Balloons Gen 1 Radio

Macaw is the first generation of a VHF/UHF transmit/receive board for Balloons Core Avionics. The purpose of the radio board is to provide an alternate means of transmitting payload vitals to the ground, in addition to RockBlock (which requires buying credits to transmit data). In addition, the radio board provides a much higher data rate than Iridium, which can be used to transmit payload vitals more often, and larger data (such as images or video).

Design

Macaw has three DORJI radios, which can be either DRA818Vs (which can transmit from 134MHz to 174MHz in the Very High Frequency(VHF) Band) or DRA818Us (which can transmit from 400-470MHz in the Ultra High Frequency (UHF) Band). Each radio also has a current sensor to monitor the amount of current being drawn from that radio. In Macaw all functionalities of the DORJI radios are supported, which means that any of the radios can be transmit or receive radios, and VHF or UHF radios, although one is designated as the APRS radio (Automatic Packet Reporting System), and the other two are designated as VHF Transmit and VHF Receive. The APRS radio is responsible for transmitting the payload vitals to the ground, while the other two are reserved for higher data-rate applications; however, in the board the hardware for each radio is identical and any VHF radio can transmit or receive any data.

Schematics

• 

  Top Sheet
• 

  Radio 1/APRS
• 

  Radio 2/RX
• 

  Radio 3/TX
• HAB Macaw StackBus.PNG

  Stackbus Connections
• 

  PCB Layout
• 

  Assembled

Pinout

-APRS = Radio 1; Bottom

-VHFRX = Radio 2; Middle

-VHFTX = Radio 3; Top

Serial Ports: The communication interface between the radio and the microcontroller; used to set settings (ie transmit frequency). RX here is in the frame of the Teensy (MCU RX; radio TX).

Pin	APRS	VHFRX	VHFTX
RX	7(RX3)	9	0
TX	8 (TX3)	10	1

MIC(Microphone): The signal input to the radio transmitter.

Pin	APRS	VHFRX	VHFTX
	DAC/A14	22/A8	32

PTT (Push-To-Talk): The signal which enables or disables transmission

Pin	APRS	VHFRX	VHFTX
	6	13	25

AF_OUT (Audio Out): The output of the radio receiver.

Pin	APRS	VHFRX	VHFTX
	14/A0	17/A3	28/A17

SQ:(Squelch): Signal for setting the squelch level (threshold for suppressing noise in received signals)

Pin	APRS	VHFRX	VHFTX
	5	12	16/A2

PD/SLP (Sleep): Signal for setting the radio to low power mode

Pin	APRS	VHFRX	VHFTX
	20/A6	23/A9	15/A1

RFPWR (RF Power): Signal for setting the radio to .5W output power or 1W output power

Pin	APRS	VHFRX	VHFTX
	21/A7	19/A5	27/A16

EN (Enable): Signal for driving a FET switch to power or unpower each radio (radio is powered if this is LOW)

Pin	APRS	VHFRX	VHFTX
	2	11	18/A4

Other System Flags

LOW PWR (Flag from Main Avionics for asserting Low Power Mode)- 24

System Ready (Flag from Main Avionics for enabling startup) - A10

F3- A11

F4- A12

F5- A13

I2C

SCL - 29/A18

SDA - 30/A19

CHIRP (Serial Communication with Main Avionics). Uses the alternate pins for HWSerial 2 on the Teensy 3.2. Radio 2 Serial will be disabled during use.

RX- 26/A15 (RX2 Alt)

TX- 31/A20 (TX2 Alt)

Issues and Future Design Ideas

• Number and Kind of Radios

One issue with having three VHF radios on one board is the potential for one to interfere with the others if the transmit and receive frequencies are too near one another (crosstalk). A filter between the antenna and radio could solve this, however it requires additional space and components. If interference is significant then a receiving radio would not be able to receive while another is transmitting. In effect this would be no different than having one radio operating half-duplex; components, space, and pins could be saved by removing one radio entirely; the second radio could be changed to UHF. Additional pins and board space allows for more interfacing with the main avionics and other functionality, such as data logging or image/video recording.