Aeroflex 3550 User manual

EXPORT CONTROL WARNING: This document contains controlled technical data under the
jurisdiction of the Export Administration Regulations (EAR), 15 CFR 730-774. It cannot be
transferred to any foreign third party without the specific prior approval of the U.S. Department of
Commerce, Bureau of Industry and Security (BIS). Violations of these regulations are punishable
by fine, imprisonment, or both.
Communications
Test Set
3550 / 3550R
Maintenance Manual
Issue-5

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MAINTENANCE MANUAL
COMMUNICATIONS TEST SET
3550 / 3550R
PUBLISHED BY
Aeroflex
COPYRIGHT Aeroflex 2014
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or
transmitted in any form or by any means, electronic, mechanical, photocopying, recording or
otherwise without the prior permission of the publisher.
Original Printing Jan 2013
Issue-2 Aug 2013
Issue-3 Sep 2013
Issue-4 Apr 2014
Issue-5 Nov 2014
10200 West York / Wichita, Kansas 67215 U.S.A. / [phone] / FAX [phone]

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Electromagnetic Compatibility:
For continued EMC compliance, all external cables must be shielded and three meters or less in
length.
Nomenclature Statement:
In this manual, 3550 / 3550R, Test Set or Unit refers to the 3550 / 3550R Communications Test Set.

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THIS PAGE INTENTIONALLY LEFT BLANK.

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SAFETY FIRST: TO ALL SERVICE PERSONNEL
REFER ALL SERVICING OF UNIT TO QUALIFIED TECHNICAL PERSONNEL.
WARNING: USING THIS EQUIPMENT IN A MANNER NOT SPECIFIED BY THE ACCOMPANYING
DOCUMENTATION MAY IMPAIR THE SAFETY PROTECTION PROVIDED BY THE EQUIPMENT.
CASE, COVER OR PANEL REMOVAL
Opening the Case Assembly exposes the operator to electrical hazards that can result in electrical shock or
equipment damage. Do not operate this Test Set with the Case Assembly open.
SAFETY IDENTIFICATION IN TECHNICAL MANUAL
This manual uses the following terms to draw attention to possible safety hazards that may exist when
operating this equipment.
CAUTION: THIS TERM IDENTIFIES CONDITIONS OR ACTIVITIES THAT, IF IGNORED, CAN RESULT IN
EQUIPMENT OR PROPERTY DAMAGE (E.G., FIRE).
WARNING: THIS TERM IDENTIFIES CONDITIONS OR ACTIVITIES THAT, IF IGNORED, CAN RESULT IN
PERSONAL INJURY OR DEATH.
SAFETY SYMBOLS IN MANUALS AND ON UNITS
CAUTION: Refer to accompanying documents. (This symbol refers to specific CAUTIONS
represented on the unit and clarified in the text.)
AC OR DC TERMINAL: Terminal that may supply or be supplied with AC or DC voltage.
DC TERMINAL: Terminal that may supply or be supplied with DC voltage.
AC TERMINAL: Terminal that may supply or be supplied with AC or alternating voltage.
EQUIPMENT GROUNDING PRECAUTION
Improper grounding of equipment can result in electrical shock.
USE OF PROBES
Check specifications for the maximum voltage, current and power ratings of any connector on the Test Set
before connecting it with a probe from a terminal device. Be sure the terminal device performs within these
specifications before using it for measurement, to prevent electrical shock or damage to the equipment.
POWER CORDS
Power cords must not be frayed, broken nor expose bare wiring when operating this equipment.
USE RECOMMENDED FUSES ONLY
Use only fuses specifically recommended for the equipment at the specified current and voltage ratings.
INTENDED USE
The 3550 is intended for indoor use only and should not be subjected to conditions which cause water or other
liquids to collect on the Touch Screen Display.
The 3550R is intended for use in both indoor and outdoor environments and remains functional in typical rain
conditions.

