Selex Sistemi Integrati DMEL2 User Manual

1. TECHNICAL DESCRIPTION

1.1 Introduction

This section contains a technical description of the single and dual 2160 and 2170 DME. This includes,
simplified system block diagram theory and block diagram and detailed circuit theory of the Circuit Card
Assemblies (CCA) contained in the system.

1.2 DME Operation Principles

Refer to Figure 1-1. The DME system requires a single-channel receiver-transmitter combination
(transponder beacon) in conjunction with a special omni-directional antenna as the ground station, and a
multichannel receiver-transmitter combination (interrogator) on board the aircraft. One multichannel
airborne receiver-transmitter (transmitting and receiving coded, pulsed information) provides both the
distance and identification functions.

The DME system has 252 operating channels, with the adjacent channels spaced one megahertz apart. For
air-to-ground transmission (interrogation), there are 126 channels within the frequency band of 1025 MHz
to 1150 MHz. For ground-to-air transmission (reply), there are 63 channels in the frequency of 960 MHz
to 1024 MHz, plus 63 channels in the frequency band of 1151 MHz to 1215 MHz.

The DME system utilizes pulse -coding techniques in the transmission of its intelligence. The transmissions
are composed of pulse groups with a prearranged spacing between the pulses of the group. For X­Channels, the interrogation pulses and the transponder reply pulses are both spaced 12 µs apart. For Y­Channels, the interrogation pulses are spaced at 36 µs; and the transponder reply pulses are spaced at 30 µs.

Both the interrogator and transponder receivers employ pulse decoders, which are set to pass only pulse
pairs of the prescribed spacing. The purpose of the two -pulse technique is to increase the signal-to-noise
ratio and to discriminate against pulse interference, such as might be produced by radar transmission and
other extraneous sources of RF energy on the frequency. The intelligence supplied to the aircraft by the
DME transponder is both identity and distance information. The identity information is necessary for the
pilot to positively identify the station that has been selected. Identity information is provided to th e aircraft
approximately every 30 seconds. The distance information, however, is provided to the aircraft only upon
demand. Each aircraft must interrogate the ground facility by means of the coded interrogation pulse pairs,
before the transponder beacon can generate and transmit distance information.

Figure 1-1 Basic X-Channel DME System Block Diagram

Refer to Figure 2-2. As stated, the transponder beacon must be interrogated by the aircraft before the
ground facility can transmit usable distance information. Assuming an aircraft has interrogated the ground
facility; the interrogation signal is received at the beacon antenna, and then routed to the receiver through
the Circulator and Preselector. The signal is then amplified and fed to the Receiver Transmitter Controller
(RTC) for verification of proper pulse spacing. Once receive pulses are validated the RTC encodes a reply
with the proper pulse spacing and delay.

The shaped pulses modulate the gated RF in the transmitter power amplifier (PA) to produce the RF output
pulses. The output pulses are then sent to the antenna and radiated to the aircraft as reply pulse pairs.

Figure 1-2 DME Transpon der Block Diagram

Three separate signals are transmitted by the beacon as a train of pulse pairs. These signals, in order of
priority, are: identification, replies to interrogations, and squitter pulse pairs (used as fill -in pulses). This
priority system prevents any interference between the three signals in the overall pulse train.

The identification of the ground facility is important to the using aircraft; therefore, it has been assigned
first priority in the priority system. The generation of identification intelligence is a function of the RTC.
Identification is transmitted periodically in International Morse Code with the characters of the code
consisting of a periodic train of pulse pairs. Identification keying occurs approximately every 30 seconds.
When keyed, the priority logic circuit input is disabled; and the circuits will not accept any decodes from
the receiver.

