TB9100 Reciter Service Manual RF Circuitry 95
© Tait Electronics Limited January 2006
Low Pass Filter
There are two output signals from the twisted ring counter. Both signals
have the sum and difference frequency contents of the TCXO and VCXO
signals, but there is a 90° phase difference between them.
I and Q low pass filters capture the difference frequency contents down to
DC and integrate them to form two triangular waves, which are 90° out of
phase with each other. This frequency is equal to a quarter of the difference
frequency content of the TCXO and VCXO signals.
The in-phase triangle frequency is referred to as “I channel” and the
quadrature-phase triangle frequency is referred to as “Q channel”.
ADC The I_Q channel triangular analog waveforms are sampled and transformed
to digits using a 16-bit ADC with a signal-to-noise ratio of 75dB.
FCL Processor and
DAC
The FCL processor runs a DSP-based algorithm which takes the digitized
signals, I_Q and transmit audio, and compares them to the transmit
modulation calibration data.
Using the compared results it attempts to lock FCL_VCXO to the TCXO,
as well as modulate the FCL_VCXO and VCO signals to achieve
modulation flatness across the transmit audio and VCO RF bands.
The FCL processor achieves this by sending the digitized modulation code
through a16-bit DAC. Here the code is translated to analog signals which
modulate FCL_VCXO and the VCO. The DAC has a signal-to-noise ratio
of 70dB.
6.3.2 Synthesizer
The exciter synthesizer consists of a programmable frequency synthesizer
IC, the exciter VCO, and a modulatable frequency reference.
Figure 6.4 Comparison of VCXO and VCO Modulation Responses
VCXO VCO
150Hz
Gain
Frequency
3kHz
linear relationship
proportianal to the RF
frequency and
FCL_GAIN, measured
during calibration
non-linear relationship,
determined by
CalVcoGain_Corrected,
calculated from the
calibration data
0
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