User's Manual

G3S-800-140 Installation & Service Manual
044-05095 Rev. A 4-2 February 2001
attenuated, phase shifted 180 degrees, then fed to the error amplifier where it is amplified to a
level identical to the sampled output from the main amplifier. The output from the error amplifier
is then coupled back and added to the output from the main amplifier. The control loops continu-
ously make adjustments to cancel out any distortion in the final output signals.
Pre
Amp
Pre
Main
Main
Amp
Error
Amp
Delay
Feed Forward Loop control
2nd Loop
Phase & Gain
1st Loop
Phase & Gain
Delay
Alarms & Display
+15 +5 -5
Power Supply
-30dB
-10dB
RF Out
RFL
PWR
FWD
PWR
Front Panel
Smart Rack
+27VDC
Figure 4-1 G3S-800-140 Power Amplifier Module Functional Block Diagram
The 2nd loop control section obtains a sample of the distortion added to the output signals by the
main amplifiers, phase shifts the signals by 180 degrees, then feeds it to the error amplifier.
There it is amplified to the same power level as the input sample and coupled on to the main out-
put signal. The final output is monitored by the 2nd loop and adjusted to ensure that the signal
distortion and IMD on the final output is canceled out.
4-4.1 Main Amplifier
The input and output of the amplifier employ two-stage, class AB amplifiers which provide ap-
proximately 32 dB of gain in the 25 MHz frequency band from 869 to 894 MHz. The amplifier op-
erates on +27 Vdc, and a bias voltage of +5 Vdc, and is mounted directly on a heat sink which is
temperature monitored by a thermostat. If the heat sink temperature exceeds 85 °C, the thermo-
stat opens and a high temperature fault occurs. The alarm logic controls the +5 Vdc bias voltage
which shuts down the amplifier.
4-4.2 Error Amplifier
The main function of the error amplifier is to sample and amplify the signal distortion level gener-
ated by the main amplifier, to a level that cancels out the distortion and IMD when the error signal
is coupled onto the main signal at the amplifier output. The error amplifier is a balanced multi-
stage, class AB amplifier, has 51 dB of gain, and produces up to an 80-watt output. The amplifier
operates on 27 Vdc and a bias voltage of +5 Vdc, and is mounted directly on a heat sink.
4-4.3 Amplifier Monitoring
In the main and error amplifier modules, all normal variations are automatically compensated for
by the feedforward loop control. However, when large variations occur beyond the adjustment
range of the loop control, a loop fault will occur. The alarms are displayed on the front panel indi-
cators and output via a 21-pin connector on the rear of the module to the subrack summary board
for subsequent remote monitoring via the ALARMS connector. Refer to paragraph 2-5 as well as
figure 2-2 and table 2-2 for a description of the ALARMS connector.