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)
    • QT Py (RP2040 based) could be used as alternative
  • 3.3V, 48 MHz
• DRIVE50 - 50 Ohm coax card
  • 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

Hardware

QTPy49 Card

• CPU choices are
  • QT Py (SAMD based)
    • Adafruit QT Py SAMD21 Product Page
    • Seeeduino XIAO
    • 48 MHz
  • QT Py (RP2040 based)
    • Adafruit QT Py RP2040 Product page
    • Seeed XIAO RP2040 Product page
    • 133 MHz
  • XIAO ESP32C3
    • SeeedStudio XIAO ESP32C3 (New 2022-07)
    • RISC-V Single Core
    • Up to 160 MHz
    • 400 KB SRAM
    • 4 MB Flash
    • 2.4GHz Wi-Fi subsystem
    • Wiki
    • ESP32C3 Datasheet
• Connectors/Headers
  • DC Power
  • AD0 GVS
  • AD1 GVS
  • AD2 GVS
  • AD3 GVS
  • I2C with ground/VCC
  • TX/RX with ground/VCC
  • SPI with ground/VCC
• QTPy49 can be powered from with either the USB C or DC jack
  • Automatic Schottky diode power switching between
    • 5V from USB C
    • 5V on external DC jack
    • 7-12V on DC jack
• 49x49mm ODAS form factor
• (4) 4-40 mounting holes

GPS Card

• GPS Satellite Positioning Module NEO7M UBLOX Module
  • EBay search for NEO7M
• 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 Card

• Uses DRIVE50 50 Ohm Driver
  • Able to drive long 50 ohm coax
  • 5 parallel 74AC14 outputs with 249 ohm series resistors on each output
  • Square wave output
  • Very fast edges
  • BNC is high quality (Amphenol) connector

Alternate (Previous) Driver Design

• Previous design used modified PulseGen card to drive 50 Ohms at 3.3V level output
  • Remove oscillator circuit parts (R/C)
  • 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

• Schematic

Wiring

• Dupont headers
  • GPS = 5-pin
  • LED = 3-pin
  • UART = 4-pin
  • DRIVE50 = 3-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	DRIVE50 J1-3 GND	Black
VCC	QTPy49 J5-2	DRIVE50 J1-2 VCC	Red
PPS	GPU pin 1	DRIVE50 J1-1 In	Purple
LED-VCC	QTPy49 J9-3	D0	Red
LED-GND	QTPy49 J9-1	GND	Black

Build V1

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

Software

• Code runs on multiple CPUs on QTPy49
  • QT Py (SAMD based) or
    • SeeedStudio XIAO or
    • Adafruit QT Py
  • QT_Py_(RP2040_based)
    • SeeedStudio XIAO
    • Adafruit QT Py
• 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

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