. 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.
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.
Figure 1-2 DME Transponder 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 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.
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 low 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 / Synthesizer (LPA).
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.
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.
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.
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.
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.
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 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.
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).
start feature that monitors the antenna VSWR while ramping up the output power. This helps protect the high power amplifiers from damage in case the antenna port is not properly terminated.
Figure 1-8 Receiver Transmitter Controller Block Diagram
Multiple synthesizers are controlled by the RTC CCA. First is the transmitter synthesizer located on the LP amplifier that is serially loaded based on the backplane frequency select switches. Second is the receiver synthesizer that is loaded to provide an IF frequency of 125MHz. Output frequencies of both these synthesizers are checked by the Monitor CCA.
1.3.2.4.2 CCA, Receiver RF Block Diagram Theory This section describes the details of the Receiver RF CCA. Throughout this section refer to Figure 1-9 and 012181-9001 schematic. Figure 1-9 Receiver RF Block Diagram The Receiver RF CCA uses a dual heterodyne receiver with a first intermediate frequency (IF) of 125MHz and second IF of 10.7 MHz.
Frequency selection of the receiver is accomplished with an independent synthesizer. Thus there are no RF cables interconnecting the interrogation synthesizer and transmitter synthesizer. This also allows use of commonly available IF filter products. The synthesizer output power is +17dBm in order to run a high IP3 mixer. A sampled output is provided on the front panel of the RTC Assembly for use with a frequency counter.
Figure 1-10 Monitor CCA Block Diagram
After a system reset, all the alarm latch outputs will are in the active state until updated by the DSP. The alarms are updated only when the DSP refreshes the voltage supervisor / watchdog. If there is a DSP failure, the alarms will remain in the last output state until watchdog time-out and the voltage supervisor / watchdog reset activates the alarms. During normal operation, the alarm outputs are read by the DSP using an input buffer to verify them.
different.
1.3.2.5.1 DME Interrogator CCA Block Diagram Theory Refer to Figure 1-11. The Interrogator (RF board) has all the necessary circuitry to modulate the interrogations from the Monitor and demodulate the replies from the RTCs. For DME systems, the ANTENNA_RF signal comes from the monitor port on the DME antenna. For dual DME systems, the monitor signal is split and run to each Monitor. For TACAN systems, this signal comes from a monitor antenna placed near the TACAN antenna.
Figure 1-11 DME Interrogator (RF) Block Diagram
1.3.2.6 RMS Processor Block Diagram Theory Refer to Figure 1-12. The Remote Monitoring System (RMS) CCA performs communications via thirteen serial ports plus a parallel port, and facilitates monitoring/control in a single or dual DME system. The RMS CCA receives battery-backed DC power from the BCPS CCAs at connector J2, through OR’d diodes, and regulated to +5V and +3.3V supplies for use by the RMS CCA to power the microcontroller and all of its associated circuitry.
Figure 1-12 RMS CCA Block Diagram
1.3.2.6.1 RMS CCA Detailed Theory Battery-backed DC power 1_+48V and 2_+48V enter via connector P2-25 and P2-26, diode-OR’D by diodes CR13 and CR14, and fused by F2. This voltage is further regulated to +5V by DC-DC converter U39, diode CR15, and inductor L3. Over-voltage protection for the +5V is provided by SCR Q5, zener diodes CR19 and CR20, capacitor C115, and resistor R65. The +5V supply is further regulated by linear regulator U40 to create DVCC (+3.3V).
become active. The reset will clear after ~DELAYED_RESET from U45:B clears latch U29-9 ARM_SYS_RES; which in turn shuts off transistor Q3 and releases ~EXT_RES.
A reset can also be initiated by voltage supervisor / watchdog U6 when the power supply voltage on U6-2 drops too low; causing U6-7 to activate. Latches U29 and U31, as well as buffer U32, establish an 8-bit parallel port for LCU communications. Latch U29 signals PWRITE, PADDR, ~PREAD_EN, and PIN/~POUT determine a read or write bus access. The PWRITE and PADDR signals are converted to RS422 by U30 before routing to connector P1.
Decoding of the address space used by the U8 microcontroller is provided by decoders U23, U24, U21, and U18. All decoder outputs are used for on-board devices except for U24-10, named ~EXT_CS. This output defines the address space that is used to decode the Facilities CCA devices. Buffers U43 and U44 establish an 8-bit asynchronous bus for communications to/from the Facilities CCA. The inputs and outputs of buffers U43 and U44 as well as ~EXT_CS route to connector P2.
U31 and U32 buffer the power OK signals of up to seven Amplifier CCAs, two Monitor CCAs, and two Receiver/Transmitter Controller CCAs as well as the A-D status signal, the INTERLOCK signal, the ~FANS_OK signal, the SMOKE_DETECTOR signal, and the INTRUSION_SENSOR signal. All of these signals originate at various cards in the Low Power and High Power Backplanes except the last four signals, which originate at the Interface CCA.
Figure 1-13 Facilities CCA Block Diagram
1.3.2.7.1 Facilities CCA Detailed Theory System1 and System2 +48V power from connector P2-25 and P2-26 are scaled down by resistor networks RN1 and RN2 for input to the A-D converter as well as diode-OR’D by CR1 and CR2 to create the facilities +48V supply. This supply (also named LED_PWR) lights the CR24 PWR_OK LED when transistor Q1 is turned on under software control by U22-19; indicating all monitored power supplies are within range.
control signals are active.
