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Technical Description
VCTCXO
The task of the VCTCXO is to supply the synthesizer with a stable, accurate refer-
ence frequency. A 14.85 MHz crystal with DAC-controlled varactor diodes and a
transistor stage within BERTINDY constitues the TCXO circuit. A software algo-
rithm is used for temperature compensation as well as an AFC function.
The operating frequency of the synthesizer reference oscillator is set by the crystal
element. The crystal is specified for a resonant frequency of 14.85 MHz at a specific
load capacitance. Tuning of the series resonance frequency is accomplished by
adjusting the total series capacitance seen by the crystal. Varactor diodes provide the
method of varying the load capacitance posed to the crystal. Both varactors are con-
trolled by DAC's within BERTINDY. DAC1 is the "fine tune" adjustment, and
DAC2 is the "coarse tune" adjustment.
Initial trimming for component tolerances is done by DAC2 in the factory. In the
field, the TCXO is adjusted mainly through DAC1 for temperature compensation
and AFC. DAC2 is only adjusted in the field when necessary to maintain the fullest
DAC1 range possible. This is where the integer ratio of tuning sensitivity is desira-
ble.
The inherent frequency deviations of the crystal when operated over the full temper-
ature range are corrected by a software controlled temperature compensation algo-
rithm. A look-up table is stored in memory which is indexed by temperature
readings from a signal originating from a thermistor voltage divider network. This is
the only means for frequency compensation in the field when the down link signal
from the base station is weak and below the AFC threshold. When the base station
signal is above the threshold (roughly -110 dBm), and when the phone is in either
idle or conversation mode, the AFC algorithm is enabled. The temperature compen-
sation table ensures that the TCXO frequency will be kept close enough to lock to
the base station signal and null out frequency error once the proper conditions are
met.
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