The replies to an interrogation signal are second in the order of priority. Their induction into the pulse train
must be controlled (to prevent interference with the identity cycle and to establish priority over the squitter
pulses). This is accomplished by allowing them to enter the pulse train only during a time interval not
occupied by the identity cycle. This is a major portion of the time, since the identity cycle only occurs
approximately every 30 seconds. Once the receiver accepts an interrogation and decodes it, a blanking gate
is generated (the so-called dead-time gate). The dead-time gate is used to inhibit the transponder decoder
for approximately 60 µs. During this period, the decoded interrogation is delayed a predetermined amount
of time and transmitted back as a reply. The total delay from the time of a received interrogation to
transmission of a reply is typically set for 50 µs for an X-Channel DME.

The squitter pulses are third in the order of priority. In the absence of interrogations or identity
information, random squitter pulses are generated to maintain an average output pulse train of 800 Pulses
Pairs Per Second (PPS). The purpose of transmitting squitter pulses is to stabilize the Automatic Gain
Control (AGC) circuits in the aircraft interrogator.

The process of distance measuring originates in the airborne unit with the generation and transmission of
pulse signals called interrogations. The airborne transmitter repeatedly initiates and transmits pulse signals
consisting of pulse pairs having 12 µs spacing, a pulse width of 3.5 µs, and a gaussian or sine-squared
shape. These pulse pairs are recovered by the transponder beacon receiver, whose output triggers the
associated transmitter into transmitting reply pulse pairs. The reply pulse pairs are received by the airborne
receiver and timing circuits, which automatically measure round-trip travel time (the time interval between
interrogation and reply pulses) and convert this time into the electrical signals that operate the distance
meter.

Using the block diagram of the system in Figure 2-2, the distance measurement function can be examined
from the system stand point. The range circuits of the airborne interrogator initiate the distance measuring
process. They formulate and transmit an interrogation pulse pair, which is received at a ground station
antenna and sent to the RTC. The RTC then triggers the encoded pulse generator where the shaped pulses
are amplified and routed to the PA for modulation of the gated RF. The output RF pulses are then radiated
into space (as replies) via the antenna. The reply pulses are received by the aircraft, decoded by the
airborne receiver, and examined by the range circuits for synchronism with the airborne unit's own
randomly generated interrogation pulses.

The airborne unit measures the elapsed time between the transmission of the interrogation pulse pair and
the receipt of the reply pulse pair. It then, converts this time into a distance indication. In other words, the
distance indication is a measurement of the range time of the pulse pairs. This timing sequence is easily
seen by means of the system timing diagram of Figure 2-3. Timing starts (in the range circuits of the
airborne unit) with the first pulse of the interrogation pulse pair. After a time delay, depending upon the
distance between the aircraft and the ground station, the interrogation pulses are received at the antenna of
the ground transponder beacon. The interrogation pulses are decoded, and the reply is encoded and
transmitted after a preset time delay (the reply delay of the ground station).

This nominal reply delay duration is 50 µs, for which the airborne range circuits automatically account.
Thus, the total time lapse for any interrogation response cycle is the sum of reply pulse spacing, the
two-way transit time (range time), and the reply delay.

1.3 Theory of Operation

1.3.1 Simplified System Block Diagram Theory

Figure 2-4 is a simplified block diagram of the 2160 DME. The 2170 DME differs only by the addition of

a high power amplifier module in the signal path between the lo w power amplifier / synthesizer and the
output circulator. The Transponder portion of the DME consists of the Directional Coupler (1A6),
Circulator, Low-Noise Amplifier (LNA), Receiver Transmitter Controller (RTC), and the Low Power
Amplifier / Synthesize r (LPA). Aircraft interrogations are picked up by the antenna and routed through the
Directional Coupler to the Circulator. Additional interrogations from the Monitor/Interrogator are injected
into the Directional Coupler. The responses to these interro gations are sampled by the monitor port within
the antenna and are used to monitor the reply delay and reply efficiency. The Directional Coupler also
provides a sample of the transponder reply to the Monitor/Interrogator. The Monitor/Interrogator also us es
these samples to provide power output measurement.

