User's Manual

1.3.2.3.2 Low Power Modulator CCA Block Diagram Theory
The Modulator CCA sends and receives control signals to the RTC (Receiver Transmitter Controller) via
the Synthesizer CCA. This board controls the voltage to the RF amplifying transistors to obtain the proper
transmitter power and pulse shape. The transmitter gate signal, supplied by the RTC, is applied to the first
two RF amplifier stages through a high side MOSFET switch. The modulating transistor switches are
controlled at two different voltage levels for a 3 dB transmitter power level change. The final two
amplifying stages are also voltage controlled by a high side MOSFET, but these transistors are linear
modulated with the signal from the RTC that is Gaussian shaped. The energy storage capacitor used to
provide the short term peak current requirements of the RF amplifier stages is contained on the Modulator
CCA board. The forward and reverse detected RF video outputs signals from the Final CCA (012184) are
routed to the Modulator CCA where the signals are used to determine the RF pulse shape and level. These
detected signals are also monitored on the Modulator CCA for excessive pulse width and high VSWR
conditions. The output of the high pulse width and high VSWR monitor circuits are stretched and sent to
the RTC for monitoring. In addition, in the event there is a detected pulse width fault or a high VSWR
fault, the stretched outputs of these detectors will disable the Low Power Amplifier RF output for
protection against damage. The forward detected video signal is also routed to the RTC to complete the
control loop that provides the proper power level and pulse shape. The RTC compares this detected signal
to the desired output pulse shape, calculates the necessary corrections, and pre-distorts the shaped pulse
control signals used by the Low Power Amplifier module.
1.3.2.3.3 RF Amplifier / Transmitter Assembly Theory
The RF Amplifier/Transmitter portion of the LPA Module provides amplification of the pulsed RF signal
from the synthesizer CCA, and is comprised of three assemblies. The first assembly is the pre-driver
amplifier stage, and the second assembly is the driver amplifier stage. The last assembly contains the final
RF amplifier stage along with an output low pass filter and a dual directional coupler. The pre-driver and
the driver stage are all square wave modulated by the modulator CCA, while the final amplification stage is
square wave modulated for TACAN and High Power DME configurations or Gaussian shape modulated
for Low Power DME configuration. All the RF amplifying transistors are bi-polar junction transistors
(BJT) and are operated in Class-C mode, common base configuration. The module is capable of
transmitting 225W peak at the output port of the module in either a Gaussian shape or square wave
modulation. The transmitter’s circuit cards are described in the following paragraphs.
1.3.2.3.4 Pre-Driver and Driver CCA Block Diagram Theory
Refer to Figure 1-7. The pre-driver is a single stage RF amplifier which receives a pulsed RF signal from
the Synthesizer CCA. The RF signal is amplified and routed to the next stage, the “driver” CCA. The
driver CCA contains two stages of RF amplification and feeds into an attenuator pad on the Final CCA.
The voltage supplies of the transistors on the pre-driver and the driver stages are all square wave modulated
in synchronization with the RF switch on the Synthesizer CCA, controlled by the gate pulses from the
RTC. The pulse width, gain, and output RF signal level are controlled via the Modulator CCA.
1.3.2.3.5 Final CCA Block Diagram Theory
Refer to Figure 1-7. The final stage of the RF transmitter includes the final RF amplifier stage, a low pass
filter, and a dual directional coupler. The final transistor receives a square wave modulation signal and
maintains a square wave output shape for the High Power DME and TACAN system configurations, or
transforms the input signal into a Gaussian shaped RF signal for the Low Power DME system
configuration. This shape is controlled by modulating the supply voltage of the final transistor. All
modulation of the supply voltages are synchronized with the RF switch on the Synthesizer CCA. The
signal shape and output level are controlled by the RTC via the Modulator CCA. An attenuator pad is used
between the driver and the final RF amplifier stage in order to minimize voltage standing waves when
modulating the input square wave into a Gaussian signal. This attenuator also provides impedance stability
between these two RF amplifier stages.
The low pass filter is a lumped element design and is optimized for minimal insertion loss across the
DME/TACAN transmitter band, while providing a nominal 40dB or more of attenuation for unwanted high
frequency spurious signals. The filter is placed in a separate cavity for shielding purposes.