48 Description TP9100 Service Manual
© Tait Electronics Limited May 2005
Bias Ramp-Up
The steady-state final-stage bias level is supplied by a 10-bit DAC
programmed prior to ramp-up but held to zero by a switch on the DAC
output under the control of a Tx inhibit signal. This signal is multiplexed
with the Rx AGC signal. Bias ramp-up begins upon release by the TX
inhibit signal with the ramping shape being determined by a low-pass filter.
Owing to power leakage through the PA chain, ramping the bias takes the
PA output power from less than –10dBm to approximately 25dB below
steady-state power.
Power Ramp-Up The power ramp signal is supplied by a 13-bit DAC that is controlled by
custom logic. The ramp is generated using a look-up table in custom logic
memory that is played back at the correct rate to the DAC to produce the
desired waveform. The ramp-up and ramp-down waveforms are produced
by playing back the look-up table in forward and reverse order respectively.
For a given power level the look-up table values are scaled by a steady-state
power constant so that the ramp waveform shape remains the same for all
power levels.
PIN Switch The RF PIN switch circuitry selects the RF path to and from the antenna
to either the Tx or Rx circuitry of the radio. In addition to the switching
functionality, the PIN switch is used to provide attenuation to the Rx front
end in high signal-strength locations.
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