IFR-2975 Training Course by variablepitch342

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									IFR-2975 Training Course
WWW.P25.COM WWW.IFRSYS.COM

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IFR-2975 Training Course Outline

 P25 Technology Basics  2975 Overview  Basic Operation  Functional Test Screens  Minimizing Measurement Errors  Spectrum Analyzer Measurements

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Section 1 - P25 Technology Basics

 Spectral Efficiency  Analog to Digital Conversion  FM vs. C4FM & CQPSK

 CAI (Common Air Interface)
 Impacts on Measurement Methods

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Effects of Modulation Techniques

FM
Analog Systems 25 kHz

C4FM
Project 25 Phase 1 12.5 kHz
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CQPSK
Project 25 Phase II 6.25 kHz

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Analog vs. Digital

Voice

2800 bits/s error correction

2400 bits/s signaling
4400 bits/s voice 20 ms of speech = 88 bits of information to be transmitted over the radio link

Signaling
1 25 kHz Channel
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Total 9600 bits/s

2 x 12.5 kHz Channels

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How is Analog Converted to Digital?
 Project 25 uses the IMBE (Improved MultiBand

Excitation) Vocoder convert from Analog to Digital.
 The Transmitter encodes analog information to digital.  The Receiver decodes digital information back to analog.  88 bits of data represent 20 ms of speech.  The Vocoder encodes all aspects of speech such as tone,

and audio level into a digital stream of bits.

 Digital Information is transmitted using C4FM

modulation format for Project 25 Phase 1  The Raised Cosine Nyquist Filter is used as a shaping filter to prevent co-channel interference

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Analog FM Modulation
 With Analog FM, the carrier is

moved in frequency with respect to the modulating source.  The signal viewed on the spectrum analyzer is constantly changing in occupied bandwidth.
300 - 3400 Hz Voice

Modulation Modulated Carrier

+

Control Data

25kHz BW

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C4FM Digital Modulation
 With Digital C4FM, the carrier is moved

to specific frequency offsets regardless of the modulating source. DiBit 10 DiBit 11  The signal viewed on the spectrum analyzer is constant in occupied bandwidth.
IMBE vocoder 300 - 3400 Hz Voice A2D 4400 bps Voice information

4800 Symbols
DiBit 00 DiBit 01

Nyquist Filter

9600 bps
2400 bps Signaling 2800 bps Error Correction

12.5kHz BW 25kHz BW

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P25 Modulation (9600 Baud / 4800 Symbols)
 C4FM Modulation for Project 25 Phase I DiBit 10  4 Level FSK (Phase Only)
Information 01 00 10 11 Symbol +3 +1 - 1 - 3 Deviation + 1.8kHz + 0.6kHz - 0.6kHz - 1.8kHz
DiBit 00 DiBit 01

DiBit 11

 CQPSK Modulation for Project 25 Phase II  Compatible Differential Quadrature Phase Shift Keying (Phase and Amplitude)
Information 01 00 10 11
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Symbol +3 +1 - 1 - 3

Phase change + 1350 + 450 - 450 - 1350

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P25 - Logical Message Structure
Header
The Header is not encrypted

LDU1

LDU2

LDUn

Terminator Terminator Data Unit -Modified Link Control Word

SUPERFRAME 360 msec Link Control Low Speed Data Encryption

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

180 msec

IMBE Frame = 20 msec w/ 88 bits of info + 56 bits parity check (=4.4bps)

Logical Link Data Unit (LDU) LDU1 Header - 648 bits w/ 120 bits of info containing - Message Indicator (Encryption Algorithm) - Manufacturer’s ID - Algorithm ID - Key ID - Talkgroup ID
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LDU2

Link Control (LC) - 240 bits w/ 72 bits of info containing - Talkgroup ID - Source ID - Destination ID - Emergency Indicator - Manufacturer ID

Encryption - 240 bits w/ 96 bits of info containing - Message Indicator - Algorithm ID for encryption - Key ID for encryption key

Low Speed Data - 89.2bps currently not in use - possible application would be transmission of accurate geographic location information

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How does Digital P25 Impact Testing Requirements
 Power  Frequency  Analog Modulation Accuracy

 Digital C4FM Modulation Accuracy
 P25 Test Signals  Analog Receiver Sensitivity  Digital P25 Receiver Sensitivity

 Protocol Testing
 Harmonics

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Power Measurements
 Power measurements for Digital

C4FM can be made with a standard peak detecting power meter the same as with analog FM.
 C4FM digital transmissions are of

constant amplitude.

