Frequency Calibration via GPS

Inexpensive Frequency Standard with output synchronized to GPS satellites.

[2 build is here]

Features

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

Parts

GPS

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

U-Center Programming Software

• TP5 sets PPS output to "any" frequency
• U-Blox website
  • U-Center software
• 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

Remote Amplified GPS Antenna

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

FTDI

• 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

50 Ohm Driver (Not Required - nice to have)

• 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

Case

• Cabinet Door Bumpers - used as inexpensive rubber feet
  • search on Amazon

Connectors

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

Calibrating Frequency Counters

• Two very cheap frequency counters

Videos

• This is the approach I followed

• GPSDO vs Rubidium Frequency Standard