Figure 1-3 System Timing Diagram

The Circulator provides isolation between the transmitted and received signals, since a common antenna is
used for both. Signals applied to any of the ports will experience the least insertion loss or minimum
resistance when traveling to the adjacent port in a clockwise direction. Signals traveling in a
counterclockwise direction will be attenuated by at least 20 dB. Aircraft interrogations and monitor
interrogations arriving from the Directional Coupler are directed to the Preselector Assembly by the
circulator.

The Preselector Assembly is a narrow-band, three-pole, mechanically-tuned filter that discriminates again st
undesired frequencies and provides additional attenuation of transmitter energy. From the Preselector, the
received interrogations are directed to the receiver input of the Receiver Transmitter Controller (RTC) after
being amplified by the Low Noise Amplifier (LNA).

Within the receiver is a low-noise high stability local oscillator that is generated by an internal synthesizer.
This signal is mixed with the incoming RF to provide a 125 MHz first IF signal that is log detected and
digitally processed fo r accurate interrogation time of arrival. The 125MHz IF is further down converted to
a 10.7 MHz IF signal. This 10.7 MHz signal is narrow-band filtered and used to determine if the received
interrogation was on-channel or from an adjacent channel.

The Low Power Amplifier Module delivers 125 watts peak Gaussian shaped pulses at the output connector
when used in the Low Power DME. This translates to a minimum 100 W output at the antenna connector
of the station allowing for normal losses in the connecting cables, directional couplers, etc. The output
power is attainable on any DME channel from 960-MHz to 1215-MHz without requiring re -tuning of the
amplifier. The Low Power Amplifier module provides in nearly 200 watts shaped pulses when used as the
driver amplifier in either the 2170 High Power DME system, or the TACAN system.

The Low Power Amplifier Assembly consists of three major circuits: the transmitter RF synthesizer, the
Modulator and the RF Amplifier. The RF synthesizer is programmed via a serial interface to the station
transmit frequency. A sample of the CW output of the synthesizer is available on the front panel of the
Low Power Amplifier module to allow verification of transmitter frequency by external test equipment.
The CW output of the synthesizer is pulse modulated by an RF switch controlled by the gate pulses from
the RTC, then amplified to 26.5 dBm at the output of the synthesizer CCA within the Low Power Amplifier
module. The synthesizer assembly also contains a DC/DC converter to provide the high voltage (~51 volt)
supply used within the Low Power Amplifier module. This DC/DC converter will operate over an input
range in excess of 40 to 60 volts DC, providing a stable supply voltage for the RF amplifiers even when the
DME system is operating on batteries and is nearing the end of the useful battery life.

The pulse modulated RF signal is then routed through a 4-stage RF amplifier to achieve the final output
power of =200 watts peak. The Modulator CCA within the Low Power Amplifie r assembly performs the
required pulse shape modulation and output power control. The Gaussian shape desired when used as a
stand-alone low power transmitter (2160 DME) is achieved by a linear modulator under control of the RTC
module within the DME system. Detected output pulses from the low power amplifier module are routed
to the RTC, where they are compared to the desired pulse shape, and pre -distorted Gaussian shaped control
pulses are sent from the RTC to the Low Power Amplifier assembly where they control the outputs of the
Modulator CCA. When the Low Power Amplifier module is used in either the 2170 High Power DME or
the TACAN, the output is amplitude controlled by the RTC, and the pulse shaping is done in the low -power
amplifier modulator.

