Tait TM8000 Series, Service Manual

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62 General Description TM8100 Mobile Radio Service Manual
May 2004 © Tait Electronics Limited
Ramping
Power ramp-up consists of two stages:
■
bias
■
power ramping
The timing between these two stages is carefully controlled in order to
achieve the correct overall wave shape and meet the specification for
transient ACP (adjacent channel power). A typical ramping waveform is
shown in
Figure 2.15 .
Bias Ramp-up
The steady-state final-stage bias level is supplied by an 8-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. 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 –10 dBm to
approximately 25 dB 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.