1. CECOM LAR Tropo Operation Course
OM-71 Tropo Modem
CECOM LAR
CONUS Branch
FT McPherson, GA
This class will cover the operation of the OM-71.
Tropo Modem

2. CECOM LAR Tropo Operation Course
Purpose
Multiplexes the Baseband Data, DVOW, and DOW orderwires.
Modulates the multiplexed data on to 2 carriers.
Alternately Transmits two carriers.
Demodulates the multiplexed data from the 2 carriers
Demultiplexes the Baseband Data, DVOW, and DOW orderwires
The Tropo modem has 5 functions
Recap Slide
Tropo Modem

3. CECOM LAR Tropo Operation Course
Block Diagram
A2 Transmit Orderwire Interface- This card has two functions;
1) A buffer for the inputs; Mission data, DVOW and DOW
2) Contains the circuitry to calculate the BER
A3 Multiplexer- Combines the inputs into a single data stream and adds framing. It then develops the I & Q Data streams for use by the PSK Modulator.
A7 Waveshape- Takes the I & Q Data and Modulates them on to two different sub-carriers, F1 and F2 approximately 1 MHz above and below 70 MHz. The modulation is gated so they are NOT transmitted at the same time. The carriers are then filtered. Note a sample of the modulated 70 MHz I.F. is taken from here and sent to the I.F. test panel.
A25 Bandpass Filter-Provides additional carrier filter.
A26 Power Divider- Two-way power divider for inputs to the U/C.
A22 Frequency Standard- A 10 MHz Rubidium Frequency Standard. A4 Timing Generator-Provides the clock signals for the operation of the transmit side of the Tropo Modem.
A6 Sub-carrier Oscillator-Provides the 4 carrier frequencies for transmission.
A1 BITE- Monitors the other cards on both the transmit and receive sections and sends fault indications to Alarm Monitor.
A9/A10 Dejitterizer- Patched in only as needed. These cards; buffer the data and clock it out to the A2. When activated the clock signal will first go through a Dejitterizer circuit.
Tropo Modem

4. CECOM LAR Tropo Operation Course
Block Diagram
A9-A12 IF Preamplifier- provides the first stage of amplification of the 70 MHz carrier. To maintain a balanced output between these cards a front panel alignment is required.
A13-A16 Channelizer- Separates the two sub-carriers.
A1-A8 IF/AGC Amplifiers- Each Sub-carrier is amplified.
A17-A20, and A22 through A25- Demodulates the sub-carriers into I & Q Video (Analog) streams.
A27 Combiner/Integrator- Accepts f1 Sub-carrier I & Q Video. Here the signals are combined. A Phase Error Estimate (PEE) is developed.
A28 Combiner/Integrator- Accepts f2 Sub-carrier I & Q Video. Here the signals are combined. A Phase Error Estimate (PEE) is developed.
A29 Analog Processor- three functions: 1) Combine I & Q Video to improve bit decisions. 2) Provide I and Q data. 3) Provide f1 and f2 AGC voltages. If bad, will cause all 4 receive channels not to Gain Equalize.
A21 Digital Data Regenerator- converts the I & Q Data back into a serial data stream.
A26 LO Distribution- provides the f1 and f2 subcarrier for demodulation.
A31 BITE- Provides monitoring of the CCAs in the 5A6 and reports to the 5A3 and Alarm Monitor.
Tropo Modem

5. CECOM LAR Tropo Operation Course
Block Diagram
The signal now goes into the 5A3 section.
A13 Demultiplexer- Demultiplexes the Mission Data, VOW and DOW.
A14 Microprocessor- Has 3 functions;
1) Controls the demultiplexing functions of the A13 CCA.
2) Examines the frame code and determines if Modem is in Frame.
3) Counts the number of errors in the frame and reports it to the A2 CCA.
A15 Timing Generator- Provides clocks for timing recovery.
A16/A17 Timing Recovery- Generates various timing signals used throughout the 5A3. Lights the RCV CLK ERROR lamp if a loss of timing recovery is detected.
A18 BITE/TLM- Provides the 5A3 with a demodulator BITE circuitry.
Tropo Modem

