Functional Description
G3L-1929-160-001 MCPA
1-4
044-05305 Rev A
Feed-forward Loop Control
The primary function of the first loop is to amplify the carrier signals and isolate an error signal for the second
loop. It does this by amplifying the carrier signals and isolating an error signal which is passed to the second
loop. The first loop control section phase shifts the main input signals by 180 degrees and constantly monitors
the output for correct phase and gain.
The primary function of the second loop is to 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 attenuator and phase shifter in the first loop.
The second loop control section obtains a sample of the distortion added to the output signals by the main
amplifiers. The signal is phase shifted 180 degrees, then fed to the error amplifier where it is amplified to the
same power level as the input sample. The signal is then coupled to the main amplifier output. The final output
is monitored by the second loop and adjusted to ensure that the signal distortion and intermodulation distortion
on the final output is cancelled out.
During routine operation, all normal variations are automatically compensated for by the feed-forward loop
control. If large variations occur beyond the adjustment range of the loop control, the controller shuts down the
MCPA RF section and a loop fault is reported to the system.
Pilot Tone Generator
The pilot tone is an internally generated signal with a predetermined frequency, phase, and amplitude is
known. If the pilot signal is suppressed at the amplifier output, then the distortion created by the main amplifier
is also suppressed.
The pilot tone signal is injected into the first loop and detected at the feedforward output of the second loop.
The pilot tone is coupled off of the main amplifier creating a second pilot tone that is attenuated and phase
shifted 180 degrees to be used as the reference. The second pilot tone is amplified in the error amplifier and
mixed with the signals from the main signal path.
The first and second pilot tones should cancel each other out in an ideal situation. If the output detector senses
that the pilot tones do not cancel each other out, the information feeds back to control the gain and phase of
both the main and error amplifier paths minimizing output distortion.
Controller
The controller constantly compares the active RF paths with internal references for dynamic correction of small
variations through the RF feedback control circuits to maintain constant gain, and also provides the alarm
monitoring and control for the MCPA.
A front panel mounted tri-color LED provides MCPA operational status to the local operator. The OFF/ON/
RESET switch allows the operator to power on, power off or reset the MCPA. System faults are sent to a
remote location through the user interface.
Cooling
The MCPA components are mounted on a heatsink and cooled by forced air flowing over the heatsink fins.
System fans draw external air through the MCPA front panel inlet and exhaust air through the MCPA rear
panel outlet. The internal temperature is monitored to keep the MCPA within the specified operating
temperature.