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SAFETY FIRST: TO ALL SERVICE PERSONNEL (cont)
INTERNAL BATTERY
This unit contains a Lithium Ion Battery, serviceable only by a qualified technician.
WARNING: THE 3550 / 3550R USES A LITHIUM ION BATTERY PACK. THE FOLLOWING WARNINGS
CONCERNING LITHIUM ION BATTERIES MUST BE HEEDED:
•DO NOT RECHARGE OUTSIDE THE 3550 / 3550R.
•DO NOT CRUSH, INCINERATE OR DISPOSE OF IN NORMAL WASTE.
•DO NOT SHORT CIRCUIT OR FORCE DISCHARGE AS THIS MIGHT CAUSE THE BATTERY TO
VENT, OVERHEAT OR EXPLODE.
CAUTION: INTEGRATED CIRCUITS AND SOLID STATE DEVICES SUCH AS MOS FETS, ESPECIALLY CMOS
TYPES, ARE SUSCEPTIBLE TO DAMAGE BY ELECTROSTATIC DISCHARGES RECEIVED FROM
IMPROPER HANDLING, THE USE OF UNGROUNDED TOOLS AND IMPROPER STORAGE AND
PACKAGING. ANY MAINTENANCE TO THIS UNIT MUST BE PERFORMED WITH THE FOLLOWING
PRECAUTIONS:
•BEFORE USE IN A CIRCUIT, KEEP ALL LEADS SHORTED TOGETHER EITHER BY THE USE
OF VENDOR-SUPPLIED SHORTING SPRINGS OR BY INSERTING LEADS INTO A
CONDUCTIVE MATERIAL.
•WHEN REMOVING DEVICES FROM THEIR CONTAINERS, GROUND THE HAND BEING USED
WITH A CONDUCTIVE WRISTBAND.
•TIPS OF SOLDERING IRONS AND/OR ANY TOOLS USED MUST BE GROUNDED.
•DEVICES MUST NEVER BE INSERTED INTO NOR REMOVED FROM CIRCUITS WITH POWER
ON.
•PC BOARDS, WHEN TAKEN OUT OF THE SET, MUST BE LAID ON A GROUNDED
CONDUCTIVE MAT OR STORED IN A CONDUCTIVE STORAGE BAG. REMOVE ANY BUILT-IN
POWER SOURCE, SUCH AS A BATTERY, BEFORE LAYING PC BOARDS ON A CONDUCTIVE
MAT OR STORING IN A CONDUCTIVE BAG.
•PC BOARDS, IF BEING SHIPPED TO THE FACTORY FOR REPAIR, MUST BE PACKAGED IN A
CONDUCTIVE BAG AND PLACED IN A WELL-CUSHIONED SHIPPING CONTAINER.
CAUTION
THIS EQUIPMENT CONTAINS PARTS
SENSITIVE TO DAMAGE
BY ELECTROSTATIC DISCHARGE (ESD)
CAUTION: SIGNAL GENERATORS CAN BE A SOURCE OF ELECTROMAGNETIC INTERFERENCE (EMI) TO
COMMUNICATION RECEIVERS. SOME TRANSMITTED SIGNALS CAN CAUSE DISRUPTION AND
INTERFERENCE TO COMMUNICATION SERVICES OUT TO A DISTANCE OF SEVERAL MILES.
USERS OF THIS EQUIPMENT SHOULD SCRUTINIZE ANY OPERATION THAT RESULTS IN
RADIATION OF A SIGNAL (DIRECTLY OR INDIRECTLY) AND ENSURE COMPLIANCE WITH
INSTRUCTIONS IN FAA CIRCULAR AC 170-6C, DATED FEBRUARY 19, 1981.

PREFACE
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SCOPE
This Manual contains instructions for maintaining the 3550 / 3550R. It is strongly recommended that
the technician be thoroughly familiar with this manual before attempting to perform maintenance on
the equipment.
ORGANIZATION
The Manual is composed of the following Chapters:
CHAPTER 1 - INTRODUCTION
Provides an Introduction and Functional Descriptions for each assembly in the 3550 / 3550R.
CHAPTER 2 - MAINTENANCE INSTRUCTIONS
Identifies and explains Routine Service, Troubleshooting, Calibration/Verification,
Remove/Replace and Shipping/Storage Procedures.