6. CECOM LAR Tropo Operation Course
Operational Faults
OPERATIONAL FAULTS (TRANSMIT)
1) INPUT ACT LED indicates no Mission Data input activity at DATA IN to the Tropo Modem. Lack of TIM IN will also light the indicator. This is caused by your equipment, not the distant end. Go straight to TM DATA IN and TIM IN at patch panel and see which one is missing.
2) INPUT FREQ LED indicates that the clock signal at the TIM IN is not within 1.5 x 10-7 of the internal RB Standard.
9) INT TMG LED indicates internal timing has switched from the TIM IN source to RB Standard. It will not light when the CLOCK SOURCE switch is set to RB position
OPERATIONAL FAULTS (RECEIVE)
3) CLK ERROR LED indicates that recovered timing, as detected by timing recovered circuits, is out of the maintenance window. It is tied to the Demux Frame lite in that it will always be on if Demux Frame is on.
4) DEMUX FRAME LED, indicates A Tropo Modem frame error has been detected and as a result a Frame Squelch is being sent to the DGM equipment. This will prevent the TGM (or LGM) from looking for framing, and they will have red lights. IGNORE THEM until the TM locks up with good BER and Demux Frame lite off.
Tropo Modem

7. CECOM LAR Tropo Operation Course
Equipment Faults
EQUIPMENT FAULTS
21- XMT LED, indicates that the modulator BITE has detected one or more faults.
19- RCV LED, indicates that the demodulator BITE has detected one or more faults.
17- RB STD LED, indicates that one or more of the six rubidium faults has been detected.
OVER TEMP
20- XMT LED, indicates that an overtemp exist in either the transmit section or the rubidium STD
18- RCV LED, indicates that an overtemp exist in the receive section.
Tropo Modem

8. CECOM LAR Tropo Operation Course
Meter Selection
6 METER SELECTOR SWITCH, 7 POSITION ROTARY
GAIN EQUALIZATION-Allows gain equalization for the four receive IF Preamp A9-A12. See Op Ver for procedure.
BER SHORT- When in the BER short position, the bit error rate of the frame data is measured and averaged over the last 100,000 bits. This is where you should be when testing system or bringing ina shot. It should go to 10 neg 5 in less than 1 minute when in loopback, or with a good receive signal.
BER LONG- When in the BER long position, the bit error rate of the frame data is measured over the last 10,000,000 bits and averaged over a ten minute sliding window
AGC-1- the processed AGC-1 for F1 is indicated on the meter for data rates of 1536 or greater at 3.5 MHz bandwidth. Otherwise it is a combination of F1 and F2 AGC.
AGC-2- the processed AGC-2 for F2 is indicated on the meter for data rates of 1536 or greater at 3.5 MHz bandwidth. Otherwise it is a combination of F1 and F2 AGC.
CLK ERROR- the meter will indicate system recovered timing offset. A zero meter reading indicates no offset. Cover relationship to Clock Error lite and Demux Frame lite.
RB VCXO- indicates the error voltage for the RB Standard 10 MHz VCXO is indicated on the meter. A normal reading is 2/3rd up-scale
Tropo Modem

9. CECOM LAR Tropo Operation Course
Modulation & Data Rate
12) MODULATION SWITCH
NORM- Normal data transmission
CW- Transmits an unmodulated lower sub-carrier ( MHz at 3.5 MHz BW) for antenna alignment.
DATA RATE KB/S
7) GROUP RATE- Used when the clock source switch is set to group. The group that you want to get timing from must be patched into the DJ and the group rate switch must be set to the data rate of that group. If the Clock Source switch is in MISSION or RB STD, these switches have no effect.
8) MISSION RATE- Used to set the mission data rate. This is ALWAYS set to the TGM Supergroup rate, or GM or LGM group rate if operating GM or LGM stand alone.
Tropo Modem