The High Power Amplifier module (used only in the 2170 High Power DME and the TACAN, not shown
in Figure 2-4) consists of three major circuits, the Modulator and the RF amplifier, and the DC/DC
converter. There is a slight difference in the TACAN version of the High Power Amplifier RF Amplifier
and the DME version of the High Power Amplifier. In the TACAN version, there is an additional driver
amplifier stage to compensate for the reduction in input power because the output of the Low Power
Amplifier is split 5-ways. In the DME application, the full output of the Low Power amplifier is applied to
the input of the High Power amplifier, hence this additional stage is not needed. The DME version of the
High Power Amplifier has two gain stages, a 500 watt ampli fier followed by a 4-wide 2000 watt peak

amplifier (4 x 500).
The modulator within the High Power Amplifier module operates in a similar fashion as the modulator in
the low power amplifier above. It receives square -pulse input signals from the RTC, and provides the
collector voltage modulation to achieve the final RF output pulse shaping. The High Power amplifier
provides in excess of 2000 watts peak power across the full DME/TACAN transmitter band with no tuning
required.

The DC/DC converter located within the High Power amplifier provides a constant high voltage power
supply (approximately 53 volts) independent of the DME/TACAN system 48 volt power supply status.
This allows full power operation, even when the system is operating on battery backup and is nearing the
end of the batteries’ useful life. Energy storage capacitance to provide the large peak current requirements
of the RF amplifier stages is also located on the DC/DC converter CCA.

The Monitor portion of the DME consists of two majo r sections: the Interrogator (for interrogating the
transponder) and the Monitor (to evaluate the reply parameters). Both of these functions are located on the
Monitor/Interrogator/Synthesizer module in the 2160/2170 DME system. The Monitor CCA is actually two
separate printed wiring boards but they are plugged into the other; forming one module. The main board is
dedicated mainly to digital circuitry and is the card -cage support of the module, going from the back plane
to the front panel. The second board is dedicated to Interrogator (RF) circuitry.

Each monitor in the system is capable of monitoring all the critical parameters of two transponders on a
dual DME, and is capable of performing monitor integrity. One monitor interrogator interrogates the
transponders 50 times per second, therefore in a dual system the total rate is 100 interrogations on each
transponder for monitor purposes. The interrogation signals are fed into the transponder via the directional
coupler in the DME/TACAN system, and the trans ponder replies are routed to the monitor by the forward
coupled transmitter RF signals from the system directional coupler. In dual-equipment stations, the Standby
RF input is obtained from the output of the attenuator load connected to the transfer switch. The monitor
can vary the signal level, the pulse shape and timing and the frequency of the interrogations, so the monitor
sends different interrogations to measure different parameters. In normal mode different interrogations are
mixed together to measure all the critical parameters, if any of these parameters are out of range for an
amount of time, the condition is reported to the LCU using the alarm signals. Upon request from the RMS
other parameters can be measured and reported to the RMS. While a monitor is disabled to interrogate the
transponders (which is half of the time), the monitor uses this time to send certification signals to itself and
verifies that the circuitry and the software are working properly.

The Monitor(s) measure the signals and compare against the operator set limits. If parameters fall outside
the preset limits the alarm indication to the Local Control Unit (LCU) is changed. The LCU examines the
outputs from the Monitor(s) and determine whether to transfer or shutdown the transmitter based on the
present settings such as station bypass, and/or voting logic and whether the equipment is single or dual
transmitter equipment.

The standard PMDT consists of a laptop computer and is the input/output device for controlling and
communicating with the TACAN system. Station control, adjustment and monitoring functions are
available through the computer, and are accessed via a Windows-based operator interface. An external
mouse is supplied with the laptop computer for ease in operation. An optional desktop PC is available as a
substitute for the standard laptop computer. Also, an optional printer is available for use with either the
laptop or desktop PC.

Station security control is provided through a four -level password system. Complete access to the system
for adjustments and measurements is provided at level 3. Modification of non-critical parameters is
available at level 2, and read-only access is available at level 1. Password and account administration is
accomplished at level 4.

All functions available on the local PMDT are available remotely via a modem and dial-up telephone line
to an optional remote laptop or desktop PC running PMDT software.