TABLE OF CONTENTS
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PARAGRAPH PAGE
CHAPTER 1 - INTRODUCTION
1-1 General Information .................................................................................. 1-1
1-1A Scope ................................................................................................. 1-1
1-1B Nomenclature Cross-Reference List........................................................ 1-1
1-2 Equipment Description .............................................................................. 1-1
1-2A Equipment Characteristics, Capabilities and Features .............................. 1-1
1-2B Equipment Data.................................................................................... 1-1
1-2C Safety, Care and Handling..................................................................... 1-1
1-3 Principles of Operation.............................................................................. 1-2
1-3-1 General Description.............................................................................. 1-2
1-3-2 Detailed Description ............................................................................. 1-3
1-3-2A Power Supply PCB Assy (1A1) ........................................................... 1-3
1-3-2B Multi-Function PCB Assy (1A2A1) ...................................................... 1-7
1-3-2C LCD Assy (1A2A2) ............................................................................ 1-8
1-3-2D RF Assy (1A3).................................................................................. 1-9
CHAPTER 2- MAINTENANCE INSTRUCTIONS
2-1 Service Upon Receipt................................................................................ 2-1
2-1-1 Service Upon Receipt of Materiel ........................................................... 2-1
2-1-1A Unpacking ....................................................................................... 2-1
2-1-1B Checking Unpacked Equipment .......................................................... 2-1
2-1-2 Preliminary Servicing and Adjustment of Equipment ................................. 2-1
2-2 Troubleshooting ....................................................................................... 2-2
2-2-1 Troubleshooting Guidelines ................................................................... 2-2
2-2-2 Equipment Inspection............................................................................ 2-2
2-2-3 Troubleshooting Precautions.................................................................. 2-3
2-2-4 EMC / Safety Compliance ...................................................................... 2-3
2-2-5 Battery Precautions .............................................................................. 2-4
2-2-6 Support Equipment ............................................................................... 2-4
2-2-7 Troubleshooting Tests........................................................................... 2-9
2-2-7A Unit Power-Up Failure....................................................................... 2-9
2-2-7B BATT Indicator Failure...................................................................... 2-11
2-2-7C Display Failure................................................................................. 2-12
2-2-7D ANT Connector Failure...................................................................... 2-14
2-2-7E T/R Connector Failure....................................................................... 2-16
2-2-7F AUDIO Connector Failure .................................................................. 2-18
2-2-7G SWR Connector Failure..................................................................... 2-22
2-2-7H AUDIO OUT Connector Failure........................................................... 2-24
2-2-7I AUDIO IN Connector Failure.............................................................. 2-27
2-2-7J DVM Connector Failure ..................................................................... 2-30
2-3 Calibration / Verification Procedures........................................................... 2-33
2-3-1 Calibration / Verification Schedule ......................................................... 2-33
2-3-2 Testing Conditions................................................................................ 2-33
2-3-3 Support Equipment ............................................................................... 2-34
2-3-4 Verification Procedures......................................................................... 2-35
2-3-4A Generator Verification....................................................................... 2-35
2-3-4B Receiver RSSI Verification ................................................................ 2-38
2-3-4C TCXO Verification............................................................................. 2-42
2-3-4D RF Power Meter Verification .............................................................. 2-43
2-3-4E SWR Meter Verification..................................................................... 2-49
2-3-4F Audio Level Meter Verification ........................................................... 2-51
2-3-4G Verification Data Sheets ................................................................... 2-55
2-3-5 Calibration Procedures.......................................................................... 2-59
2-3-5A Calibration Setup ............................................................................. 2-59
2-3-5B TXCO Calibration ............................................................................. 2-61
2-3-5C Generator Calibration ....................................................................... 2-63

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PARAGRAPH PAGE
CHAPTER 2- MAINTENANCE INSTRUCTIONS (cont)
2-3-5D Receiver Calibration......................................................................... 2-66
2-3-5E RF Power Meter Calibration............................................................... 2-69
2-3-5F SWR Phase Detector Calibration........................................................ 2-73
2-3-5G Audio Level Meter Calibration............................................................ 2-74
2-4 Remove/Replace Procedures ..................................................................... 2-75
2-4A Battery............................................................................................ 2-76
2-4B Fuse ............................................................................................... 2-79
2-4C Open Case Assembly........................................................................ 2-82
2-4D Power Supply PCB Assy (1A1) ........................................................... 2-84
2-4E Multi-Function PCB Assy (1A2A1) ...................................................... 2-86
2-4F RF Assy (1A3).................................................................................. 2-90
2-4G ANT Connector................................................................................. 2-92
2-4H T/R Connector.................................................................................. 2-93
2-4I DC IN Connector .............................................................................. 2-94
2-4J AUDIO Connector............................................................................. 2-95
2-4K SWR Connector................................................................................ 2-96
2-4L DVM Connector................................................................................ 2-97
2-4M AUDIO IN Connector......................................................................... 2-98
2-4N AUDIO OUT Connector...................................................................... 2-99
2-5 Preparation for Storage or Shipment........................................................... 2-100
2-5A Packaging............................................................................................ 2-100
2-5B Environment......................................................................................... 2-100
APPENDICES
A Replacement Parts.................................................................................... A-1
B Connector Pin-Out Tables.......................................................................... B-1
C Support Equipment Requirements............................................................... C-1
D Abbreviations........................................................................................... D-1
E Assembly Drawings................................................................................... E-1