10. Clock & Bandwidth Controls
CECOM LAR Tropo Operation Course
Clock & Bandwidth Controls
9) CLOCK SOURCE SWITCH AND INDICATOR
GROUP- The shelter timing is extracted from the cable side input through the DJ patch from a GM or LSCD. The group rate switch position on the TROPO Modem matches that GP rate, and the desired group must be patched through the DJ.
MISSION- When in the MISSION position the timing signal at the Tropo Modem TIM IN jack on pp is used as the timing source. This is recovered clock from distant end when in slave mode and RO timing is patched.
RB STD When in the RB STD position, transmit timing is derived from the local rubidium standard.
INT TMG- Indicates that selected group or mission timing has failed and the rubidium standard has automatically switched into service. NOTE: will not come on if RB STD has been selected.
11) BANDWIDTH SWITCH
7 MHz- Sets the transmission bandwidth to 7 MHz wide . The sub-carriers will be at and MHz.
3.5 MHz- Sets the transmission bandwidth to 3.5 MHz wide. The sub-carrier will be at and MHz.
Tropo Modem

11. CECOM LAR Tropo Operation Course
Gain Equalization
Refer to Handbook page 22-10
Step 1
Cover procedure in Handbook
Tropo Modem

12. CECOM LAR Tropo Operation Course
Troubleshooting
Look at the front panel indicators
Open the front panels and look at the LEDs
Remember, TM BER depends on receiving TM frame bits at correct data rate (whatever you have your MISSION RATE switches set to), NOT data or framing bits from the DGM data link. Leave the DGM off until you get the shot in. THEN turn on and work the link in.
If you have DGM red lights, first look at TM and see if you have Demux Frame lite on and no BER. If so, IGNORE the DGM lites and work the shot in.
Tropo Modem

13. Rear Panel Transmit Section
CECOM LAR Tropo Operation Course
Rear Panel Transmit Section
J MHz OUTPUT TO SYNTHESIZER NUMBER 1
J MHz OUTPUT TO SYNTHESIZER NUMBER 2 in V2
J15- MODULATED 70 MHz I.F. TO IF TEST PANEL
J13- MODULATED 70 MHz I.F. TO UP CONVERTER NUMBER 1
J14- MODULATED 70 MHz I.F. TO UP CONVERTER NUMBER 2
J32- TG ULO TO RECEIVE TROPO MODEM
J33- TG LLO TO RECEIVE TROPO MODEM
J6- OUTPUTS TO BASEBAND AND DOW PATCH PANELS
J5- INPUTS FROM BASEBAND AND DOW PATCH PANELS
J4- DVOW TO/FRM BASEBAND PATCH PANEL
J9- TX CLOCK TO DOW
J35- INTERFACE CABLE TO RECEIVE TROPO MODEM
J1- MISSION DATA AND TIMING TO/FRM BASEBAND
Tropo Modem

14. Rear Panel Receive Section
CECOM LAR Tropo Operation Course
Rear Panel Receive Section
J22-70 MHz I.F RECEIVER 1 INPUT FROM THE I.F. TEST PANEL
J MHz I.F RECEIVER 2 INPUT FROM THE I.F. TEST PANEL
J MHz I.F RECEIVER 3 INPUT FROM THE I.F. TEST PANEL
J MHz I.F RECEIVER 4 INPUT FROM THE I.F. TEST PANEL
J26- TGULO FROM TX TM SUB CARRIER OSCILATOR A6
J27- TGLLO FROM TX TM SUB CARRIER OSCILATOR A6
J7- TGM FRAME SQUELCH TO TGM
J8- LGM 1 FRAME SQUELCH TO LGM 1
J9- LGM 2 FRAME SQUELCH TO LGM 2
J3- GAIN ADJUST FOR RX CHANELS 1-4, V TUNE 1-4, V REF, RELAY DRIVE, AND CHANNEL DISABE FROM TX TM GAIN ADJUST POTS. RSL TO METER IN TX TM. MISSION RATE CODE FROM TX TM. RX FAULT TO TX TM.
J6- SQUELCH FROM FROM THE A13 DEMUX CCA IN THE TX TM. BITE FAULT TO BITE CCA IN TX TM.
JI- POWER INPUT
J29- ALL SIGNALS ARE FROM THE TX TM SUCH AS SAMPLE, DUMP, PEE CLOCK, SER CLK, 13.8 MHz FROM THE A17 TIMING RECOVERY C CCA. PHASE ERROR SIGNALS ARE OUTPUTED TO THE A17 TIMING RECOVERY C CCA.
Tropo Modem

15. CECOM LAR Tropo Operation Course
Summary
BPSK
QPSK
DAR
Controls & Indicators
Gain Equalization
Troubleshooting
Recap slide
Tropo Modem