisted ring counter. Both signalshave the sum and difference frequency contents of the TCXO and VCXOsignals, but there is a 90° phase difference between them.I and Q low pass filters capture the difference frequency contents down toDC and integrate them to form two triangular waves, which are 90° out ofphase with each other. This frequency is equal to a quarter of the differencefrequency content of the TCXO and VCXO signals.The in-phase triangle frequency is referred to as “I channel” and thequadrature-phase triangle frequency is referred to as “Q channel”.