 Power measurements for Digital

CQPSK cannot be made with a standard power meter. The signal must be decoded through Digital Signal Processing and calculated.

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Frequency measurements for Digital modulation
 C4FM Frequency measurements can be

made with a standard frequency counter but may appear to be unstable unless the radio is configured to transmit a “LOW DEVIATION PATTERN” as described in the ANSI/TIA/EIA102CAAA standard.  CQPSK frequency measurements must be made through Digital Signal Processing where the signal is decoded and the frequency is calculated. A standard frequency counter is not usable for this type of signal.
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Measuring Analog Modulation Accuracy
 A deviation meter is used to measure

analog FM Modulation Accuracy.
 The deviation meter monitors peak

carrier movement above and below the carrier center frequency and displays the average offset.
 Various IF Bandwidth filters are

required to ensure accurate measurements.

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Selecting an appropriate IF Bandwidth
 With a Spectrum Analyzer

measure the bandwidth of the modulated signal 26 dB down from the signal peak and add 20%.
 The test receiver’s IF bandwidth

-26 dBc

should be at least this wide.  IF BW too narrow causes distortion or compression.  IF BW too wide allows excessive noise.

Required BW + 20%

 With constant rate use

Carsons Rule: 2 * (Rate + Deviation)
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Measuring P25 C4FM Modulation Accuracy
 Testing modulation accuracy of a P25

digital signal can be done with two methods.  Deviation Meter Method:
 Measurements can be made with a deviation

meter if the radio is configured to a defined test pattern where the radio is transmitting all high or all low bits.  Problems arise because a deviation meter measures only peak deviation and the measurement is not synchronized with the symbols.

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Measuring P25 C4FM Modulation Accuracy
 A Sampling Method takes deviation

measurements at specific time intervals.
 Measurements are taken synchronously

+1.8

+0.6
-0.6 -1.8

with the decoded symbol clock.  Measurements are averaged over a group of symbols and a percentage of error is calculated.  The measured deviation is only important at symbol time.  The sampling method requires no special setups.

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Analog Receiver Sensitivity
 Receiver sensitivity in Analog

systems is a measure of the amount of RF signal level that is required to produce an intelligible audio when demodulated.
 The measurement device used

in analog systems is typically the SINAD meter which shows a ratio in dB of:
Signal + Noise + Distortion Noise + Distortion.
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P25 Digital Receiver Sensitivity
 The term used to quantify a digital

receivers performance is BER (Bit Error Rate).
 The test must be conducted with a known

test signal such as the 1011 pattern.  Radio specific software and a computer may be required to evaluate a decoded 1011 tone. The radio is placed into a special test mode for this test.  Another method is to listen to the decoded audio.  A digital signal will stop decoding abruptly when the BER becomes high.

The IFR 2975 provides a SPEECH encoder with defined messages to test the vocoder at any TGID, SID or NAC configuration.

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Testing Protocol (CAI)
 Protocol testing requires the ability to

decode the 9600 baud data into voice and data information.
 After decoding, the data bits are

reconstructed into their original message elements.

 Decoding allows quick verification of

the radio programming.
 Allows testing of EMERGENCY facility

off the air.

 Decoding can also be used to

evaluate Transmit BER.

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IFR-2975 Overview

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Powerful Processing Features
 Pentium Processor  Internal Storage  Hard Drive  3.5” Floppy  Display  Color LCD  Sunlight Readable  Portable Rugged Design  All Digital System (DSP)  Field Upgradeable

33 lbs.

