GPS Disciplined Oscillator

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					               par projekta 2010/0283/2DP/ “Daudzfunkcionāla
               signālu laika analizatora eksperimentāla izstrāde" tekošajiem rezultātiem

     Augstas stabilitātes GPS pieskaņojamās atbalsta
     frekvences ģeneratora moduļa izstrāde un izpēte

Projekta zinātniskais vadītājs Jurijs Artjuhs                                            Referents Armands Mezeriņš

                                    Elektronikas un    Institute of
                                       Datorzinātņu    Electronics and
                                           Institūts   Computer Science

Informatīvais seminārs, Rīga, 2012. gada 29.maijs                                                1
  Subject and objectives of research

                          (detector of zero crossings,

                            controllable prescaler)

                              Signal Conditioner

                                                         EVENT TIMER

                                                         Time Analyser

               The performance of Frequency Standard directly defines
                           accuracy of the Time Analyser

  Research Objectives:
  • Comparative analysis of available high-stability clock oscillators
  • Design and investigation of Frequency Standard module

Informatīvais seminārs, Rīga, 2012. gada 29.maijs                                   2

Commonly used built-in clock oscillators:

• Temperature Compensated Crystal Oscillators
  (Stability: 1 ppm - over operating temperature range)
• Oven Controlled Crystal Oscillators
  (Stability: 10 ppb)
• Rubidium Oscillators
  (Stability:  0.05 ppb)

  Main drawbacks:
  • Noticeable long-term instability
  • Aging
  •Requires regular calibration against a traceable reference
  with a high long-term accuracy

      Currently GPS disciplining the clock oscillators allows considerable
                           reducing these drawback

Informatīvais seminārs, Rīga, 2012. gada 29.maijs                            3
  GPS Disciplined Oscillator Basics

    Space based navigation system (such as NAVSTAR GPS) provides 3D position,
    velocity, precise time (traceable to UTC) and high accuracy frequency worldwide.

    GPS disciplining technique uses the signals obtained from a GPS receiver as a time
    and frequency reference to synchronize, calibrate or control a frequency source.

    In a simple GPS Disciplined Oscillator the voltage controlled oscillator VCO (10 MHz)
    signal is divided to a lower frequency prior to phase comparison with 1PPS derived
    from GPS receiver. A microcontroller monitors the phase difference, and software
    changes the control voltage applied to the VCO to held phase difference within an
    appropriate range.

Informatīvais seminārs, Rīga, 2012. gada 29.maijs                                        4
       Commercially available GPS Disciplined Oscillators

 Trimble Mini-T GPS Disciplined Clock board        ULN-2550 GPSDO                           FTS375 Disciplined Reference                                   

Trimble Thunderbolt E GPS Disciplined Clock       Fury GPS Disciplined Frequency Standard             FTS500 XenithTBR

                                                         GPS-12R Portable GPS Rubidium Frequency Standard


    Informatīvais seminārs, Rīga, 2012. gada 29.maijs                                                                5
   Frequency stability of some commercial GPS Disciplined Oscillators

                                    Allan Deviation
 0.01     0.1              1   10     100      1000     10000

                                                                Trimble Thunderbolt E

                                                                Trimble MINI T GPS Disciplined
                1.00E-10                                        Clock Board
                                                                Fury GPSDO frequency standard


                                                                FireFly-IIA GPSDO DOCXO
                                                                FTS375, FTS500


• Allan Deviation σ(τ) is a stability metric which shows the
 RMS frequency change we can expect over a τ second interval.

• Frequency deviations measured at 10 MHz relative
 to an atomic frequency standard.

• A stability of 1 × 10-13 or less after one day of averaging
indicates a device of very high quality.

Informatīvais seminārs, Rīga, 2012. gada 29.maijs                                                5
  GPS Disciplined Oscillator Module Design

 The module consists of :
 • FTS375-010.0M hybrid circuit – 10MHz CMOS and 10MHz phase-lock loop circuit
   synchronized with GPS signal
 • PIC18F24K20 microcontroller unit - provide initial setup and monitoring functions;
 • MAX202 transceiver circuit for RS232 to TTL level conversion

Informatīvais seminārs, Rīga, 2012. gada 29.maijs                                       6
  Experimental estimation of the module stability
                                                                         FREQUENCY STABILITY
                                                                                                         GPS Disciplined State
                                                                                                         Unlocked State (Holdover)

         Allan Deviation


                                       -2              -1                  0                    1              2                      3
                                  10                 10                  10                   10             10                      10
                                                                       Averaging Time tau, seconds

                                                Locked versus unlocked state of GPSDO module

                                                                          FREQUENCY STABILITY

           Allan Deviation


                                            Pendulum GPS-12R Rubidium Frequency Standard
                                            GPS Disciplined Frequency Reference module
                                            Trimble Thunderbolt GPS Disciplined Clock
                                    -2                    -1                   0                     1              2
                                  10                  10                  10                     10                10
                                                                        Averaging Time tau, seconds

                                             GPSDO module versus Trimble Thunderbolt E GPS Clock

Informatīvais seminārs, Rīga, 2012. gada 29.maijs                                                                                         7
  GPS receiver data using NS3Kview software

    NMEA Command Monitor Window - all NMEA data sentences received from the GPS receiver

     Navigation window – current time, clock drift, longitude, latitude and altitude or elevation

                                        Satellite Azimuth & Elevation Window – azimuth and
                                        elevation for each satellite that is in view

Informatīvais seminārs, Rīga, 2012. gada 29.maijs                                                   8

   GPSDO is self calibrating reference with an excellent long-term stability.

   Application of GPS Disciplined Oscillator Module as time-base in
  analyzer provides possibility for accurate and stable long-term
  measurements (frequency stability is in the range from 4·10-11 at 300s to
  10-13 at 1 day).

   For short-term measurements GPS disciplining may be turned off (time
  intervals less than 300 seconds) .

   Timing data from GPS receiver and synchronization with UTC may
  provide time tagging of measured events in real time scale.

Informatīvais seminārs, Rīga, 2012. gada 29.maijs                                9

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