LIST OF ILLUSTRATIONS / TABLES
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LIST OF ILLUSTRATIONS
TITLE PAGE
System Block Diagram ................................................................................................... 1-2
Power Supply PCB Assy Block Diagram ........................................................................... 1-5
Multi-Function PCB Assy Block Diagram .......................................................................... 1-6
RF Assy Block Diagram.................................................................................................. 1-11
3550 / 3550R................................................................................................................ E-3
Power Supply PCB Assy................................................................................................. E-9
Chassis Assy................................................................................................................ E-11
Multi-Function PCB Assy................................................................................................ E-13
RF Assy ....................................................................................................................... E-16
LIST OF TABLES
TITLE PAGE
I/O Connectors.............................................................................................................. B-1
AUDIO Connector Pin-Out Table ..................................................................................... B-2
REMOTE Connector Pin-Out Table .................................................................................. B-3
ETHERNET Connector Pin-Out Table............................................................................... B-4
USB Connector Pin-Out Table......................................................................................... B-5

CHAPTER 1 - INTRODUCTION
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1-1. GENERAL INFORMATION
A. Scope
Type of Manual: Maintenance Manual
Equipment Name and Model Number: 3550 / 3550R Communications Test Set
Purpose of Equipment: The 3550 / 3550R Communications Test Set is used for
testing radios and related equipment.
B. Nomenclature Cross-Reference List
COMMON NAME OFFICIAL NOMENCLATURE
3550 / 3550R 3550 / 3550R Communications Test Set
Test Set or Unit 3550 / 3550R Communications Test Set
1-2. EQUIPMENT DESCRIPTION
A. Equipment Characteristics, Capabilities and Features
Refer to the 3550 / 3550R Operation Manual for this information.
B. Equipment Data
Refer to the 3550 / 3550R Operation Manual for this information.
C. Safety, Care and Handling
Observe all WARNINGS, CAUTIONS and NOTES in this manual. This equipment can be extremely
dangerous if these instructions are not followed.
CAUTION
THIS EQUIPMENT CONTAINS PARTS
SENSITIVE TO DAMAGE
BY ELECTROSTATIC DISCHARGE (ESD)

1-2 Subject to Export Control, see Cover Page for details.
1-3. PRINCIPLES OF OPERATION
1-3-1 GENERAL DESCRIPTION
LCD T/R
POWER
SUPPLY
BATTERY
MULTI-
FUNCTION
RF SWR
ANT
REMOTE
DC IN
DVM
AUDIO IN
AUDIO OUT
ETHERNET
USB
AUDIO
Figure 1-1. System Block Diagram

