User's Manual

1.3.2.3 Low Power Amplifier Block Diagram Theory
Refer to Figure 1-7 for a block diagram of the Low Power Amplifier module. The 030802-0001 Low
Power Module is used in the 2160 Low Power DME as the complete transmitter module, and in the 2170
High Power DME and the TACAN as the transmitter RF signal source/driver amplifier. It is comprised of
three major sections. Within the module are the Synthesizer CCA which generates a pulsed RF signal for
the DME/TACAN transmitters, the RF amplifier/transmitter assembly boards which provide the necessary
amplification of the Synthesizer signal, and the Modulator CCA which processes the control signals from
the RTC (Receiver Transmitter Controller) module to properly control the desired output RF pulse shape
and amplitude. The synthesizer CCA also contains the DC/DC power supply which regulates the various
supply voltages used within the module. Analog control signals from the RTC are routed via high speed
differential analog paths through the low power backplane to the Low Power Amplifier. Differential
analog signals are used to suppress the effects of common mode noise on the signal paths and to maintain
the integrity of the analog control signals. Similarly, the detected video outputs of the Low Power
Amplifier assembly are routed through similar high speed differential analog paths back to the RTC, for the
same reasons.
1.3.2.3.1 Synthesizer CCA Block Diagram Theory
Refer to Figure 2-7. The Synthesizer Assembly contains only one CCA which generates a pulsed RF signal
across the full DME/TACAN transmit band. This CCA also provides a sample of the RF signal to the
RTC for BITE considerations. The circuit card contains the Low Power Module’s temperature sensor and
all of the input/output control signals for this module.
The Synthesizer CCA contains a DC/DC converter which accepts an input voltage range of 40Vdc –
60VDC and generates a nominal output voltage of 51VDC using the converters output “trim” pin to alter
the output voltage. This nominal voltage is passed to the Modulator CCA for modulation and power
control of the RF transistors. This nominal voltage is also DC/DC converted to 20VDC using an additional
switching regulator. The other required DC voltages used within the module are linear regulated from the
20VDC voltage. The DC/DC converter and switching regulator voltages are monitored for proper DC
levels using a window comparator circuit. This output signal from this comparator is sent to the RTC for
BITE monitoring with a logic level “0” indicating a “POWER GOOD” condition. A front panel status LED
is also illuminated green to indicate this power good condition. The LED can also be illuminated by the
RMS for lamp test and troubleshooting purposes.
The RF synthesizer portion of this CCA generates a CW or pulsed RF output power of 26.5dBm ±0.5dB on
any DME ground station transmit frequency from 960 MHz to 1215 MHz. Synthesis of the transmit
frequencies is accomplished by controlling the tuning voltage on a Voltage Controlled Oscillator (VCO).
A phase lock loop is used to control the tuning voltage. The PLL uses an active gain loop filter and is
frequency referenced to a 10 MHz standard provided by a temperature compensated crystal oscillator
(TCXO). This TCXO reference provides the required transmitter frequency stability over all environmental
conditions. Programming of the desired frequency is done through serial control lines from the RTC. The
output RF signal from the VCO is buffered, amplified, and split three ways using a resistive divider. One
of these paths is used as feedback for the phase lock loop. The second path routes the signal to a fixed
frequency divider. The output of this divider is monitored by the DME system for frequency integrity. The
final RF signal path is amplified to the proper signal level, pulse modulated and is fed to the power
amplifier. The CW RF signal is pulse modulated using a non-reflective switch by the gate controls from the
RTC, and is fed to the Pre-Driver CCA (012175) of the power amplifier. Prior to the RF switch and the
final gain block, a sample of the CW signal is provided for use by external test equipment via a resistive
coupler. This port is AC coupled and is properly terminated internally (50 ohm load) to provide sufficient
isolation to avoid disruption of the transmitted signal by external influences, and made available on a front
panel connector.
The low power amplifier temperature sensor is mounted on the back side of the final RF amplifier stage
transistor. It is in close proximity to the transistor in order to maximize heat transfer to the sensor. The
sensor is equipped with a serial programmable interface (SPI). The temperature data from this sensor is
processed by the RMS and used for amplifier protection in the event of over temperature.