Section
PRINCIPLES OF OPERATION
4-1 . INTRODUCT ION
This section contains a functional des c riptio n of the Multic arrie r Cellular Amplifier.
4-2. RF INPUT SIGNAL
The maximum input power for all carrier frequencies should not exceed the limits s pecified in table
1-2. For pr oper amplifier loop balance, the out of band components of t he input signals should not
exceed -40 dBc. The input VSWR should be 2:1 maximum (or bet ter).
4-3. RF OUTPUT LOAD
The load impedance should be as good as possible (1. 5:1 or better ) in the working band for good
power t ransfer to the load. If the amplifier is oper ated into a filter, it w ill maintain its distortion
characteristics outside the signal band even if the VSWR is infinite, provided the reflected pow er
does not exceed one watt. A parasitic signal of less than one watt incident on the output will not
cause distortion at a higher level than the normal forw ar d dist or t ion (i.e. - 65 dBc) .
4-4. SYST EM FUNCT IO NAL DESCRIPTION
The G3S-800-150 amplifier is a linear, f eed-f orwar d power amplifier that operat es in the 25 MHz
frequency band from 869 MHz to 894 MHz. A typical t wo-module system is shown in figure 4- 1.
The power output specification is listed in table 1-2. Each amplifier is a self-contained plug-in
module and is functionally independent of t he other amplifier module. The amplifier modules are
designed for parallel operation to achieve high peak power output, and for redundancy in
unmanned remote locations. Each amplifier in the system can simultaneously transmit multiple
carrier frequencies, at an average total power output of 135 watts (one amplifier module in a
subrack unit) to 270 watts (two amplifier modules), with -65 dBc third order intermodulation
distort ion (IMD).
4
The output fr om each amplifier is an amplif ied composite signal of appr oximately 150 watt s before
combiner losses. All phase and gain corrections are perf ormed on the signal(s) in t he individual
amplifier modules. In a two-module system, the amplifier outputs ar e f ed to a power combiner and
combined to form a composite RF out put of up to 270 watt s. Each amplif ier module has an alarm
and display board that monitors the amplifier performance. If a failure or fault occurs in an
amplifier module, it is displayed on the individual amplifier front panel.
4-5. G 3S- 800- 150 AMPLIFIER MO DULE
The amplifier module, figure 4-2, has an average output of 150 watts power (1500 watts peak
power) with inter modulation products suppressed to better t han -65 dBc below carrier levels. The
amplifier provides an amplified output signal with constant gain and phase by adding approximately
30 dB of distort ion cancellation on the output signal. Constant gain and phase is maintained by
continuously comparing active paths with passive references, and correcting for small variations
through the RF feedback controls. All gain and phase variations, for example those due to
temperature, ar e reduced to the passive reference variations. The amplifier module is comprised
of:
044-05073 Rev. A 4-1
Preamplifiers
Main amplif ie r
Error amplifier
Two feed- forwar d loops with phase-shift and gain controls
DC/DC power regulator
Alarm monitoring, control and display panel
COMBINER
ACTIVE POWER COMBINER
SPLITTER &
SIGNAL DIST.
ASSY.
500-001095
RF IN J9 VVA COUPLER BPF
POWER SPLITTER
MCA 1
17
ASSY
500-001094
RF OUT J2
J3/J4
J5/J6
DC_CH1
DC_CH2
2
SIGNAL DISTRIBUTION
5 5
IN-RUSH
IN-RUSH
80A MAX
80A MAX
100 30
CONTROL
ASSEMBLY
500-001105
MCA 2
17
4
12
INTERFACE
ASSY.
500-1107
Figure 4-1. G3S-800- 150 Two Module Amplifier Syst em
RMS DET.
500-001090
2X3
2X3
DB9
DB9
DB9
ALC_LED
MOD PORT
BDM PORT
FORM C ALARMS
RS485/ADD.
RS485/RS232
J1
J7
J8
The main amplifier employs class AB amplification for maximum ef f iciency. The er r or amplifier and
feed for ward loops ar e employed to correct signal nonlinearities introduced by t he class AB main
amplifier. The error amplifier operates in class AB mode. The RF input signals are amplified by a
preamp and coupled to an attenuator and phase shifter in the first feed- forward loop. The main
signal is phase shifted by 180 degrees and amplif ied in the premain amplifier. The output f r om t he
premain amplifier is fed to the class AB main amplifier. The output from the main amplifier is
typically 220 watts. The signal is output to several couplers and a delay line.