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1-3-2 DETAILED DESCRIPTION
The System Block Diagram (Figure 1-1) and the System Interconnect Diagram (Figure E-1) shows the
relationship of all assemblies. The following paragraphs are detailed descriptions of the 3550 /
3550R assemblies.
A. Power Supply PCB Assy (1A1) (Figure 1-2)
The Power Supply PCB Assy is responsible for supplying power to the internal modules for operation
and for charging the internal batteries. The Power Supply PCB Assy operates from externally
supplied DC power and provides simultaneous run and battery charge, or battery charge only. The
battery charge time increases when in the run and charge mode. The Power Supply PCB Assy
consists of a DC-DC Converter, ON/OFF Control circuitry and the Battery Charger circuitry. The
external DC input is supplied from an External DC Power Supply (supplied).
The internal batteries are removable/replaceable Li Ion battery packs with an internal "gas-gauge"
feature that allows accurate determination of remaining battery life. Maximum operating and storage
temperature for Li Ion batteries is -20°C to +60°C and the maximum charging temperature is 0°C to
+45°C.
The Power Supply PCB Assy contains a synchronous buck converter to convert the input voltage to a
fixed output voltage (+10 Vdc). The Power Supply PCB Assy also contains a low-pass filter to reduce
the amount of internal emissions. The Input Converter Assembly is capable of providing enough
output current to charge the battery and run the Test Set at the same time, as long as the input
voltage is within range.
Battery Charger
The battery charger is a boost type converter. The battery charger monitors the battery voltage and
temperature to determine if the battery is capable of being recharged, and if safe to attempt to
recharge the battery. The battery must be at least at a 9.2 V level and the temperature must be
between 0°and 45°C before a charge cycle initiates.
Output Circuitry
The output converters are comprised of a dual-phase synchronous buck converter for developing
+3.3 and +5 V outputs. The converter also has an auxiliary output that is used to generate +18 V
from the +VS source (either the battery or input converter). A separate buck/boost converter is used
to generate the -5 V output. A separate buck converter is used to generate the +5.5 V output. The
primary converter provides dual phase control, as well as gate drive for the switching MOSFETS and
over-current protection. The main converter runs at 220 kHz, while the auxiliary converter runs at
1.2 MHz.
Both +3.3 and +5 V outputs are capable of delivering up to 3 A of current and the 18 V output can
deliver up to 80 mA. If any of these three outputs experience a severe over-current, the supply turns
OFF.
The -5 V converter is a stand-alone buck/boost converter that runs at approximately 220 kHz and can
deliver up to 400 mA of current before starting to fold back. If the supply experiences a severe over-
current condition, the supply stays in fold-back mode until the short is removed.
The +5.5 V converter is a stand-alone buck converter that runs at approximately 500 kHz and can
deliver up to 500 mA of current before starting to fold back. If the supply experiences a severe over-
current condition, the supply stays in fold-back mode until the short is removed.

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A. Power Supply PCB Assy (1A1) (Figure 1-2) (cont)
ON/OFF Control
The ON/OFF control circuit provides debounce for the POWER Key and turn off delay, as well as
monitoring the protection circuitry.
Pressing the POWER Key shorts that point to ground providing a low to the ON/OFF control circuit
input, which turns the power supply circuits ON or OFF. To turn the unit ON, the POWER Key must
be pressed for a minimum of 128 ms. To turn the unit OFF, the PWR button must be pressed for
approximately 250 ms. After the 250 ms, an interrupt signal is sent by the ON/OFF circuit to the
software, indicating that the power supply circuits are shutting down and allowing the software to
perform an orderly shutdown. Approximately 5 sec after the generation of the interrupt the power
supply circuits shuts down.
The ON/OFF circuit also monitors the protection circuitry for a +3.3 V or +5 V over current condition
or a low battery condition, and shuts down the power supply circuits if any of these conditions are
present.
Source Select Circuitry
The source select circuitry is designed as a low-loss switch that selects the appropriate source when
the output of the ON/OFF circuitry is high. The external DC (+Vdc) is selected if present. If external
DC is not present, the battery (VBAT+) connects to +VS. If the Test Set is running the power source,
either external DC or battery may be switched without interrupting system operation.
The second function of the switching circuitry is to prevent battery current from flowing into the input
converter section when the Test Set is running on battery power.
Protection Circuitry
The protection circuitry has four basic functions related to the DC input. The battery has internal
protection circuitry. The external DC input has a fuse, an over-voltage crowbar and reverse
protection diodes. The fuse is the primary disconnect to protect against any of the fault conditions.
If the DC input is too high (>32 V), the over-voltage crowbar triggers and the SCR opens the fuse. If
the DC input polarity is incorrect, the input clamp diodes forward bias and open the fuse. If the input
converter fails and the output of the converter is above 12 V, the SCR triggers and opens the fuse.

