Tait TM8100 mobiles Service Manual page 63

Ramping
Bias Ramp-Up
>25W 25W
Power Ramp-Up
PIN Switch
TM8100/TM8200 Service Manual
© Tait Electronics Limited November 2007
Power ramp-up consists of two stages:
bias
■
power ramping.
■
The timing between these two stages is critical to achieving the correct
overall wave shape in order to meet the specification for transient ACP
(adjacent channel power). A typical ramping waveform is shown in
Figure 2.17.
Figure 2.17 Typical ramping waveforms
Power
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
release by the
TX INHIBIT
a low-pass filter. Owing to power leakage through the PA chain, ramping
the bias takes the PA output power from
less than –20dBm for the 40/50W radios
■
–20dBm to approximately 40dB below steady-state power for the
■
30/35W K5 band radios
–10dBm to approximately 25dB below steady-state power for the 25W
■
radios.
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.
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.
Bias Power Power
ramp ramp ramp
High power
Low power
signal. Bias ramp-up begins upon
TX INHIBIT
signal with the ramping shape being determined by
Bias
ramp
Time
Description 63