User's Manual

TB8100 Calibration Kit User’s Manual Calibrating the PA Bias 27
Calibrating the Power Amplifier
The PA is fully calibrated in the factory, but if the PA is serviced you may need
to perform the following procedures:
■ Calibrating the PA Bias
■ Calibrating the PA Power
■ Calibrating the Forward and Reverse Detector Bias Voltages
Important: It is recommended that only accredited service centers
and Tait engineers perform these procedures.
Note: The PA is calibrated by the microprocessor via the Calibration
Kit. You do not need to make any manual adjustments.
The power amplifier (PA) receives the RF signal from the reciter
and amplifies it to the required level, in watts, as requested by the
reciter. The desired output power is determined by the reference
voltage for the power control loop.
Calibrating the PA Bias
The driver and final transistors of the power amplifier must be biased at a
constant current. Since the characteristics of individual transistors vary slightly,
the bias current is calibrated for each device.
If either the driver or final transistor, or the PCB modules themselves, are
replaced during servicing, you should perform this procedure to calibrate the
bias current for the new device.
The bias current required for each amplifier stage is stored within the PA.
During the calibration process, the microprocessor adjusts the gate bias voltage
to obtain the required bias current for each stage.
The stage bias calibration sets up the amplifier’s DC operating conditions.
These DC conditions will be upset if there is RF present during calibration. It
is important to disconnect the RF cable from the PA input to avoid this.
The other conditions which must be met to ensure a successful calibration are
as follows:
■ No other Calibration Wizards are running
■ The supply voltage is within the range 27-29V
■ The temperature of each stage is within the range 5-50°C
To calibrate the PA stage bias current
1. Ensure you are already connected to the base station.
2. Select the PA Calibration tab, and double-click Calibrate PA Bias.
3. Ensure that the exciter RF is isolated from the PA by disconnecting the
SMA connector on the PA front panel.
4. Click Calibrate to calibrate the PA bias.
When you have finished calibrating the power amplifier bias, the icon on
the PA Calibration tab indicates that this task is complete.