Figure 1-2. Power Supply PCB Assy
Block Diagram
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Figure 1-3. Multi-Function PCB Assy
Block Diagram
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B. Multi-Function PCB Assy (1A2A1) (Figure 1-3)
The Multi-Function PCB Assy provides processor control of various digital, analog and RF sub-
circuits used to generate or receive RF signaling normally used in mobile radio communication. The
Multi-Function PCB Assy also contains the application specific circuits to control the other
assemblies. These circuits include interfaces to the LCD Assy, Power Supply PCB Assy and RF
Assembly.
The user interface consists of the LCD Panel and Touch Screen, along with the Power and Menu
Buttons. The ETHERNET, USB and REMOTE Connectors provide digital connectivity to the external
devices. (Refer to Appendix B for the Connector Pin-Out Tables.)
AUDIO I/O
The Audio I/O circuitry provides the DVM, Audio In and Audio Out signals to the 3550 / 3550R Front
Panel.
The DVM Connector has an input range of ±2.5 Vdc in divide-by-1 mode and an input range of
±40 Vdc in divide-by-20 mode. AC signals are limited to be 1/2 the 50 kHz bandwidth of the TDM
audio digitizing bus.
The Audio In Connector is specified at 3 Vrms with High-Z, 1 kΩ, 600 Ωor 150 Ωloads available.
There is also a divide-by-10 mode with a fixed 1 kΩload that allows input signals up to 30 Vrms.
AM/FM external signals are limited from 300 Hz to 5 kHz.
The Audio Out Connector is rated to deliver 1.57 Vrms into a 600 Ωload with an output impedance of
100 Ω. Driven by a sample rate of 50 kHz, the specified frequency range is 30 Hz to 5 kHz. The
usable frequency range is 0 to 20 kHz.

1-8 Subject to Export Control, see Cover Page for details.
B. Multi-Function PCB Assy (1A2A1) (Figure 1-3) (cont)
RF DIGITIZER
The RF Digitizer circuitry is used for generating the source 10.5 MHz (±500 kHz) output at
approximately -10 dBm from the DUC (Digital Up Converter), while the receive input to the ADC is
optimized for an input level of -10 dBm at 13 MHz (±500 kHz). All other RF signaling, filtering,
scaling and up/down conversion methodologies are accomplished in the RF Assy.
TX
DAC
RCV
ADC
13 MHz BPF
3 dB PAD
10.5 MHz BPF
TX IF
RCV IF
For the Generator, Audio I/O and modulation signals are handled by the FPGA`s TDM measurement
bus. This bus runs at a 50 kHz-sample rate capable of delivering high quality audio to the DUC
(Digital Up Converter). The output of the DUC feeds a 14-Bit DAC for the digital-to-analog
conversion needed to form the output source RF signal.
For the Receiver, the output of the 14-Bit ADC is passed to the FPGA. The FPGA sends the data to
the DDC (Digital Down Converter) IC which processes the demodulation schemes needed and passes
the resultant data to the TDM audio bus for final interpretation by one of the system processors. The
system processors move the resultant modulation data to the LCD where the user can view the final
measurements.
C. LCD Assy (1A2A2)
The LCD Assy is composed of a Color LCD Panel, Touch Screen Sensor and Touch Screen
Controller. The Touch Screen is the User Interface. The Touch Screen input is detected and
processed by the Touch Screen Controller The Multi-Function PCB Assy receives signals and data
from the external connectors and the RF Assy. The processed signals and data are sent to the LCD
Assy to display on the various function screens.