The tip and ring signals from dedicated modem U14 are isolated, TVS-protected, and filtered by T1, CR8, FL4, and FL5 before exiting connector P2-A14 and P2-A15. Audio from U14-64 is scaled by R28 and R30 before audio header JP1-3. Three more sources of audio (other than the modems) are controlled by analog multiplexer U16. MON1_AUDIO_ID and MON2_AUDIO_ID from P1-C1 and P1-C2 (originally from the Monitor CCAs) connect to multiplexer U16-15 and U16-12.
converted to TTL by U34. The U38 output latch directs activity of the TACAN antenna controller after the U38 latch signals are converted to RS422 by U35 and U36. The U38-19 loop-back signal may be used for fault-isolation purposes to buffer U37-9. Latch U38-16 also controls ~TACAN_RESET though transistor Q5 and TVS diode CR42 before routing to P1-C8.
1.3.2.8 Interface CCA Theory Refer to Figure 1-14. The Interface CCA provides interface connections between the RMS/Facilities/Low Power Backplane CCAs and the outside world. Examples include spare analog and digital inputs, spare digital outputs, temperature sensors, smoke detector, intrusion sensor, and a TACAN antenna controller. RS232 communications are provided to RCSU and PMDT terminals as well as an optional Ethernet module.
Figure 1-14 Interface CCA Block Diagram
1.3.2.8.1 Interface CCA Detailed Theory The Interface CCA provides interface connections between the Backplane CCA and the outside world. All signals are protected by transient voltage suppression (TVS) devices. All connections between the Interface CCA and the Backplane CCA are accomplished via headers J1 and J2. The J2 and J3 connections will not be used in a DME system since there will not be a TACAN antenna system controller.
The EXT_KEY_OUT signal is TVS-protected by diode CR35 before routing to pull-down resistor R9 and transistor Q1. Resistors R10 and R11 bias the LED of opto-coupler U2 while diodes CR33 and CR34 transient protect the transistor outputs of opto-coupler U2. The transistor outputs are labeled EXT_KEY_OUT+ and EXT_KEY_OUT- before routed to terminal block TB1. The MON1_AUDIO_ID and MON2_AUDIO_ID are transient protected by diodes CR30 and CR31 before being voltage-divided by resistors R1/R3 and R2/R4.
Figure 1-15 AC Power Monitor Block Diagram
1.3.2.10 Local Control Unit Simplified Theory Refer to Figure 1-16. The Local Control Unit (LCU) controls the normal operation of the DME/TACAN. All operational functions are performed by the LCU and are controlled by either front panel keyboard when in the local mode or by the Remote Maintenance Subsystem (RMS) through the parallel interface.
1.3.2.11.1.3 Key Switch Registers Front panel switches are de-bounced and held in the Key Switch Registers pending processing by the LCU transfer state machines. Commands received from the RMS via the parallel interface also control the contents of the Key Switch Registers. The registers will hold the last command received until the LCU transfer state machine processes the command. 1.3.2.11.1.
1.3.2.11.2 Positive Alarm Register This register receives the positive (high True) alarms from the two potential monitors within a system. Depending on the configuration of the alarm voting and bypass logic, the Alarm Register will report an alarm to the transfer state machines if reported by the enabled monitors. 1.3.2.11.3 Negative Alarm Register This register receives the negative (low True) alarms from the two potential monitors within a system.
1.3.2.11.9 Station Control Logic The station control logic is duplicated in both U1 and U3. The logic responds to local operator control through the pushbutton switch inputs as well as remote control through the parallel interface. The local operator can perform the following functions: a. b. c. d. e. f. g. Specify which DME/TACAN transmitter is to be designated as main. Turn either transmitter ON and connect it to the antenna. Turn either transmitter ON and connect it to the load.
1.3.2.11.10 System Configuration Inputs In order to reduce the amount of effort required to program various modules within the DME/TACAN for the proper configuration, there are eight logic signals that are sent from the RMS to each module to specify the system configuration. 1.3.2.12 High Power Amplifier Theory Refer to Figure 1-18 for a block diagram of the High Power Amplifier module. The 030804-0001 High Power Module has three major assemblies.
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 High Power Amplifier module. 1.3.2.12.
circuit and lumped elements in the design. The coupler is a printed circuit “micro-strip” component with a nominal coupling of 30 dB and a minimum directivity of 15dB. The coupler is used to sample the transmitted signal and detect any reflected signals due to load mismatches. Both the forward and reflected signals are further attenuated by 10dB and are converted to video signals before being passed to the Modulator CCA. Both the forward and reflected detectors have about 25dB of linearity.
Figure 1-18 High Power Amplifier Block Diagram
1.3.2.13 Fan Control CCA Block Diagram Theory Refer to Figure 1-19. The Fan Control CCA provides control and monitoring of up to two fans in a Model 2170 High Power DME System. The Facilities CCA has an open-drain FANS_ON signal which turns the fans on when high. The Facilities CCA monitors the active-low ~FANS_OK signal to verify both fans are operating properly. If ~FANS_OK is high, one or both of the fans need servicing. The Fan Control CCA has three connectors.
1.3.2.14 BCPS Block Diagram Theory Refer to Figure 1-20. The Battery Charger Power Supply (BCPS) monitors various system power levels and switches the sourced power between the commercial power and batteries to assure continued service. There are three basic blocks used to illustrate the functions within the BCPS; the Analog Multiplexer, PIC processor with A/D converter and the DC to DC converter.
1.3.2.15 Extender Board Block Diagram Theory Refer to Figure 1-21. The Extender (Logic) CCA provides the means for a service technician to extend an RMS, Facilities, Monitor/Interrogator, or RTC CCA outside of the Low Power Backplane card cage so that measurements can be taken. The Extender (Logic) CCA extends a 96 pin DIN41612, a 60 pin hybrid DIN41612, and four RF signals through conformable conductors. The Extender (Logic) CCA can only extend the RMS, Facilities, Monitor/Interrogator, or RTC CCAs.