The signal output from the main amplifier is sampled using a coupler, and the sample signal is
combined wit h the main input signal and input to the second f eed- forwar d loop. The err or signal is
attenuated, phase shift ed 180 degrees, then fed to the error amplifier where it is amplified t o a
level identical to the sampled output fr om the main amplifier. The output from t he er r or amplifier is
then coupled back and added to the output fr om the main amplifier. The control loops continuously
make adjustments to cancel out any distortion in the final output signals.
044-05073 Rev. A 4-2
The primar y f unction of t he first loop is to pr ovide an error signal for the second loop. The pr imary
funct ion of the second loop is t o amplify the error signal to cancel out spurious products developed
in the main amplifier. The input signal is amplified by a preamplifier and fed to a coupler and delay
line. The signal from the coupler is fed to the att enuator and phase shifter in the 1st loop. The
first loop control section phase shifts the main input signals by 180 degrees and constantly
monitor s t he output for corr ect phase and gain.
Figure 4-2. G3S-800- 150 Pow er Amplifier Module Functional Block Diagram
The 2nd loop control section obtains a sample of t he dist ortion added to the output signals by t he
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
output signal. The final out put is monitored by t he 2nd loop and adjusted to ensure that the signal
distort ion and I M D on the final output is canceled out.
4-5.1. M AIN AMPLIFIER
The input and output of the amplifier employ two-stage, class AB amplifiers which provide
approximately 25 dB of gain in the 25 MHz frequency band from 869 to 894 MHz. The amplifier
operates 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
thermostat opens and a high temperature fault occurs. The alarm logic controls the + 5 Vdc bias
voltage which shuts down the amplifier.
4-5.2. ERROR AMPLIFIER
The main function of the error amplifier is to sample and amplify the signal distortion level
generated by the main amplifier, to a level that cancels out the distortion and IMD when the err or
signal is coupled onto t he main signal at the amplifier output. The er ror amplifier is a balanced
multistage, 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.
4-5 .3. AMPL IFIER MONITORING
044-05073 Rev. A 4-3
In the main and err or amplifier modules, all normal variations are automatically compensat ed f or by
the feedforward loop control. However, when large variations occur beyond the adjustment
044-05073 Rev. A 4-4
range of the loop control, a loop fault will occur. The alarms are displayed on the front panel
indicator s and output via a 21-pin connector on the rear of t he module to the subrack summary
board for subsequent remote monitor ing via the ALARMS connector. Ref er to paragraph 2-5 as
well as figure 2-2 and table 2-2 for a description of the ALARMS connector.
4-5.4. AMPLIFIER MO DULE COOLING
Although each amplifier module contains its own heat sink, it is cooled w ith for ced air. Four fans
are used for for ced air cooling and redundancy. The fans, locat ed on the f ront and rear of t he
amplifier module, draw air in through the front of the amplifier and exhaust hot air out the back of
the module. The fans are field replaceable.
4-6 . POW ER DISTRIBUTION
Primary DC power for the system is provided by the host system to the MCR20XX Series
(NTL107AC) subrack. The subrack supplies each amplifier module with +27 Vdc directly and via
the RF power splitter /combiner. The amplifier module has a DC/DC converter that convert s the
+27 Vdc to + 15 Vdc, + 5 Vdc and -5 Vdc.
4-7 . INTERMODUL ATION
The G3X-800 amplifier is designed to deliver a 150-watt composite average power, multicarrier
signal, occupying a bandwidth less than or equal to 25 MHz, in the bandwidth from 869 to 894
MHz. The maximum average power for linear operation, and thus the amplifier efficiency, will
depend on the type of signal amplified.
4-7 .1 T W O T ONE INT ERMODUL ATION
When measured with two equal CW tones spaced anywhere from 30 kHz to 20 MHz apart, and at
any power level up to the average power, the t hird order intermodulation products w ill be be low
-65 dBc
4-7.2 M ULT ITONE INTERMODULATION
Adding more tones t o the signal w ill lower individual inter modulation prod uct s. I f the f requencies
are not equally spaced, t he level of intermodulation products gets very low . W hen the fr equencies
are equally spaced, those products fall on top of each other on the same frequency grid. The
average power of all intermodulation beats falling on the sa me freq uency is called t he co mposite
intermodulation; it is -65 dBc or bet t er .
4-8. ALARMS
The presence of several plug-in amplifier alarms can be detected at t he DC and logic c onnector on
the amplifier rear panel. Refer to t able 2-1 and figure 2-2 for a descr ipt ion of the connector .
044-05073 Rev. A 4-5
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