Rev0Trac UTx

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rev0Trac VTx, as modeled in Autodesk Inventor.

The rev0Trac UTx APRS Transceiver is an all-in-one unit containing a high-altitude capable GPS, 1W transceiver covering the US 70cm amateur band, LCD and user interface, xxxxmAH Lithium Polymer battery, and USB interface for charging and firmware updates. The transceiver measures 73 x 41 x xxmm without antenna and weighs xxxg.

rev0Trac VTx v1.0 Schematic.
rev0Trac VTx v1.0 PCB Layout.

Features

  • Based around the DRA808M 400-470MHz 1W UHF transceiver
  • Covers US 70cm band, supporting 38 CTCSS codes, and selectable Tx/Rx split over any two frequencies in the operating range
  • 128*64 Graphic LCD
  • Full 16-key interface (0-9 plus *, #, A-D) for ease of interface including direct frequency entry
  • Ambidextrous (menu-selectable) PTT and Monitor buttons
  • Re-programmable and rechargable via micro-USB
  • Battery gauge
  • 100-Lumen Cree LED flashlight
  • Four programmable IO pins: 2 Analog/Digital IO, 2 Digital IO
  • Software DTMF encoding/decoding for OTA control
  • Internal 1000mAH LiPo battery

3D Model

The design of the rev0Trac VTx involved the creation and evolution of a 3D model, made in Autodesk Inventor.

Electrical Design

The system design follows the core design of the rev0Trac VTx, utilizing the DRA808M 1W UHF transceiver module for the RF functions. The microcontroller is an MSP430F5508, which takes in data from a U-blox 6 based GP-635T GPS with a 16-key/graphic LCD user interface. The battery is charged through a dedicated LiPo charger IC at 500mA (standard USB port) or up to 1.6A (dedicated charging port), determined automatically. The analog modulation signal is controlled through a 12-bit TI DAC7311 digital to analog converter, or an electret microphone.

Testing/Debug

5/17/2012 - The microcontroller section was assembled and the device was plugged into a computer in bootloader mode to verify correct operation of the MCU section.

5/30/2012 - The transmitter was tested at a battery voltage of 7.78V on the CXA N9000A spectrum analyzer to measure harmonics, bandwidth, and output power. The results are summarized below:

Specified power Carrier power 2nd Harmonic power 3rd Harmonic power 4th Harmonic power
5W 37dBm -30.00dBm -46.94dBm -53.72dBm
1W 30.42dBm -39.54dBm -56.64dBm -52.51dBm
.5W 27.81dBm -45.39dBm <-60dBm <-60dBm
.1W 20.91dBm -51.40dBm <-60dBm <-60dBm

Harmonics are more than -67dBc, easily meeting the FCC specification of -40dBc and spurious power <25uW for a <25W transmitter.

5/31/2012 - The program was modified to allow control of the output power via the push switch, cycling through 0.1, 0.5, 1, and 5W. Pre-emphasis was added to the 2200Hz tone to improve the copy on standard FM receivers. The amplitudes of the two waveforms are: .217V 1200Hz, .52V 2200Hz. The program was hard-coded to transmit on 144.390MHz so the packets can be put on APRS-IS via a temporary IGate using AGWPE and UI-View32. The LCD displays the current frequency (as a variable, so changes to frequency will be reflected on screen).

6/2/2012 - A new revision PCB was routed to include the fixes outlined below:

  • Added trace from MCU to LCD RST line
  • Added trace from MCU to LCD D/C line
  • Added trace from MCU to LCD backlight PMOS
  • Added 1k resistor to ground on VUSBR line
  • Added tri-state buffer and header to accommodate an external GPS or serial UART connection
Revised rev0Trac VTx schematic.
Revised rev0Trac VTx PCB layout.

Photos