GPS Frequency Standard

Build V2 Features

Stand-alone design does not require PC. Generates a 10 MHz output after satellite lock.

• Hackaday project page
• NEO-7M GPS
• QTPy49 card
  • CircuitPython code fits into QT Py (SAMD based) but a QT Py (RP2040 based) could be used
  • 3.3V, 48 MHz
• Modified PulseGen card
  • Oscillator circuit removed (not used as pulse generator)
  • Receives input from GPS output and drives 50 Ohm output on BNC
• Status LED
  • Blinks in for 1 second at power up
  • Goes off until serial communications are working
  • Blinks on-off as data is received from GPS while waiting for Satellite signal lock
  • LED is on solid once output is enabled
• Fits into Aluminum Project Box Enclosure DIY 100*76*35mm - ebay search
• Inspired by Scullcom design

Wiring

• Dupont headers
  • GPS = 5-pin
  • LED = 3-pin
  • UART = 4 pin

Signal	From	To	Color
GND	QTPy49 J4-1	GPS pin 4	Black
VCC	QTPy49 J4-2	GPS pin 5	Red
QTPy>GPS	QTPy49 J4-3	GPS pin 2	Blue
GPS>QTPy	QTPy49 J4-4	GPS pin 3	White
GND	QTPy49 J5-1	PulseGen GND	Black
VCC	QTPy49 J5-2	PulseGen VCC	Red
PPS	GPU pin 1	PulseGen In	Purple
LED-VCC	QTPy49 J9-3	D0	Red
LED-GND	QTPy49 J9-1	GND	Black

Code

• Code runs on either QT Py (SAMD based) or QT_Py_(RP2040_based) (either SeeedStudio XIAO or Adafruit QT Py cards
• FreqStdCap.ino - Tool that runs on Arduino to generate frequency bytestrings to send to GPS
• GPS_004.py - Final working code
  • Saved as code.py to auto-run
• GPS_003.py - Capture data from GPS module a line at a time - helpful for debugging
• GPS_002.py - Read 32 bytes at a time
• GPS_001.py - Formatted stings output

Build V1

• Build Version 1
• Requires PC running ublox software on PC to set output.

GPS

• GPS Satellite Positioning Module NEO7M UBLOX
• Specifications:
• Working Voltage: 3.3v-5.0v
• Color: As Picture Shown
• Description:
• With micro USB interface, you can debug GPS module with ordinary phone data cable, no longer need to use USB-TTL and other tools.
• TTL interface is reserved to facilitate the use of USB-TTL or external microcontroller to control and receive GPS information.
• Ceramic antenna on module board, no GPS external antenna can be used to search for stars.
• Reserved SMA antenna interface, can use external antenna, star search ability is stronger.

50 Ohm Driver

• Uses modified PulseGen card to drive 50 Ohms at 3.3V level output
  • Remove oscillator circuit parts (R/C)
  • 5 parallel 74AC14 outputs with 249 ohm series resistors on each output
  • Only 1 BNC high quality (Amphenol) connector
  • Wire from GPS PPS pin attached as below
  • Use 1x2 pin header for power instead of 5MM Terminal Blocks
• Able to drive long 50 ohm coax
• Square wave output
• Very fast edges

• Schematic

Enclosure

• Fits into Aluminum Project Box Enclosure DIY 100*76*35mm - ebay search
• White-on-Black P-Touch labels

Reference Materials

• Design & Build a GPS locked Frequency Standard- Scullcom
• SIMPLE 10 MHZ GPS FREQUENCY STANDARD AND RF GENERATOR
  • uses Arduino Nano as FTDI replacement
• An Arduino-based GPS Disciplined Oscillator
• An Arduino Version of Brooks Shera's GPSDO
• Simulating the Brooks Shera (W5OJM) GPSDO Algorithm