Figure 1-4 DME Simplified Block Diagram

1.3.2 Detailed Theory of Operation

1.3.2.1 Low Power Backplane CCA Block Diagram Theory

Refer to Figure 1-5. The Low Power Backplane CCA provides interconnection and configuration for a
Model 2160/2170 DME System. The Low Power Backplane is an 84HP (approximately 16.8”) wide, 9 slot
card cage intended to fit in a standard 19” rack. The Low Power Backplane will accommodate both a single
or dual controlled/monitored DME system. It’s referred to as a Low Power Backplane because only low
power amplifiers may be inserted, while a high power DME or TACAN will have a separate, dedicated
High Power Backplane.

Configuration of the DME system resides on the Low Power Backplane CCA in the form of DIP switches.
Sixteen individual switches determine system configuration and eight switches set operating frequency.

System1 and System2 Low Power Amplifiers (LPA) connect to the backplane via J1 and J16 DB37
connectors with blind mate adapters (BMA). Each LPA also has an RF_OUT signal which eventually
connects to couplers for further processing.

The remaining circuit cards that insert into slots of the Low Power Backplane utilize both 96 pin and 60 pin
DIN41612 connectors. The 60 pin connectors have openings for RF connectors which carry RF signals to
and from the Monitor CCAs and RTC CCAs. For circuit cards such as the RMS and Facilities CCAs that
do no have RF signals, these RF connector positions are not populated.

System1 and System2 Receiver/Transmitter Controller (RTC) CCAs connect via J2/J3 and J14/J15 as well
as their RF connectors.

System1 and System2 Monitor / Interrogator CCAs connect to the backplane via J4/J5 and J12/J13 as well
as their RF connectors.

The RMS CCA connects to the backplane via J8/J9 and the Facilities CCA connects to the backplane via
J10/J11.

Connections between the Low Power Backplane CCA and a High Power Backplane CCA are accomplished
by 50 pin headers P5 and P6.

The J17 DB9 connector facilitates interface between the Low Power Backplane CCA and the Battery
Charge Power Supply (BCPS) modules for System1 and System2.

Power for the backplane enters via terminals E1 through E4 block TB1.

The LCU CCA is attached via 60 pin header P4.

The Interface CCA has two possible connections; the 60 pin header P1 for general purpose signals and the
40 pin header P1 if a TACAN antenna system must be controlled.

The Fan Control CCA is connected by 14 pin header P7.

Figure 1-5 Low Power Backplane CCA Block Diagram

1.3.2.2 High Power Backplane CCA Block Diagram Theory

Refer to Figure 1-6. The High Power Backplane CCA provides interconnection and configuration for a
Model 2170 DME System. The High Power Backplane is an 84HP (approximately 16.8”) wide, 5 slot card
cage intended to fit in a standard 19” rack. The High Power Backplane will accommodate up to five High
Power Amplifier Assemblies.

Connections between the High Power Backplane CCA and the Low Power Backplane CCA are
accomplished by 50 pin headers A2P1 and A2P2. These connectors are keyed to prevent incorrect
installation.

The control signals of A2P1 and A2P2 are distributed to the five possible amplifier connectors A3P1
through A7P1. Connectors A3P1 through A7P1 are DB37 female connectors with blind-mate adapters
(BMA).

+48 volts and ground power for each of the five possible amplifiers is routed via screw terminals A2E1
through A2E10. These terminals are each rated for 30 amps of continuous current.

Each amplifier has an RF_OUT signal, routed via connectors A3J2 through A7J2, which eventually
connect to couplers for further processing.

Figure 1-6 High Power Backplane CCA Block Diagram

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.

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.

The output directional coupler is a discrete component with a nominal coupling of -20 dB and a minimum
directivity of 20dB. The coupler is used to sample the transmitted RF signal and detect any reflected
signals due to load mismatches. Both the forward and reflected signals are further attenuated by 10dB
attenuators and are converted to video signals by differential diode detectors before being passed to the
Modulator CCA. Both the forward and reflected detectors have 25dB of linearity. The coupler/detector
section of this CCA is placed in a separate cavity for shielding purposes.

