Tait TM8235, Руководство по обслуживанию

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60 Description TM8100/TM8200 Service Manual
© Tait Electronics Limited June 2006
Power Control
The steady-state power output of the transmitter is regulated using a
hardware control loop. With the 40W/50W radio, the sum of the forward
power output from the RF power amplifier and reverse power reflected from
the load is sensed by the directional coupler and fed back to the power
control loop. With the 25W radio, the forward power output from the RF
power amplifier is sensed by the directional coupler and fed back to the
power control loop. The PA output power is controlled by varying driver
gate bias voltage that has a calibrated maximum limit to prevent overdrive.
The power control signal is supplied by a 13-bit DAC driven by
custom logic.
Ramping
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 .
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 –20dBm for the
40W/50W or –10dBm for the 25W radio 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
Figure 2.17 Typical ramping waveforms
Power
ramp
High power
power Low
Power
Time
Bias
ramp
Bias
ramp
Power
ramp