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D. RF Assy (1A3) (Figure 1-4)
The RF Assy consists of the RF Converter PCB Assy and the RF Controller PCB Assy. The RF Assy
receives power and control via the Multi-Function PCB Assy.
The RF Converter PCB Assy contains the RF termination for the T/R Connector, ANT Connector and
SWR Connector, as well as the frequency translation circuitry and step attenuator for both the
receive and generate paths. The RF Controller PCB Assy interfaces directly with the Multi-Function
PCB Assy to control the RF Assy. The RF Controller PCB Assy contains the 80 MHz TCXO system
clock and the RF synthesizers that generate the required Local Oscillator (LO) signals.
An 80 MHz TCXO with a stability of 1 ppm provides the reference signal for the RF Assy as well as
the Multi-Function PCB Assy. The 80 MHz TCXO provides the reference signal to the five Phase
Lock Loops (PLL) used to generate the required Local Oscillator (LO) signals.
The Generate path of the RF Assy receives a 10.5 MHz IF signal from the Multi-Function PCB Assy.
This signal is mixed with the 80 MHz Gen. 3rd LO to create a 69.5 MHz IF that is filtered to remove
unwanted mixing products and amplified before being input to the Generate leveler circuit. The
Generate leveler circuit is used to set the desired output signal level by compensating for variations
in flatness over the 2 to 1000 MHz output band. The 69.5 MHz IF is then mixed with the 1330 MHz
Generate 2nd LO to produce a 1399.5 MHz IF. The output of the mixer is filtered to remove unwanted
mixing products and amplified before being input to the last mixer. The Generate 1st LO, which
covers 1401.5 to 2399.5 MHz, is mixed with the 1399.5 MHz IF to produce an output frequency of 2 to
1000 MHz. Following the mixer the signal passes through a low-pass filter and into an amplifier. The
output of the amplifier is tapped off to the Generate Level detector. The output of the detector is
feed back to the Generate Leveler to level the generate system. The RF signal out of the amplifier
then passes through the step attenuators. The step attenuators provide 60 dB of attenuation in 1 dB
steps to set the signal level out of the Unit. The signal is then input to a RF switch. One port of the
switch goes to the T/R Connector via a 3 dB pad, 3 way resistive splitter, and the 20 dB power pad.
This attenuator allows the T/R Connector to accept signal levels up to 20 W directly into the Test Set.
The other port of the RF switch is input to a second RF switch which steers the signal to either the
ANT Connector or the SWR circuit. When the ANT Connector is selected, the signal passes through
another RF switch to insure sufficient isolation and is coupled to the ANT Connector through the ANT
Connector protection relay.
Connected to the ANT and SWR Connectors are the connector protection circuits. These circuits are
designed to protect the rest of the circuitry from damage if a high power signal is inadvertently input
to either of these connectors. If the signal level input to either connector is too high, the signal level
is detected by diode detector and the mechanical relay at the connector is opened. Additionally a
signal is sent to the software via the Multi-Function PCB Assy that alerts the operator that an
overload condition exists.
The SWR Connector can be used to measure SWR and Distance to Fault (DTF) or as a generate
connector providing up to -5 dBm output. When SWR or DTF testing is selected, the signal is
amplified and passes through a 6 dB pad to insure a good match at the SWR Connector. The signal
then goes through two directional couplers and the SWR connector protection relay. The coupler
couples off the forward and reverse power at the SWR Connector. The coupled signals are input to a
dual Log Amp that outputs video signal proportional to the amplitude and phase difference of the
forward and reverse power. From these measurements the software can calculate the SWR of the
UUT. It is necessary to calibrate the SWR circuitry by connecting a short, an open and a 50 Ω
termination to the end of the cable connected to the UUT.
When the 3550 / 3550R is in a Receive Mode, either Transmitter Test or Duplex Test, the signal may
be input to the T/R or ANT.

1-10 Subject to Export Control, see Cover Page for details.
D. RF Assy (1A3) (Figure 1-4) (cont)
When the T/R Connector is selected, the signal is routed through a 20 dB high power attenuator.
This attenuator allows the T/R Connector to accept signal levels up to 20 W directly into the Test Set.
A temperature sensor mounted next to the attenuator is monitored by software. If the temperature
rise is too great, the operator is alerted. Following the attenuator are resistive splitter feeds, part of
the received signal to the power detect circuit. The power detect circuit is a diode detector used to
measure the input power for the Power Meter function. The resistive splitter also feeds the signal
through a 10 dB pad to the receive connector selection switch.
When the ANT Connector is selected, a signal input to the ANT Connector passes through the
protection circuitry to the receive connector selection switch. From this point on both the ANT and
T/R Connectors share a common path. The signal is input to the step attenuator that provides 0 to
30 dB of attenuation in 10 dB steps. Following the attenuator part of the signal is tapped of to feed
the log amp detector. The detected level out of the log amp is used to insure that the receiver does
not go into compression.
The received signal passes through a low-pass filter to remove out-of-band signal to the switchable
pre-amp. The pre-amp is selected by software to increase sensitivity for low-level signals. The
signal is then input to a mixer where the signal is mixed with the Receive 1st LO. The LO is tunable
from 1405 to 2403 MHz to produce an IF signal at 1403 MHz. The IF signal is filtered to remove
unwanted mixing products and amplified before being input to a second mixer. The signal is then
mixed with the Receive 2nd LO of 1310 MHz to down-convert the signal to a 93 MHz IF. The 93 MHz
IF is amplified and filtered then input to the 3rd mixer. The signal is mixed with the Receive 3rd LO
of 80 MHz to down-convert the signal to the final IF of 13 MHz. The IF then passes through a low-
pass filter and an amplifier before being input to a switchable filter network. At this point the signal
either passes through a 1 MHz wide bandpass filter or a 3 dB pad that compensates for the insertion
loss of the filter. Following the switchable filter, the signal then passes through several selectable
gain amplifiers before being output to the Multi-Function PCB Assy for digital processing. The
amplifiers are selectable in gain increments of 10 dB and controlled by the AGC software.