Frequency Calibration via GPS

From Land Boards Wiki
Jump to navigation Jump to search

Inexpensive Frequency Standard with output synchronized to GPS satellites,

FreqSrc-Front-01-720px.jpg

Features[edit]

  • Generates very accurate frequency output
  • U-Blox NEO-7M GPS Module
  • Remote Amplified GPS Antenna
  • FTDI-49MM
  • 50 Ohm Output
  • Metal enclosure

Parts[edit]

GPS[edit]

  • U-Blox NEO-7M GPS Module
    • Programmable frequency from USB
  • SMA Female connector
  • 5V operation (power)
  • 3.3V Rx/Tx/PPS levels

NEO-7M-02.PNG

NEO-7M-04.PNG

NEO-7M-05.PNG

U-Center Programming Software[edit]

  • TP5 sets PPS output to "any" frequency
  • U-Blox website
  • Set to 8 MHz on GPS PPS pin when locked
    • Other frequencies have some "jitter" but are still GPS accurate
  • Set to 2 Hz on GPS PPS pin when not locked

U-center-10MHz.PNG

Remote Amplified GPS Antenna[edit]

  • 20 dB amplified output
  • SMA male connector connects to GPS module without adapter
  • Magnetic mount
  • Locate outside

GPS Antenna.jpg

FTDI[edit]

  • FTDI-49MM USB to serial interface
  • U-Center communicates with the GPS over this interface
  • Tx/Rx LEDs
  • USB signal voltage level jumper set to 3.3V to match Tx/Rx on GPS
  • 5V header provides 5V to GPS card
  • 49x49mm

FTDI-P1746-CROPPED-720PX.jpg

50 Ohm Driver (Not Required - nice to have)[edit]

  • Used modified PulseGen card to drive 50 Ohms, 5V level output
    • Remove oscillator circuit parts (R/C)
    • 5 parallel 74AC14 outputs with 249 ohm series resistors on each output
  • Able to drive long 50 ohm coax
  • Square wave output
  • Very fast edges

PulseGen 5242-720px.JPG

Case[edit]

FreqSrc-Top-01-720px.jpg

FreqSrc-Back-01-720px.jpg

Connectors[edit]

  • OUT = Frequency source output
    • 5V, 50 ohm
  • USB
  • ANT - external amplified GPS antenna

Calibrating Frequency Counters[edit]

  • Two very cheap frequency counters

FreqSrc-Counter-01-720px.jpg

Videos[edit]

  • This is the approach I followed

  • GPSDO vs Rubidium Frequency Standard