Figure 1-7 Low Power DME Amplifier Block Diagram

1.3.2.4 Receiver Transmitter Controller Theory

The Receiver Transmitter Controller (RTC) is an integral part of the DME dedicated to receiving aircraft
interrogations and controlling the transmitter replies. All of the receiver hardware is contained on the RTC
assembly except for the pre-selector filter that is tuned to the station frequency.

The RTC Assembly (030805-0001) consists of two circuit card assemblies (CCA). First is the RTC CCA
(012168-1001) that slides into the card cage assembly and plugs into the motherboard for power and signal
connections. The Receiver RF CCA (012180-0001) is a second circuit card that plugs into the side of the
RTC CCA. Power and signal lines come from the stacked circuit card connectors while the receiver RF
signal enters through a SMA connector. The RF signal is routed from the backplane connector on the RTC
CCA to the Receiver RF CCA using conformable RF cable. The Receiver RF CCA is housed in a
completely shielded enclosure consisting of a backing plate, fence, and cover.

1.3.2.4.1 CCA, Receiver Transmitter Controller Block Diagram Theory

This section describes the details of the RTC CCA. Throughout this section refer to Figure 1-8 and
012169-9001 schema tic.

A Digital Signal Processor (DSP) and two Field Programmable Gate Arrays (FPGA) comprise the heart of
the RTC CCA. All of the timing critical functions of pulse reception and transmitter control are located in
the FPGA hardware. Non critical tasks such as identification control, squitter/transmitter rate control, and
transmitter pulse shaping are handled by the RTC software in the DSP. The DSP uses its speed to help the
hardware handle tasks such as short and long distance echo suppression, decoder correlation, and CW
desensitization.

Control of the transmitter is accomplished by the RTC CCA. LVDS hardware on the board provides
differential gate pulses that enable the RF modulators and apply the synthesizer RF output to the transistor
amplifiers. Additionally the RTC CCA provides differential Gaussian shaped pulses used to control the RF
modulator amplitudes. With each pulse transmitted the RTC CCA samples the detected RF output and
determines what, if any, errors exist in the pulse shape parameters. The RTC CCA then modifies the output
waveform before transmission of the next pulse. This N-1 pulse shaping algorithm is used to ensure the
transmitter meets critical spectral requirements. Pulse shaping is one of the transmitter control loops in the
DME/TACAN equipment and does not require any user intervention.

Each RTC CCA has an associated low power amplifier and synthesizer. For high power DME operation
the LP amp is followed by a HP amplifier. In this case the RTC CCA drives the LP amp wit h a trapezoidal
shape pulse and performs the final pulse shaping in the high power amplifier. When operating as a
TACAN, up to five HP amps are summed together with a RF combiner. In this case the RTC CCA shapes
each HP amp individually and monitors the composite detected RF envelop for pulse corrections using the
forward power detector input signal.

In addition to driving the RF modulators, the RTC CCA controls the transmitter pulse pair spacing and
transmission rate. The transmission rate is monitore d in the FPGA hardware and randomly spaced pulse
pairs are generated if necessary in order to meet a minimum transmission rate. When overloaded by too
many aircraft interrogations, the RTC CCA limits the transmitter rate by reducing the receiver sensitivity.

Functions also accomplished by the RTC CCA include the decoder that correlates each received pulse with
previous pulses to find a pair that meets the decoder aperture for the DME operation mode. Received pulse
widths are monitored such that narrow radar pulses are rejected and wide out of tolerance pulses are
rejected. The receiver also provides continuous wave (CW) interference rejection and suppression of
transmission due to interrogation echoes.

Communications between the RTC CCA and the RMS is accomplished with a serial data link. Transmitter
and receiver configurations are received from the RMS while operating data and maintenance alert signals
are sent to the RMS for display on the PMDT. A serial data link between the RTC CCA and the Monitor
CCA is provided so the RTC software can control the transmitter delay as well as provide a transmitter soft