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Rear Panel I/O
 Ethernet (10T/100T)  2 USB ports

 Printer port
 RS-232  GPIB  VGA Output

 Keyboard input
 Mouse port  Universal Power Supply  110/220 operation

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Remote Access

 X-Windows operating under VX Works  IP Addressable  Network Accessible  Route Display and Keyboard to a remote computer  TCL / TK scripting language support

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Legacy Support

 Power Meter (Wide and Narrow band)
 Deviation, Distortion, SINAD  RF and Audio Counters  Digital AC/DC Voltmeter

 Quad Function Generators
 FM Modulation  600 Ohm Balanced / Unbalanced Audio Input / Output

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Extended Features
 2.7 GHz frequency coverage (1 Hz resolution)
 Generate (+10 to -137 dBm Output level)  Receive (Protected Input)  Crossband Duplex (Encode / Decode

Simultaneously)  Signal Tracing  High Stability Reference Oscillator 0.01 PPM

 High Resolution Spectrum Analyzer  +10 dBm Tracking Generator  Dual Input Digital Storage Scope

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Project 25 Unique Features
 C4FM Modulation Analyzer  DES OFB Type III  Encryption / Decryption  IMBE Vocoder (Encode /

Decode)
 Embedded Voice Speech patterns
 Allows receiver testing without special test modes.

 C4FM Modulation Source  Common Air Interface Testing  Decode Uplink Data  Encode Downlink Data

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Flexible Operator Interface
 The operator interface

allows the flexibility to choose what meters are available on each screen  Both the Spectrum and Scope can be viewed simultaneously

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IFR-2975 Functional Test Screens

 Basic Operation  Generate (RX Test)  Receive (TX Test)  Duplex  Function Generator

 Scope
 Spectrum Analyzer  Project 25 Repeater Simulator  System Configuration

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Functional Test Screen Selections
 Operational Mode selections

are made by pressing the MODE key to access a pull down selection menu.  OR position the cursor to the Function Selector and press the ENTER key.  OR mouse click on the Function Selector.  Enter a menu item number or use data scroll features and the ENTER key.

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Selecting Function Options
 Function Option selections

are made by pressing the SHIFT + MODE keys to access a pull down selection menu.  OR position the cursor to the Function Option Selector and press the ENTER key.  OR mouse click on the Function Option Selector.  Enter a menu item number, use data scroll features and the ENTER key.
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Generate (RX Test)
 Signal Generator  FM / C4FM (P25)

 Receiver Stimulus
 Dual Modulation sources  Distortion / SINAD meter  Scope

 DVM

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Receiver Tests

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Receive (TX Test)
 Off Air Monitor  Transmitter Test

 Power Meter  50 W Cont.  125 W 1 on / 4 off
 Deviation Meter

 Distortion Meter
 Scope  DVM  FM / C4FM (P25)

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Transmitter Tests

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Duplex (RX / TX Test)
 Simultaneous Generate and   



Receive operation. Cross-band Repeater testing Signal Tracing All in one radio test screen FM / C4FM (P25)

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Function Generator
 Base-band Audio Tests  Dual Function Generators

 DVM
 AF Counter  Distortion Meter  SINAD Meter

 Balanced / Unbalanced

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Scope
 Dual channel Ext. Input  Monitor Audio In

 Monitor MIC

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Spectrum Analyzer
 Full Span 2.7 GHz  Independent controls

 Video Filters
 Tracking Generator

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Cable Fault measurement
 The Tracking Generator

level may be adjusted to place the dips closely to the center horizontal line to allow use of the grid markings.
 The Center Frequency may
20 MHz / Div.

200 MHz

5.4Divisions x 20 MHz per division = 108 MHz

be adjusted to position one of the dips closely to a vertical grid line.

Distance feet =

492 * Velocity Factor Delta Freq. in MHz 492 * 0.659 108

3.03 ft =

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P25 Repeater Simulator
 Encode UPLINK data  Decode DOWNLINK data

 Receiver test patterns
 Transmitter Modulation

Accuracy test.

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System Configuration
 Configure IP address  Redirect 2975

operations to a network computer.  RS-232 settings  GPIB address  Internal / External Reference.

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