THEORY OF OPERATION: Transmitter Path Section
12
Level Set and Phase Training
Level set training is performed to ensure that the RF PA is not driven into clip,
which would result in excess splatter and out-of-band spurious emissions. During
training, the DSP signal is disconnected from the forward path and an internal
analog ramp generator is connected. The feedback is monitored and compared to
the analog ramp. As the ramp amplitude increases and the RF PA begins to clip,
the error voltage increases. See Figure 6 on page 12.
When the error signal exceeds a pre-determined threshold, the ramp level is held
and step attenuators are set to prevent the maximum-input signal level from
exceeding the ramp level, which caused the clip. The ramp is allowed to decay to
zero.
Figure 6. Level Set Training
Negative feedback is required to maintain system stability. Phase training is done
to ensure that the feedback is negative (180
o
). The phase shift of the loop consists
of the sum of the delays of several modules and components, such as SAW filters
and amplifiers.
The Q channel is set to zero and a positive signal is sent to the I channel. If there is
negative feedback, the I feedback channel is negative and no signal should be
present in the Q channel. The phase of the LO down mixer is adjusted to maxi-
mum negative feedback. The phase-training algorithm is iterative. The I signal
•
Error
Voltage
Predetermined
Threshold
Voltage
Comparator
Trigger
Sample
and Hold
In
Out
+
+
--
+
--
Software
Controlled
Switch
Ramp
Generator
Attenuator
From
ADDAG
