Harris Atlas DVL400, Technical Manual

Page: 120 / 180

4-20 888-2473-001 2/25/03
WARNING: Disconnect primary power prior to servicing.
Section 4 Theory of Operation ATLAS Series™
4.4.4.3 RF Pallets
The PA module utilises several LDMOS RF amplifier pallets. The pallet is actually
made up of 2 push-pull amplifiers with a hybrid splitter on the input and a hybrid
combiner on the output. Eight (8) pallets are used as the main power amplifiers or
PA pallets, two (2) are used as Driver Pallets and 1/2 of one pallet is used for a
Predriver. A simplified diagram of the pallet is given in Figure 4-5.
Figure 4-5 Pallet Simplified Diagram
4.4.4.4 Auto Bias Circuit
As LDMOS devices age, their biasing requirements change. To account for this, the
module controller has the ability to automatically re-bias the pallets for the correct
idle current by adjusting the gate bias voltages. This Auto Bias procedure should be
done at regularly scheduled intervals and is initiated manually using the GUI.
NOTE:
When the biasing procedure is initiated, the module will be muted and trans-
mitter RF output will be affected. This procedure takes about 5 seconds.
The pallets get their bias voltages from 12 bit DACs (Digital to Analog Converters)
on the Module Controller. Each pallet is divided into a Side A and Side B with each
side having one Push-Pull Pair (PPP) LDMOS device. Each PPP has its own bias
signal from a DAC output, but there is only one current sensor for each pallet.
Therefore, the Auto Bias adjustment will be carried out on one side of all 8 pallets at
a time, then it will do the other side to assure that each half is drawing its share of
the idle current.
To bias the module, the controller will disable the ALC and mute the RF by opening
the RF switch on the Phase and gain board. Then it will zero the bias currents for
AB
+32 VDC Bias
AB
RF IN 1
RF OUT 1
3dB Hybrid
3dB Hybrid
Combiner
Isolation Load
Splitter
Isolation Load
Side A
Side B